The second-largest extinction event in the evolutionary history of planktic foraminifera occurred at the Aptian-Albian boundary. This extinction may reflect ocean acidification (OA) associated with Oceanic Anoxic Event 1b. As calcium isotope ratios (δ44/40Ca) can track how biocalcification rates respond to OA, we measured δ44/40Ca records for planktic and benthic foraminifera, bulk carbonates, and authigenic calcite across the Aptian-Albian boundary in the South Atlantic. Benthic and bulk δ44/40Ca data display a distinct sequence of negative and positive excursions, similar to δ44/40Ca variations across other OA events. Planktic δ44/40Ca values increase markedly, tracking a reduction in calcification rates coincident with decreases in the size, diversity, and shell thickness of planktic foraminifera. These results suggest that OA drove extinctions of planktic foraminifera at the Aptian-Albian boundary.

Triple oxygen isotope compositions of sedimentary sulfate (triangle ' 17O) have been applied as a tracer of past primary productivity, pO2 and pCO2. However, large intraformational variability and debate surrounding how such signatures are produced and preserved has limited the understanding of this record. Here we explore what controls triangle ' 17O signatures of mid-Proterozoic (ca. 2.0-1.0 Ga) sulfates. We identify a clear depositional control on the magnitude of preserved sulfate triangle ' 17O signatures, with Proterozoic terrestrial environments preserving the most negative and variable signatures and marine environments displaying values near modern marine sulfate. Our results strongly suggest that local depositional setting influences the magnitude of preserved triangle ' 17O signals, and that the processes within local environments drive much of the observed intraformational variability. Finally, this analysis suggests that triangle ' 17O signatures of carbonate associated sulfate in concert with sedimentological observations may offer a powerful tool to identify basin restriction and non-marine settings throughout the geologic record.

Despite their importance for long-term climate regulation, the rates and mechanisms of seafloor carbonate dissolution are poorly understood, especially with respect to calcite saturation and the role of sedimentary metabolic CO2 production. Here, we present results from an in situ porewater sampler deployed at the Cocos Ridge in the eastern equatorial Pacific, where we examine seafloor carbonate dissolution in locations with bottom water Omega(calcite) ranging from 1.0 to 0.84 (1600-3200 m). With cm-scale resolution from the sediment-water interface to 35 cm, we present porewater profiles of total alkalinity, pH, dissolved inorganic carbon (DIC), delta C-13 of DIC, Omega(calcite), [Mn], [Ca], and [Sr], as well as solid phase porosity, % CaCO3, and % organic C. These profiles provide evidence that deep-sea sedimentary carbonate dissolution occurs via sediment-side control, wherein dissolution is dominated by sedimentary processes rather than strictly bottom water saturation state. We estimate dissolution fluxes using three independent approaches: alkalinity fluxes, delta C-13 of DIC combined with DIC fluxes, and [Ca] fluxes. We report seafloor dissolution fluxes with uncertainties < 38 %: 40 +/- 15, 98 +/- 20, 100 +/- 32, and 89 +/- 27 mu mol CaCO3/m(2)/day at sites 3200, 2900, 2700, and 1600 m deep, respectively. The magnitude of dissolution fluxes is a function of bottom water saturation state (Omega(calcite)), bottom water dissolved oxygen, and sedimentary CaCO3 content, but not correlated with any of these parameters independently. We observe dissolution occurring at all stations, including where bottom water is saturated with respect to calcite, and present evidence that this occurs through respiration-driven dissolution within the sediment. At all sites, porewater Omega(calcite) decreases below bottom water values before increasing toward saturation deeper in the sediment. Using the delta C-13 of DIC, we partition the DIC fluxes across the sediment-water interface and find 21-48 % of DIC is sourced from CaCO3 dissolution, with the remainder sourced from organic matter respiration. We present a sedimentary mass balance, assembled with dissolution rates and mass accumulation rates obtained through Delta C-14 of foraminiferal calcite, and calculate CaCO3 burial efficiencies between 2 and 67 %, inversely correlating with water depth. Our results also provide evidence that net chemical erosion of 5,000--10,000 year old carbonate is occurring at the deepest site. Aerobic organic C respiration coupled with sedimentary CaCO3 dissolution, as documented here, will provide more alkalinity to bottom waters than from undersaturation-driven dissolution alone. This process can neutralize anthropogenic CO2 at the seafloor in a larger range of saturation states than previously estimated.

