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Abstract Estimating dissolved oxygen (O2) concentrations in seawater during the Neoproterozoic is central to testing hypotheses about the role of O2 in animal evolution. Here we apply the thallium (Tl) isotope redox proxy to samples stratigraphically below the ca. 810-million-year-old (Ma) Bitter Springs Carbon Isotope Excursion and spanning the interval between the two Snowball Earth glaciations (ca. 662–650 Ma) to constrain the evolution of Neoproterozoic bottom water redox conditions. Thallium isotopes can be used to reconstruct the global extent of oxygenated oceanic bottom waters because the primary control on seawater Tl isotope compositions (ε205Tl) over million-year time scales is changes in the amount of 205Tl removal by Mn oxides on the seafloor. Samples spanning an ~20-m.y. period preceding the Bitter Springs excursion from the Tonian Reefal Assemblage (n = 18/30) yield ε205Tlauth values lower than global oceanic inputs (ε205Tl ~–2±), with some samples approaching the modern seawater ε205Tl value of –6±. These sustained low ε205Tlauth values require enhanced burial of Mn oxides elsewhere on the seafloor, which we interpret as evidence for the oxygenation of the deep ocean in the Tonian. In contrast, the majority of samples from the Cryogenian Hay Creek Group (n = 13/16) yield ε205Tlauth values similar to global oceanic inputs, suggesting that the deep ocean was not ventilated at this time. This indicates that Earth’s deep ocean was not gradually oxygenated throughout the Neoproterozoic, but rather experienced intervals of increased and decreased O2 concentrations. 

The concentration of H2OT (i.e., total H; assumed here to be the sum of hydroxyl and molecular water) in silicate minerals and melts exerts a primary control on the physicochemical properties of Earth’s crust and mantle. The partitioning of H2OT between minerals and melts is a key parameter used to model the H2OT contents of magmatic source regions in terrestrial and planetary systems. In mafic systems, olivine is frequently the first crystallizing phase, which makes it a useful tracer of primitive melt composition. Previously, the partitioning of H2OT between olivine and melt was only experimentally constrained at H2O-undersaturated conditions and pressures ≥500 MPa, which are broadly applicable to Earth’s mid- to lower-crust and uppermost mantle. However, we have few constraints at upper crustal pressures, limiting our ability to model pre-eruptive H2OT contents of magmas, decompression rates, and ultimately volcanic hazards. Here we present the first experimental determination of the partitioning of H2OT between olivine and melt at pressures relevant to Earth’s upper crust (10–200 MPa). Contrary to predictions based upon the extrapolation of experimental results from pressures ≥500 MPa, we find that the olivine-melt H2OT partition coefficient (DH2OTol/melt ranges from 0.0011 to 0.00033) decreases with increasing melt H2OT and increasing pressure from 10–200 MPa. One explanation for the observed relationship between DH2OTol/melt and melt H2OT concentration is that DH2OTol/melt is controlled by the speciation of H (e.g., hydroxyl and molecular water) in silicate melts. We calculate the concentration of hydroxyl (OH) dissolved in the melt, and assuming H is only incorporated into olivine as OH, calculate an olivine-melt OH partition coefficient (DOHOl/melt). Our data indicate that DOHOl/melt is constant (DOHOl/melt=0.0011±0.0002; 1 St.dev.) and that the proportion of molecular water (H2Om) to OH in the melt controls the variation of DH2OTOl/melt with melt H2OT concentration for our experiments. We also compared San Carlos olivine seed crystals to olivine crystallized in the same experimental charges. Our data indicate that San Carlos olivine seed crystals have lower H2OT concentrations than olivine crystallized during the experiments, which may be explained by differences in their compositions and, therefore, extrinsic point defect populations. Our results demonstrate that at low pressures (≤200 MPa), the partitioning of H2OT between olivine and melt is primarily dependent upon the speciation of H in silicate melts. Similarly, in combination with prior experimental work on clinopyroxene and plagioclase, our results suggest that the speciation of H in silicate melts may be a primary control on the partitioning of H2OT between all nominally anhydrous minerals and melts, at least at H2OT contents up to ∼5 wt%. We apply our results to prior estimates for magma decompression rates from the 1977 fire fountain eruption at Seguam volcano and find that, for the variable DH2OTOl/melt determined in our study, median magma decompression rates are slightly slower (a factor of ∼2.5) but within the uncertainty of models using a constant DH2OTol/melt=0.0009±0.0003 (1 St.dev.) (Towbin et al. 2023). Therefore, prior estimates of magma decompression rates based upon H+ diffusion in olivine may be slightly overestimated if the dependence of DH2OTol/melt on the speciation of H in the melt is unaccounted for. 

