| 1. |
- Gill, Benjamin C., et al.
(författare)
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Redox dynamics of later Cambrian oceans
- 2021
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Ingår i: Palaeogeography, Palaeoclimatology, Palaeoecology. - : Elsevier BV. - 0031-0182. ; 581
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Tidskriftsartikel (refereegranskat)abstract
- A growing body of evidence suggests that the deep oceans during the early Paleozoic Era were widely oxygen deficient, despite evidence for increased marine oxygenation during the Neoproterozoic. However, the temporal and geographic extents and dynamics of reducing marine conditions within these oceans are not well understood. Here, we investigate marine redox history during the Drumian through the earliest Jiangshanian International Stages of the Cambrian Period, using concentrations of redox-sensitive metals (vanadium, uranium, and molybdenum), iron speciation, and Mo isotope stratigraphy of the Alum Shale Formation of Scandinavia. These data suggest a major perturbation occurred in trace metal cycling during the later Cambrian Period that was linked to a transient change in marine redox conditions coincident with the well-known Steptoean Positive Isotope Excursion or SPICE. The δ98Mo measurements of the Alum shale show systematic variations during the interval that contains the SPICE which are broadly consistent with a transient expansion of sulfidic, reducing marine environments — indicating a significant exacerbation of an already-common condition during the Cambrian Period. Additionally, iron speciation data record a local transition from predominantly anoxic, ferruginous (Fe+2 containing) to anoxic, euxinic (sulfide containing) water column conditions near the initiation of the SPICE. Trace metal abundances, however, appear to decline well before the start of the SPICE, suggesting an earlier initiation of the global expansion of reducing environments. More broadly, our data and modeling support the notion that significant portions of the oceans remained oxygen deficient throughout the later portion of the Cambrian, and that these oceans were also prone to transient intervals of more reducing conditions similar to the Oceanic Anoxic Events of the Mesozoic.
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| 2. |
- 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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| 3. |
- 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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| 4. |
- Lu, Xinze, et al.
(författare)
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Marine redox conditions during deposition of Late Ordovician and Early Silurian organic-rich mudrocks in the Siljan ring district, central Sweden
- 2017
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Ingår i: Chemical Geology. - : Elsevier BV. - 0009-2541 .- 1872-6836. ; 457, s. 75-94
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Tidskriftsartikel (refereegranskat)abstract
- The Late Ordovician Period witnessed the second largest mass extinction in the Phanerozoic Eon and the Hirnantian glaciation. To infer ocean redox conditions across the Ordovician-Silurian transition, we measured the U (as δ238U relative to standard CRM145 = 0‰) and Mo (as δ98Mo relative to standard NIST SRM 3134 = +0.25‰) isotope compositions of 26 organic-rich mudrock samples from the Late Ordovician (Katian) Fjacka Shale and the Early Silurian (Aeronian-Telychian) Kallholn Formation (Siljan ring district, Sweden). The magnitude of Re, Mo, and U enrichments, ReEF/MoEF and UEF/MoEF ratios, and sedimentary Fe speciation point to locally euxinic bottom water conditions during deposition of the Fjacka Shale. The same proxies suggest that black shales of the Kallholn Formation were deposited under transiently euxinic conditions with the chemocline situated near the sediment-water interface, whereas gray shales stratigraphically equivalent to the upper Kallholn Formation were deposited from oxygenated bottom waters. These observations are consistent with higher δ98Mo and δ238U in the Fjacka Shale compared with the Kallholn Formation.Because the Fjacka Shale was deposited from persistently euxinic bottom waters, the Mo and U isotope compositions from these rocks can be used to estimate the extent of global ocean euxinia and ocean anoxia (euxinic plus ferruginous conditions), respectively. Elevated MoEF and Mo/TOC ratios in the euxinic Fjacka Shale suggest no more than moderate basin restriction from the open ocean as well as non-quantitative removal of Mo from the euxinic bottom waters, thus pointing