| 1. |
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| 2. |
- Björck, Svante, et al.
(författare)
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Anomalously mild Younger Dryas summer conditions in southern Greenland
- 2002
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 30:5, s. 427-430
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Tidskriftsartikel (refereegranskat)abstract
- The first late-glacial lake sediments found in Greenland were analyzed with respect to a variety of environmental variables. The analyzed sequence covers the time span between 14400 and 10500 calendar yr B.P., and the data imply that the conditions in southernmost Greenland during the Younger Dryas stadial, 12800-11550 calendar yr B.P., were characterized by an arid climate with cold winters and mild summers, preceded by humid conditions with cooler summers. Climate models imply that such an anomaly may be explained by local climatic phenomenon caused by high insolation and Fohn effects. It shows that regional and local variations of Younger Dryas summer conditions in the North Atlantic region may have been larger than previously found from proxy data and modeling experiments.
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| 3. |
- Felitsyn, S, et al.
(författare)
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Nd isotope composition and rare earth element distribution in early Paleozoic biogenic apatite from Baltoscandia : A signature of Iapetus ocean water
- 1998
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 26:12, s. 1083-1086
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Tidskriftsartikel (refereegranskat)abstract
- Analyses of the Nd isotopic composition and REE distribution in biogenic apatite (organophosphatic brachiopods and conodont elements) from the Cambrian and Ordovician sequences of the Baltic plate give new insights into the development of the southeastern segment of the continental margin bounding the Iapetus ocean. The Nd isotope analyses show ϵNd(t) of ∼−8.0 for the Cambrian, indicating that the main source of the sedimentary deposition came from weathered sedimentary rocks of Vendian and Cambrian age. The increase of ϵNd(t) to ∼−5.0 for the Early Ordovician indicates the appearance of a new source of radiogenic Nd in the surrounding area—most likely a volcanic arc along the western borderland of Baltic plate from Arenigian time. Samples of Cambrian biogenic apatite show significantly lower total amounts of REE than do the Ordovician samples, and this is probably due to a shorter exposure to seawater before burial during Ordovician sedimentary accumulation in Baltoscandia. These preliminary results suggest that biogenic apatite from the Baltoscandian basin preserves geochemical signatures of the water masses that will be important for understanding the evolution of the Iapetus ocean during the early Paleozoic.
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| 4. |
- Henriksson, Anders S., et al.
(författare)
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Dimethylsulfide production variations over the past 200 k.y. in the equatorial Atlantic: A first estimate
- 2000
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 28:6, s. 499-502
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Tidskriftsartikel (refereegranskat)abstract
- Dimethylsulfide (DMS) released from phytoplankton produces cloud condensation nuclei in the marine troposphere, thereby providing a climate-regulating mechanism by increasing cloud albedo. A 200-k.y.-long sedimentary record from the equatorial Atlantic of DMS-producing phytoplankton suggests that DMS production increased during the glacial stages, thus potentially working to further cool the glacial world. Important factors limiting the operation of this possible feedback mechanism for planetary cooling may be the strength of trade winds and the availability of fine, iron-rich dust needed to fertilize growth of DMS producers.
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| 5. |
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| 6. |
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| 7. |
- Mulugeta, Genene, et al.
(författare)
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Three-dimensional geometry of piggyback thrusting
- 1987
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Ingår i: Geology. - : Geological Society of America. - 0091-7613 .- 1943-2682. ; 15:11, s. 1052-1056
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Tidskriftsartikel (refereegranskat)abstract
- The three-dimensional geometry and kinematics of piggyback stacks of imbricate thrust sheets are illustrated and discussed using a single model shortened in a squeeze box. Strike-parallel geometric elements simulated include lateral ramps, eyed sheath folds, splays, and thrust/thrust interference. Fine details of these structures were exposed by eroding a shortened wedge of sand using a newly developed vacuum-eroding technique. A kinematic analysis of the model shows a stepwise increase in imbricate thrust spacing and/or a decrease in rate of nucleation of imbricate thrusts in the direction of thrust transport. Despite the steady forward advance of a rear wall, the piggyback wedge accreted episodically, recording different strain domains in longitudinal cross sections. Strain partitioning in single layers by bed-length balancing showed an increase in layer shortening with volume loss and a corresponding decrease in imbricate thrusting and ramp folding with depth.
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| 8. |
- Rönnlund, p., et al.
(författare)
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Fry spacing of deformed and undeformed modelled and natural salt domes
- 1988
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 16:5, s. 465-468
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Tidskriftsartikel (refereegranskat)abstract
- Fry's center-point spacing strain analysis is applied to experiments of gravity-driven overturn of horizontal fluid layers on scales of centimetres or decimetres, and to natural salt diapirs on scales of kilometres to tens of kilometres. Laboratory experiments in which about 100 diapirs formed from initially plane horizontal layers resulted in laterally isotropic spacing and yielded an open circle on a Fry plot with a radius equal to the wavelength (W). Lateral deformation and initial departures from horizontal layers with uniform properties may influence W. Sample experiments of diapirism with and without lateral deformation are compared to natural equivalents.The spacing of post-Late Triassic diapirs in the Zechstein salt of Germany and of post-Jurassic diapirs in the Hormuz salt of Arabia yields circles on Fry plots which indicate that these gravity structures developed without additional lateral forces. Fry plots of the spacing of salt diapirs in the Zagros Mountains define a strain ellipse with the long axis (= W in Arabia) parallel and the short axis perpendicular to post-Miocene regional fold axes. The ratio of the Zagros strain ellipse is about 1.7, which suggests a northeast-southwest shortening of 41%, rather than 10%-21% as estimated from folding alone. This mismatch may be a result of unquantified shortening due to thrusts and/or layer-parallel shortening. Center-point spacing of post-Late Triassic domes of Zechstein salt in the central North Sea also gives an ellipse. Here, the long axis coincides with the direction of Middle Jurassic to Cretaceous extension. The axial ratio of the North Sea strain ellipse is about 2.8, compared to estimates of about 1.5-2.0 by Sclater and Christie, and 1.8 by Wood and Barton.
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| 9. |
- Almqvist, Bjarne, et al.
(författare)
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Bulk strain in orogenic wedges based on insights from magnetic fabrics in sandbox models
- 2018
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 46:6, s. 483-486
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Tidskriftsartikel (refereegranskat)abstract
- Anisotropy of magnetic susceptibility (AMS) analysis is used as a petrofabric indicator for a set of four identical-setup sandbox models that were shortened by different amounts and simulate contraction in a fold-and-thrust belt. During model shortening, a progressive reorientation of the initial magnetic fabric occurs due to horizontal compaction of the sand layers. At the early stages of shortening, magnetic lineation (k(1) axis) rotates parallel to the model backstop with subhorizontal orientation, whereas the minimum susceptibility (k(3) axis) is subvertical, which indicates a partial tectonic overprint of the initial fabric. With further shortening, the k(3) axis rotates to subhorizontal orientation, parallel to shortening direction, marking the development of a dominant tectonic magnetic fabric. A near-linear transition in magnetic fabric is observed from the initial bedding to tectonic fabric in all four models, which reflects a progressive transition in deformation from foreland toward hinterland. Model results confirm a long-held hypothesis where the AMS pattern and degree of anisotropy have been suggested to reflect the amount of layer-parallel shortening, based on field observations in many mountain belts. Results furthermore indicate that grain rotation may play a significant role in low-grade compressive tectonic regimes. The combination of analogue models with AMS enables the possibility to predict magnetic fabrics in different tectonic settings and to develop quantitative links between AMS and strain.
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| 10. |
- Alwmark, C., et al.
