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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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| 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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