Hydrothermal seafloor massive sulfide deposition at ocean spreading centers modifies the flux of iron from vents while producing an archive of these processes in both active and fossil hydrothermal complexes. Despite hopes of stable iron isotopes tracing mineral formation and iron cycling at these sites, the competing fractionations accompanying various mineralization processes have presented an obstacle to confidently interpreting iron isotopic datasets. We have developed a triple iron isotope proxy that can resolve some of these previously indistinguishable mineral formation histories, and applied it to a suite of samples from the Trans-Atlantic Geotraverse (TAG) active hydrothermal mound on the Mid-Atlantic Ridge. We show that massive pyrite-dominated sulfides formed near the TAG mound surface retain primary kinetic isotope signatures indicative of their rapid formation, likely from an iron monosulfide intermediate. Pyrite-anhydrite breccias retain mixed isotopic signatures of reworked primary massive sulfides together with secondary pyrite, grown in confined conditions, in some cases in equilibrium with subsurface hydrothermal fluid. We also suggest that iron oxyhydroxide-rich chert precipitation across parts of the mound surface trapped isotopically evolved fluid within the mound, and a later generation of sulfides precipitated from this evolved fluid. Metalliferous sediments from a core recovered near the base of the TAG mound mostly fall along a primary mass fractionation law defined by mound sulfides. This is consistent with a dominant contribution of collapsed mound debris and/or black smoker sulfide fallout to the seafloor near the base of the TAG mound. However, the triple iron isotopic composition of surface sediment from this core suggests it may also contain iron oxyhydroxide fallout from the dispersing non-buoyant plume that had already undergone extensive sulfide precipitation. Triple iron isotope studies of non-buoyant plume sediments may be used to quantify the mineral fate of iron vented to the oceans, estimate variations in iron to sulfide ratios of primary vent fluids, and resolve the relative importance of low and high temperature hydrothermal flow to basin-scale iron-rich plumes in the global oceans. 

Abstract Reconstructing past oxygen fluctuations in oxygen minimum zones (OMZs) is crucial for understanding their response to climate change. Numerous studies suggest better oxygenation in the Arabian Sea OMZ during the Last Glacial Maximum (LGM) compared to the Holocene. However, bottom water oxygen (BWO) variability during the Penultimate Glacial Cycle (Marine Isotope Stage [MIS] 6 to MIS 5e, ∼140–115 ka B.P.) remains poorly constrained. This study reconstructs BWO variations during this period from sediment core TN041‐8JPC in the western Arabian Sea OMZ, utilizing proxies including benthic foraminiferal surface porosity, redox‐sensitive trace metal enrichment factors (e.g., U_EF), and U/Ba ratios. Bottom water oxygen concentrations were 24.4 ± 5.9 μmol/kg during MIS 6 and 16.8 ± 6.5 μmol/kg during MIS 5e, with all proxies indicating higher BWO in MIS 6 than in MIS 5e. However, these proxies show different patterns within MIS 5e, indicating that U_EFand U/Ba ratios may be limited to recording average BWO in glacial and interglacial (quasi)steady states. We propose that the intensified OMZ during MIS 5e, relative to MIS 6, was driven by higher productivity, temperature‐induced reductions in oxygen solubility, and reduced delivery of Southern‐sourced intermediate waters. In contrast, the intensified OMZ during the Holocene, compared to the LGM, was likely influenced by lower oxygen solubility, reduced Southern water delivery, and winter convective mixing rather than productivity. This study highlights a general trend of weaker OMZs in glacial than interglacial periods, though the mechanisms may not be identical, offering insights into OMZ dynamics under climate change in the past. 

The Early Paleozoic radiation of diverse animal life is commonly connected to a well-ventilated global ocean. Yet the oxygenation history of Paleozoic deep oceans remains debated. Using thallium (Tl) isotope ratios in deep-marine mudrocks, we reconstruct the history of deep marine oxygenation from ~485 to 380 million years ago. Thallium isotopes can track bottom water oxygenation indirectly through their sensitivity to seafloor Mn oxide burial. We apply Tl isotopes to a global set of mudrocks, placing a particular focus on the Road River Group of Yukon, Canada. Our data reveal an oscillatory pattern in seawater Tl isotope ratios and, in turn, a dynamic ocean ventilation history. A long-lived deep ocean oxygenation episode is identified between ~405 and 386 million years ago. These short-term dynamics are superimposed on a muted positive ocean oxygenation trend over the entire Early and Middle Paleozoic. Sustained O₂accumulation in global marine bottom waters occurred sometime after ~380 million years ago according to our dataset.