| 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. |
- Lenton, Timothy M., et al.
(författare)
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Earliest land plants created modern levels of atmospheric oxygen
- 2016
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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. ; 113:35, s. 9704-9709
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Tidskriftsartikel (refereegranskat)abstract
- The progressive oxygenation of the Earth's atmosphere was pivotal to the evolution of life, but the puzzle of when and how atmospheric oxygen (O-2) first approached modern levels (similar to 21%) remains unresolved. Redox proxy data indicate the deep oceans were oxygenated during 435-392 Ma, and the appearance of fossil charcoal indicates O-2 > 15-17% by 420-400 Ma. However, existing models have failed to predict oxygenation at this time. Here we show that the earliest plants, which colonized the land surface from similar to 470 Ma onward, were responsible for this mid-Paleozoic oxygenation event, through greatly increasing global organic carbon burialthe net long-term source of O-2. We use a trait-based ecophysiological model to predict that cryptogamic vegetation cover could have achieved similar to 30% of today's global terrestrial net primary productivity by similar to 445 Ma. Data from modern bryophytes suggests this plentiful early plant material had a much higher molar C:P ratio (similar to 2,000) than marine biomass (similar to 100), such that a given weathering flux of phosphorus could support more organic carbon burial. Furthermore, recent experiments suggest that early plants selectively increased the flux of phosphorus (relative to alkalinity) weathered from rocks. Combining these effects in a model of long-term biogeochemical cycling, we reproduce a sustained +2% increase in the carbonate carbon isotope (delta C-13) record by similar to 445 Ma, and predict a corresponding rise in O-2 to present levels by 420-400 Ma, consistent with geochemical data. This oxygen rise represents a permanent shift in regulatory regime to one where fire-mediated negative feedbacks stabilize high O-2 levels.
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| 5. |
- Dahl, Tais W., et al.
(författare)
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Do large predatory fish track ocean oxygenation?
- 2011
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Ingår i: Communicative & Integrative Biology. - : Informa UK Limited. - 1942-0889. ; 4:1, s. 92-94
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Tidskriftsartikel (refereegranskat)abstract
- The Devonian appearance of 1-10 meter long armored fish (placoderms) coincides with geochemical evidence recording a transition into fully oxygenated oceans. A comparison of extant fish shows that the large individuals are less tolerant to hypoxia than their smaller cousins. This leads us to hypothesize that Early Paleozoic O2 saturation levels were too low to support >1 meter size marine, predatory fish. According to a simple model, both oxygen uptake and oxygen demand scale positively with size, but the demand exceeds supply for the largest fish with an active, predatory life style. Therefore, the largest individuals may lead us to a lower limit on oceanic O2 concentrations. Our presented model suggests 2-10 meter long predators require >30-50% PAL while smaller fish would survive at <25% PAL. This is consistent with the hypothesis that low atmospheric oxygen pressure acted as an evolutionary barrier for fish to grow much above ~1 meter before the Devonian oxygenation.
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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. |
- del Rey, Álvaro, et al.
(författare)
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Mid-Ludfordian uranium isotope records distinguish the role of expansive marine anoxia in global carbon cycle dynamics during the late Silurian Lau/Kozlowskii bioevent
- 2023
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Ingår i: Global and Planetary Change. - 0921-8181. ; 229
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Tidskriftsartikel (refereegranskat)abstract
- The late Silurian Lau/Kozlowskii bioevent marks a time interval with substantial loss in marine biodiversity linked to the largest positive carbon isotope excursion (Mid-Ludfordian CIE; MLCIE) recorded in the Phanerozoic (δ13Ccarb peaks at +8–10‰). The positive δ13C excursion and the extinctions have been linked to increased marine productivity (ocean eutrophication), leading to increased organic carbon burial, and expansion of marine anoxia in shallow continental seas. To explore this idea, we reconstruct the contemporaneous global extent of marine anoxia using uranium (U) isotopes from Ludfordian marine carbonates sampled from two widely spaced paleogeographic locations. Our δ238U results demonstrate that the overall MLCIE was not coupled to expanding marine anoxia and a presumed associated increased marine primary productivity. Instead, a time interval of widespread global anoxia occurred only before and during the onset of the MLCIE (up to end of P. siluricus conodont zone; ca. δ13C ∼ 2.6‰), which is when the extinctions took place. Strong climatic variability at this time suggests anoxia and climate change led to the extinctions of the Lau/Kozlowskii bioevent. The major part of the rise to the peak (δ13C ∼ 10‰) and falling limb of the MLCIE was not coupled with corresponding changes in δ238U values. Thus, increased marine primary productivity and marine anoxia may have caused the initial carbon cycle disturbance and the extinctions but was not a main driver for the continued growth and exceptional magnitude of the MLCIE.
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| 8. |
- del Rey, Álvaro, et al.
