| 1. |
- Gaines, Robert R., et al.
(författare)
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Mechanism for Burgess Shale-type preservation
- 2012
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 109:14, s. 5180-5184
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Tidskriftsartikel (refereegranskat)abstract
- Exceptionally preserved fossil biotas of the Burgess Shale and a handful of other similar Cambrian deposits provide rare but critical insights into the early diversification of animals. The extraordinary preservation of labile tissues in these geographically widespread but temporally restricted soft-bodied fossil assemblages has remained enigmatic since Walcott’s initial discovery in 1909. Here, we demonstrate the mechanism of Burgess Shale-type preservation using sedimentologic and geochemical data from the Chengjiang, Burgess Shale, and five other principal Burgess Shale-type deposits. Sulfur isotope evidence from sedimentary pyrites reveals that the exquisite fossilization of organic remains as carbonaceous compressions resulted from early inhibition of microbial activity in the sediments by means of oxidant deprivation. Low sulfate concentrations in the global ocean and low-oxygen bottom water conditions at the sites of deposition resulted in reduced oxidant availability. Subsequently, rapid entombment of fossils in fine-grained sediments and early sealing of sediments by pervasive carbonate cements at bed tops restricted oxidant flux into the sediments. A permeability barrier, provided by bed-capping cements that were emplaced at the seafloor, is a feature that is shared among Burgess Shale-type deposits, and resulted from the unusually high alkalinity of Cambrian oceans. Thus, Burgess Shale-type preservation of soft-bodied fossil assemblages worldwide was promoted by unique aspects of early Paleozoic seawater chemistry that strongly impacted sediment diagenesis, providing a fundamentally unique record of the immediate aftermath of the “Cambrian explosion.”
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| 2. |
- Canfield, Donald E., et al.
(författare)
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A Mesoproterozoic iron formation
- 2018
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424. ; 115:17, s. 3895-3904
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Tidskriftsartikel (refereegranskat)abstract
- We describe a 1,400 million-year old (Ma) iron formation (IF) from the Xiamaling Formation of the North China Craton. We estimate this IF to have contained at least 520 gigatons of authigenic Fe, comparable in size to many IFs of the Paleoproterozoic Era (2,500–1,600 Ma). Therefore, substantial IFs formed in the time window between 1,800 and 800 Ma, where they are generally believed to have been absent. The Xiamaling IF is of exceptionally low thermal maturity, allowing the preservation of organic biomarkers and an unprecedented view of iron-cycle dynamics during IF emplacement. We identify tetramethyl aryl isoprenoid (TMAI) biomarkers linked to anoxygenic photosynthetic bacteria and thus phototrophic Fe oxidation. Although we cannot rule out other pathways of Fe oxidation, iron and organic matter likely deposited to the sediment in a ratio similar to that expected for anoxygenic photosynthesis. Fe reduction was likely a dominant and efficient pathway of organic matter mineralization, as indicated by organic matter maturation by Rock Eval pyrolysis combined with carbon isotope analyses: Indeed, Fe reduction was seemingly as efficient as oxic respiration. Overall, this Mesoproterozoic-aged IF shows many similarities to Archean-aged (>2,500 Ma) banded IFs (BIFs), but with an exceptional state of preservation, allowing an unprecedented exploration of Fe-cycle dynamics in IF deposition.
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| 3. |
- Canfield, Donald E., et al.
(författare)
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Oxygen dynamics in the aftermath of the Great Oxidation of the Earth’s atmosphere.
- 2013
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Ingår i: Proceedings of the National Academy of Sciences. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 110:42, s. 16736-16741
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Tidskriftsartikel (refereegranskat)abstract
- The oxygen content of Earth’s atmosphere has varied greatly through time, progressing from exceptionally low levels before about 2.3 billion years ago, to much higher levels afterward. In the absence of better information, we usually view the progress in Earth’s oxygenation as a series of steps followed by periods of relative stasis. In contrast to this view, and as reported here, a dynamic evolution of Earth’s oxygenation is recorded in ancient sediments from the Republic of Gabon from between about 2,150 and 2,080 million years ago. The oldest sediments in this sequence were deposited in well-oxygenated deep waters whereas the youngest were deposited in euxinic waters, which were globally extensive. These fluctuations in oxygenation were likely driven by the comings and goings of the Lomagundi carbon isotope excursion, the longest–lived positive ?13C excursion in Earth history, generating a huge oxygen source to the atmosphere. As the Lomagundi event waned, the oxygen source became a net oxygen sink as Lomagundi organic matter became oxidized, driving oxygen to low levels; this state may have persisted for 200 million years.
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| 4. |
- Dahl, Tais W., et al.
