Revisiting marine redox conditions during the Ediacaran Shuram carbon isotope excursion.

The Neoproterozoic carbonate record contains multiple carbon isotope anomalies, which are the subject of intense debate. The largest of these anomalies, the Shuram excursion (SE), occurred in the mid-Ediacaran (~574-567 Ma). Accurately reconstructing marine redox landscape is a clear path toward making sense of the mechanism that drives this δ13 C anomaly. Here, we report new uranium isotopic data from the shallow-marine carbonates of the Wonoka Formation, Flinders Ranges, South Australia, where the SE is well preserved. Our data indicate that the δ238 U trend during the SE is highly reproducible across globally disparate sections from different depositional settings. Previously, it was proposed that the positive shift of δ238 U values during the SE suggests an extensive, near-modern level of marine oxygenation. However, recent publications suggest that the fractionation of uranium isotopes in ferruginous and anoxic conditions is comparable, opening up the possibility of non-unique interpretations of the carbonate uranium isotopic record. Here, we build on this idea by investigating the SE in conjunction with additional geochemical proxies. Using a revised uranium isotope mass balance model and an inverse stochastic carbon cycle model, we reevaluate models for δ13 C and δ238 U trends during the SE. We suggest that global seawater δ238 U values during the SE could be explained by an expansion of ferruginous conditions and do not require a near-modern level of oxygenation during the mid-Ediacaran.

Significant fluctuation in the global sulfate reservoir and oceanic redox state during the Late Devonian event.

Ocean sulfate concentration might have fluctuated greatly throughout the Earth's history and may serve as a window into perturbations in the ocean-atmosphere system. Coupling high-resolution experimental results with an inverse modeling approach, we, here, show an unprecedented dynamic in the global sulfate reservoir during the Frasnian-Famennian (F-F) boundary event, as one of the "Big five" Phanerozoic biotic crises. Notably, our results indicate that, in a relatively short-time scale (∼200 thousand years), seawater sulfate concentration would have dropped from several mM before the Upper Kellwasser Horizon (UKH) to an average of 235 ± 172 µM at the end of the UKH (more than 100 times lower than the modern level) as the result of evaporite deposition and euxinia, and returned to around mM range after the event. Our findings indicate that the instability in the global sulfate reservoir and nutrient-poor oceans may have played a major role in driving the Phanerozoic biological crises.

A largely invariant marine dissolved organic carbon reservoir across Earth's history.

Marine dissolved organic carbon (DOC), the largest pool of reduced carbon in the oceans, plays an important role in the global carbon cycle and contributes to the regulation of atmospheric oxygen and carbon dioxide abundances. Despite its importance in global biogeochemical cycles, the long-term history of the marine DOC reservoir is poorly constrained. Nonetheless, significant changes to the size of the oceanic DOC reservoir through Earth's history have been commonly invoked to explain changes to ocean chemistry, carbon cycling, and marine ecology. Here, we present a revised view of the evolution of marine DOC concentrations using a mechanistic carbon cycle model that can reproduce DOC concentrations in both oxic and anoxic modern environments. We use this model to demonstrate that the overall size of the marine DOC reservoir has likely undergone very little variation through Earth's history, despite major changes in the redox state of the ocean-atmosphere system and the nature and efficiency of the biological carbon pump. A relatively static marine DOC reservoir across Earth's history renders it unlikely that major changes in marine DOC concentrations have been responsible for driving massive repartitioning of surface carbon or the large carbon isotope excursions observed in Earth's stratigraphic record and casts doubt on previously hypothesized links between marine DOC levels and the emergence and radiation of early animals.

Biogeochemical and physical controls on methane fluxes from two ferruginous meromictic lakes.

