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In the modern climate, the ocean below 2 km is mainly filled by waters sinking into the abyss around Antarctica and in the North Atlantic. Paleoproxies indicate that waters of North Atlantic origin were instead absent below 2 km at the Last Glacial Maximum, resulting in an expansion of the volume occupied by Antarctic origin waters. In this study we show that this rearrangement of deep water masses is dynamically linked to the expansion of summer sea ice around Antarctica. A simple theory further suggests that these deep waters only came to the surface under sea ice, which insulated them from atmospheric forcing, and were weakly mixed with overlying waters, thus being able to store carbon for long times. This unappreciated link between the expansion of sea ice and the appearance of a voluminous and insulated water mass may help quantify the ocean's role in regulating atmospheric carbon dioxide on glacial-interglacial timescales. Previous studies pointed to many independent changes in ocean physics to account for the observed swings in atmospheric carbon dioxide. Here it is shown that many of these changes are dynamically linked and therefore must co-occur. [ABSTRACT FROM AUTHOR]

We have developed a simple, rapid method to screen carbonates for survey applications, which provides radiocarbon dates with decreased precision at lower cost. The method is based on previous work by Longworth et al. (2011) and involves mixing pulverized CaCO3 with Fe powder, followed by pressing into aluminum target holders for direct 14C accelerator mass spectrometry (AMS) measurements. An optimum beam current averaging ~10% of those produced by >0.7 mg C graphite targets was obtained for carbonate samples of 0.3-0.5 mg (0.04-0.06 mg C). The precision of the method was evaluated by measuring triplicates of 14C reference materials, as well as by comparing results from this rapid method with results from high-precision AMS measurements on graphite (typically 0.2-0.3%). Measurement reproducibility was ~1.8% (1σ) for samples <10 ka BP, and it increased drastically for older samples. However, t tests on paired samples resulted in p values greater than 0.05, indicating a good correlation between this survey method and the conventional one. An average blank (calcite) of 0.0075 Fm (~39 ka BP) was achieved. The simplicity of the technique allowed us to process and measure 72 deepsea coral samples in less than 25 hr. [ABSTRACT FROM AUTHOR]

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The article focuses on the distribution of deep-sea corals in the Northwest Atlantic. According to the author, there have been thousands of fossil scleractinian corals collected from cruises between 2003 and 2005, from which the live ones are reported to be rare. It mentions that the depth distribution of fossil Desmophyllum dianthus corals in the New England Seamounts in the Northwest Atlantic id different, as compared to the colonial scleractinian corals. The author suggests that the ocean circulation has an effect on the distribution of corals, though the cause of the link is unknown.

We use pore fluid measurements of the chloride concentration and the oxygen isotopic composition from Ocean Drilling Program cores to reconstruct salinity and temperature of the deep ocean during the Last Glacial Maximum (LGM). Our data show that the temperatures of the deep Pacific, Southern, and Atlantic oceans during the LGM were relatively homogeneous and within error of the freezing point of seawater at the ocean's surface. Our chloride data show that the glacial stratification was dominated by salinity variations, in contrast with the modern ocean, for which temperature plays a primary role. During the LGM the Southern Ocean contained the saltiest water in the deep ocean. This reversal of the modern salinity contrast between the North and South Atlantic implies that the freshwater budget at the poles must have been quite different. A strict conversion of mean salinity at the LGM to equivalent sea-level change yields a value in excess of 140 meters. However, the storage of fresh water in ice shelves and/or groundwater reserves implies that glacial salinity is a poor predictor of mean sea level. [ABSTRACT FROM AUTHOR]

Sulfur isotope fractionation resulting from microbial sulfate reduction (MSR) provides some of the earliest evidence of life, and secular variations in fractionation values reflect changes in biogeochemical cycles. Here we determine the sulfur isotope effect of the enzyme adenosine phosphosulfate reductase (Apr), which is present in all known organisms conducting MSR and catalyzes the first reductive step in the pathway and reinterpret the sedimentary sulfur isotope record over geological time. Small fractionations may be attributed to low sulfate concentrations and/or high respiration rates, whereas fractionations greater than that of Apr require a low chemical potential at that metabolic step. Since Archean sediments lack fractionation exceeding the Apr value of 20‰, they are indicative of sulfate reducers having had access to ample electron donors to drive their metabolisms. Large fractionations in post-Archean sediments are congruent with a decline of favorable electron donors as aerobic and other high potential metabolic competitors evolved. Changes in S-isotope ratios over time provide clues to understanding the co-evolution of Earth and its biosphere. Here the authors determine the isotope effect of the first reductive enzyme in the sulfate respiration pathway and reinterpret sedimentary S-isotope records based on this biochemical constraint. [ABSTRACT FROM AUTHOR]

Sedimentary rocks from the early Eocene Green River Formation comprise the largest known lacustrine oil shale deposits, contain remarkably well-preserved fossils, and provide a unique record of climate evolution across the Early Eocene Climate Optimum, a period of high atmospheric CO 2. The depositional environment of these intermountain lakes spanned from relatively fresh and fluvially influenced to expanded and stratified saline closed basin lakes under the influence of the Laramide orogeny and alternating humid and arid climatic conditions. As the surface area of the lakes expanded, alkalinity and salinity increased, with depositional cycles that linked to evaporative cycles and marked by an increasing abundance of organic matter-rich shales (TOC > 10 wt%). Simultaneously, an intriguing 20 ‰ positive shift in the sulfur isotope composition of sulfide minerals and organic sulfur is observed. Given that this trend cannot be simply explained by a change in the source of sulfate delivered to the basin, the evolution of biogeochemical sulfur cycling and the balance of fluxes in response to basin evolution remain unresolved. Here, we combine the sulfur isotope compositions of pyrite, organic sulfur, and carbonate-associated sulfate and molecular proxies of euxinia in samples from the Uinta basin's depocenter. We find that organic matter-rich sediments reflect deposition in a stratified water column with enhanced burial of pyrite and sulfurized organic matter, while organic-lean facies present evidence of, at least transiently, euxinic conditions reaching the photic zone during arid conditions presumably because of evaporation. As the lake became both saline-stratified and euxinic, we observe that δ34S values of all measured sulfur-bearing sedimentary proxies increase and evolve along a 1:1 line, a trend independent of facies that we interpret as reflecting a sulfate-limited system despite saline conditions. The isotopic mass balance of sulfur fluxes implies the existence of a sink of sulfur depleted in 34S that is spatially decoupled from burial in the depocenter. Modeling sulfur biogeochemical processes in a saline stratified lake system allows us to estimate that at least 50 % of the sulfur entering the lake could have been lost from the upper part of the euxinic water column where the fractionation factor imparted by microbial sulfate reduction is expressed. We propose that the overall isotopic enrichment of the system was caused by H 2 S degassing during arid climate intervals, presumably enhanced by transient water column mixing events. Further, episodic intrusion of euxinic bottom waters into the upper part of the water column might have triggered mass fish and plankton mortality, consequently facilitating the formation of these exceptionally fossiliferous and organic matter-rich rocks. Our study finds that volatile outgassing may be an underappreciated mechanism for the sulfur mass balance of stratified lacustrine systems. [ABSTRACT FROM AUTHOR]