The triple oxygen isotope composition of sulphate minerals has been used to constrain the evolution of Earth's surface environment (e.g., pO2, pCO2 and gross primary productivity) throughout the Proterozoic Eon. This approach presumes the incorporation of atmospheric O2 atoms into riverine sulphate via the oxidative weathering of pyrite. However, this is not borne out in recent geological or modern sulphate records, where an atmospheric signal is imperceptible and where terrestrial pyrite weathering occurs predominantly in bedrock fractures that are physically more removed from atmospheric O2. To better define the transition from a Proterozoic to a modern-like weathering regime, here we present new measurements from twelve marine evaporite basins spanning the Phanerozoic. These data display a step-like transition in the triple oxygen isotope composition of evaporite sulphate during the mid-Paleozoic (420 to 387.7 million years ago). We propose that the evolution of early root systems deepened the locus of pyrite oxidation and reduced the incorporation of O2 into sulphate. Further, the early Devonian proliferation of land plants increased terrestrial organic carbon burial, releasing free oxygen that fueled increased redox recycling of soil-bound iron and resulted in the final rise in pO2 to modern-like levels.

Seawater sulfate is a major carrier of oxidizing capacity and influences the redox budget of the Earth's surface on geologic timescales. Records of oxygen and sulfur isotopes in seawater sulfate are used to track changes in sulfate cycling through geologic time. Interpretations of these records are typically based on models that describe the seawater sulfate reservoir as a mass balance between microbial, riverine and sedimentary sulfate fluxes. Here, we investigate the influence of hydrothermal sulfate cycling, which remains an unconstrained but potentially significant additional flux in this mass balance. We find that anhydrite (CaSO4) from eight submarine hydrothermal vent fields is consistently offset from seawater sulfate in δ18O but not Δ'17O or δ34S. Experiments at hydrothermal pressure–temperature conditions indicate that this δ18O offset is driven by oxygen isotope exchange between sulfate and hydrothermal H2O at high temperature. An updated isotope mass balance model shows that a flux of hydrothermal sulfate into seawater, derived from retrograde dissolution of hydrothermal anhydrite, could cause high-temperature oxygen isotope exchange to buffer seawater sulfate δ18O and Δ'17O by as much as 25%. Hydrothermal sulfate preserved in the Troodos Ophiolite (Cretaceous) also records a δ18O and Δ'17O signal of high-temperature oxygen isotope exchange, supporting the conclusion that geologic records of seawater sulfate oxygen isotopes may include a hydrothermal component.

The triple oxygen isotope composition of seawater sulfate, as recorded in marine sulfate evaporites and barites, is commonly used to interpret past changes in atmospheric pO(2)/pCO(2) and gross primary production (GPP). In practice, the most-negative measured triple oxygen isotope value (Delta'O-17) of sulfate from a marine evaporite deposit is thought to most closely represent contemporaneous seawater sulfate and is used to calculate atmospheric composition. However, a range of triple oxygen isotope compositions are typically measured within a single marine evaporite basin. Here, we characterize in detail the variability in the triple oxygen isotope composition of the sulfate in gypsum sampled from three Messinian (5-6 Ma) marine evaporite sub-basins from the Western Mediterranean Basin. Evaporite sulfate is offset from contemporaneous seawater sulfate and reflects mixing between two end-member sulfate populations: the original seawater sulfate and sulfate that has been isotopically reset after basin restriction. The combined Delta'O-17 and delta O-18 compositions of sulfate within a stratigraphic context offer the opportunity to better constrain the degree to which marine sulfate evaporites preserve the original isotopic composition of open ocean seawater sulfate. This study encompasses an exploration of mass-dependent fractionation, isotope equilibrium with water, and various scenarios of mixing. Our results calibrate the utility of marine sulfate evaporites in constraining the contemporaneous, open ocean triple oxygen isotope composition of seawater sulfate. (C) 2021 Elsevier B.V. All rights reserved.