(författare)
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Impact origin for the Hummeln structure (Sweden) and its link to the Ordovician disruption of the L chondrite parent body
- 2015
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Ingår i: Geology. - : Geological Society of America. - 0091-7613 .- 1943-2682. ; 43:4, s. 279-282
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Tidskriftsartikel (refereegranskat)abstract
- Several studies of meteorites show that a large disruption of an asteroid occurred ca. 470 Ma in our solar system's asteroid belt. As a consequence, a large number of meteorite impacts occurred on Earth during the following few million years. The finding and characterization, for the first time, of planar deformation features in quartz grains from rocks collected at the Middle Ordovician Hummeln structure (Sweden) prove the hypervelocity impact origin of the structure. The unambiguous shock features allow us to close an similar to 200-yr-old discussion about its origin, and further the hypothesis of enhanced asteroid bombardment during the Middle Ordovician, adding an impact crater to the increasing number confirmed and properly dated from this period. Despite its relatively small size (similar to 1.2 km in diameter), similar to the young Meteor Crater (Arizona, USA), and its old age, the Hummeln structure is remarkably well preserved, contradicting the general assumption that small craters are not preserved on Earth for more than a few tens of thousands to a couple of million years.
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| 11. |
- Bacon, Christine D., et al.
(författare)
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Quaternary glaciation and the Great American Biotic Interchange
- 2016
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Ingår i: Geology. - : Geological Society of America. - 0091-7613 .- 1943-2682. ; 44:5, s. 375-378
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Tidskriftsartikel (refereegranskat)abstract
- Recent geological studies demonstrate that the Isthmus of Panama emerged some 10 m.y. earlier than previously assumed. Although absent today in Panama, Central American savanna environments likely developed in connection with the onset of Northern Hemisphere glaciations. As is widely recognized, most of the mammals crossing the isthmus since 2.5 Ma lived in savannas. Could climate-induced vegetational changes across Panama explain the delayed migration of mammals, rather than terrestrial connectivity? We investigate the congruence between cross-continental mammal migration and climate change through analysis of fossil data and molecular phylogenies. Evidence from fossil findings shows that the vast majority of mammals crossed between South and North America after ca. 3 Ma. By contrast, dated mammal phylogenies suggest that migration events started somewhat earlier, ca. 4-3 Ma, but allowing for biases toward greater ages of molecular than geologic dating and uncertainties in the former, we consider this age range not to be significantly earlier than 3 Ma. We conclude that savanna-like environments developed in response to the vast Laurentide ice sheet at the first Quaternary glaciation triggered the initiation of the Great American Biotic Interchange in mammals.
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| 12. |
- Balthasar, Uwe, et al.
(författare)
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Homologous skeletal secretion in tommotiids and brachiopods
- 2009
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 37:12, s. 1143-1146
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Tidskriftsartikel (refereegranskat)abstract
- Tommotiids are distinctive components of the early Cambrian small shelly fauna, almost invariably represented by isolated phosphatic sclerites derived from a multielement protective cover (scleritome). The unusual range of tommotiid sclerite morphologies and unknown construction of the scleritome have severely hampered our understanding of their phylogenetic affinities. However, recent description of rare, articulated scleritome material belonging to the tommotiid genera Eccentrotheca and Paterimitra support the hypothesis that some tommotiids fall within the stem group of the lophophorate phyla Phoronida and Brachiopoda and that at least some tommotiid sclerites are homologous precursors of the shells of organophosphatic brachiopods. Here we show that the shell microstructure of Eccentrotheca and Paterimitra share substantial similarities with paterinid brachiopods. While paterinids possess an overall brachiopod morphology, their microstructure appears more similar to Eccentrotheca and Paterimitra than to nonpaterinate lingulids. These findings strongly support the existence of a brachiopod total group that is solidly rooted within tommotiids, and identify the organophosphatic skeletal composition as plesiomorphic with calcareous shells as derived. The microstructural changes of the proposed tommotiid-brachiopod transition probably reflect an adaptation to fluctuating food and phosphorous intake that came with the switch to a sessile life style at the base of the tommotiid clade.
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| 13. |
- Barker, Abigail, et al.
(författare)
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Direct observation of a fossil high-temperature, fault-hosted, hydrothermal upflow zone in crust formed at the East Pacific Rise
- 2010
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 38:4, s. 379-382
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Tidskriftsartikel (refereegranskat)abstract
- Fault zones in the ocean crust are commonly hypothesized to act as high-permeability conduits that focus fluid flow in oceanic hydrothermal systems. However, there has been little direct study of faults in crust formed at fast-spreading ridges. Here we describe the geology and geochemistry of an ∼40-m-wide fault zone within the uppermost sheeted dike complex exposed at Pito Deep (northeastern Easter microplate). Titanium in quartz thermometry gives temperatures of 392 ± 33 °C for quartz precipitation, indicating that this fault zone focused upwelling fluids at temperatures similar to those of black-smoker vent fluids. Correlated enrichment in 87Sr/86Sr and MgO in fault breccias, along with 87Sr/86Sr ratios higher than in average vent fluids, provide evidence for mixing between high-temperature upwelling fluids and a seawater-like fluid within the fault zone. Large high-temperature fluid fluxes are required to maintain high temperatures during mixing. If this fault zone is representative of upflow zones beneath hydrothermal vents on the East Pacific Rise, then it is possible that vent fluids evolve thermally and chemically during their ascent and may not record the precise conditions at the base of the hydrothermal system.
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| 14. |
- Barker, Abigail, et al.
(författare)
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Interaction of the rifting East Greenland margin with a zoned ancestral Iceland plume.
- 2006
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 34:6, s. 481-484
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Tidskriftsartikel (refereegranskat)abstract
- Neodymium and high-precision lead isotopic data are presented for Paleogene East Greenland flood basalts that erupted during an early phase of magmatic activity associated with the Iceland hotspot. The 6-km-thick volcanic sequence shows marked chemostratigraphic variations: lavas in the lower half of the sequence (Milne Land and Geikie Plateau Formations) have low 206Pb/204Pb values (17.8–18.4), abruptly changing to high 206Pb/ 204Pb values (18.8–19.3) in the overlying Rømer Fjord Formation, followed by intermediate 206Pb/204Pb values (18.6–18.8) in the uppermost Skrænterne Formation. These three isotopic groups of crustally uncontaminated lavas are broadly similar to spatially distinct isotopic domains found in present-day Iceland. The East Greenland data indicate that the same mantle domains present beneath Iceland today were present in the ancestral Iceland hotspot at 55 Ma, and were sequentially tapped during continental break-up as the spatially zoned mantle interacted with the rifting continental margin. The compositional domains within the Iceland hotspot appear to be long-lived features that, given estimates of Icelandic mantle-upwelling velocities, have vertical length-scales of at least ∼500 km.
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| 15. |
- Baucon, A., et al.
(författare)
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Small-world dynamics drove Phanerozoic divergence of burrowing behaviors
- 2021
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Ingår i: Geology. - : Geological Society of America. - 0091-7613 .- 1943-2682. ; 49:6, s. 748-752
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Tidskriftsartikel (refereegranskat)abstract
- Species of burrowing animals have changed substantially over evolutionary time scales, but, surprisingly, burrows display persisting morphological patterns throughout the Phanerozoic. Deep-sea burrows are geometrically patterned, whereas shallow-marine burrows display simpler morphologies. This divergence between burrow associations is one of the central conundrums of paleontology, but it has never been quantitatively demonstrated, and the organizing principles responsible for this structure remain unknown. We show that the divergence of burrow associations has been shaped by small-world dynamics, which is proposed as a major macroevolutionary force in marine environments. Using network analysis, our study reveals that the association patterns between burrow morphotypes in 45 paleontological sites span similar to 500 m.y. Strong statistical support is demonstrated for a surprising association pattern, according to which the data set is optimally partitioned into two subgroups of tightly associated burrow types. These groups correspond to shallow- and deep-marine biomes. Our analysis demonstrates that across the Phanerozoic Eon, burrows did not assemble randomly nor regularly, following instead small-world assembly rules remarkably similar to those that shape human social networks. As such, small-world dynamics deeply influenced gene flow and natural variation in heritable behavior across evolutionary time.
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| 16. |
- Birch, Heather S., et al.