We present the first examination of calcite dissolution in seawater using Atomic Force Microscopy (AFM). We quantify step retreat velocity and etch pit density to compare dissolution in seawater to low ionic strength water, and also to compare calcite dissolution under AFM conditions to those conducted in bulk solution experiments (e.g. Subhas et al., 2015, Dong et al., 2018). Bulk dissolution rates and step retreat velocities are slower at high and mid-saturation state (Ω) values and become comparable to low ionic strength water rates at low Ω. The onset of defect-assisted etch pit formation in seawater is at Ω ∼ 0.85 (defined as Ω critical), higher than in low ionic strength water (Ω ∼ 0.54). There is an abrupt increase in etch pit density (from ∼106 cm−2 to ∼108 cm−2) occurring when Ω falls below 0.7 in seawater, compared to Ω ∼ 0.1 in low ionic strength water, suggesting a transition from defect-assisted dissolution to homogeneous dissolution much closer to equilibrium in seawater. The step retreat velocity (v) does not scale linearly with undersaturation (1-Ω) across an Ω range of 0.4 to 0.9 in seawater, potentially indicating a high order correlation between kink rate and Ω for non-Kossel crystals such as calcite, or surface complexation processes during calcite dissolution in seawater. [ABSTRACT FROM AUTHOR]

The last glacial period is punctuated by abrupt changes in Northern Hemisphere temperatures that are known as Dansgaard–Oeschger (DO) events. A striking and largely unexplained feature of DO events is an interhemispheric asymmetry characterized by cooling in Antarctica during periods of warming in Greenland and vice versa—the bipolar seesaw. Methane-synchronized ice core records indicate that the Southern Hemisphere lags the Northern Hemisphere by approximately 200 years. Here, we propose a mechanism that produces observed features of both the bipolar seesaw and the phasing of DO events. The spatial pattern of sea ice formation and melt in the Southern Ocean imposes a rigid constraint on where water masses are modified: waters are made denser near the coast where ice forms and waters are made lighter farther north where ice melts. This pattern, coupled to the tilt of density surfaces across the Southern Ocean and the stratification of the ocean basins, produces two modes of overturning corresponding to different bipolar seesaw states. We present evolution equations for a simplified ocean model that describes the transient adjustment of the basin stratification, the Southern Ocean surface density distribution, and the overturning strength as the ocean moves between these states in response to perturbations in North Atlantic Deep Water formation, which we take as a proxy for Greenland temperatures. Transitions between different overturning states occur over a multicentennial time scale, which is qualitatively consistent with the observed Southern Hemisphere lag. The volume of deep density layers varies inversely with the overturning strength, leading to significant changes in residence times. Evidence of these dynamics in more realistic circulation models is discussed. [ABSTRACT FROM AUTHOR]

Abstract We measured U/Ca ratios, 4He concentrations, 234U/238U, and 238U/235U in a subset of well-preserved aragonitic scleractinian fossil corals previously described by Gothmann et al. (2015). Comparisons of measured fossil coral He/U ages with the stratigraphic age demonstrate that well-preserved coral aragonite retains most or all of its radiogenic He for 10's of millions of years. Such samples must be largely or entirely free of alteration, including neomorphism. Measurements of 234U/238U and 238U/235U further help to characterize the fidelity with which the original U concentration has been preserved. Analyses of fossil coral U/Ca show that the seawater U/Ca ratio rose by a factor of 4–5 between the Early Cenozoic and today. Possible explanations for the observed increase include (1) the stabilization of U in seawater due to an increase in seawater [CO 3 2−], and a resulting increase in UO 2 -CO 3 complexation as originally suggested by Broecker (1971); (2) a decrease in the rate of low-temperature hydrothermal alteration from Early Cenozoic to present, leading to a diminished U sink and higher seawater [U]; or (3) a decrease in uranium removal in reducing sediments, again leading to higher seawater [U]. [ABSTRACT FROM AUTHOR]

The stable isotope compositions of biogenic carbonates have been used for paleoceanographic and paleoclimatic reconstructions for decades, and produced some of the most iconic records in the field. However, we still lack a fully mechanistic understanding of the stable isotope proxies, especially the biological overprint on the environmental signals termed “vital effects”. A ubiquitous feature of stable isotope vital effects in marine calcifying organisms is a strong correlation between δ 18 O and δ 13 C in a range of values that are depleted from inorganic calcite/aragonite. Two mechanisms have been proposed to explain this correlation, one based on kinetic isotope effects during CO 2 (aq)-HCO 3 − inter-conversion, the other based on equilibrium isotope exchange during pH dependent speciation of the dissolved inorganic carbon (DIC) pool. Neither mechanism explains all the stable isotope features observed in biogenic carbonates. Here we present a fully kinetic model of biomineralization and its isotope effects using deep-sea corals as a test organism. A key component of our model is the consideration of the enzyme carbonic anhydrase in catalyzing the CO 2 (aq)-HCO 3 − inter-conversion reactions in the extracellular calcifying fluid (ECF). We find that the amount of carbonic anhydrase not only modulates the carbonate chemistry of the calcifying fluid, but also helps explain the slope of the δ 18 O-δ 13 C correlation. Differences in CA activity in the biomineralization process can possibly explain the observed range of δ 18 O-δ 13 C slopes in different calcifying organisms. A mechanistic understanding of stable isotope vital effects with numerical models can help us develop better paleoceanographic tracers. [ABSTRACT FROM AUTHOR]