(författare)
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Stable ocean redox during the main phase of the Great Ordovician Biodiversification Event
- 2022
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Ingår i: Communications Earth and Environment. - : Springer Science and Business Media LLC. - 2662-4435. ; 3:1
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Tidskriftsartikel (refereegranskat)abstract
- The Great Ordovician Biodiversification Event (GOBE) represents the greatest increase in marine animal biodiversity ever recorded. What caused this transformation is heavily debated. One hypothesis states that rising atmospheric oxygen levels drove the biodiversification based on the premise that animals require oxygen for their metabolism. Here, we present uranium isotope data from a Middle Ordovician marine carbonate succession that shows the steepest rise in generic richness occurred with global marine redox stability. Ocean oxygenation ensued later and could not have driven the biodiversification. Stable marine anoxic zones prevailed during the maximum increase in biodiversity (Dapingian–early Darriwilian) when the life expectancy of evolving genera greatly increased. Subsequently, unstable ocean redox conditions occurred together with a marine carbon cycle disturbance and a decrease in relative diversification rates. Therefore, we propose that oceanic redox stability was a factor in facilitating the establishment of more resilient ecosystems allowing marine animal life to radiate.
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| 9. |
- Hammarlund, Emma U., 1971-, et al.
(författare)
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A sulfidic driver for the end-Ordovician mass extinction
- 2012
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Ingår i: Earth and Planetary Science Letters. - : Elsevier BV. - 0012-821X .- 1385-013X. ; 331, s. 128-139
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Tidskriftsartikel (refereegranskat)abstract
- The end-Ordovician extinction consisted of two discrete pulses, both linked, in various ways, to glaciation at the South Pole. The first phase, starting just below the Normalograptus extraordinarius Zone, particularly affected nektonic and planktonic species, while the second pulse, associated with the Normalograptus persculptus Zone, was less selective. Glacially induced cooling and oxygenation are two of many suggested kill mechanisms for the end-Ordovician extinction, but a general consensus is lacking. We have used geochemical redox indicators, such as iron speciation, molybdenum concentrations, pyrite framboid size distribution and sulfur isotopes to analyze the geochemistry in three key Hirnantian sections. These indicators reveal that reducing conditions were occasionally present at all three sites before the first pulse of the end-Ordovician extinction, and that these conditions expanded towards the second pulse. Even though the N. extraordinarius Zone appears to have been a time of oxygenated deposition, pyrite is significantly enriched in 34S in our sections as well as in sections reported from South China. This suggests a widespread reduction in marine sulfate concentrations, which we attribute to an increase in pyrite burial during the early Hirnantian. The S-isotope excursion coincides with a major positive carbon isotope excursion indicating elevated rates of organic carbon burial as well. We argue that euxinic conditions prevailed and intensified in the early Hirnantian oceans, and that a concomitant global sea level lowering pushed the chemocline deeper than the depositional setting of our sites. In the N. persculptus Zone, an interval associated with a major sea level rise, our redox indicators suggests that euxinic conditions, and ferruginous in some places, encroached onto the continental shelves. In our model, the expansion of euxinic conditions during the N. extraordinarius Zone was generated by a reorganization of nutrient cycling during sea level fall, and we argue, overall, that these dynamics in ocean chemistry played an important role for the end-Ordovician mass extinction. During the first pulse of the extinction, euxinia and a steepened oxygen gradient in the water column caused habitat loss for deep-water benthic and nektonic organisms. During the second pulse, the transgression of anoxic water onto the continental shelves caused extinction in shallower habitats.
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| 10. |
- Zhao, Zhengfu, et al.
(författare)
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High-resolution carbon isotope chemostratigraphy of the middle Cambrian to lowermost Ordovician in southern Scandinavia : Implications for global correlation
- 2022
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Ingår i: Global and Planetary Change. - : Elsevier BV. - 0921-8181. ; 209
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Tidskriftsartikel (refereegranskat)abstract
- Continuous and biostratigraphically well-constrained carbon isotope (δ13Corg) profiles through the upper Miaolingian (middle Cambrian) to Tremadocian (Lower Ordovician) part of the Alum Shale Formation in four drill cores from southernmost Scandinavia are described. The sections record seven Furongian (upper Cambrian) excursions/spikes, of which three appear valuable as auxiliary markers for identifying global stage boundaries. The Steptoean Positive Carbon Isotope Excursion (SPICE) event with a magnitude of up to 2.8‰ is located mainly within the Olenus Superzone. The onset, as specified here, coincides with the end-Marjuman extinction and is marked by a minor negative excursion (up to −1.6‰) right below the Miaolingian–Furongian boundary. The end of the SPICE event approximates the upper boundary of the Olenus Superzone and has potential for correlation of the Paibian–Jiangshanian boundary. The Top of Cambrian Excursion (TOCE) with a magnitude of about −1.6‰ straddles the Parabolina lobata–Parabolina heres megalops zonal boundary in Baltoscandia. This chemostratigraphic excursion, considered as a synonym of the Hellnmaria-Red Tops Boundary excursion (HERB), represents a secondary marker for correlating the base of Stage 10, if defined by the FAD of Eoconodontus notchpeakensis. A positive spike at the Cambrian–Ordovician boundary, here named COBS, recognized in the stratotype section at Green Point (Canada; δ13Ccarb magnitude +5.3‰), Scandinavia (δ13Corg magnitude up to +2.2‰) and elsewhere, appears as a valuable supplementary marker for recognizing this important boundary. The described δ13Corg profile from the Albjära-1 core provides a standard reference for Furongian carbon isotope chemostratigraphy in Baltica, refines the correlation between Baltica and other palaeocontinents, and indicates new possibilities for pinpointing international stage boundaries within the Furongian based on carbon isotope records.
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