(författare)
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Partial collapse of the marine carbon pump after the Cretaceous-Paleogene boundary
- 2016
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 44:4, s. 287-290
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Tidskriftsartikel (refereegranskat)abstract
- The impact of an asteroid at the end of the Cretaceous caused mass extinctions in the oceans. A rapid collapse in surface to deep-ocean carbon isotope gradients suggests that transfer of organic matter to the deep sea via the biological pump was severely perturbed. However, this view has been challenged by the survival of deep-sea benthic organisms dependent on surface-derived food and uncertainties regarding isotopic fractionation in planktic foraminifera used as tracers. Here we present new stable carbon (delta C-13) and oxygen (delta O-18) isotope data measured on carefully selected planktic and benthic foraminifera from an orbitally dated deep-sea sequence in the southeast Atlantic. Our approach uniquely combines delta O-18 evidence for habitat depth of foraminiferal tracer species with species-specific delta C-13 eco-adjustments, and compares isotopic patterns with corresponding benthic assemblage data. Our results show that changes in ocean circulation and foraminiferal vital effects contribute to but cannot explain all of the observed collapse in surface to deep-ocean foraminiferal delta C-13 gradient. We conclude that the biological pump was weakened as a consequence of marine extinctions, but less severely and for a shorter duration (maximum of 1.77 m.y.) than has previously been suggested.
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| 17. |
- Brandes, Christian, et al.
(författare)
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Intraplate seismicity in northern Central Europe is induced by the last glaciation
- 2015
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 43:7, s. 611-614
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Tidskriftsartikel (refereegranskat)abstract
- There is growing evidence that climate-induced melting of large ice sheets has been able to trigger fault reactivation and earthquakes around the migrating ice limit. Even today, the stress due to glacial isostatic adjustment can continue to induce seismicity within the onceglaciated region. Northern Central Europe lies outside the former ice margin and is regarded as a low-seismicity area. However, several historic earthquakes with intensities of up to VII occurred in this region during the past 1200 years. Here we show with numerical simulations that the seismicity can potentially be explained by the decay of the Scandinavian ice sheet after the Weichselian glaciation. Combination of historic earthquake epicenters with fault maps relates historic seismicity to major reverse faults of Late Cretaceous age. Mesozoic normal faults remained inactive in historic times. We suggest that many faults in northern Central Europe are active during postglacial times. This is a novelty that sheds new light on the distribution of postglacial faulting and seismicity. In addition, we present the first consistent model that can explain both the occurrence of deglaciation seismicity and the historic earthquakes in northern Central Europe.
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| 18. |
- Büntgen, Ulf, et al.
(författare)
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Multi-proxy dating of Iceland's major pre-settlement Katla eruption to 822-823 CE
- 2017
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 45:9, s. 783-786
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Tidskriftsartikel (refereegranskat)abstract
- Investigations of the impacts of past volcanic eruptions on climate, environment, and society require accurate chronologies. However, eruptions that are not recorded in historical documents can seldom be dated exactly. Here we use annually resolved radiocarbon (C-14) measurements to isolate the 775 CE cosmogenic C-14 peak in a subfossil birch tree that was buried by a glacial outburst flood in southern Iceland. We employ this absolute time marker to date a subglacial eruption of Katla volcano at late 822 CE to early 823 CE. We argue for correlation between the 822-823 CE eruption and a conspicuous sulfur anomaly evident in Greenland ice cores, which follows in the wake of an even larger volcanic signal (ca. 818-820 CE) as yet not attributed to a known eruption. An abrupt summer cooling in 824 CE, evident in tree-ring reconstructions for Fennoscandia and the Northern Hemisphere, suggests a climatic response to the Katla eruption. Written historical sources from Europe and China corroborate our proposed tree ring-radiocarbon-ice core linkage but also point to combined effects of eruptions occurring during this period. Our study describes the oldest precisely dated, high-latitude eruption and reveals the impact of an extended phase of volcanic forcing in the early 9th century. It also provides insight into the existence of prehistoric woodland cover and the nature of volcanism several decades before Iceland's permanent settlement began.
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| 19. |
- Byrne, P. K., et al.
(författare)
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A sagging-spreading continuum of large volcano structure
- 2013
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 41:3, s. 339-342
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Tidskriftsartikel (refereegranskat)abstract
- Gravitational deformation strongly influences the structure and eruptive behavior of large volcanoes. Using scaled analog models, we characterize a range of structural architectures produced by volcano sagging and volcano spreading. These arise from the interplay of variable basement rigidity and volcano-basement (de-)coupling. From comparison to volcanoes on Earth (La Reunion and Hawaii) and Mars (Elysium and Olympus Montes), the models highlight a structural continuum in which large volcanoes throughout the Solar System lie.
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| 20. |
- Caracciolo, A., et al.
(författare)
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Oxygen isotope evidence for progressively assimilating trans-crustal magma plumbing systems in Iceland
- 2022
-
Ingår i: Geology. - : Geological Society of America. - 0091-7613 .- 1943-2682. ; 50:7, s. 796-800
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Tidskriftsartikel (refereegranskat)abstract
- The oxygen isotope composition of mantle-derived melts can place important constraints on how magmas are processed as they traverse the crust. Assimilation of crustal material is a crucial aspect of basalt petrogenesis, as it affects the chemical and rheological characteristics of eruptive magmas at active volcanoes. We report oxygen isotope (δ18O) and trace element (TE) data from a suite of well-characterized basaltic melt inclusions and groundmass glasses from the Bárðarbunga volcanic system in Iceland to assess how and where in the plumbing system crustal rocks interact with ascending magmas. While both melt inclusions and groundmass glasses record a large range in δ18O values (+3.2‰ to +6.4‰ and +2.6‰ to +5.5‰, respectively) groundmass glasses record lower values on average. Relationships between incompatible trace element (e.g., Zr/Nb) and oxygen isotope ratios are best explained with three-component mixing, where primary melts derived from depleted and enriched mantle components with distinct δ18O values mix and acquire a low-δ18O character upon progressive contamination with altered Icelandic crust. The majority (60%) of melt inclusions require 10–30% exchange of oxygen with the Icelandic crust. In addition, for the first time, we link the extent of oxygen isotope exchange with melt equilibration depths, showing that most of the contamination occurs at 1–2 kbar (3–7 km depth). We propose that a progressively assimilating, multi-tiered plumbing system is a characteristic feature of the Bárðarbunga volcanic system, whereby chemical modifications resulting from interaction with the crust systematically increase as melts migrate through higher crustal levels. We show that similar processes may also occur across the active rift zone in Iceland.
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| 21. |
- Caron, Jean-Bernard, et al.
(författare)
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A new Burgess Shale-type assemblage from the "thin" Stephen Formation of the Southern Canadian Rockies
- 2010
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 38:9, s. 811-814
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Tidskriftsartikel (refereegranskat)abstract
- A new Burgess Shale–type assemblage, from the Stephen Formation of the southern Canadian Rocky Mountains, is described herein. It occurs near Stanley Glacier in Kootenay National Park, 40 km southeast of the type area near Field, British Columbia. While at least a dozen Burgess Shale localities are known from the “thick” Stephen Formation, the Stanley Glacier locality represents the first discovery of Burgess Shale–type fossils from the “thin” Stephen Formation. The Cathedral Escarpment, an important regional paleotopographic feature, has been considered important to the paleoecologic set- ting and the preservation of the Burgess Shale biota. However, the Stanley Glacier assemblage was preserved in a distal ramp setting in a region where no evidence of an escarpment is present. The low- diversity assemblage contains eight new soft-bodied taxa, including the anomalocaridid Stanleycaris hirpex n. gen., n. sp. (new genus, new species). Pelagic or nektobenthic predators represent the most diverse group, whereas in relative abundance, the assemblage is dominated by typical Cambrian shelly benthic taxa. The low diversity of both the benthic taxa and the ichnofauna, which includes diminutive trace fossils associated with carapaces of soft-bodied arthropods, suggests a paleoenvironment with restrictive conditions. The Stanley Glacier assemblage expands the temporal and geographic range of the Burgess Shale biota in the southern Canadian Rockies, and suggests that Burgess Shale–type assemblages may be common in the “thin” Ste- phen Formation, which is regionally widespread.