Deep-sea corals are a useful archive of thermocline, intermediate, and deep waters in past oceans. However, application of traditional oceanographic tracers to deep-sea corals remains a challenge due to our insufficient understanding of their "vital effects". Deep-sea corals are ideal test organisms to study the mechanism underlying vital effects generally, due to the large tracer gradients in individual corals living under relatively constant environmental conditions. Lessons learned from these corals might apply to other scleractinia and to marine calcifiers more generally. Here we present stable isotope, minor and trace metal (Me/Ca ratios) data in a suite of modern Desmophyllum dianthus specimens, collected over multiple spatial scales in individual corals (bulk, micromill, SIMS, NanoSIMS), with multi-proxy analyses made on the same material whenever possible. Spatially coherent Me/Ca correlations are observed in the fibrous aragonite of individual corals, including positive correlations between Mg/Ca, Li/Ca and B/Ca, as well as negative correlations between Mg/Ca and Sr/Ca, consistent with previous studies. We also for the first time document strong correlations between the isotopic (δ18O and δ13C) and elemental compositions of the skeletons, most notably a negative correlation between δ18O and Mg/Ca. The centers of calcification (COCs) in the coral skeletons show distinct tracer correlations from the aragonite fibers that possibly reflect a more complicated formation mechanism. We interpret the spatially coherent tracer correlations in deep-sea corals with a numerical model of coral calcification previously developed for stable isotopes that considers the role of the enzyme carbonic anhydrase in the calcification processs. With the carbonate chemistry in the model constrained by the stable isotope data, we are able to explain the observed Me/Ca correlations as well as their range of variability, as a result of internal pH elevation in the extracellular calcifying fluid (ECF) of the corals with limited Ca-pumping through the calicoblastic membrane. In particular, the positive Mg/Ca–B/Ca correlation in the fibrous aragonite suggests a borate (B(OH) 4 –) substitution for carbonate ion (CO 3 2–) incorporation mechanism in biogenic aragonite. We also suggest the growth rate dependence of the incorporation of minor and trace elements based conceptually on an ion-by-ion growth model may help explain the absolute Me/Ca values in biogenic aragonites. Finally, we generally find more limited tracer variability in corals from undersaturated seawater compared to their counterparts from supersaturated conditions, suggesting a limit to their internal pH elevation in response to this environmental stress. Understanding the biomineralization mechanisms underlying the vital effects is important for better use of these tracers for paleoceanographic applications, and may shed light on the response of marine calcification to future ocean acidification. [ABSTRACT FROM AUTHOR]

Organic matter (OM) burial in marine sediments is a potentially important control on global climate and the long-term redox state of the earth’s surface. Still, we have only a limited understanding of the processes that stabilize OM and facilitate its preservation in the geologic record. Abiotic reactions with (poly)sulfides can enhance the preservation potential of OM, but for this process to be significant it needs to compete with OM remineralization, the majority of which occurs before sinking particles reach the sea floor. Here we investigate whether OM sulfurization occurs within sinking particles in the Cariaco Basin, a modern sulfidic marine environment with high rates of OM burial. Proto-kerogen in sinking particles is frequently more sulfur-rich and 34 S-depleted than expectations for biomass, with a composition that is difficult to explain by mixing with resuspended or terrigenous material. Instead, it appears that sulfur is being incorporated into OM on a timescale of days in sinking particles. The flux of this abiogenic organic S from particles is equivalent to approximately two-thirds of the total amount of proto-kerogen S at 10 cm depth in underlying sediments (ODP Core 1002B); after 6000 years of more gradual sulfurization reactions, potential water column sources are still equivalent to nearly half of the total proto-kerogen S in Cariaco sediments. Water column sulfurization is most extensive during periods of upwelling and high primary productivity and appears to involve elemental S, possibly via polysulfides. This process has the potential to deliver large amounts of OM to the sediments by making it less available for remineralization, generating OM-rich deposits. It represents a potentially dynamic sink in the global carbon cycle that can respond to changes in environmental conditions, including the size and intensity of O 2 -depleted environments. Water column OM sulfurization could also have played a more significant role in the carbon cycle during ocean anoxic events, for example during the Cretaceous. [ABSTRACT FROM AUTHOR]

• Porewater Ni isotope composition (δ60Ni) can be heavier than deep seawater. • Dissolution of detrital materials in marine sediments may supply dNi to pore fluids. • In marine sediments, porewater Ni isotopes are fractionated by active Mn cycling. • Continental margin sediments can supply dissolved Ni with heavy δ60Ni to the ocean. • Heavy benthic δ60Ni may help to resolve the global ocean mass balance of Ni. Nickel (Ni) is a bio-essential element for phytoplankton in the modern oceans, and yet, the global ocean mass balance of Ni has puzzled scientists for decades. Many estimates of total Ni output flux are larger than the total Ni input flux to the ocean. The measurement of Ni stable isotopes (δ60Ni) in earth materials may inform our understanding of ocean biogeochemistry and redox conditions in both the modern ocean and the geological past. An ocean Ni stable isotope imbalance has also concerned scientists, with a global ocean δ60Ni of +1.4 ‰ which is notably heavier than the major source of Ni to the oceans from rivers. Recent efforts to resolve Ni and δ60Ni imbalances have focused on the role which manganese (Mn) oxides play in marine Ni cycling, both in the water column and in marine sediments. Manganese oxide rich marine sediments can supply isotopically heavy dissolved Ni to porewater fluids and seawater, but the source of the isotopically heavy porewater Ni remains unclear. Here we present porewater trace metal concentrations and δ60Ni from two sites from the continental margin off southern California. Porewater Ni concentrations are up to roughly 100-fold higher than deep seawater concentrations (∼1 μM compared to 10 nM, respectively). Porewater δ60Ni is near 0 ‰ in the subsurface region where Ni concentrations are highest, suggesting dissolution of lithogenic material from sediments. Porewater δ60Ni increases dramatically towards the sediment-water interface with values of up to +2.66 ‰, which to our knowledge are the heaviest Ni isotope compositions ever reported for natural materials. Measurements of porewater and sediment Ni and Mn concentrations suggest that Ni is released to porewaters in the Mn-reducing zone, and then removed by newly precipitated Mn oxides as Mn and Ni move towards the oxygenated zone of sediments. A simple Rayleigh model suggests a Ni isotope fractionation of factor of -0.61 ‰ for Ni sorption onto Mn oxides, while a diffusion-reaction model suggests a Ni isotope fractionation of -1.80 to -0.96 ‰, always with preferential adsorption of lighter Ni isotopes. This flux of Ni toward the sediment-water interface, coupled with preferential removal of lighter Ni isotopes in marine sediments, provides evidence supporting an isotopically heavy benthic source of new Ni to the oceans, which we estimate has a magnitude of 0.65 × 108 to 1.66 × 108 moles/year. We find that a benthic flux such as measured here over just 0.27-2.70 % of the ocean seafloor could mix with the river δ60Ni of +0.8 ‰ to achieve the observed deep ocean δ60Ni of +1.4 ‰. This study reveals the similarities in how rivers and continental margin sediments supply heavy Ni isotopes to the ocean. In both continental rivers and margin sediments, terrigenous Ni is released with an δ60Ni near 0.1 ‰, but lighter isotopes are captured by precipitation onto oxides so that the dissolved flux to the ocean is isotopically heavier than the terrigenous material from which Ni was released, and in the case of margin sediments may be heavier than even bulk seawater δ60Ni. We thus recognize continental margin sediments with active Mn cycling as a 'new' source of isotopically heavy dissolved Ni to the ocean. [ABSTRACT FROM AUTHOR]

