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Present estimates of the biogeochemical cycles of calcium, strontium, and potassium in the ocean reveal large imbalances between known input and output fluxes. Using pore fluid, incubation, and solid sediment data from North Pacific multi‐corer cores we show that, contrary to the common paradigm, the top centimeters of abyssal sediments can be an active site of authigenic precipitation of clay minerals. In this region, clay authigenesis is the dominant sink for potassium and strontium and consumes nearly all calcium released from benthic dissolution of calcium carbonates. These observations support the idea that clay authigenesis occurring over broad regions of the world ocean may be a major buffer for ocean chemistry on the time scale of the ocean overturning circulation, and key to the long‐term stability of Earth's climate., Key Points: North Pacific red clay sediments are a sink for marine calcium, strontium, and potassium. Authigenic formation of clay minerals is prevalent in pelagic sediments throughout the North Pacific. The main mechanism for clay formation is recrystallization of aluminosilicates, neoformation can occur in biogenic silica rich sediments., EC H2020 PRIORITY “Excellent science” H2020 European Research Council http://dx.doi.org/10.13039/100010663, Blavatnik Family Foundation http://dx.doi.org/10.13039/100011643, Isaac Newton Trust http://dx.doi.org/10.13039/501100004815, Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659, National Science Foundation http://dx.doi.org/10.13039/100000001, https://doi.pangaea.de/10.1594/PANGAEA.946881

To date Indian summer monsoon (ISM) dynamics have been assessed by changes in stalagmite δ18O. However, stalagmite δ18O is influenced by multiple environmental factors (e.g., atmospheric moisture transport, rainfall amount at the study site, and ISM seasonality), precluding simple and clear reconstructions of rainfall amount or variability. This study aims to disentangle these environmental factors by combining δ18O, δ44Ca, and elemental data from a stalagmite covering Termination II and the last interglacial from Mawmluh Cave, NE India, to produce a semiquantitative reconstruction of past ISM rainfall. We interpret δ18O as a mixed signal of rainfall source dynamics and rainfall amount and coupled δ44Ca and X/Ca ratios as indicators of local infiltration rate and prior calcite precipitation in the karst zone. The wettest conditions in our studied interval (135 and 100 kyrs BP; BP = before present, with the present being 1950 CE) occurred during Marine Isotope Stage 5e. Our multiproxy data set suggests a likely change in seasonal distribution of Marine Isotope Stage 5e rainfall compared to the Holocene; the wet season was longer with higher‐than‐modern dry season rainfall. Using the last interglacial as an analogue for future anthropogenic warming, our data suggest a more erratic ISM behavior in a warmer world.

Plain Language Summary\ud Modeling of future climate suggests that California may experience increased frequency of both extremely wet and extremely dry periods in the 21st century, leading to the emergence of “climate whiplash” phenomena which would significantly stress the state's water‐sensitive infrastructure. Understanding hydroclimate changes in California's past can help planners prepare for extremes that may be more severe than those of the historical record. However, existing paleoclimate records are often limited to qualitative interpretations of hydroclimate change, restricting their usefulness. We present new calcium isotope measurements from a California stalagmite that grew from 6,900 to 8,600 years ago, revealing variability in rainfall amounts on the California coast during and surrounding the 8.2 kyr event, an abrupt cold snap noted in other global paleoclimate records 8,200 years ago. We generate semiquantitative estimates of annual rainfall rates during the 8.2 kyr event period and compare them with modern annual rainfall amounts, finding that the magnitude of rainfall variability during and surrounding the 8.2 kyr event approaches and in some cases exceeds that of California today. This work indicates that California may have experienced even more intense “climate whiplash” phenomena in the past than during recent decades, suggesting that future planning may need to consider greater wet and dry extremes.\ud Abstract\ud A multiproxy record from a fast‐growing stalagmite reveals variable hydroclimate on the California coast across the 8.2 kyr event and a precursor event likely caused by initial drainage of proglacial Lake Agassiz. Using speleothem δ44Ca, we develop the first semiquantitative estimates of paleorainfall variability for California through calibration with measurements of the modern climate and cave environment. We find that the magnitude of rainfall variability during the 8.2 kyr event approached the multiyear variability observable in the recent past (1950–2019) and the magnitude of variability during the precursor event likely exceeded this range. Additionally, we observe other instances of multidecadal variability comparable in magnitude to the precursor event during the record. Our work suggests that speleothem calcium isotope ratios are a powerful semiquantitative means to reconstruct paleorainfall, although numerous factors must be assessed in each cave system before applying this approach.

We present calcium isotope data from pore fluids and solids from Ocean Drilling Program Leg 175: Sites 1081 and 1086 (off the coast of West Africa). These sites are similar with respect to geographic location, sediment age (from modern to 8 Myr), and water depth (800 m), but Site 1081 is carbonate-poor, whereas Site 1086 is carbonate-rich. Therefore, these sites are suited for the exploration of the influence of sediment type on carbonate dissolution, precipitation, and recrystallization. We use two numerical modelling approaches to explore the rates of carbonate dissolution and precipitation in the sediment column. The first is the standard diffusion-reaction approach, using the strontium concentration within the pore fluid to ascertain a dissolution rate for the carbonate, which is then applied to a second model of calcium isotopes within the pore fluid to calculate precipitation rates. Given the high sedimentation rates we also apply an advection-reaction model (Huber et al., 2017) which results in the same depth distribution of carbonate precipitation but significantly higher overall rates, which is discussed. Calcium isotope ratios in pore fluid calcium increase in zones where our model predicts carbonate precipitation, and approach isotopic equilibrium with the solid in zones where our model predicts equivalent rates of dissolution and precipitation, similar to previous findings. Contrary to previous findings in marine sediments, our model requires a calcium isotope fractionation on carbonate precipitation to fit the data, as there is an offset between the δ44Ca of the fluid and the solid. Using the zones of carbonate precipitation determined from the models and previously published carbon isotope profiles of the dissolved inorganic carbon from these sites, we suggest that the δ13C of the authigenic carbonate is uniformly lower than biogenic carbonate but by a wider range than was previously suggested.