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| 22. |
- Cassidy, Mike, et al.
(författare)
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Volatile dilution during magma injections and implications for volcano explosivity
- 2016
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 44:12, s. 1027-1030
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Tidskriftsartikel (refereegranskat)abstract
- Magma reservoirs underneath volcanoes grow through episodic emplacement of magma batches. These pulsed magma injections can substantially alter the physical state of the resident magma by changing its temperature, pressure, composition, and volatile content. Here we examine plagioclase phenocrysts in pumice from the 2014 Plinian eruption of Kelud (Indonesia) that record the progressive capture of small melt inclusions within concentric growth zones during crystallization inside a magma reservoir. High-spatial-resolution Raman spectroscopic measurements reveal the concentration of dissolved H2O within the melt inclusions, and provide insights into melt-volatile behavior at the single crystal scale. H2O contents within melt inclusions range from ∼0.45 to 2.27 wt% and do not correlate with melt inclusion size or distance from the crystal rim, suggesting that minimal H2O was lost via diffusion. Instead, inclusion H2O contents vary systematically with anorthite content of the host plagioclase (R2 = 0.51), whereby high anorthite content zones are associated with low H2O contents and vice versa. This relationship suggests that injections of hot and H2O-poor magma can increase the reservoir temperature, leading to the dilution of melt H2O contents. In addition to recording hot and H2O-poor conditions after these injections, plagioclase crystals also record relatively cold and H2O-rich conditions such as prior to the explosive 2014 eruption. In this case, the elevated H2O content and increased viscosity may have contributed to the high explosivity of the eruption. The point at which an eruption occurs within such repeating hot and cool cycles may therefore have important implications for explaining alternating eruptive styles.
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| 23. |
- Cohen, Timothy, et al.
(författare)
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Continental aridification and the vanishing of Australia's mega-lakes
- 2011
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 39, s. 167-170
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Tidskriftsartikel (refereegranskat)abstract
- The nature of the Australian climate at about the time of rapid megafaunal extinctions and humans arriving in Australia is poorly understood and is an important element in the contentious debate as to whether humans or climate caused the extinctions. Here we present a new paleoshoreline chronology that extends over the past 100 k.y. for Lake Mega-Frome, the coalescence of Lakes Frome, Blanche, Callabonna and Gregory, in the southern latitudes of central Australia. We show that Lake Mega-Frome was connected for the last time to adjacent Lake Eyre at 50–47 ka, forming the largest remaining interconnected system of paleolakes on the Australian continent. The fi nal disconnection and a progressive drop in the level of Lake Mega-Frome represents a major climate shift to aridifi cation that coincided with the arrival of humans and the demise of the megafauna. The supply of moisture to the Australian continent at various times in the Quaternary has commonly been ascribed to an enhanced monsoon. This study, in combination with other paleoclimate data, provides reliable evidence for peri-ods of enhanced tropical and enhanced Southern Ocean sources of water fi lling these lakes at different times during the last full glacial cycle.
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| 24. |
- Cohen, Tim J., et al.
(författare)
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Hydrological transformation coincided with megafaunal extinction in central Australia
- 2015
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 43:3, s. 195-198
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Tidskriftsartikel (refereegranskat)abstract
- Central to the debate over the extinction of many of Australia's last surviving megafauna is the question: Was climate changing significantly when humans arrived and megafauna went extinct? Here we present a new perspective on variations in climate and water resources over the last glacial cycle in arid Australia based on the study of the continent's largest lake basin and its tributaries. By dating paleoshorelines and river deposits in the Lake Eyre basin, we show that major hydrological change caused previously overflowing megalakes to enter a final and catastrophic drying phase at 48 +/- 2 ka just as the giant bird, Genyornis newtoni, went extinct (50-45 ka). The disappearance of Genyornis and other megafauna has been previously attributed to ecosystem collapse coincident with the spread of fire-wielding humans. Our findings suggest a climate-driven hydrological transformation in the critical window of human arrival and megafaunal extinction, and the results call for a re-evaluation of a human-mediated cause for such extinctions in arid Australia.
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| 25. |
- Dessandier, P-A., et al.
(författare)
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Ice-sheet melt drove methane emissions in the Arctic during the last two interglacials
- 2021
-
Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 49:7, s. 799-803
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Tidskriftsartikel (refereegranskat)abstract
- Circum-Arctic glacial ice is melting in an unprecedented mode, and release of currently trapped geological methane may act as a positive feedback on ice-sheet retreat during global warming. Evidence for methane release during the penultimate (Eemian, ca. 125 ka) interglacial, a period with less glacial sea ice and higher temperatures than today, is currently absent. Here, we argue that based on foraminiferal isotope studies on drill holes from offshore Svalbard, Norway, methane leakage occurred upon the abrupt Eurasian ice-sheet wastage during terminations of the last (Weichselian) and penultimate (Saalian) glaciations. Progressive increase of methane emissions seems to be first recorded by depleted benthic foraminiferal δ13C. This is quickly followed by the precipitation of methane-derived authigenic carbonate as overgrowth inside and outside foraminiferal shells, characterized by heavy δ18O and depleted δ13C of both benthic and planktonic foraminifera. The similarities between the events observed over both terminations advocate for a common driver for the episodic release of geological methane stocks. Our favored model is recurrent leakage of shallow gas reservoirs below the gas hydrate stability zone along the margin of western Svalbard that can be reactivated upon initial instability of the grounded, marine-based ice sheets. Analogous to this model, with the current acceleration of the Greenland ice melt, instabilities of existing methane reservoirs below and nearby the ice sheet are likely.
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| 26. |
- Dunk, M., et al.
(författare)
-
Evidence for a late Cambrian juvenile arc and a buried suture within the Laurentian Caledonides of Scotland : Comparisons with hyperextended Iapetan margins in the Appalachian Mountains (North America) and Norway
- 2019
-
Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 47:8, s. 734-738
-
Tidskriftsartikel (refereegranskat)abstract
- Uranium-lead (U-Pb) zircon dating establishes a late Cambrian (Drumian) protolith age of 503 ± 2 Ma for a trondhjemitic gneiss of the calc-alkaline Strathy Complex, northern Scottish Caledonides. Positive εHf and εNd values from trondhjemitic gneisses and co-magmatic amphibolites, respectively, and an absence of any inheritance in zircon populations support published geochemistry that indicates a juvenile origin distal from Laurentia. In order to account for its present location within a stack of Laurentia-derived thrust sheets, we interpret the complex as allochthonous and located along a buried suture. We propose that a microcontinental ribbon was detached from Laurentia during late Neoproterozoic to Cambrian rifting; the intervening oceanic tract closed by subduction during the late Cambrian and formed a juvenile arc, the protolith of the Strathy Complex. The microcontinental ribbon was reattached to Laurentia during the Grampian orogeny, which transported the Strathy Complex as a tectonic slice within a nappe stack. Peak metamorphic conditions for the Strathy Complex arc (650–700 °C, 0.6–0.75 GPa) are intermediate in pressure between those published previously for Grampian mineral assemblages in structurally overlying low-pressure migmatites (670–750 °C, <0.4 GPa) that we deduce to have been derived from an adjacent backarc basin, and structurally underlying upper amphibolite rocks (650–700 °C, 1.1–1.2 GPa) that we interpret to represent the partially subducted Laurentian margin. This scenario compares with that of the northern Appalachian Mountains and Norway where microcontinental blocks are interpreted to have their origins in detachment from passive margins of the Iapetus Ocean during Cambrian rifting and to have been re-amalgamated during Caledonian orogenesis.
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| 27. |
- Evans, Katy, et al.