The article presents a study which established several benchmarks for Atlantic, Southern, and Pacific deep-sea circulation and ventilation since the last ice age. Topics discussed include a flipped Pacific and slower deep overturning circulation during the Last Glacial Maximum (LGM), Southern, Atlantic, and Pacific deep-sea ventilation during the LGM, and deep-sea reorganization and carbon ventilation during the deglaciation.

Uranium isotope ratios are widely utilized in paleoceanography. The 238U/235U ratio (expressed as δ238U) is leveraged as a proxy for the areal extent of anoxic seafloor, and the 234U/238U ratio (expressed as δ234U sec) tracks riverine and estuarine inputs to the ocean, in addition to featuring prominently in U-series geochronology. Both of these ratios are thought to be recorded by biological carbonates precipitating from seawater, with corals serving as one of the most commonly-used archives of seawater U isotope ratios in the past. The utility of the U isotope proxy in biological carbonate archives relies not only on this faithful archiving of ambient seawater signatures, but also on the homogeneity of the seawater U isotope composition, which enables samples to be leveraged as proxy for the entire ocean. Here we revisit the foundational assumption of homogeneity of the marine U reservoir, and the capacity of deep-sea corals to record the U isotopic composition of ambient seawater. We begin by evaluating the analytical limits of precision and accuracy achievable for both δ238U and δ234U sec analysis by MC-ICP-MS. We then report data for 45 seawater and 26 deep-sea coral samples from multiple sites around the world. We find subtle δ238U and δ234U sec heterogeneity that correlates with U concentrations, which allows us to calculate new salinity-normalized global mean seawater values for δ238U (−0.379 ± 0.023 ‰) and δ234U sec (+145.55 ± 0.28 ‰). At each site, biological carbonates act as precise archives of the seawater δ238U value. The same is true for δ234U sec , with a few exceptions where samples appear to show vital effects that cause intra-sample 234U/238U re-partitioning. In sum, these observations support deep-sea corals as a robust archive of seawater U isotope ratios, but highlight the importance of utilizing multiple sample sites and replicate analyses to overcome coral vital effects (for δ234U sec) and subtle marine U isotopic heterogeneity. [ABSTRACT FROM AUTHOR]

Paleoceanographic proxies indicate that the ocean state during the Last Glacial Maximum (LGM) differed from the modern ocean state. Depth profiles of ocean sediment pore fluid δ18O and [Cl−] have been used to reconstruct the δ18O and salinity at the LGM. Here, it is investigated whether pore fluid profiles can constrain ocean δ18O and salinity at other times and, simultaneously, their ability to constrain the LGM δ18O and salinity. An inverse framework is developed that relies on Bayesian parameter estimation, thus allowing formal separation of prior assumptions from the information in observations. Synthetic problems are used to explore the information about past ocean tracers that can be recovered from pore fluid profiles. It is concluded that prior knowledge of deep ocean mixing time scales is essential to an accurate inverse estimate of LGM ocean salinity and δ18O from modern pore fluid profiles. The most recent 10 000 years of ocean salinity and δ18O and the error in their estimates are better constrained by the pore fluid profiles than are the LGM values. The inverse estimate of salinity and δ18O is strongly correlated with the estimate of diffusivity of oxygen isotopes and [Cl−] in sediment pore fluids. Uncertainty on the diffusivity of oxygen isotopes and chloride in sediments is reduced through inversion of the pore fluid profiles, but simultaneous estimation of δ18O or salinity and diffusivity increases the total uncertainty. The error reported in previous work may underestimate the true uncertainty of LGM deep ocean salinity and δ18O. [ABSTRACT FROM AUTHOR]

The article presents a study which explores an integrated understanding of carbonate biomineralization as necessary to illuminate this evolutionary record and to understand how modern organisms will respond to 21st century global change. It mentions that biomineralization evolved independently but convergently across phyla, suggesting a unity of mechanism that transcends biological differences.

The relationship between climate variability and rainfall oxygen isotopic (δ18O) variability is poorly constrained, especially in the tropics, where many key paleoclimate records rely on past rainfall isotopes as proxies for hydroclimate. Here we present a daily-resolved, 5-yr-long timeseries of rainfall δ18O from Gunung Mulu National Park, located in northern Borneo (4°N, 114°E) in the heart of the West Pacific Warm Pool, and compare it to local and regional climatic variables. Daily rainfall δ18O values range from +0.7‰ to −18.5‰ and exhibit a weak but significant inverse relationship with daily local precipitation amount (R=−0.19, p<0.05), consistent with the tropical amount effect. Day-to-day δ18O variability at Mulu is best correlated to regional precipitation amount averaged over the preceding week (R=−0.64, p<0.01). The inverse relationship between Mulu rainfall δ18O and local (regional) precipitation amount increases with increased temporal averaging, reaching R=−0.56 (R=−0.72) on monthly timescales. Large, negative, multi-day rainfall δ18O anomalies of up to 16‰ occur every 30–90 days and are closely associated with wet phases of the intraseasonal Madden–Julian Oscillation. A weak, semi-annual seasonal cycle in rainfall δ18O of 2–3‰ bears little resemblance to seasonal precipitation variability, pointing to a complex sequence of moisture sources and/or trajectories over the course of the year. Interannual rainfall δ18O variations of 6–8‰ are significantly correlated with indices of the El Niño Southern Oscillation, with increased rainfall δ18O during relatively dry El Niño conditions, and vice versa during La Nina events. We find that Mulu rainfall δ18O outperforms Mulu precipitation amount as a tracer of basin-scale climate variability, highlighting the time- and space-integrative nature of rainfall δ18O. Taken together, our results suggest that rainfall δ18O variability at Mulu is significantly influenced by the strength of regional convective activity. As such, our study provides further empirical support for the interpretation of δ18O-based paleo-reconstructions from northern Borneo stalagmites as robust indicators of regional-scale hydroclimate variability, where higher δ18O reflects regional drying. [ABSTRACT FROM AUTHOR]

