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The stable isotope ratios of meteoric waters change predictably over orographic barriers. We present a new approach to determine stable isotope ratios in ancient meteoric waters from spatial patterns of hydrothermal alteration in continental volcanic fields. In the San Juan Volcanic Field, Colorado, USA we reconstruct water δ18O values feeding hydrothermal systems of −7 to −10‰ from 35 to 20 Ma, followed by a drop to −17 to −18‰ between 20 and 12 Ma. This drop is consistent with greater magmatic water input to older hydrothermal systems, ∼1 km of rock exhumation, ∼2–3 km of surface uplift, or a combination of all three. Our approach returns water isotope compositions integrated over spatial (kms to 10s kms) and temporal (104 − 107 years) scales of continental hydrothermal systems. Such length‐ and time‐scales approach those of continental tectonics, potentially alleviating issues with diagenetic space and time limitations associated with other paleo‐meteoric water proxies. Plain Language Summary: Past surface elevations are commonly reconstructed from the stable isotope ratios of oxygen and hydrogen in paleo‐precipitation. These ratios change systematically as air lifts up and over mountains and other topographic barriers. Direct samples of ancient precipitation do not exist so we rely on proxies instead. The most common proxies used are individual minerals, such as calcium carbonate, or fossils, such as leaves. We present a new approach, where we instead use the isotopic "footprint" of large hydrothermal cells associated with ancient volcanoes as a record of precipitation isotope ratios. By processing these large patterns with an inverse model to "undo" the alteration, we find that there was a large change in oxygen isotope values in precipitation between 20 and 12 million years ago in the San Juan Volcanic Field, Colorado, USA. This is consistent with surface uplift of 2–3 km over this timeframe. Unlike other isotopic proxies for paleo‐precipitation our technique captures an isotopic signal integrated over the space‐ and timescales of hydrothermal alteration, which approach those of the tectonic processes that make and break mountains. Key Points: Continental hydrothermal systems record meteoric water oxygen isotope compositions over broad space‐ and time‐scalesIsotopic alteration in hydrothermal systems in the San Juan Volcanic Field, Colorado, USA provides a test of this approachInverse modeling suggests a decrease in meteoric oxygen isotope ratios between 23 and 15 Ma, consistent with 2–3 km of surface uplift [ABSTRACT FROM AUTHOR]

• Extensive new Ba isotopic survey of modern barites. • First Proterozoic Ba isotopic results. • Provides genetic model for enigmatic post-Marinoan barite formation. • Provides evidence for stratified post-Marinoan ocean. • Provides arguments for why barite horizons are confined to Marinoan and not Sturtian or Paleoproterozoic strata. Measurements of triple oxygen isotope ratios in barite horizons within post-Marinoan cap carbonates have provided some of the most compelling evidence that the Marinoan glaciation was a Snowball Earth event. However, the origin of these barite horizons remains unresolved. To constrain the Ba sources, and thus formation mechanisms of these horizons, we analyzed the Ba isotope composition of post-Marinoan barite deposits from Northwest Canada, Northern Norway, Brazil and South China. We augment these analyses with a Ba isotope survey of almost 100 modern and ancient additional barite measurements, including samples from pelagic (or 'marine'), hydrothermal, terrestrial, Proterozoic stratiform and cold seep environments. Unlike modern cold seep or terrestrial barites, we find that globally-distributed post-Marinoan barites exhibit a relatively narrow isotopic range, suggesting a well-mixed, effectively limitless Ba source. Moreover, post-Marinoan deposits exhibit a similar mean Ba isotope composition to modern marine barites, which we interpret as evidence of a marine Ba source. Considered alongside existing geochemical, geological, and new Ba isotope survey data, we conclude that Ba in barite horizons was sourced from a well-mixed, Ba-replete but SO 4 -poor reservoir that accumulated during the Marinoan Snowball Earth interval. This deep Ba reservoir was then transported upward—either by ocean circulation or dolomitization of underlying cap carbonates—and was brought into contact with continental weathering-derived sulfate in a post-glacial meltwater surface layer. Thus, in addition to providing a plausible mechanism for generating globally-synchronous deposition of post-Marinoan barite horizons that reconciles all existing geochemical and geological data, our results demonstrate the utility of Ba isotopes to interrogate the origin of enigmatic barite deposits throughout the sedimentary record. [ABSTRACT FROM AUTHOR]