Meromictic lakes with anoxic bottom waters often have active methane cycles whereby methane is generally produced biogenically under anoxic conditions and oxidized in oxic surface waters prior to reaching the atmosphere. Lakes that contain dissolved ferrous iron in their deep waters (i.e., ferruginous) are rare, but valuable, as geochemical analogues of the conditions that dominated the Earth's oceans during the Precambrian when interactions between the iron and methane cycles could have shaped the greenhouse regulation of the planet's climate. Here, we explored controls on the methane fluxes from Brownie Lake and Canyon Lake, two ferruginous meromictic lakes that contain similar concentrations (max. >1 mM) of dissolved methane in their bottom waters. The order Methanobacteriales was the dominant methanogen detected in both lakes. At Brownie Lake, methanogen abundance, an increase in methane concentration with respect to depths closer to the sediment, and isotopic data suggest methanogenesis is an active process in the anoxic water column. At Canyon Lake, methanogenesis occurred primarily in the sediment. The most abundant aerobic methane-oxidizing bacteria present in both water columns were associated with the Gammaproteobacteria, with little evidence of anaerobic methane oxidizing organisms being present or active. Direct measurements at the surface revealed a methane flux from Brownie Lake that was two orders of magnitude greater than the flux from Canyon Lake. Comparison of measured versus calculated turbulent diffusive fluxes indicates that most of the methane flux at Brownie Lake was non-diffusive. Although the turbulent diffusive methane flux at Canyon Lake was attenuated by methane oxidizing bacteria, dissolved methane was detected in the epilimnion, suggestive of lateral transport of methane from littoral sediments. These results highlight the importance of direct measurements in estimating the total methane flux from water columns, and that non-diffusive transport of methane may be important to consider from other ferruginous systems.

Organic sulfur was integral to the Archean sulfur cycle.

The chemistry of the Early Earth is widely inferred from the elemental and isotopic compositions of sulfidic sedimentary rocks, which are presumed to have formed globally through the reduction of seawater sulfate or locally from hydrothermally supplied sulfide. Here we argue that, in the anoxic Archean oceans, pyrite could form in the absence of ambient sulfate from organic sulfur contained within living cells. Sulfides could be produced through mineralization of reduced sulfur compounds or reduction of organic-sourced sulfite. Reactive transport modeling suggests that, for sulfate concentrations up to tens of micromolar, organic sulfur would have supported 20 to 100% of sedimentary pyrite precipitation and up to 75% of microbial sulfur reduction. The results offer an alternative explanation for the low range of δ34S in Archean sulfides, and raise a possibility that sulfate scarcity delayed the evolution of dissimilatory sulfate reduction until the initial ocean oxygenation around 2.7 Ga.

Sedimentary sulfur isotopes and Neoarchean ocean oxygenation.

Abrupt disappearance of mass-independent fractionation of sulfur isotopes (MIF-S) from the geologic record and an apparent ingrowth in seawater sulfate around 2.45 billion years ago (Ga) signal the first large-scale oxygenation of the atmosphere [the Great Oxygenation Event (GOE)]. Pre-GOE O2 production is evident from multiple other terrestrial and marine proxies, but oceanic O2 concentrations remain poorly constrained. Furthermore, current interpretations of S isotope records do not explain a concurrent expansion in the range of both MIF-S-diagnostic for low atmospheric O2-and δ34S beginning at 2.7 Ga. To address these unknowns, we developed a reaction-transport model to analyze the preservation patterns of sulfur isotopes in Archean sedimentary pyrites, one of the most robust and widely distributed proxies for early Earth biogeochemistry. Our modeling, paradoxically, reveals that micromolar levels of O2 in seawater enhance the preservation of large MIF-S signals, whereas concomitant ingrowth of sulfate expands the ranges in pyrite δ34S. The 2.7- to 2.45-Ga expansion in both Δ33S and δ34S ranges thus argues for a widespread and protracted oxygenation of seawater, at least in shallow marine environments. At the micromolar levels predicted, the surface oceans would support a strong flux of O2 to the atmosphere, where O2 sinks balanced these fluxes until the GOE. This microoxic seawater would have provided habitat for early aerobic microorganisms and supported a diversity of new O2-driven biogeochemical cycles in the Neoarchean.