The abundance of 18O isotopes and 13C-18O isotopic "clumps" (measured as δ18O and Δ 47 , respectively) in carbonate minerals have been used to infer mineral formation temperatures. An inherent requirement or assumption for these paleothermometers is mineral formation in isotopic equilibrium. Yet, apparent disequilibrium is not uncommon in biogenic and abiogenic carbonates formed in nature and in synthetic carbonates prepared under laboratory settings, as the dissolved carbonate pool (DCP) from which minerals precipitate is often out of δ18O and Δ 47 equilibrium. For this, a complete understanding of both equilibrium and kinetics of isotopic partitioning and 13C-18O clumping in DCP is crucial. To this end, we analyzed Δ 47 of inorganic BaCO 3 samples from Uchikawa and Zeebe (2012) (denoted as UZ12), which were quantitatively precipitated from NaHCO 3 solutions at various times over the course of isotopic equilibration at 25 °C and pH NBS of 8.9. Our data show that, although the timescales for δ18O and Δ 47 equilibrium in DCP are relatively similar, their equilibration trajectories are markedly different. As opposed to a simple unidirectional and asymptotic approach toward δ18O equilibrium (first-order kinetics), Δ 47 equilibration initially moves away from equilibrium and then changes its course towards equilibrium. This excess Δ 47 disequilibrium is manifested as a characteristic "dip" in the Δ 47 equilibration trajectory, a feature consistent with an earlier study by Staudigel and Swart (2018) (denoted as SS18). From the numerical model of SS18 , the non-first-order kinetics for Δ 47 equilibration can be understood as a result of the difference in the exchange rate for oxygen isotopes bound to 12C versus 13C, or an isotope effect of ~25‰. We also developed an independent model for the Ex change and Clump ing of 13 C and 18 O in DCP (ExClump38 model) to trace the evolution of singly- and doubly-substituted isotopic species (i.e., δ13C, δ18O and Δ 47). The model suggests that the dip in the Δ 47 equilibration trajectory is due largely to kinetic carbon isotope fractionation for hydration and hydroxylation of CO 2. We additionally examined the BaCO 3 samples prepared from NaHCO 3 solutions supplemented with carbonic anhydrase (CA), an enzyme known to facilitate δ18O equilibration in DCP by catalyzing CO 2 hydration (UZ12). These samples revealed that, while CA effectively shortens the time required for Δ 47 equilibrium in DCP, the overall pattern and magnitude of the dip in the Δ 47 equilibration trajectory remain unchanged. This suggests no additional isotope effects due to the CA enzyme within the tested CA concentrations. With the ExClump38 model, we test various physicochemical scenarios for the timescales and trajectories of isotopic equilibration in DCP and discuss their implications for the Δ 47 paleothermometry. [ABSTRACT FROM AUTHOR]

An important factor for predicting the effect of increased CO 2 on future acidification of the ocean is a proper understanding of the interactions controlling production and dissolution of calcium carbonate minerals (CaCO 3). The production and dissolution of CaCO 3 in the ocean can be assessed over large spatial scales by measuring seawater calcium concentrations and total alkalinity (A T), yet past studies suggest that there could be large discrepancies between calcium and A T -based balances of the CaCO 3 cycle in the North Pacific and Indian Oceans. Here, we analyse water column samples collected along transects in the North Pacific, Southern Ocean, tropical Indian Ocean and Red Sea for their concentrations of calcium, nutrients, and A T. We find that there is an excess calcium over A T anomaly in the top 1000 m of the tropical Indian Ocean water-column. The source of this anomaly is the dissolution of subsurface gypsum deposits in the Red Sea. We find no evidence for calcium-over-A T anomalies in the North Pacific, in contrast to previous studies. Our results show that, in most cases, calcium and A T data agree well and can be used to reconstruct the marine CaCO 3 cycle. [ABSTRACT FROM AUTHOR]

Anomalous sulfur isotope compositions present in Archean rocks have been intensely scrutinized over the last 20 years because they record key aspects of Earth's atmospheric composition prior to the appearance of free molecular oxygen ca. 2.3 billion years ago. These isotopic compositions can be described as mass anomalous fractionations (MAF) and are produced in the atmosphere as UV light interacts with SO 2 molecules. Most interpretations suggest that atmospheric processes generate a reduced S-phase with a positive (33S-enriched) MAF signature, as measured in pyrites, and an oxidized S-phase with a negative anomaly, as measured in bedded barite deposits. However, recent data for carbonate-associated sulfate (CAS) — a direct proxy for the isotopic composition of sulfur from seawater sulfate — in Neoarchean rocks showed no such negative values, but rather the opposite. To understand if the positive MAF anomalies we measured in Neoarchean CAS reflect secondary processes (diagenetic, metamorphic, handling) instead of original signals of Archean seawater sulfate, we collected additional sample suites with various degrees of preservation and metamorphic alteration across the Campbellrand-Malmani platform in South Africa. Results illustrate that within this comprehensive suite, less-altered samples all contain positive MAF values while secondary processes tend to either remove CAS from the sample and/or decrease the 33S-enrichment. This positive MAF signal in sulfate is therefore reasonably interpreted as a primary depositional origin, and implies that the assumption that sulfate always carries a negative MAF anomaly throughout the Archean rock record needs to be reconsidered. Our CAS observations suggest that future experiments and calculations should also consider atmospheric and/or sulfur cycling processes that can produce oxidized sulfur with a positive MAF signature. [ABSTRACT FROM AUTHOR]