(författare)
-
The origin of platinum group minerals in oceanic crust
- 2023
-
Ingår i: Geology. - : Geological Society of America. - 0091-7613 .- 1943-2682. ; 51:6, s. 554-558
-
Tidskriftsartikel (refereegranskat)abstract
- Highly siderophile elements (HSEs), including Re and Os, are used extensively as geochemical tracers and geochronometers to investigate the formation and evolution of Earth's crust and mantle. Mantle rocks are commonly serpentinized, but the effect of serpentinization on the distribution of HSEs is controversial because HSEs are commonly hosted by rare, micrometer to sub-micrometer-scale grains of platinum group minerals (PGMs) of ambiguous origin that are challenging to identify, characterize, and interpret. In this study, atom probe tomography (APT) is used to characterize two spatially close PGM grains hosted by a partially serpentinized harzburgite from Macquarie Island, Australia. The APT data reveal an extraordinary level of detail that provides insights into the origin of a complex Cu-Pt alloy grain (average composition similar to Cu4Pt). The grain hosts Fe-, Ni-, and Pt-rich sub-grains associated with Rh, variably overlapping networks of Pd-and Cd-enrichment, and OH-rich volumes identified as fluid inclusions. Osmium and Ru are hosted by an idioblastic laurite (RuS2) grain. Compositional, textural, and phase-diagram constraints are consistent with a modified pre-serpentinization origin for the PGMs, and a comparison between observed and calculated grain distributions indicate that while Os isotope ratios were probably unaffected by serpentinization, whole-rock and grain-scale HSE and isotopic ratios may have been decoupled during serpentinization.
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| 28. |
- Frank, T.D., et al.
(författare)
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Pace, magnitude, and nature of terrestrial climate change through the end-Permian extinction in southeastern Gondwana
- 2021
-
Ingår i: Geology. - : Geological Society of America. - 0091-7613 .- 1943-2682. ; 49:9, s. 1089-1095
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Tidskriftsartikel (refereegranskat)abstract
- Rapid climate change was a major contributor to the end-Permian extinction (EPE). Although well constrained for the marine realm, relatively few records document the pace, nature, and magnitude of climate change across the EPE in terrestrial environments. We generated proxy records for chemical weathering and land surface temperature from continental margin deposits of the high-latitude southeastern margin of Gondwana. Regional climate simulations provide additional context. Results show that Glossopteris forest-mire ecosystems collapsed during a pulse of intense chemical weathering and peak warmth, which capped ∼1 m.y. of gradual warming and intensification of seasonality. Erosion resulting from loss of vegetation was short lived in the low-relief landscape. Earliest Triassic climate was∼10–14 °C warmer than the late Lopingian and landscapes were no longer persistently wet. Aridification, commonly linked to the EPE, developed gradually, facilitating the persistence of refugia for moisture-loving terrestrial groups.
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| 29. |
- Froitzheim, Nikolaus, et al.
(författare)
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Devonian subduction and syncollisional exhumation of continental crust in Lofoten, Norway
- 2016
-
Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 44:3, s. 223-226
-
Tidskriftsartikel (refereegranskat)abstract
- When continents collide, continental crust of the lower plate may be subducted to mantle depth and return to the surface to form eclogite facies metamorphic terranes, as typified by the Western Gneiss Complex of the Scandinavian Caledonides. Proterozoic basement of the Lofoten Islands, located northeast and along strike of the Western Gneiss Complex, contains Caledonian eclogite, although Caledonian deformation is only minor. Previous dating suggested that Lofoten eclogites formed at ca. 480 Ma, i.e., similar to 50 Ma before the collision between the major continents Baltica and Laurentia, and that the Lofoten basement may not originate from Baltica but rather represents a stranded microcontinent. Newly discovered kyanite eclogites from the Lofoten Islands record deep subduction of continental crust during the main (Scandian) stage of Baltica-Laurentia collision ca. 400 Ma. Thermobarometry and thermodynamic modeling yield metamorphic conditions of 2.5-2.8 GPa and similar to 650 degrees C. Lu-Hf geochronology yields 399 +/- 10 Ma, corresponding to the time of garnet growth during subduction. Our results demonstrate that the Lofoten basement belonged to Baltica, was subducted to similar to 90 km depth during the collision with Laurentia, and was exhumed at an intermediate to high rate (>6 mm/yr) while thrusting of a Caledonian allochthon (Leknes Group) was still ongoing. This supports the challenging conclusions that (1) subducted continental crust may stay rigid down to a depth of similar to 90 km, and (2) it may be exhumed during ongoing collision and crustal shortening.
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| 30. |
- Gaines, Robert R., et al.
(författare)
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Burgess Shale-type biotas were not entirely burrowed away
- 2012
-
Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 40:3, s. 283-286
-
Tidskriftsartikel (refereegranskat)abstract
- Burgess Shale–type biotas occur globally in the Cambrian record and offer unparalleled insight into the Cambrian explosion, the initial Phanerozoic radiation of the Metazoa. Deposits bearing exceptionally preserved soft-bodied fossils are unusually common in Cambrian strata; more than 40 are now known. The well-documented decline of soft-bodied preservation following the Middle Cambrian represents the closure of a taphonomic window that was only intermittently open in marine environments thereafter. The prevailing hypothesis for this secular shift in taphonomic conditions of outer shelf environments is that soft-bodied biotas were literally burrowed away from the fossil record by increasing infaunal activity in muddy substrate environments; this would have affected geochemical gradients and increased the efficiency of organic matter recycling in sediments. New and recently published data, however, suggest a more complex scenario. Ichnologic and microstratigraphic data from Burgess Shale–type deposits indicate that (1) bioturbation exerts a limiting effect on soft-bodied preservation; (2) the observed increase in the depth and extent of bioturbation following the Middle Cambrian would have restricted preservation of Burgess Shale?type biotas in a number of settings; but (3) increasing depth and extent of bioturbation would not have affected preservation in many other settings, including the most richly fossiliferous portions of the Chengjiang (China) deposit and the Greater Phyllopod Bed of the Burgess Shale (Canada). Therefore, increasing bioturbation cannot account for the apparent loss of this pathway from the fossil record, and requires that other circumstances, including, but not limited to, widespread benthic anoxia, facilitated widespread exceptional preservation in the Cambrian.
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| 31. |
- Ge, Rongfeng, et al.
(författare)
-
A 4463 Ma apparent zircon age from the Jack Hills (Western Australia) resulting from ancient Pb mobilization
- 2018
-
Ingår i: Geology. - : Geological Society of America. - 0091-7613 .- 1943-2682. ; 46:4, s. 303-306
-
Tidskriftsartikel (refereegranskat)abstract
- Hadean (≥4.0 Ga) zircon grains provide the only direct record of the first half-billion years of Earth’s history. Determining accurate and precise crystallization ages of these ancient zircons is a prerequisite for any interpretation of crustal evolution, surface environment, and geodynamics on the early Earth, but this may be compromised by mobilization of radiogenic Pb due to subsequent thermal overprinting. Here we report a detrital zircon from the Jack Hills (Western Australia) with 4486–4425 Ma concordant ion microprobe ages that yield a concordia age of 4463 ± 17 Ma (2σ), the oldest zircon age recorded from Earth. However, scanning ion imaging reveals that this >4.4 Ga apparent age resulted from incorporation of micrometer-scale patches of unsupported radiogenic Pb with extremely high 207Pb/206Pb ratios and >4.5 Ga 207Pb/206Pb ages. Isotopic modeling demonstrates that these patches likely resulted from redistribution of radiogenic Pb in a ca. 4.3 Ga zircon during a ca. 3.8 Ga or older event. This highlights that even a concordia age can be spurious and should be carefully evaluated before being interpreted as the crystallization age of ancient zircon.
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| 32. |
- Grazhdankin, Dmitriy V., et al.