to Mo isotope fractionation between seawater and the euxinic sediments. Hence, we infer that even the highest δ98Mo (+1.28‰) preserved in the Fjacka Shale is only a minimum estimate for the Mo isotope composition of coeval global seawater. Correcting for seawater-sediment Mo isotope fractionation, the δ98Mo of late Katian seawater may have been +1.4-2.1%0, which corresponds to similar to 10-70% Mo removal into the euxinic sink. The average authigenic δ238U of the Fjacka Shale is 0.05‰ to +0.02‰ after correcting for a range of possible detrital δ238U values, thus yielding an overall average of-0‰. Taking into account isotope fractionation during U removal to euxinic sediments, we infer that late Katian seawater δ238U was about 0.85‰ to 0.60‰. A steady-state U isotope mass balance model reveals that 46-63% of riverine U input was removed in anoxic settings. Based on the Mo and U isotope data, we infer that euxinic and anoxic waters may have covered <1% and at least 5% (potentially tens of percent) of the total seafloor area during the late Katian, respectively, based on previously published models that relate the magnitude of Mo and U burial fluxes to the areal extent of euxinic and anoxic seafloor. By comparison, only 021-035% and <1% of the total seafloor area was covered by anoxic waters today and during the Cenozoic, respectively. The difference between the estimated extent of ocean anoxia (euxinic plus ferruginous) and ocean euxinia points to an appreciable extent of ferruginous water masses during the late Katian. Integration of our data with previous studies thus supports the hypothesis that ocean oxygenation intensified during the subsequent Hirnantian glaciation (when seawater δ98Mo temporarily reached values similar to today). Hence, environmental stresses related to glaciation, not an expansion of ocean anoxia, may have triggered the first phase of the Hirnantian mass extinction.
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| 5. |
- Lu, Xinze, et al.
(författare)
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Reconstruction of local and global marine redox conditions during deposition of Late Ordovician and Early Silurian organic-rich mudrocks in the Siljan ring district, central Sweden
- 2015
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Ingår i: Geological Society of America Abstracts with Programs. Vol. 47, No. 7. ; , s. 698-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The Siljan ring district in central Sweden was created by a bolide impact at 377±2 Ma that triggered oil generation from organic-richmudrocks (ORM) of the Late Ordovician (Katian) Fjäcka Shale and/or the Early Silurian (Rhuddanian-Telychian) Kallholn Formation.New drill cores obtained by Swedish private company IGRENE AB in 2011 provide an opportunity to significantly improve constraintson the global ocean redox conditions before and after the Late Ordovician Hirnantian glaciation using the U and Mo isotopepaleoredox proxies. Here, we analyzed δ238U (relative to standard CRM145 = 0‰) and δ98Mo (relative to standard NIST SRM 3134 =+0.25‰) of 26 ORM samples from the Fjäcka Shale, Kallholn Formation, and latter deposited Nederberga Formation. The extent ofRe, Mo, and U enrichment, Re/Mo and U/Mo ratios, and Fe speciation indicate euxinic and oxygenated bottom water conditions duringdeposition of the Fjäcka Shale and Nederberga Formation, respectively. The same proxies suggest that the Kallholn Formation wasdeposited under transiently euxinic conditions with the chemocline situated near the sediment-water interface.The most euxinic shales provide the most relevant estimates of global redox conditions. As expected, the euxinic Fjäcka Shale yieldsthe highest δ98Mo (~1.3‰) and δ238U (~0.1‰) of the studied units. High Mo/TOC ratios (>30 ppm/wt%) of the Fjäcka Shale indicateweak basin restriction and large amounts of Mo in the euxinic bottom waters, which could lead to Mo isotope fractionations betweenseawater and sediments due to incomplete formation/removal of tetrathiomolybdate. This interpretation is further supported by high Uisotope composition in the Fjäcka Shale, which is only slightly lower than the modeled value of 0.2‰ for modern open ocean euxinicsediments. Expanded ocean anoxia should lead to deposition of ORMs with low δ238U (<0‰) as observed during the Cenomanian-Turonian OAE2. Hence, the relatively high δ238U coupled with high Mo, Re, and U enrichments and Mo/TOC ratios in the Fjäcka Shalesuggest a more oxygenated ocean prior to the Hirnantian glaciation than previously thought, though the extent of oxygenation was lessthan today.
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