Sulfate (SO 4 2−) incorporated into calcium carbonate minerals enables measurements of sulfur (S) isotope ratios in carbonate rocks. This Carbonate Associated Sulfate (CAS) in marine carbonate minerals is thought to faithfully represent the S isotope composition of the seawater sulfate incorporated into the mineral, with little or no S isotope fractionation in the process. However, comparison between different calcifying species reveals both positive and negative S isotope fractionation between CAS and seawater sulfate, and a large range of S isotope ratios can be found within a single rock sample, depending on the component measured. To better understand the isotopic effects associated with sulfate incorporation into carbonate minerals, we precipitated inorganic calcite and aragonite over a range covering more than two orders of magnitude of sulfate concentration and precipitation rate. Coupled measurements of CAS concentration, S isotope composition and X-ray absorption near-edge spectra (XANES) permit characterization and explanation of the observed dependence of S isotope fractionation between CAS and aqueous sulfate (CAS-SO 4 2− isotope fractionation) on sulfate concentration and precipitation rate. In aragonite, the CAS-SO 4 2− isotope fractionation is 1.0 ± 0.3‰ and independent of the sulfate (and CAS) concentration. In contrast, the CAS-SO 4 2− isotope fractionation in calcite covaries strongly with the sulfate concentration and weakly with the precipitation rate, between values of 1.3 ± 0.1 and 3.1 ± 0.6‰. We suggest that the correlation between aqueous sulfate concentration and CAS-SO 4 2− isotope fractionation in calcite reflects a dependence of the equilibrium S isotope fractionation on the concentration of CAS, through the effect of the sulfate impurity on the carbonate mineral's energetic state. [ABSTRACT FROM AUTHOR]

Diatoms are the dominant algal group that cycles dissolved silicic acid in the ocean; they also play an important role in the oceanic carbon cycle. It is therefore important to quantify the spatial distribution of silica cycling for defining global ocean biogeochemical cycles. On the research cruise CDisK-IV, water samples and sediment cores were collected at 5 stations along a North Pacific transect near 150°W from 22°N to 50°N to evaluate benthic remineralization rates of biogenic silica (bSi). Two independent methods, core incubation and diffusive transport based on porewater profiles, were utilized to estimate benthic silicic acid fluxes, and these independent methods yield fluxes that agree within uncertainties. The benthic fluxes are reported as 0.04 ± 0.01, 0.04 ± 0.01, 0.05 ± 0.01, 0.67 ± 0.14, 0.40 ± 0.08 mmol Si m−2 day−1 for Stations 1 to 5, south to north, respectively. Burial fluxes were estimated using measurements of solid phase bSi in sediments and literature values of sediment accumulation rate. Burial efficiencies of bSi at all stations were <5% and show reasonable agreement with previous estimates. When burial rates were added to benthic fluxes to calculate rain rates, the rain observed under the subarctic gyre (Stations 4–5), was far larger than in the lower latitudes of the subtropics (Stations 1–3), corresponding to higher surface diatom productivity at higher latitudes. At the two northern stations, the bottom 500 m of the water column shows a near-bottom increase in silicic acid that is consistent with the measured benthic flux and the estimated vertical eddy diffusivity. Above this horizon, water column density stratification increases and vertical diffusivity decreases, but the silicic acid gradient decreases. This reduction in gradient indicates that above this horizon, horizontal transport by deep waters, rather than vertical diffusion, becomes the dominant process removing the silicic acid released by benthic remineralization. • Benthic Si fluxes in the North Pacific were estimated with two independent methods. • Fluxes were 10 times higher in the subarctic gyre than in the subtropical gyre. • Burial efficiency of biogenic silica was low at all study sites. • The spatial pattern of benthic Si fluxes was similar to that of upper ocean chlorophyll. • The benthic flux was sufficient to sustain the near-bottom Si increase in the water column. [ABSTRACT FROM AUTHOR]

Abstract Sulfide (H 2 S, HS−, and S2−) is ubiquitous in marine porewaters as a result of microbial sulfate reduction, constituting the reductive end of the biogeochemical sulfur cycle. Stable isotopes have been widely used to constrain the sulfur cycle, because the redox transformations of sulfur compounds, such as microbial sulfate reduction, often exhibit sizable kinetic isotope effects. In contrast to sulfate ion (SO 4 2−), the most abundant form of dissolved sulfur in seawater, H 2 S is volatile and also deprotonated at near neutral pH. Equilibrium isotope partitioning between sulfide species can therefore overlap with kinetic isotope effects during reactions involving sulfide as either reactant or intermediate. Previous experimental attempts to measure equilibrium fractionation between H 2 S and HS− have reached differing results, likely due to solutions of widely varying ionic strength. In this study, we measured the sulfur isotope fractionation between total dissolved sulfide and gaseous H 2 S at 20.6 ± 0.5 °C over the pH range from 2 to 8, and calculated the equilibrium isotope effects associated with deprotonation of dissolved H 2 S. By using dilute solutions of Na 2 S, made possible by the improved sensitivity of mass spectrometric techniques, uncertainty in the first dissociation constant of H 2 S due to ionic strength could be better controlled. This in turn allowed us to close sulfur isotope mass balance for our experiments and increase the accuracy of the estimated fractionation factor. At equilibrium, aqueous H 2 S was enriched in 34S by 0.7‰ and 3.1‰ relative to gaseous H 2 S and aqueous HS−, respectively. The estimated fractionation between aqueous H 2 S and HS− lies between two earlier experimental reports, but agrees within the uncertainty of the measurements with a recent theoretical calculation. [ABSTRACT FROM AUTHOR]

Abstract Knowledge of the mechanism of calcite dissolution in seawater is a critical component of our understanding of the changing global carbon budget. Towards this goal, we provide the first measurements of the temperature dependence of seawater calcite dissolution kinetics. We measured the dissolution rates of 13C-labeled calcite in seawater at 5, 12, 21, and 37 °C across the full range of saturation states 0 < Ω = C a 2 + [ C O 3 2 - ] K sp ' < 1 . We show that the dissolution rate is non-linearly dependent on Ω and that the degree of non-linearity both increases with temperature, and changes abruptly at "critical" saturation states (Ω crit). The traditional exponential rate law most often utilized in the oceanographic community, R = k (1 − Ω) n , requires different fits to k and n depending upon the degree of undersaturation. Though we calculate a similar activation energy to other studies far from equilibrium (25 ± 2 kJ/mol), the exponential rate law could not be used to mechanistically explain our near equilibrium results. We turn to an alternative framework, derived from crystal nucleation theory, and find that our results are consistent with calcite dissolution kinetics in seawater being set by the retreat of pre-existing edges/steps from Ω = 1–0.9, defect-assisted etch pit formation from Ω = 0.9–0.75, and finally homogenous etch pit formation from Ω = 0.75–0. The Ω crit s for each mechanism are shifted significantly closer to equilibrium than they occur in dilute solutions, such that ocean acidification may cause marine carbonates to enter faster dissolution regimes more readily than would be expected from previous studies. We use the observed temperature dependence for each dissolution mechanism to calculate step kinetic coefficients (β , cm/s), densities of active nucleation sites (n s , sites/m2), and step edge free energies (α, mJ/m2). Homogenous dissolution is well explained within the surface nucleation framework, but defect-assisted dissolution is not. Dissolution is initiated via step-propagation at all temperatures, but the defect-assisted mechanism is skipped over at 5 °C, potentially due to a lack of nucleation sites. The surface nucleation framework enhances our understanding of calcite dissolution in seawater, but our results suggest that a complete theory will also need to incorporate the role of solution/surface speciation and complexation. [ABSTRACT FROM AUTHOR]