Previous attempts to quantify the amount of sedimentary carbonate precipitation in modern marine sediments have been derived from the flux of calcium into the sediments due to diffusion under assumed steady state, the application of Fick's first law, and then extrapolation of these site-specific rates to the global ocean sediment column. This approach is limited, however, as much of the ocean floor has not been sampled. We take a machine learning approach to update and refine the estimate of the amount of sedimentary carbonate precipitation, as well as define whether sedimentary carbonate precipitation is driven by organoclastic microbial sulfate reduction or anaerobic methane oxidation. We identify areas where there is sedimentary carbonate formation using machine learning, based upon oceanic physical and chemical properties including bathymetry, temperature, water depth, distance from shore, and tracers of primary production, and data from the global ODP/IODP database. Our results suggest that the total amount of sedimentary carbonate formation is much lower than previous estimates, at 1.35 ± 0.5 × 10 11 mol C/yr. We suggest that this rate is a lower estimate and discuss why machine-learning approaches may always produce lower-bound estimates of global processes. Our calculations suggest that the formation of sedimentary carbonate today is mainly driven by anaerobic methane oxidation (77%), with the remainder attributed to organoclastic sulfate reduction. We use our machine-learning results to speculate the impact that sedimentary carbonate precipitation may have had on the carbon isotope composition of the surface dissolved inorganic carbon reservoir over Earth history.

The measurement of isotope ratios in sedimentary rocks deposited over geological time can provide key insights to past environmental change over important intervals in the past. However, it is important to be aware that secondary alteration can overprint the original isotopic records. We demonstrate this principle using high‐resolution carbon, sulfur, and oxygen isotope measurements in organic carbon, pyrite, and carbonate minerals (δ13Corg, δ34Spyr, δ34SCAS, δ13Ccarb, and δ18Ocarb) and kerogen analyses (HI and OI) from the Wordie Creek Formation, East Greenland. These sediments were initially deposited across the Permo‐Triassic transition, but as we will show, the carbonate record has been altered by interaction with meteoric water significantly after initial deposition. Comparison of the better preserved organic carbon and pyrite records with a proximal Permo‐Triassic sequence reveals significant pyrite‐sulfur isotope variability across the Permo‐Triassic transition. This regional heterogeneity argues against basin‐wide euxinia and instead suggests localized changes in sulfur fractionation in response to variations in organic carbon flux. This hypothesis can be used to explain seemingly inconsistent regional trends in other sulfur isotopes across the Permo‐Triassic transition.\ud \ud Plain Language Summary\ud Much of what is known about environmental change in the past has been determined through measuring the chemical properties of rocks. In this study, we demonstrate how those chemical properties may have been altered by processes occurring millions of years later. We show new chemical records from rocks deposited during across the Permo‐Triassic boundary (the world's largest mass extinction event) in East Greenland and discuss how some of the records have been altered by interaction with rain water millions of years after they were deposited. Finally, we talk about the records that have been unaffected by alteration, and what they imply for environmental changes occurring across the Permo‐Triassic boundary.

We report the calcium isotope fractionation during the microbially-induced precipitation of calcium carbonate minerals in pure cultures of the marine sulfate-reducing bacterium Desulfovibrio bizertensis. These data are used to explore how the calcium isotope fractionation factor during microbially-induced carbonate mineral precipitation differs from the better-constrained calcium isotope fractionation factors during biogenic or abiotic carbonate mineral precipitation. Bacterial growth was then modulated with antibiotics, and the evolution of δ44Ca in solution was monitored under different microbial growth rates. The faster the microbial growth rate, the larger the calcium isotope fractionation during carbonate mineral precipitation, with Δ44Ca(s-f) ranging from −1.07‰ to −0.48‰. The reported calcium isotope fractionation can help us understand the link between calcium isotope fractionation and microbial metabolism in carbonate minerals precipitated during sedimentary diagenesis.

Calcium isotope ratios in ancient carbonate rocks can provide insight into the global marine calcium cycle as well as local conditions during carbonate mineral precipitation and diagenesis. We compare two extraction techniques for the separation of calcium from other ions before δ44Ca analysis, using an automated ion chromatograph and using manual gravity columns. The two techniques produce the same δ44Ca within error (2σ). We present 31 δ44Ca analyses of carbonate rocks from the Nama Group, Namibia, which record a negative shift in δ44Ca of 0.35‰ between ∼550 and ∼547 Ma, from −1.25‰ to −1.60‰, followed by persistently low δ44Ca (−1.48 ± 0.06‰) between ∼547 and 539 Ma. Very low δ44Ca (