The fundamental questions of “Where do we come from?” and “How did life begin?” date back millennia. Yet, the scientific community still seeks to understand abiogenesis, the origin of life. It is now generally agreed that any answer to this question must involve an explanation for the emergence of biological homochirality, that naturally appearing biomolecules from organisms occur with a particular handedness (enantiomeric form), for example, L-amino acids and D-sugars. A unifying concept among deterministic theories for homochirality is the presence of a chiral bias, which breaks the symmetry for driving the formation of molecules with a given handedness over that of the other. The bias has previously been attributed to circularly polarized light, fluid dynamics, and magnetic fields, among many others (1). In PNAS, Ozturk and Sasselov (2) approach this age-old question by proposing that the symmetry breaking involves a phenomenon known as chiral-induced spin selectivity (CISS). A central feature of the CISS effect is the coupling between the intrinsic angular momentum of an electron, or its spin, and the molecular frame of a chiral molecule. Electrons exist in one of two possible angular momentum states and are commonly referred to as either spin-up or spin-down. Multiple studies have shown that electrons of one spin type favorably transmit through an assembly composed of left-handed molecules, but the other spin type does not (3). Conversely, the opposite preference for spin transmission becomes true when the assembly is composed of right-handed molecules. A chiral molecule’s spin preference manifests for electrons transmitted through the molecules, for charge exchange between two chiral molecules, or for a chiral molecule and a magnetized surface (3, 4). The latter example forms the basis of Ozturk and Sasselov’s (2) conjecture that the spin of electrons can impart enantioselectivity for the production of chiral molecules from achiral precursors under conditions that are believed to be consistent with that of prebiotic Earth. Precedent exists for their hypothesis that magnetized surfaces can lead to enantioselective reactions and the formation of chiral molecules from achiral precursors (5, 6), and others have considered the implications of CISS for the origin of life (7). Experimental studies have shown that CISS operates on chemical processes of multiple length scales and of varying complexity; see the summary in Fig. 1. Experiments involving more-complex multistep reactions have shown that a preferred molecular handedness can emerge for systems that do not initially possess any chirality (8). Features of CISS have also proven to be influential in biological processes, such as electron transport in proteins and across cell surfaces, as well as in allosteric regulation (9–11). In previous work, Sasselov et al. (12) proposed chemical pathways for the origin of biomolecular building blocks that are consistent with prebiotic Earth conditions. This picture involves the accumulation of reactants (precursor chemicals) in shallow subaqueous basins that undergo photochemical reactions through UV radiation, believed to be extant at that time undefined. That picture did not account for molecular chirality, however; and Ozturk and Sasselov (2) extend that model to account for chirality, by invoking the CISS effect and magnetite as the origin of a chiral bias. Magnetite, a ferrimagnetic material, is an abundant constituent of subaqueous sedimentary mineral deposits on the anoxic Earth, circa 1.8 billion to 3.7 billion years ago (13). Ozturk and Sasselov propose that UV irradiation generates spin-polarized photoelectrons from uniformly magnetized magnetite, which then initiate enantioselective chemical reactions near the magnetite surface because of the CISS effect. Studies have shown that a spin selectivity dependence exists between the molecular frame of a chiral molecule and the direction of an Fig. 1. Examples of CISS-related processes in chemical and biological systems which may have contributed to the evolutionary progression responsible for the origin of life. Spin effects for facilitating chemical reactions are discussed in refs. 5, 6, and 8, spin interactions among chiral molecules are reviewed in ref. 9, the spin-filtering capabilities of proteins are reviewed in refs. 7 and 9, CISS-based allostery is described in ref. 11, and the effect of CISS on extracellular respiration is discussed in ref.10. Note that ET stands for electron transfer and red arrows indicate an electron with a defined spin direction.