This study provides laboratory data of calcite dissolution rate as a function of seawater undersaturation state (1 −  Ω ) under variable pressure. 13 C-labeled calcite was dissolved in unlabeled seawater and the evolving δ 13 C composition of the fluid was monitored over time to evaluate the dissolution rate. Results show that dissolution rates are enhanced by a factor of 2–4 at 700 dbar compared to dissolution at the same Ω under ambient pressure (10 dbar). This dissolution rate enhancement under pressure applies over an Ω range of 0.65–1 between 10 dbar and 700 dbar. Above 700 dbar (up to 2500 dbar), dissolution rates become independent of pressure. The observed enhancement is well beyond the uncertainty associated with the thermodynamic properties of calcite under pressure (partial molar volume ΔV), and thus should be interpreted as a kinetic pressure effect on calcite dissolution. Dissolution at ambient pressure and higher pressures yield non-linear dissolution kinetics, the pressure effect does not significantly change the reaction order n in Rate = k(1 −   Ω ) n , which is shown to vary from 3.1 ± 0.3 to 3.8 ± 0.5 from 10 dbar to 700 dbar over Ω  = 0.65–0.9. Furthermore, two different dissolution mechanisms are indicated by a discontinuity in the rate-undersaturation relationship, and seen at both ambient and higher pressures. The discontinuity, Ω critical = 0.87 ± 0.05 and 0.90 ± 0.03 at 10 dbar and 1050 dbar respectively, are similar within error. The reaction order, n, at Ω  > 0.9 is 0.47 ± 0.27 and 0.46 ± 0.15 at 10 dbar and 700 dbar respectively. This Ω critical is considered to be the threshold between step retreat dissolution and defect-assisted dissolution. The kinetic enhancement of dissolution rates at higher pressures is related to a decrease in the interfacial energy barrier at dissolution sites. The impact of pressure on the calcite dissolution kinetics implies that sinking particles would dissolve at shallower depths than previously thought. [ABSTRACT FROM AUTHOR]

Abstract We present the dissolution kinetics of mixed planktic foraminifera, the benthic foraminifera Amphistegina , the coccolithophore Emiliania huxleyi , and the soft coral Rhythismia fulvum in seawater. Dissolution rates were measured across a large range of saturation states (Ω = 0.99–0.2) by dissolving 13C-labeled calcites in natural seawater undersaturated with respect to calcite. 13C-label was incorporated into biogenic calcite by culturing marine calcifiers in 13C-labeled natural seawater. Net dissolution rates were calculated as the slope of seawater δ 13C versus time in a closed seawater-calcite system. All calcites show distinct, nonlinear, dependencies on seawater saturation state when normalized by mass or by specific surface area. For example, coccolith calcite dissolves at a similar rate to inorganic calcite near equilibrium when normalized by surface area, but dissolves much more slowly far from equilibrium. Mass loss from foraminiferal tests is correlated with a decrease in Mg/Ca of the solid, indicating that Mg-rich phases are preferentially leached out at even mild undersaturations. Dissolution also appears to strongly affect test B/Ca. Finally, we provide an interpretation of surface area-normalized biogenic calcite dissolution rates as a function of their Mg and organic carbon content. Near-equilibrium dissolution rates of all calcites measured here show a strong, nonlinear dependence on Mg content. Far-from-equilibrium dissolution rates decrease strongly as a function of organic carbon content. These results help to build a framework for understanding the underlying mechanisms of rate differences between biogenic calcites, and bear important implications for the dissolution of high-Mg calcites in view of ocean acidification. [ABSTRACT FROM AUTHOR]

Little is known about the fractionation of Li isotopes during formation of biogenic carbonates, which form the most promising geological archives of past seawater composition. Here we investigated the Li isotope composition (δ 7 Li) and Li/Ca ratios of organisms that are abundant in the Phanerozoic record: mollusks (mostly bivalves), echinoderms, and brachiopods. The measured samples include (i) modern calcite and aragonite shells from various species and natural environments (13 mollusk samples, 5 brachiopods and 3 echinoderms), and (ii) shells from mollusks grown under controlled conditions at various temperatures. When possible, the mollusk shell ultrastructure was micro-sampled in order to assess intra-shell heterogeneity. In this paper, we systematically characterize the influence of mineralogy, temperature, and biological processes on the δ 7 Li and Li/Ca of these shells and compare with published data for other taxa (foraminifera and corals). Aragonitic mollusks have the lowest δ 7 Li, ranging from +16 to +22‰, echinoderms have constant δ 7 Li of about +24‰, brachiopods have δ 7 Li of +25 to +28‰, and finally calcitic mollusks have the largest range and highest δ 7 Li values, ranging from +25‰ to +40‰. Measured brachiopods have similar δ 7 Li compared to inorganic calcite precipitated from seawater (δ 7 Li of +27 to +29‰), indicating minimum influence of vital effects, as also observed for other isotope systems and making them a potentially viable proxy of past seawater composition. Calcitic mollusks, on the contrary, are not a good archive for seawater paleo–δ 7 Li because many samples have significantly higher δ 7 Li values than inorganic calcite and display large inter-species variability, which suggests large vital effects. In addition, we observe very large intra-shell variability, in particular for mixed calcite-aragonite shells (over 20‰ variability), but also in mono-mineralic shells (up to 12‰ variability). Aragonitic bivalves have less variable δ 7 Li (7‰ variability) compared to calcitic mollusks, but with significantly lower δ 7 Li compared to inorganic aragonite, indicating the existence of vital effects. Bivalves grown at various temperatures show that temperature has only a minor influence on fractionation of Li isotopes during shell precipitation. Interestingly, we observe a strong correlation ( R 2  = 0.83) between the Li/Mg ratio in bivalve Mytilus edulis and temperature, with potential implications for paleo-temperature reconstructions. Finally, we observe a negative correlation between the δ 7 Li and both the Li/Ca and Mg/Ca ratio of calcite mollusks, which we relate to biomineralization processes. To explain this correlation, we propose preferential removal of 6 Li from the calcification site of calcite mollusks by physiological processes corresponding to the regulation of the amount of Mg in the calcifying medium. We calculate that up to 80% of the initial Li within the calcification site is removed by this process, leading to high δ 7 Li and low Li/Ca in some calcite mollusk specimens. Collectively, these results suggest that Mg (and thus [Li]) is strongly biologically controlled within the calcifying medium of calcite mollusks. Overall, the results of this study show that brachiopods are likely to be suitable targets for future work on the determination of paleo-seawater Li isotope composition—an emerging proxy for past weathering and hydrothermal processes. [ABSTRACT FROM AUTHOR]

