This content will become publicly available on March 29, 2023
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- Proceedings of the National Academy of Sciences
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- National Science Foundation
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Triple oxygen isotope constraints on atmospheric O 2 and biological productivity during the mid-ProterozoicReconstructing the history of biological productivity and atmospheric oxygen partial pressure ( p O 2 ) is a fundamental goal of geobiology. Recently, the mass-independent fractionation of oxygen isotopes (O-MIF) has been used as a tool for estimating p O 2 and productivity during the Proterozoic. O-MIF, reported as Δ′ 17 O, is produced during the formation of ozone and destroyed by isotopic exchange with water by biological and chemical processes. Atmospheric O-MIF can be preserved in the geologic record when pyrite (FeS 2 ) is oxidized during weathering, and the sulfur is redeposited as sulfate. Here, sedimentary sulfates from the ∼1.4-Ga Sibley Formation are reanalyzed using a detailed one-dimensional photochemical model that includes physical constraints on air–sea gas exchange. Previous analyses of these data concluded that p O 2 at that time was <1% PAL (times the present atmospheric level). Our model shows that the upper limit on p O 2 is essentially unconstrained by these data. Indeed, p O 2 levels below 0.8% PAL are possible only if atmospheric methane was more abundant than today (so that p CO 2 could have been lower) or if the Sibley O-MIF data were diluted by reprocessing before the sulfates weremore »
Sulfur belongs among H2O, CO2, and Cl as one of the key volatiles in Earth’s chemical cycles. High oxygen fugacity, sulfur concentration, and δ34S values in volcanic arc rocks have been attributed to significant sulfate addition by slab fluids. However, sulfur speciation, flux, and isotope composition in slab-dehydrated fluids remain unclear. Here, we use high-pressure rocks and enclosed veins to provide direct constraints on subduction zone sulfur recycling for a typical oceanic lithosphere. Textural and thermodynamic evidence indicates the predominance of reduced sulfur species in slab fluids; those derived from metasediments, altered oceanic crust, and serpentinite have δ34S values of approximately −8‰, −1‰, and +8‰, respectively. Mass-balance calculations demonstrate that 6.4% (up to 20% maximum) of total subducted sulfur is released between 30–230 km depth, and the predominant sulfur loss takes place at 70–100 km with a net δ34S composition of −2.5 ± 3‰. We conclude that modest slab-to-wedge sulfur transport occurs, but that slab-derived fluids provide negligible sulfate to oxidize the sub-arc mantle and cannot deliver34S-enriched sulfur to produce the positive δ34S signature in arc settings. Most sulfur has negative δ34S and is subducted into the deep mantle, which could cause a long-term increase in the δ34S of Earth surface reservoirs.
Photic zone euxinia (PZE) is a condition where anoxic, H2S-rich waters occur in the photic zone (PZ). PZE has been invoked as an impediment to the evolution of complex life on early Earth and as a kill mechanism for Phanerozoic mass extinctions. Here, we investigate the potential application of mercury (Hg) stable isotopes in marine sedimentary rocks as a proxy for PZE by measuring Hg isotope compositions in late Mesoproterozoic (∼1.1 Ga) shales that have independent evidence of PZE during discrete intervals. Strikingly, a significantly negative shift of Hg mass-independent isotope fractionation (MIF) was observed during euxinic intervals, suggesting changes in Hg sources or transformations in oceans coincident with the development of PZE. We propose that the negative shift of Hg MIF was most likely caused by (
i) photoreduction of Hg(II) complexed by reduced sulfur ligands in a sulfide-rich PZ, and ( ii) enhanced sequestration of atmospheric Hg(0) to the sediments by thiols and sulfide that were enriched in the surface ocean as a result of PZE. This study thus demonstrates that Hg isotope compositions in ancient marine sedimentary rocks can be a promising proxy for PZE and therefore may provide valuable insights into changes in ocean chemistry and its impactmore »
Oxygen isotopic investigation of silicic magmatism in the Stillwater caldera complex, Nevada: Generation of large volume, low-Œ¥18O rhyolitic tuffs and assessment of their regional contextABSTRACT 18 Successive caldera-forming eruptions from ~30-25 Ma generated a large nested 19 caldera complex in western Nevada that was subsequently dissected by Basin and Range 20 extension, providing extraordinary cross-sectional views through a diverse range of 21 eruptive and intrusive products. A high-resolution oxygen isotopic study was conducted 22 on units that represent all major parts of the Job Canyon, Poco Canyon, Elevenmile 23 Canyon, and Louderback Mountains caldera cycles (29.2-25.1 Ma), and several 24 Cretaceous basement plutons that flank the Stillwater caldera complex. We also provide 2 25 new oxygen and strontium isotope data for regional caldera centers in the Great Basin, 26 which are synthesized with published oxygen and strontium isotope data for regional 27 Mesozoic basement rocks. Stillwater zircons span a large isotopic range (d18Ozircon of 3.6 28 to 8.2‰), and all caldera cycles possess low-d18O zircons. In some cases, they are a small 29 proportion of the total populations, and in others they dominate, such as in the low-d18O 30 rhyolitic tuffs of Job Canyon and Poco Canyon (d18Ozircon=4.0-4.3‰ and calculated d18O 31 magma=5.5-6‰). These are the first low-d18O rhyolites documented in middle Cenozoic 32 calderas of the Great Basin, adding to the global occurrencemore »
The mass-independent minor oxygen isotope compositions (Δ′17O) of atmospheric O2and
are primarily regulated by their relative partial pressures, / . Pyrite oxidation during chemical weathering on land consumes and generates sulfate that is carried to the ocean by rivers. The Δ′17O values of marine sulfate deposits have thus been proposed to quantitatively track ancient atmospheric conditions. This proxy assumes direct incorporation into terrestrial pyrite oxidation-derived sulfate, but a mechanistic understanding of pyrite oxidation—including oxygen sources—in weathering environments remains elusive. To address this issue, we present sulfate source estimates and Δ′17O measurements from modern rivers transecting the Annapurna Himalaya, Nepal. Sulfate in high-elevation headwaters is quantitatively sourced by pyrite oxidation, but resulting Δ′17O values imply no direct tropospheric incorporation. Rather, our results necessitate incorporation of oxygen atoms from alternative,17O-enriched sources such as reactive oxygen species. Sulfate Δ′17O decreases significantly when moving into warm, low-elevation tributaries draining the same bedrock lithology. We interpret this to reflect overprinting of the pyrite oxidation-derived Δ′17O anomaly by microbial sulfate reduction and reoxidation, consistent with previously described major sulfur and oxygen isotope relationships. The geologic application of sulfate Δ′17O as a proxy for past / should consider both 1) alternative oxygen sources during pyrite oxidation and 2) secondary overprinting by microbial recycling.