(författare)
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Devonian rise in atmospheric oxygen correlated to the radiations of terrestrial plants and large predatory fish
- 2010
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 107:42, s. 17911-17915
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Tidskriftsartikel (refereegranskat)abstract
- The evolution of Earth's biota is intimately linked to the oxygenation of the oceans and atmosphere. We use the isotopic composition and concentration of molybdenum (Mo) in sedimentary rocks to explore this relationship. Our results indicate two episodes of global ocean oxygenation. The first coincides with the emergence of the Ediacaran fauna, including large, motile bilaterian animals, ca. 550-560 million year ago (Ma), reinforcing previous geochemical indications that Earth surface oxygenation facilitated this radiation. The second, perhaps larger, oxygenation took place around 400 Ma, well after the initial rise of animals and, therefore, suggesting that early metazoans evolved in a relatively low oxygen environment. This later oxygenation correlates with the diversification of vascular plants, which likely contributed to increased oxygenation through the enhanced burial of organic carbon in sediments. It also correlates with a pronounced radiation of large predatory fish, animals with high oxygen demand. We thereby couple the redox history of the atmosphere and oceans to major events in animal evolution.
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| 5. |
- Dahl, Tais W., et al.
(författare)
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Reply to Butterfield : The Devonian radiation of large predatory fish coincided with elevated athospheric oxygen levels
- 2011
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 108:9, s. E29-
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Tidskriftsartikel (refereegranskat)abstract
- We welcome this opportunity to clarify the conclusions and implications of our recent publication in PNAS. Butterfield (1) raises four issues regarding the oxygenation of the Paleozoic Earth's surface and its correlation to animal evolution. Our geochemical and paleontological data supported ocean oxygenation in the Silurian-Early Devonian (2), a critical transition in Earth history that influenced biogeochemical cycles and biological systems. First, Butterfield suggests that evidence of charcoal in late Silurian rocks is incompatible with our claim that the earlier Paleozoic atmosphere had oxygen levels below 50% PAL (present-day atmospheric level). This counterargument rests on the assumption that the “fire window” of 62–166% PAL oxygen is well defined, but this is not the case (3).
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| 6. |
- Dahl, Tais W., et al.
(författare)
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Sulfidic anoxia in the oceans during the Late Ordovician mass extinctions – insights from molybdenum and uranium isotopic global redox proxies
- 2021
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Ingår i: Earth-Science Reviews. - : Elsevier BV. - 0012-8252. ; 220
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Forskningsöversikt (refereegranskat)abstract
- The Late Ordovician Mass Extinction wiped out 85% of animal species in two phases (LOME1 and LOME2). The kill mechanisms for the extinction phases are debated, but deteriorating climate and the expansion of marine anoxia appear to have been important factors. Nevertheless, the spatial extent and intensity of marine anoxia and its temporal relationship with the extinctions are not well understood. Here, we review existing global paleoredox proxy data based on molybdenum (Mo) and uranium (U) isotopes from four paleocontinents combined with new Mo isotope data from Dob's Linn, Scotland. Individually, these sedimentary records demonstrate significant redox fluctuations, but our coupled dynamic oceanic mass balance model for the evolution of the marine Mo and U cycles reveals that globally expansive ocean anoxia is best constrained by δ238U in carbonates from Anticosti Island that record expansive anoxia during LOME2. In addition, we consider periodic sulfidic anoxia developing in well-ventilated parts of the shallow oceans (e.g. during warmer periods with greater solar insolation) to have produced temporarily high seawater δ98Mo values during LOME1 in accordance with trends to high values observed in the sedimentary records. In this view, oceanic oxygen loss had a causal role during both extinction phases in the Late Ordovician.
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| 7. |
- El Albani, Abderrazak, et al.
(författare)
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Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago
- 2010
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 466:7302, s. 100-104
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Tidskriftsartikel (refereegranskat)abstract
- The evidence for macroscopic life during the Palaeoproterozoic era (2.5-1.6 Gyr ago) is controversial(1-5). Except for the nearly 2-Gyr-old coil-shaped fossil Grypania spiralis(6,7), which may have been eukaryotic, evidence for morphological and taxonomic bio-diversification of macroorganisms only occurs towards the beginning of the Mesoproterozoic era (1.6-1.0 Gyr)(8). Here we report the discovery of centimetre-sized structures from the 2.1-Gyr-old black shales of the Palaeoproterozoic Francevillian B Formation in Gabon, which we interpret as highly organized and spatially discrete populations of colonial organisms. The structures are up to 12 cm in size and have characteristic shapes, with a simple but distinct ground pattern of flexible sheets and, usually, a permeating radial fabric. Geochemical analyses suggest that the sediments were deposited under an oxygenated water column. Carbon and sulphur isotopic data indicate that the structures were distinct biogenic objects, fossilized by pyritization early in the formation of the rock. The growth patterns deduced from the fossil morphologies suggest that the organisms showed cell-to-cell signalling and coordinated responses, as is commonly associated with multicellular organization(9). The Gabon fossils, occurring after the 2.45-2.32-Gyr increase in atmospheric oxygen concentration(10), may be seen as ancient representatives of multicellular life, which expanded so rapidly 1.5 Gyr later, in the Cambrian explosion.