(författare)
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Carbonate-hosted Avalon-type fossils in arctic Siberia
- 2008
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 36:10, s. 803-806
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Tidskriftsartikel (refereegranskat)abstract
- Avalon-type fossils are crucial to understanding the origin of Phanerozoic ecosystems, but their usual occurrence in volcaniclastic and siliciclastic facies greatly limits their paleobiological resolution. The unique carbonate-hosted. assemblage of the Khatyspyt Formation, on the Olenek uplift of north-central Siberia, promises a much enhanced anatomical and paleoecological view of these enigmatic organisms. Avalon-type fossils are preserved by authigenic carbonate cementation in intervals of finely laminated nodular limestones (Khatyspyt-type taphonomic window). Interbedded silicified calcareous mudstones yield diverse carbonaceous compressions, occasionafly with taphonomic phantoms of Avalon-type taxa (Miaohetype taphonomic window). Styles of moldic preservation do not appear to be taxon selective, and therefore cannot alone be responsible for the morphological distinctiveness of Ediacaran macrofossils and the profound disparity in the taxonomic composition between fossil assemblages. On the other hand, the exclusion of Avalon-type fossils from carbonaceous compressions (Miaohe-type preservational window) is a real taphonomic signal that provides an important constraint on the properties of certain Ediacaran tissues.
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| 33. |
- Guo, Benhong, et al.
(författare)
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Dominant precessional forcing of the East Asian summer monsoon since 260 ka
- 2022
-
Ingår i: Geology. - : Geological Society of America. - 0091-7613 .- 1943-2682. ; 50:12, s. 1372-1376
-
Tidskriftsartikel (refereegranskat)abstract
- One of the most perplexing problems in paleoclimate research is how orbital cyclicities force East Asian summer monsoon (EASM) precipitation variation over the middle to late Quaternary. Chinese loess records suggest that EASM precipitation was dominated by 100 k.y. cycles controlled by Northern Hemisphere ice sheet forcing. In contrast, speleothem records suggest that EASM precipitation was dominated by 23 k.y. cycles caused by Northern Hemisphere summer insolation forcing. In order to resolve this inconsistency, we present high-resolution paleoclimate records from Xijin drill cores on the western Chinese Loess Plateau for the past 260 k.y., the rough upper limit of luminescence dating. Magnetic susceptibility (χ) shows clear 23 k.y. precessional cycles over interglacials but has constant low values over glacials. This is interpreted as indicating a lack of pedogenesis, such that χ cannot record EASM precipitation variations, rather than an absence of EASM variation itself. To circumvent this issue, we use inversed sand content as an alternative proxy for EASM precipitation over glacials and splice this with the interglacial logarithmic χ from Xijin drill cores. This new record reveals dominant 23 k.y. cycles over both interglacials and glacials, consistent with speleothem δ18O data and dominant insolation forcing. These findings allow a consistent understanding of EASM variability and forcing mechanisms from both loess and speleothem archives, resolving one of the largest debates in past monsoon research. These results challenge suggestions of high-latitude ice sheet forcing of the EASM based on slowly accumulated loess records from the central Loess Plateau.
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| 34. |
- Hall, Adrian, et al.
(författare)
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Selective glacial erosion on the Norwegian passive margin
- 2013
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 41:12, s. 1203-1206
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Tidskriftsartikel (refereegranskat)abstract
- Glaciated passive margins display dramatic fjord coasts, but also commonly retain plateau fragments inland. It has been proposed recently that such elevated, low-relief surfaces on the Norwegian margin are products of highly efficient and extensive glacial and periglacial erosion (the glacial buzzsaw) operating at equilibrium line altitudes (ELAs). We demonstrate here that glacial erosion has acted instead to dissect plateaus in western Norway. Low-relief surfaces are not generally spatially associated with cirques, and do not correlate regionally with modern and Last Glacial Maximum ELAs. Glacier dynamics require instead that glacial erosion is selective, with low-relief surfaces representing islands of limited Pleistocene erosion. Deep glacial erosion of the coast and inner shelf has provided huge volumes of sediment (70,000 km3), largely resolving apparent mismatches (65–100,000 km3) between fjord and valley volumes and Pliocene–Pleistocene sediment wedges offshore. Nonetheless, as Pleistocene glacial valleys and cirques are cut into preexisting mountain relief, tectonics rather than isostatic compensation for glacial erosion have been the main driver for late Cenozoic uplift on the Norwegian passive margin.
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| 35. |
- Hansman, Reuben J., et al.
(författare)
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Absolute ages of multiple generations of brittle structures by U-Pb dating of calcite
- 2018
-
Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 46:3, s. 207-210
-
Tidskriftsartikel (refereegranskat)abstract
- Direct dating of brittle structures is challenging, especially absolute dating of diagenesis followed by a series of superimposed brittle deformation events. We report 22 calcite U-Pb ages from tectonites and carbonate host rocks that date 3 diagenetic and 6 brittle deformation events. Results show that U-Pb dating of calcite fibers from these structures is compatible with overprinting relationships. Ages indicate that diagenesis occurred between 147 +/- 6 Ma and 103 +/- 34 Ma, and was followed by top-to-the-south, layer-parallel shearing due to ophiolite obduction at 84 +/- 5 Ma (2 sigma errors). Sheared top-to-the- northeast, layer-parallel veins were dated as 64 +/- 4 Ma and are interpreted to have developed during postobduction exhumation. After this event, a series of strike-slip structures, which crosscut and reactivated older faults due to northwest-southeast horizontal shortening, were dated as 55 +/- 22 Ma and 43 +/- 6 Ma. Eight ages from strike-slip faults and thrusts resulting from northeast-southwest shortening range from 40.6 +/- 0.5 Ma to 16.1 +/- 0.2 Ma. The youngest ages are from minor overprinting fibers ranging in age between 7.5 +/- 0.9 Ma and 1.6 +/- 0.6 Ma. Our results show that U-Pb dating of calcite fibers can be successfully used to constrain a complicated succession of brittle deformation structures that encompasses two orogenies and an intervening extension period.
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| 36. |
- Harnett, Claire E., et al.
(författare)
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Large-scale lava dome fracturing as a result of concealed weakened zones
- 2022
-
Ingår i: Geology. - : Geological Society of America. - 0091-7613 .- 1943-2682. ; 50:12, s. 1346-1350
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Tidskriftsartikel (refereegranskat)abstract
- Mechanically weakened alteration zones in lava domes are thought to jeopardize their stability. Such zones can be hazardous when concealed within the dome, either because they formed by subsurface hydrothermal circulation or because they formed on the surface but were subsequently buried by renewed lava extrusion. We present a new suite of computa-tional models showing how the size and position of a weakened brittle zone within a dome can affect large-scale fracture formation, displacement, and the collapse mechanism. By combining recent laboratory data for the mechanical behavior of dome rocks with discrete element method models, we show (1) the presence of a weak zone increases instability, which is exacerbated when the size of the zone increases or the zone is positioned off-center; (2) the position of the weak zone changes the deformation mechanism from slumping-type slope de-formation when the zone is positioned centrally, compared with deep-seated rotational slope failure when the zone is positioned toward the dome flank; and finally, (3) dome-cutting tensile fractures form in the presence of a small weak zone (60 m diameter, -14% of dome width), whereas large weak zones (120 m diameter, -27% of dome width) promote the formation of longer and deeper fractures that jeopardize larger dome volumes. Our results corroborate previous field observations at lava domes and indicate that large fracture formation, which greatly influences dome stability and outgassing, can be explained by the presence of con-cealed alteration zones. This improved understanding of the mechanisms responsible for dome instability enables better hazard assessment at volcanoes worldwide.
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| 37. |
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| 38. |
- Jakobsson, Martin, et al.
