Global marine redox changes drove the rise and fall of the Ediacara biota

This is the final version. Available on open access from Wiley via the DOI in this recordThe role of O2 in the evolution of early animals, as represented by some members of the Ediacara biota, has been heavily debated because current geochemical evidence paints a conflicting picture regarding global marine O2 levels during key intervals of the rise and fall of the Ediacara biota. Fossil evidence indicates that the diversification the Ediacara biota occurred during or shortly after the Ediacaran Shuram negative C-isotope Excursion (SE), which is often interpreted to reflect ocean oxygenation. However, there is conflicting evidence regarding ocean oxygen levels during the SE and the middle Ediacaran Period. To help resolve this debate, we examined U isotope variations (δ238U) in three carbonate sections from South China, Siberia, and USA that record the SE. The δ238U data from all three sections are in excellent agreement and reveal the largest positive shift in δ238U ever reported in the geologic record (from ~ −0.74‰ to ~ −0.26‰). Quantitative modeling of these data suggests that the global ocean switched from a largely anoxic state (26%–100% of the seafloor overlain by anoxic waters) to near-modern levels of ocean oxygenation during the SE. This episode of ocean oxygenation is broadly coincident with the rise of the Ediacara biota. Following this initial radiation, the Ediacara biota persisted until the terminal Ediacaran period, when recently published U isotope data indicate a return to more widespread ocean anoxia. Taken together, it appears that global marine redox changes drove the rise and fall of the Ediacara biota.NASADanish Agency for Science, Technology and InnovationNational Science Foundation (NSF)National Key Basic Research Program of ChinaNatural Environment Research Council (NERC)Natural Science Foundation of Chin

Extensive marine anoxia associated with the Late Devonian Hangenberg Crisis

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this record The global Hangenberg Crisis near the Devonian-Carboniferous boundary (DCB) represents one of the major Phanerozoic mass extinction events, which shaped the roots of modern vertebrate biodiversity. Marine anoxia has been cited as the proximate kill mechanism for this event. However, the detailed timing, duration, and extent of global marine redox chemistry changes across this critical interval remain controversial because most of the studies to date only constrain changes in local or regional redox chemistry. Thus, opinions on the significance of anoxia as a kill mechanism are variable—from anoxia being a primary driver to being relatively unimportant. In this study, we explore the evolution of global marine redox chemistry using U isotopes of marine limestones. The δ238U trends at Long'an section in South China document systematic oscillations with three negative shifts punctuated by two positive events in between. The magnitude of the δ238U oscillations implies that the sediments do not record contemporaneous seawater with a constant offset at all times. The lack of covariation between δ238U data and diagenetic indicators (e.g., Mn and Sr contents, Mn/Sr ratio, δ18O) suggests that the δ238U trends are not produced by the same post-depositional diagenetic processes. Instead, trace-metal enrichments suggest that more reducing conditions prevailed during the deposition of the two positive events. We present plausible model scenarios that fit the observed δ238U trends in the context of redox-sensitive trace metal data suggesting marine anoxia expanded in the latest Devonian oceans to cover >5% of the continental shelf seafloor area. The rapid expansion of marine anoxia coincident with the onset of the Hangenberg Crisis supports marine anoxia as an important kill mechanism. Biogeochemical modeling of the coupled C-P-U cycles suggests that intensified continental weathering, for example, assisted by the spread of seed plants with deeper root systems at this time, could have triggered expansion of marine anoxia and other global changes (e.g., positive excursion in δ13Ccarb and decrease in sea surface temperature) in the latest Devonian. The anoxic event is inferred to have been transient as climatic cooling would have reduced weathering fluxes.Natural Environment Research Council (NERC

The terrestrial uranium isotope cycle

Changing conditions on the Earth’s surface can have a remarkable influence on the composition of its overwhelmingly more massive interior. The global distribution of uranium is a notable example. In early Earth history, the continental crust was enriched in uranium. Yet after the initial rise in atmospheric oxygen, about 2.4 billion years ago, the aqueous mobility of oxidized uranium resulted in its significant transport to the oceans and, ultimately, by means of subduction, back to the mantle1, 2, 3, 4, 5, 6, 7, 8. Here we explore the isotopic characteristics of this global uranium cycle. We show that the subducted flux of uranium is isotopically distinct, with high 238U/235U ratios, as a result of alteration processes at the bottom of an oxic ocean. We also find that mid-ocean-ridge basalts (MORBs) have 238U/235U ratios higher than does the bulk Earth, confirming the widespread pollution of the upper mantle with this recycled uranium. Although many ocean island basalts (OIBs) are argued to contain a recycled component9, their uranium isotopic compositions do not differ from those of the bulk Earth. Because subducted uranium was probably isotopically unfractionated before full oceanic oxidation, about 600 million years ago, this observation reflects the greater antiquity of OIB sources. Elemental and isotope systematics of uranium in OIBs are strikingly consistent with previous OIB lead model ages10, indicating that these mantle reservoirs formed between 2.4 and 1.8 billion years ago. In contrast, the uranium isotopic composition of MORB requires the convective stirring of recycled uranium throughout the upper mantle within the past 600 million years

A molybdenum-isotope perspective on Phanerozoic deoxygenation events

The expansion and contraction of sulphidic depositional conditions in the oceans can be tracked with the isotopic composition of molybdenum in marine sediments. However, molybdenum isotope data are often subject to multiple, conflicting interpretations. Here I present a compilation of molybdenum isotope data from three time intervals: the Toarcian Oceanic Anoxic Event about 183 million years ago, Oceanic Anoxic Event 2 about 94 million years ago, and two early Eocene hyperthermal events from 56 to 54 million years ago. A comparison of data from sites located in different hydrographic settings tightly constrains the molybdenum cycle for these intervals, allowing a direct comparison of the expanse of sulphidic conditions in each interval compared to today. Nonetheless, tracing rates of redox change over such rapid climatic events using molybdenum-isotopes remains challenging. Future efforts to achieve this goal might be accomplished by analysing specific mineral phases, using complementary redox-sensitive geochemical techniques, and by linking isotopic observations with Earth System modelling. Such improvements will make it possible to more fully assess the links between ocean deoxygenation, climatic and oceanographic changes, and biotic turnover