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| 8. |
- El Albani, Abder, et al.
(författare)
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Organism motility in an oxygenated shallow-marine environment 2.1 billion years ago
- 2019
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Ingår i: Proceedings of the National Academy of Sciences. - : Proceedings of the National Academy of Sciences. - 1091-6490 .- 0027-8424. ; 116:9, s. 3431-3436
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Tidskriftsartikel (refereegranskat)abstract
- The 2.1 billion-year-old sedimentary strata contain exquisitely preserved fossils that provide an ecologic snapshot of the biota inhabiting an oxygenated shallow-marine environment. Most striking are the pyritized string-shaped structures, which suggest that the producer have been a multicellular or syncytial organism able to migrate laterally and vertically to reach for food resources. A modern analogue is the aggregation of amoeboid cells into a migratory slug phase in modern cellular slime molds during time of food starvation. While it remains uncertain whether the amoeboidlike organisms represent a failed experiment or a prelude to subsequent evolutionary innovations, they add to the growing record of comparatively complex life forms that existed more than a billion years before animals emerged in the late Neoproterozoic.Evidence for macroscopic life in the Paleoproterozoic Era comes from 1.8 billion-year-old (Ga) compression fossils [Han TM, Runnegar B (1992) Science 257:232–235; Knoll et al. (2006) Philos Trans R Soc Lond B 361:1023–1038], Stirling biota [Bengtson S et al. (2007) Paleobiology 33:351–381], and large colonial organisms exhibiting signs of coordinated growth from the 2.1-Ga Francevillian series, Gabon. Here we report on pyritized string-shaped structures from the Francevillian Basin. Combined microscopic, microtomographic, geochemical, and sedimentologic analyses provide evidence for biogenicity, and syngenicity and suggest that the structures underwent fossilization during early diagenesis close to the sediment–water interface. The string-shaped structures are up to 6 mm across and extend up to 170 mm through the strata. Morphological and 3D tomographic reconstructions suggest that the producer may have been a multicellular or syncytial organism able to migrate laterally and vertically to reach food resources. A possible modern analog is the aggregation of amoeboid cells into a migratory slug phase in cellular slime molds at times of starvation. This unique ecologic window established in an oxygenated, shallow-marine environment represents an exceptional record of the biosphere following the crucial changes that occurred in the atmosphere and ocean in the aftermath of the great oxidation event (GOE).
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| 9. |
- El Albani, Abderrazak, et al.
(författare)
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The 2.1 Ga old Francevillian biota: biogenicity, taphonomy and biodiversity.
- 2014
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Ingår i: PLOS ONE. - : Public Library of Science (PLoS). - 1932-6203. ; 9:6:e99438, s. 1-18
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Tidskriftsartikel (refereegranskat)abstract
- The Paleoproterozoic Era witnessed crucial steps in the evolution of Earth’s surface environments following the first appreciable rise of free atmospheric oxygen concentrations ~2.3 to 2.1 Ga ago, and concomitant shallow ocean oxygenation. While most sedimentary successions deposited during this time interval have experienced thermal overprinting from burial diagenesis and metamorphism, the ca. 2.1 Ga black shales of the Francevillian B Formation (FB2) cropping out in southeastern Gabon have not. The Francevillian Formation contains centimeter-sized structures interpreted as organized and spatially discrete populations of colonial organisms living in an oxygenated marine ecosystem. Here, new material from the FB2 black shales is presented and analyzed to further explore its biogenicity and taphonomy. Our extended record comprises variably sized, shaped, and structured pyritized macrofossils of lobate, elongated, and rodshaped morphologies as well as abundant non-pyritized disk-shaped macrofossils and organic-walled acritarchs. Combined microtomography, geochemistry, and sedimentary analysis suggest a biota fossilized during early diagenesis. The emergence of this biota follows a rise in atmospheric oxygen, which is consistent with the idea that surface oxygenation allowed the evolution and ecological expansion of complex megascopic life.
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| 10. |
- Gaines, Robert R., et al.
(författare)
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Burgess Shale-type biotas were not entirely burrowed away
- 2012
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 40:3, s. 283-286
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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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