(författare)
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Geological record of ice shelf break-up and grounding line retreat, Pine Island Bay, West Antarctica
- 2011
-
Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 39:7, s. 691-694
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Tidskriftsartikel (refereegranskat)abstract
- The catastrophic break-ups of the floating Larsen A and B ice shelves (Antarctica) in 1995 and 2002 and associated acceleration of glaciers that flowed into these ice shelves were among the most dramatic glaciological events observed in historical time. This raises a question about the larger West Antarctic ice shelves. Do these shelves, with their much greater glacial discharge, have a history of collapse? Here we describe features from the seafloor in Pine Island Bay, West Antarctica, which we interpret as having been formed during a massive ice shelf break-up and associated grounding line retreat. This evidence exists in the form of seafloor landforms that we argue were produced daily as a consequence of tidally influenced motion of mega-icebergs maintained upright in an iceberg armada produced from the disintegrating ice shelf and retreating grounding line. The break-up occurred prior to ca. 12 ka and was likely a response to rapid sea-level rise or ocean warming at that time.
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| 39. |
- Jakobsson, Martin, et al.
(författare)
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Major earthquake at the Pleistocene-Holocene transition in Lake Vattern, southern Sweden
- 2014
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 42:5, s. 379-382
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Tidskriftsartikel (refereegranskat)abstract
- Lake Vattern, Sweden, is within a graben that formed through rifting along the boundary between two Precambrian terrains. Geophysical mapping and geological coring show that substantial tectonic movements along the Lake Vattern graben occurred at the very onset of the Holocene. This is evident from deformation structures in the soft sediment accumulated on the lake floor. Our interpretation of these structures suggests as much as 13 m of vertical tectonic displacements along sections of a >80-km-long fault system. If these large displacements are from one tectonic event, Lake Vattern must have had an earthquake with seismic moment magnitudes to 7.5. In addition, our geophysical mapping shows large landslides along sections of the steep lake shores. Pollen analysis of sediment infillings of some of the most prominent sediment deformation structures places this major seismic event at the Younger Dryas-Preboreal transition, ca. 11.5 ka. We suggest that this event is mainly related to the rapid release of ice-sheet load following the deglaciation. This paleoseismic event in Lake Vattern ranks among the larger known intraplate tectonic events in Scandinavia and attests to the significance of glacio-isostatic unloading.
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| 40. |
- Jansen, John D., et al.
(författare)
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Does decreasing paraglacial sediment supply slow knickpoint retreat?
- 2011
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 39:6, s. 543-546
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Tidskriftsartikel (refereegranskat)abstract
- In four rivers in western Scotland for which there is a well-constrained record of relative base-level fall, the rate of postglacial bedrock erosion is quantified by measuring the concentration of in situ cosmogenic (10)Be on strath terraces downstream of headward-retreating knickpoints. Along-channel gradients in (10)Be exposure age show two distinct trends: upstream younging and constant age, which we interpret as diagnostic of knickpoint retreat and diffusive transport-limited incision, respectively. We show that bedrock channel incision and regional formation of strath terraces began shortly after deglaciation (ca. 11.5 ka), and that knickpoint retreat rates peaked in the early to mid-Holocene. Erosion rates have since decreased by two orders of magnitude, converging in the late Holocene to low rates independent of stream power per unit channel area. We infer this regional slowing in postglacial knickpoint retreat to be the result of the depletion of paraglacial sediment supply over the Holocene, leading to a deficiency in tools for bedrock erosion. Our results imply that episodes of major fluvial erosion may be in tune with glacial cycles, and that sediment depletion following glacial-interglacial transitions may be an important cause of bedrock erosion rate variations in rivers draining glaciated landscapes.
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| 41. |
- Jarochowska, Emilia, et al.
(författare)
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Conodonts in Silurian hypersaline environments : specialized an dunexpectedly diverse
- 2017
-
Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 45:1, s. 3-6
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Tidskriftsartikel (refereegranskat)abstract
- Hypersaline environments are commonly assumed to be barren of metazoans and therefore are avoided by paleontologists, yet a number of early Paleozoic jawless vertebrate groups specialized to live in such settings. Sampling bias against restricted settings resulted in substantial underestimation of their diversity. Rare studies venturing into such environments yielded multiple new species of conodonts, suggesting that the diversity and habitat range of these hyperdiverse predators of the early oceans are equally underestimated. We describe here autochthonous conodont fauna from evaporite-bearing horizons from the middle Silurian of Estonia that provide evidence for efficient osmoregulation in this group. Based on a global compilation of coeval conodont assemblages, we show that marginal-marine, periodically emergent environments were characterized by higher conodont diversity than open-marine shallow settings. This diversity is due to a high number of species occurring in these environments only. The high degree of specialization is also reflected by the highest within-habitat variability (β diversity) in marginal settings. Most conodont species had narrow environmental niches and, unlike in marine invertebrates, extreme environments were inhabited by the most specialized taxa. Such environments represent a large proportion of early Paleozoic tropical epicratonic basins. Our analysis allows quantification of the degree to which mid-Silurian conodont diversity is underestimated as a result of sampling bias against marginal-marine settings.
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| 42. |
- Jiang, Peilin, et al.
(författare)
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Water in omphacite fingerprints the thermal history of eclogites
- 2022
-
Ingår i: Geology. - : Geological Society of America. - 0091-7613 .- 1943-2682. ; 50:3, s. 316-320
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Tidskriftsartikel (refereegranskat)abstract
- Omphacite is a diagnostic mineral of high- and ultrahigh-pressure rocks, and its association with garnet is characteristic of eclogites from subduction-related massifs and volcano-entrained xenoliths. Omphacite can accommodate significant amounts of water as structurally bound hydroxyl (OH) groups, and is able to convey water into Earth's interior. We show, for the first time, experimental evidence that the infrared absorption patterns of water in omphacite are temperature sensitive. This provides a new framework for discriminating water in natural omphacite equilibrated at different temperatures. We also demonstrate that in low-temperature omphacite, the integral absorbance ratio between the infrared OH absorption bands at 3620 cm−1 and 3450 cm−1 is linearly related to temperature. Water in omphacite of massif eclogites records the temperature of OH equilibrium, allowing reconstruction of the fluid-involved thermal history and tectonics of rock evolution.
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| 43. |
- Jiang, Qiang, et al.
(författare)
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Longest continuously erupting large igneous province driven by plume-ridge interaction
- 2021
-
Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 49:2, s. 206-210
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Tidskriftsartikel (refereegranskat)abstract
- Large igneous provinces (LIPs) typically form in one short pulse of ∼1–5 Ma or several punctuated ∼1–5 Ma pulses. Here, our 25 new 40Ar/39Ar plateau ages for the main construct of the Kerguelen LIP—the Cretaceous Southern and Central Kerguelen Plateau, Elan Bank, and Broken Ridge—show continuous volcanic activity from ca. 122 to 90 Ma, a long lifespan of >32 Ma. This suggests that the Kerguelen LIP records the longest, continuous high-magma-flux emplacement interval of any LIP. Distinct from both short-lived and multiple-pulsed LIPs, we propose that Kerguelen is a different type of LIP that formed through long-term interactions between a mantle plume and mid-ocean ridge, which is enabled by multiple ridge jumps, slow spreading, and migration of the ridge. Such processes allow the transport of magma products away from the eruption center and result in long-lived, continuous magmatic activity.
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| 44. |
- Jilbert, Tom, et al.
(författare)
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Glacio-isostatic control on hypoxia in a high-latitude shelf basin
- 2015
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 43:5, s. 427-430
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Tidskriftsartikel (refereegranskat)abstract
- In high-latitude continental shelf environments, late Pleistocene glacial overdeepening and early Holocene eustatic sea-level rise combined to create restricted marine basins with a high vulnerability to oxygen depletion. Here we show that ongoing glacio-isostatic rebound during the Holocene may have played an important role in determining the distribution of past hypoxia in these environments by controlling the physical exchange of water masses and the distribution of large-scale phosphorus (P) sinks. We focus on the Baltic Sea, where sediment records from a large, presently oxic sub-basin show evidence for intense hypoxia and cyanobacteria blooms during the Holocene Thermal Maximum. Using paleobathymetric modeling, we show that this period was characterized by enhanced deep-water exchange, allowing widespread phosphorus regeneration. Intra-basin sills then shoaled over a period of several thousand years, enhancing P burial in one of the sub-basins. Together with climate forcing, this may have caused the termination of hypoxia throughout the Baltic Sea. Similar rearrangements of physical and chemical processes likely occurred in response to glacio-isostatic rebound in other high-latitude shelf basins during the Holocene.