The triple oxygen isotope composition (Δ’ 17 O) of sulfate minerals is widely used to constrain ancient atmospheric p O 2 / p CO 2 and rates of gross primary production. The utility of this tool is based on a model that sulfate oxygen carries an isotope fingerprint of tropospheric O 2 incorporated through oxidative weathering of reduced sulfur minerals, particularly pyrite. Work to date has targeted Proterozoic environments (2.5 billion to 0.542 billion years ago) where large isotope anomalies persist; younger timescale records, which would ground ancient environmental interpretation in what we know from modern Earth, are lacking. Here we present a high-resolution record of the δ 18 O and Δ’ 17 O in marine sulfate for the last 130 million years of Earth history. This record carries a Δ’ 17 O close to 0o, suggesting that the marine sulfate reservoir is under strict control by biogeochemical cycling (namely, microbial sulfate reduction), as these reactions follow mass-dependent fractionation. We identify no discernible contribution from atmospheric oxygen on this timescale. We interpret a steady fractional contribution of microbial sulfur cycling (terrestrial and marine) over the last 100 million years, even as global weathering rates are thought to vary considerably.

The majority of anaerobic biogeochemical cycling occurs within marine sediments. To understand these processes, quantifying the distribution of active cells and gross metabolic activity is essential. We present an isotope model rooted in thermodynamics to draw quantitative links between cell-specific sulfate reduction rates and active sedimentary cell abundances. This model is calibrated using data from a series of continuous culture experiments with two strains of sulfate reducing bacteria (freshwater bacterium Desulfovibrio vulgaris strain Hildenborough, and marine bacterium Desulfovibrio alaskensis strain G-20) grown on lactate across a range of metabolic rates and ambient sulfate concentrations. We use a combination of experimental sulfate oxygen isotope data and nonlinear regression fitting tools to solve for unknown kinetic, step-specific oxygen isotope effects. This approach enables identification of key isotopic reactions within the metabolic pathway, and defines a new, calibrated framework for understanding oxygen isotope variability in sulfate. This approach is then combined with porewater sulfate/sulfide concentration data and diagenetic modeling to reproduce measured 18O/16O in porewater sulfate. From here, we infer cell-specific sulfate reduction rates and predict abundance of active cells of sulfate reducing bacteria, the result of which is consistent with direct biological measurements.

Significance A geochemical model shows that discrete volcanic perturbations, coupled to long-term changes in sulfur biogeochemistry result in apparent stepwise changes in the isotopic composition of marine sulfur, similar to the patterns seen in the marine barite record of the Cenozoic and Late Mesozoic. The perturbations required to reproduce this isotope record in the model correlate with the timing and approximate size of large igneous provinces, particularly those intruding volatile-rich rock, suggesting that these events are responsible for much of the enigmatic structure in the sulfur isotope record of the last 120 My.

Atmospheric O2 and CO2 levels inform us of the changes in chemical and biological environments, yet the history of atmospheric compositions, and pO2 in particular, is not well-constrained. The triple oxygen isotope (16,17,18O) composition of marine SO42− has been proposed to directly record the ratio pO2/pCO2 in the contemporaneous atmosphere. To resolve this atmospheric signal, both a precise measurement of the 17O composition of sulfate and a model with which to interpret the measurement are needed. Here we present precise measurements of the triple oxygen isotope composition of modern marine sulfate and then forward a novel sulfur cycle model that deconvolves the potential atmospheric and microbial inputs to this signal. Our interpretation of marine sulfate oxygen isotope composition provides a framework for calculating atmospheric composition, relative rates of biogeochemical activity, and can be applied to geologic records of marine sulfate to constrain the pO2/pCO2 ratio over time.