Quartz–calcite sandstones experienced the reaction calcite + quartz = wollastonite + CO2 during prograde contact metamorphism at P = 1500 bars and T = 560°C. Rocks were in equilibrium during reaction with a CO2–H2O fluid with XCO2 = 0·14. The transition from calcite-bearing, wollastonite-free to wollastonite-bearing, calcite-free rocks across the wollastonite isograd is only several millimeters wide. The wollastonite-forming reaction was driven by infiltration of quartz–calcite sandstone by chemically reactive H2O-rich fluids, and the distribution of wollastonite directly images the flow paths of reactive fluids during metamorphism. The mapped distribution of wollastonite and modeling of an O-isotope profile across a lithologic contact indicate that the principal direction of flow was layer-parallel, directed upward, with any cross-layer component of flow <0·1% of the layer-parallel component. Fluid flow was channeled at a scale of 1–100 m by pre-metamorphic dikes, thrust and strike-slip faults, fold hinges, bedding, and stratigraphic contacts. Limits on the amount of fluid, based on minimum and maximum estimates for the displacement of the wollastonite reaction front from the fluid source, are (0·7–1·9) × 105 cm3 fluid/cm2 rock. The sharpness of the wollastonite isograd, the consistency of mineral thermobarometry, the uniform measured 18O–16O fractionations between quartz and calcite, and model calculations all argue for a close approach to local mineral–fluid equilibrium during the wollastonite-forming reaction. [ABSTRACT FROM PUBLISHER]

The Proterozoic Eon spans Earth's middle age during which many important transitions occurred. These transitions include the oxygenation of the atmosphere, emergence of eukaryotic organisms and growth of continents. Since the sulfur and oxygen cycles are intricately linked to most surface biogeochemical processes, these transitions should be recorded in changes to the isotopic composition of marine and terrestrial sulfate minerals. Here we present oxygen (∆17O, δ18O) and sulfur (∆33S, δ34S) isotope records of Proterozoic sulfate from currently available data together with new measurements of 313 samples from 33 different formations bearing Earth's earliest unambiguous evaporites at 2.4 Ga through to Ediacaran aged deposits. This record depicts distinct intervals with respect to the expression of sulfate isotopes that are not completely captured by established intervals in the geologic timescale. The most salient pattern is the muted ∆17O signatures across the GOE , late Proterozoic and Ediacaran with values that are only slightly more negative than modern marine sulfate, contrasting with highly negative values across the mid-Proterozoic and Cryogenian. We combine these results with estimates of atmospheric composition to produce a gross primary production (GPP) curve for the Proterozoic. Through these results we argue that changes in GPP across Earth history likely help account for many of the changes in the Proterozoic Earth surface environment such as rising atmospheric oxygen, large fluctuations in the size of the marine sulfate reservoir and variations in the isotopic composition of sedimentary sulfate. [ABSTRACT FROM AUTHOR]

Triple oxygen isotopes within post-Marinoan barites have played an integral role in our understanding of Cryogenian glaciations. Reports of anomalous Δ O 17 values within cap carbonate hosted barites however have remained restricted to South China and Mauritania. Here we extend the Δ O 17 anomaly to northwest Canada with our new measurements of barites from the Ravensthroat cap dolostone with a minimum Δ O 17 value of − 0.75 ‰ . For the first time we pair triple oxygen with multiple sulfur isotopic data as a tool to identify the key processes that controlled the post-Marinoan sulfur cycle. We argue using a dynamic 1-box model that the observed isotopic trends both in northwest Canada and South China can be explained through the interplay between sulfide weathering, microbial sulfur cycling and pyrite burial. An important outcome of this study is a new constraint placed on the size of the post-Marinoan sulfate reservoir (≈0.1% modern), with a maximum concentration of less than 10% modern. Through conservative estimates of sulfate fluxes from sulfide weathering and under a small initial sulfate reservoir, we suggest that observed isotopic trends are the product of a dynamic sulfur cycle that saw both the addition and removal of the Δ O 17 anomaly over four to five turnovers of the post-Marinoan marine sulfate reservoir. [ABSTRACT FROM AUTHOR]