Connections between glaciation, chemical weathering, and the global carbon cycle could steer the evolution of global climate over geologic time, but even the directionality of feedbacks in this system remain to be resolved. Here, we assemble a compilation of hydrochemical data from glacierized catchments, use this data to evaluate the dominant chemical reactions associated with glacial weathering, and explore the implications for long-term geochemical cycles. Weathering yields from catchments in our compilation are higher than the global average, which results, in part, from higher runoff in glaciated catchments. Our analysis supports the theory that glacial weathering is characterized predominantly by weathering of trace sulfide and carbonate minerals. To evaluate the effects of glacial weathering on atmospheric pCO2, we use a solute mixing model to predict the ratio of alkalinity to dissolved inorganic carbon (DIC) generated by weathering reactions. Compared with nonglacial weathering, glacial weathering is more likely to yield alkalinity/DIC ratios less than 1, suggesting that enhanced sulfide oxidation as a result of glaciation may act as a source of CO2 to the atmosphere. Back-of-the-envelope calculations indicate that oxidative fluxes could change ocean–atmosphere CO2 equilibrium by 25 ppm or more over 10 ky. Over longer timescales, CO2 release could act as a negative feedback, limiting progress of glaciation, dependent on lithology and the concentration of atmospheric O2. Future work on glaciation– weathering–carbon cycle feedbacks should consider weathering of trace sulfide minerals in addition to silicate minerals. [ABSTRACT FROM AUTHOR]

In the tropics, geochemical records from stalagmites have so far mainly been used to qualitatively reconstruct changes in precipitation, but several new methods to reconstruct past temperatures from stalagmite material have emerged recently: i) liquid–vapor homogenization of fluid inclusion water ii) noble gas concentrations in fluid inclusion water, iii) the partitioning of oxygen isotopes between fluid inclusion water and calcite, and iv) the abundance of the 13 C 18 O 16 O (‘clumped’) isotopologue in calcite. We present, for the first time, a direct comparison of these four paleo-thermometers by applying them to a fossil stalagmite covering nearly two glacial–interglacial cycles (Marine Isotope Stages (MIS) 12–9) and to two modern stalagmites, all from northern Borneo. The temperature estimates from the different methods agree in most cases within errors for both the old and recent samples; reconstructed formation temperatures of the recent samples match within 2-sigma errors with measured cave temperatures. However, slight but systematic deviations are observed between noble gas and liquid–vapor homogenization temperatures. Whereas the temperature sensitivity of fluid inclusion δ 18 O and clumped isotopes is currently debated, we find that the calibration of Tremaine et al. (2011) for fluid inclusion δ 18 O and a synthetic calcite-based clumped isotope calibration (Ziegler et al., in prep.) yield temperature estimates consistent with the other methods. All methods (with the potential exception of clumped isotopes) show excellent agreement on the amplitude of glacial–interglacial temperature change, indicating temperature shifts of 4–5 °C. This amplitude is similar to the amplitude of Mg/Ca-based regional sea surface temperature records, when correcting for sea level driven changes in cave elevation. Our reconstruction of tropical temperature evolution over the time period from 440 to 320 thousand years ago (ka) adds support to the view that climate sensitivity to varying greenhouse forcing is substantial also in the deep tropics. [ABSTRACT FROM AUTHOR]

In the low-oxygen Archean world (>2400 million years ago), seawater sulfate concentrations were much lower than today, yet open questions frustrate the translation of modern measurements of sulfur isotope fractionations into estimates of Archean seawater sulfate concentrations. In the water column of Lake Matano, Indonesia, a low-sulfate analog for the Archean ocean, we find large (>20 per mil) sulfur isotope fractionations between sulfate and sulfide, but the underlying sediment sulfides preserve a muted range of δ34 S values. Using models informed by sulfur cycling in Lake Matano, we infer Archean seawater sulfate concentrations of less than 2.5 micromolar. At these low concentrations, marine sulfate residence times were likely 10³to 104 years, and sulfate scarcity would have shaped early global biogeochemical cycles, possibly restricting biological productivity in Archean oceans. [ABSTRACT FROM AUTHOR]

Abstract: Over the last decade, the use of multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) has significantly lowered the detection limit of sulfur isotope analyses, albeit typically with decreased precision. Moreover, the presence of isobaric interferences for sulfur prevented accurate analysis of the minor isotopes 33S and 36S. In the present study, we report improved techniques for measuring sulfur isotopes on the MC-ICP-MS Neptune Plus (Thermo Fischer Scientific) using a heated spray chamber coupled to a desolvating membrane (Aridus, Cetac). Working at high mass resolution, we measured δ34S values of natural samples with a typical reproducibility of 0.08–0.15‰ (2sd) on 5 to 40nmol sulfur introduced into the instrument. We applied this method to two seawater profiles, using 25μl of sample (700nmol of sulfate). The average δ34SVCDT value is 20.97±0.10‰ (2sd, n=25). We show that the amount of sulfate required for an analysis can be decreased to 5nmol. Because the plasma is sustained by Ar, measurement of 36S is impossible at the current mass resolution due to the presence of 36Ar+, but a reproducibility of 0.1–0.3‰ (2sd) is achieved on the measurement of mass independent fractionations (Δ33S). This is the first time such precision has been achieved on samples of this size. [Copyright &y& Elsevier]