Algal blooms in lakes are often associated with anthropogenic eutrophication; however, they can occur without the human introduction of nutrients to a lake. A rare bloom of the alga Picocystis sp. strain ML occurred in the spring of 2016 at Mono Lake, a hyperalkaline lake in California, which was also at the apex of a multiyear-long drought. These conditions presented a unique sampling opportunity to investigate microbiological dynamics and potential metabolic function during an intense natural algal bloom. We conducted a comprehensive molecular analysis along a depth transect near the center of the lake from the surface to a depth of 25 m in June 2016. Across sampled depths, rRNA gene sequencing revealed that Picocystis-associated chloroplasts were found at 40 to 50% relative abundance, greater than values recorded previously. Despite high relative abundances of the photosynthetic oxygenic algal genus Picocystis, oxygen declined below detectable limits below a depth of 15 m, corresponding with an increase in microorganisms known to be anaerobic. In contrast to previously sampled years, both metagenomic and metatranscriptomic data suggested a depletion of anaerobic sulfate-reducing microorganisms throughout the lake's water column. Transcripts associated with photosystem I and II were expressed at both 2 m and 25 m, suggesting that limited oxygen production could occur at extremely low light levels at depth within the lake. Blooms of Picocystis appear to correspond with a loss of microbial activity such as sulfate reduction within Mono Lake, yet microorganisms may survive within the sediment to repopulate the lake water column as the bloom subsides.IMPORTANCE Mono Lake, California, provides a habitat to a unique ecological community that is heavily stressed due to recent human water diversions and a period of extended drought. To date, no baseline information exists from Mono Lake to understand how the microbial community responds to human-influenced drought or algal bloom or what metabolisms are lost in the water column as a consequence of such environmental pressures. While previously identified anaerobic members of the microbial community disappear from the water column during drought and bloom, sediment samples suggest that these microorganisms survive at the lake bottom or in the subsurface. Thus, the sediments may represent a type of seed bank that could restore the microbial community as a bloom subsides. Our work sheds light on the potential photosynthetic activity of the halotolerant alga Picocystis sp. strain ML and how the function and activity of the remainder of the microbial community responds during a bloom at Mono Lake.

Phosphate oxygen isotope ratios provide a powerful tool for paleoclimate reconstruction and the study of phosphorus biogeochemistry in aquatic systems. The temperature dependent offset between the oxygen isotope composition of phosphate and water in biogenic minerals and in solutions has been used extensively in paleoclimate research and the study of phosphorus cycling in aquatic and soil environments. Sample sizes are small in many applications, e.g., serial sampling of mammalian tooth enamel for paleoseasonality studies, the use of conodont elements for climate and ocean delta O-18 reconstruction, or the isolation and analysis of dissolved phosphate from marine and freshwater environments in biogeochemistry studies. This has pushed the development of techniques that allow for processing and analysis of a few micromoles of phosphate.Current approaches to phosphate oxygen isotopic composition (delta O-18(p)) determination require purification of phosphate as silver phosphate (Ag3PO4). The techniques for the final precipitation of Ag3PO4 fall into two categories: slow vs. rapid microprecipitations. We have tested both and identify artifacts that impact measured delta O-18(p) values resulting from commonly used methods for rapid Ag3PO4 precipitation. In particular, commonly used methods are prone to (a) incomplete precipitation of phosphate, with associated isotopic fractionation, and (b) the production of silver oxide contaminants whose oxygen isotope composition is sensitive to the delta O-18 of water used to prepare the precipitation reagent solution. These artifacts are commonly on the order of 0.5 to 2 parts per thousand, which is subtle enough to go undetected in many studies, but large enough to cause errors in temperature estimates on the order of 2-8 degrees C.Slow microprecipitations lead to complete precipitation of phosphate without contaminating O-bearing phases. We develop a rapid precipitation technique that avoids the problems noted above. Both the slow microprecipitation and the rapid microprecipitation developed herein lead to delta O-18(p) measurements that are indistinguishable from those made on conventional large-batch precipitations of Ag3PO4.