Abstract: This paper describes a comparison of ultraviolet photolysis experiments undertaken with SO2 (oxygen with isotopes at natural abundance levels) and S18O2 (18O-substituted oxygen). Experiments were conducted in a closed photocell using a deuterium lamp (principally 190–235nm) under pressure regimes (5–25Torr) that produced optically thick conditions for 32SO2 and variable optical depths for other isotopologues. The experiments, which were designed to examine the effects of intramolecular isotopic substitution of oxygen atoms on the S-MIF produced during UV photolysis of SO2, reveal generally reduced sulfur fractionation for 18O-rich SO2 as compared to 16O-rich SO2. Model shielding calculations were undertaken using spectra that were shifted due to changes in rotational and vibrational energy levels. The model calculations suggest that processes in addition to rotational and vibrational shifts in absorption spectra play a role in the experimentally produced isotope effects. Such additional processes may include differences in primary photoexcitation arising from smaller peak-to-valley amplitudes for fine structure of 18O-rich SO2 absorption spectra or an isotopically selective process associated with transitions between excited states. [Copyright &y& Elsevier]

Dissolved sulfate is a crucial electron acceptor for the subsurface biosphere, particularly for the living microbial ecosystems in the long-isolated (on the order of millions to billions of years) deep subsurface fracture waters in Precambrian cratons, e.g., in the Witwatersrand Basin of the Kaapvaal Craton, South Africa. Aiming to understand the role of sulfate in the sustainability of the subsurface habitat and the spatial extent of the terrestrial subsurface biosphere, we carried out a basin-scale examination on the sources and producing mechanisms of dissolved sulfur (sulfate and sulfide) in the subsurface fracture waters in the Witwatersrand Basin using multiple sulfur isotopes (32,33,34,36S) and oxygen isotopes (16,18O). Dissolved sulfates in 14 fracture water samples collected from 900 to 3,413 meters below the surface at 6 sites show isotopic ranges from −5.3‰ to +19.4‰ for δ 34S, −0.40‰ to +0.50‰ for Δ33S, and −1.1‰ to +10.9‰ for δ 18O. These isotopic signatures are distinct to those of the sulfate minerals (e.g., the carbonate-associated sulfate in local Transvaal Supergroup dolomite sequences: δ 34S = +31.4‰ to +39.2‰; Δ33S = −0.01‰ to +0.16‰), but identical to those of the sulfide minerals in the host rocks. This indicates that the dissolved sulfate in the fracture waters were dominantly generated by in situ oxidation of sulfide minerals at basin scale, although mixing of a small amount of surface sulfate at some sites cannot be completely ruled out. The dissolved sulfates in the less deep and less saline fracture waters are in oxygen isotope disequilibrium with their host waters, a surprising result given that the water residence times are orders of magnitude longer than the time required for oxygen isotope exchange to reach equilibrium. This implies that vigorous in situ sulfate production has occurred after the fracture waters were isolated. In contrast, the dissolved sulfate in the deeper, more saline waters are in apparent oxygen isotope equilibration or close to equilibration with their host waters. This might be attributed to a combined effect of faster oxygen isotope exchange between sulfate and water at higher temperatures, larger extent of sulfate reduction, and/or less efficient sulfate production. The dissolved sulfide in the fracture waters has similar Δ33S values but is 3.0‰ to 26.4‰ lower in δ 34S than coexisting sulfate, suggesting that the dissolved sulfide is mostly generated from bacterial sulfate reduction, which is consistent with the widespread existence of sulfate-reducing bacteria down to >3.4 km below surface in the Witwatersrand Basin. Overall, these novel isotopic results demonstrate that geological processes can provide a steady long-term sulfate source in deep fracture fluids by in situ oxidation of sulfide minerals in their host rocks, and thereby a mechanism to sustain the terrestrial subsurface biosphere, even in deep, high-temperature, long-isolated water systems. Thus, sulfate as a terminal electron acceptor is not the limiting factor for the spatial expansion of terrestrial subsurface biosphere. • Dissolved sulfates in subsurface fracture waters across the basin show large S-MIF. • Coexisting dissolved sulfide has similar S-MIF but much lower δ 34S values. • The S-MIF are not consistent with host-rock sulfate minerals but sulfide minerals. • Dissolved sulfate is formed by long-standing in situ oxidation of sulfide minerals. • The in situ sulfate production facilitates expansion of the biosphere to great depth. [ABSTRACT FROM AUTHOR]