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1. Triple oxygen isotope constraints on atmospheric O ₂ and biological productivity during the mid-Proterozoic

   https://doi.org/10.1073/pnas.2105074118  

   Liu, Peng ; Liu, Jingjun ; Ji, Aoshuang ; Reinhard, Christopher T. ; Planavsky, Noah J. ; Babikov, Dmitri ; Najjar, Raymond G. ; Kasting, James F. ( December 2021 , Proceedings of the National Academy of Sciences)

   Reconstructing 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 »deposited. Our model also shows that, contrary to previous assertions, marine productivity cannot be reliably constrained by the O-MIF data because the exchange of molecular oxygen (O 2 ) between the atmosphere and surface ocean is controlled more by air–sea gas transfer rates than by biological productivity. Improved estimates of p CO 2 and/or improved proxies for Δ′ 17 O of atmospheric O 2 would allow tighter constraints to be placed on mid-Proterozoic p O 2 .« less
2. Uncovering and quantifying the subduction zone sulfur cycle from the slab perspective

   https://doi.org/10.1038/s41467-019-14110-4  

   Li, Ji-Lei ; Schwarzenbach, Esther M. ; John, Timm ; Ague, Jay J. ; Huang, Fang ; Gao, Jun ; Klemd, Reiner ; Whitehouse, Martin J. ; Wang, Xin-Shui ( January 2020 , Nature Communications)

   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.
3. Mercury isotope signatures record photic zone euxinia in the Mesoproterozoic ocean

   https://doi.org/10.1073/pnas.1721733115  

   Zheng, Wang ; Gilleaudeau, Geoffrey J. ; Kah, Linda C. ; Anbar, Ariel D. ( October 2018 , Proceedings of the National Academy of Sciences)

   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 »on the evolution of life.

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4. 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 context

   Watts KE, John DA ( April 2019 , Geological Society of America bulletin)

   ABSTRACT 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 »of these important magma 33 types that fingerprint recycling of shallow crust altered by low-d18O meteoric waters. The 34 appearance of low-d18O rhyolites in the Stillwater caldera complex is overprinted on a 35 Great Basin-wide trend of miogeoclinal sediment contribution to silicic magmas that 36 elevates d18O compositions, making identification of 18O depletions difficult. Oxygen and 37 strontium isotopic data and U-Pb zircon age inheritance points to derivation of the low38 d18O tuff of Job Canyon from melting and assimilation of hydrothermally altered 39 Cretaceous granitic basement. In contrast, oxygen and strontium isotopic modeling points 40 to derivation of the low-d18O tuff of Poco Canyon from melting and assimilation of 41 hydrothermally altered intracaldera Job Canyon rocks. Though not a nominally low-d18O 42 rhyolite, the tuff of Elevenmile Canyon possesses both low-d18O and high-d18O zircon 43 cores that are overgrown by homogenized zircon rims that approximate the bulk zircon 44 average, pointing to batch assembly of isotopically diverse upper crustal melts to 45 generate one of the most voluminous (2,500-5,000 km3) tuff eruptions in the Great Basin. 46 Low-d18O zircons in the tuff of Elevenmile Canyon are inherited from the Poco Canyon 47 cycle based on their unique trace and rare earth element chemistry. The tuffs of Poco and 3 48 Elevenmile Canyon both support a caldera cannibalization model in which 49 hydrothermally altered extrusive and intrusive rocks from previous caldera cycles are 50 recycled into later magmas. Though overlapping in space and time, each caldera-forming 51 cycle of the Stillwater complex had a unique oxygen isotope record as retained in single 52 zircons. Even plutons that were spatially and temporally coincident with calderas diverge 53 from the caldera-forming tuffs and cannot be their cogenetic remnants in most cases.« less
5. Triple oxygen isotope insight into terrestrial pyrite oxidation

   https://doi.org/10.1073/pnas.1917518117  

   Hemingway, Jordon D. ; Olson, Haley ; Turchyn, Alexandra V. ; Tipper, Edward T. ; Bickle, Mike J. ; Johnston, David T. ( March 2020 , Proceedings of the National Academy of Sciences)

   The mass-independent minor oxygen isotope compositions (Δ′¹⁷O) of atmospheric O₂and${\mathrm{C}\mathrm{O}}_2$are primarily regulated by their relative partial pressures,${\mathrm{p}\mathrm{O}}_2$/${\mathrm{p}\mathrm{C}\mathrm{O}}_2$. Pyrite oxidation during chemical weathering on land consumes${\mathrm{O}}_2$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${\mathrm{O}}_2$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${\mathrm{O}}_2$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 pastmathvariant='normal'>O2/${\mathrm{p}\mathrm{C}\mathrm{O}}_2$should consider both 1) alternative oxygen sources during pyrite oxidation and 2) secondary overprinting by microbial recycling.

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