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| 45. |
- Kamenos, Nicholas A., et al.
(författare)
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Reconstructing Greenland ice sheet runoff using coralline algae
- 2012
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 40:12, s. 1095-1098
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Tidskriftsartikel (refereegranskat)abstract
- The Greenland ice sheet (GrIS) contains the largest store of fresh water in the Northern Hemisphere, equivalent to similar to 7.4 m of eustatic sea-level rise, but its impacts on current, past, and future sea level, ocean circulation, and European climate are poorly understood. Previous estimates of GrIS melt, from 26 yr of satellite observations and temperature-driven melt models over 48 yr, show increasing melt trends. There are, however, no runoff data of comparable duration with which to validate the relationship between the spatial extent of melting and runoff or temperature-based runoff models. Further, longer runoff records are needed to extend the melt pattern of Greenland to centennial timescales, enabling recent observations and trends to be put into a better historical context. We have developed a new GrIS runoff proxy by extracting information on relative salinity changes from annual growth bands of red coralline algae. We observed significant negative relationships between historic runoff, relative salinity, and marine summer temperature in Sondre Stromfjord, Greenland. We produce the first reconstruction of runoff from a section of the GrIS that discharges into Sondre Stromfjord over several decades (1939-2002) and record a trend of increasing reconstructed runoff since the mid 1980s. In situ summer marine temperatures followed an equivalent trend. We suggest that since A. D. 1939, atmospheric temperatures have been important in forcing runoff. These results show that our technique has significant potential to enhance understanding of runoff from large ice sheets as it will enable melt reconstruction over centennial to millennial timescales.
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| 46. |
- Kelly, Meredith A., et al.
(författare)
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Late glacial fluctuations of Quelccaya Ice Cap, southeastern Peru
- 2012
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 40:11, s. 991-994
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Tidskriftsartikel (refereegranskat)abstract
- The last glacial-interglacial transition (ca. 18-11 ka) was interrupted by abrupt climate events that differed in each hemisphere. During the Antarctic Cold Reversal (ca. 14.5-12.9 ka), the Southern Hemisphere high and mid latitudes cooled, while the Northern Hemisphere warmed. The pattern of change then reversed during the Younger Dryas (ca. 12.9-11.7 ka), which was characterized by cold conditions in much of the Northern Hemisphere. Well-dated paleoclimate records serve to reveal the possible mechanisms for these events. Here we present a reconstruction of the late glacial fluctuations of Quelccaya Ice Cap, located in the southern tropics, based on 38 new radiocarbon ages. Quelccaya was retreating from its Last Glacial Maximum (ca. 21 ka) extent by ca. 17.2 ka, and was located upvalley from its late glacial moraines by 13.6-12.8 ka. Quelccaya experienced a significant readvance that culminated at 12.5-12.4 ka, and then receded several kilometers to near, or within, its late Holocene extent by ca. 11.6 ka. This record provides the most detailed evidence yet of glacier fluctuations in the southern tropics during late glacial time.
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| 47. |
- Kenny, G.G., et al.
(författare)
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Differentiated impact melt sheets may be a potential source of Hadean detrital zircon.
- 2016
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 44, s. 431-434
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Tidskriftsartikel (refereegranskat)abstract
- Constraining the origin and history of very ancient detrital zircons has unique potential for furthering our knowledge of Earth’s very early crust and Hadean geodynamics. Previous applications of the Ti-in-zircon thermometer to >4 Ga zircons have identified a population with relatively low crystallization temperatures () of ~685 °C. This could possibly indicate wet minimum-melting conditions producing granitic melts, implying very different Hadean terrestrial geology from other rocky planets. Here we report the first comprehensive ion microprobe study of zircons from a transect through the differentiated Sudbury impact melt sheet. The new zircon Ti contents and corresponding fully overlap with those of the Hadean zircon population. Previous studies, which measured Ti in impact melt sheet zircons did not find this wide range because they analyzed samples only from a restricted portion of the melt sheet and because they used laser ablation analyses that can overestimate true Ti content. It is important to note that internal differentiation of the impact melt is likely a prerequisite for the observed low in zircons from the most evolved rocks. On Earth, melt sheet
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| 48. |
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| 49. |
- Kenny, Gavin, et al.
(författare)
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Recrystallization and chemical changes in apatite in response to hypervelocity impact
- 2020
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 48:1, s. 19-23
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Tidskriftsartikel (refereegranskat)abstract
- Despite the wide utility of apatite, Ca5(PO4)3(F,Cl,OH), in the geosciences, including tracing volatile abundances on the Moon and Mars, little is known about how the mineral responds to the extreme temperatures and pressures associated with hypervelocity impacts. To address this deficiency, we here present the first microstructural analysis and chemical mapping of shocked apatite from a terrestrial impact crater. Apatite grains from the Paasselkä impact structure, Finland, display intragrain crystal-plastic deformation as well as pervasive recrystallization—the first such report in terrestrial apatite. A partially recrystallized grain offers the opportunity to investigate the effect of shock recrystallization on the chemical composition of apatite. The recrystallized portion of the fluorapatite grain is depleted in Mg and Fe relative to the remnant non-recrystallized domain. Strikingly, the recrystallized region alone hosts inclusions of (Mg,Fe)2(PO4)F, wagnerite or a polymorph thereof. These are interpreted to be a product of phase separation during recrystallization and to be related to the reduced abundances of certain elements in the recrystallized domain. The shock-induced recrystallization of apatite, which we show to be related to changes in the mineral’s chemical composition, is not always readily visible in traditional imaging techniques (such as backscattered electron imaging of polished interior surfaces), thus highlighting the need for correlated microstructural, chemical, and isotopic studies of phosphates. This is particularly relevant for extraterrestrial phosphates that may have been exposed to impacts, and we urge the consideration of microstructural data in the interpretation of the primary or secondary nature of elemental abundances and isotopic compositions.
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| 50. |
- Kiel, Steffen, et al.
(författare)
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Late Triassic mollusk-dominated hydrocarbon-seep deposits from Turkey
- 2017
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Ingår i: Geology. - Boulder : Geological Society of America. - 0091-7613 .- 1943-2682. ; 44, s. 751-754
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Tidskriftsartikel (refereegranskat)abstract
- Deep-sea hydrothermal vents and hydrocarbon seeps host unique ecosystems relying on geochemical energy rather than photosynthesis. Whereas the fossil and evolutionary history of these ecosystems is increasingly well known from the Cretaceous onward, their earlier history remains poorly understood and brachiopods are considered to have played a dominant role during the Paleozoic and Mesozoic. Here we report five new hydrocarbon-seep deposits from the Upper Triassic Kasımlar shales in southern Turkey. The pyritiferous seep limestones predominantly consist of 13C-depleted micrite with δ13C values as low as −10.4‰, and contain only sparse 13C-depleted rim cement (δ13C as low as −12.0‰), interpreted to result from the recrystallization of banded and botryoidal crystal aggregates of fibrous cement. The geologic ages of the studied seep deposits were determined as late Carnian and early Norian using conodonts. The associated fauna is dominated by modiomorphid and anomalodesmatan bivalves, and also includes a diversity of gastropods and the dimerelloid brachiopod Halorella. These faunal assemblages allow a comparison between seep faunas from the two major Triassic ocean basins—the present assemblages being from Tethys, and the only previously known examples being from eastern Panthalassa—and indicate that a cosmopolitan, seep-restricted fauna as in the present-day oceans has existed since the Late Triassic. With almost 20 species, the seep fauna of the Kasımlar shales approaches the diversity of Cretaceous to present-day seep faunas, further emphasizing the ecological similarity of seep faunas since the early Mesozoic. Our findings also highlight that brachiopods and bivalves had a more complex history of coexistence at seeps than currently appreciated.
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