Potassium isotopic compositions measured by MC-ICP-MS in cold-plasma, high-resolution mode indicate terrestrial variability in 41K/39K ratios.

Groundwater-derived solute fluxes to the ocean have long been assumed static and subordinate to riverine fluxes, if not neglected entirely, in marine isotope budgets. Here we present concentration and isotope data for Li, Mg, Ca, Sr, and Ba in coastal groundwaters to constrain the importance of groundwater discharge in mediating the magnitude and isotopic composition of terrestrially derived solute fluxes to the ocean. Data were extrapolated globally using three independent volumetric estimates of groundwater discharge to coastal waters, from which we estimate that groundwater-derived solute fluxes represent, at a minimum, 5% of riverine fluxes for Li, Mg, Ca, Sr, and Ba. The isotopic compositions of the groundwater-derived Mg, Ca, and Sr fluxes are distinct from global riverine averages, while Li and Ba fluxes are isotopically indistinguishable from rivers. These differences reflect a strong dependence on coastal lithology that should be considered a priority for parameterization in Earth-system models.

Authigenic clay minerals formed on or in the seafloor occur in every type of marine sediment. They are recognized to be a major sink of many elements in the ocean but are difficult to study directly due to dilution by detrital clay minerals. The extremely low dust fluxes and marine sedimentation rates in the South Pacific Gyre (SPG) provide a unique opportunity to examine relatively undiluted authigenic clay. Here, using Mg isotopes and element concentrations combined with multivariate statistical modeling, we fingerprint and quantify the abundance of authigenic clay within SPG sediment. Key reactants include volcanic ash (source of reactive aluminium) and reactive biogenic silica on or shallowly buried within the seafloor. Our results, together with previous studies, suggest that global reorganizations of biogenic silica burial over the Cenozoic reduced marine authigenic clay formation, contributing to the rise in seawater Mg/Ca and decline in atmospheric CO2 over the past 50 million years.

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Alteration of ultramafic rocks is ubiquitous in near-surface environments, both on land and below the seafloor. Mantle olivine and pyroxene are unstable at near-surface conditions and undergo hydration (serpentinization) and carbonation when fluids are present (e.g., Moody, 1976). These reactions result in the formation of serpentine minerals, carbonates, brucite, magnetite, and other Fe-oxides and hydroxides. Serpentinization and carbonation reactions are often nearly isochemical apart from the addition of H 2 O and CO 2 (e.g., Coleman & Keith, 1971). Both observations and thermodynamic modeling suggest that changes in major element ratios such as Si/Mg are minor (e.g., ≤10% for low-temperature reaction with seawater,

The use of carbon isotope stratigraphy to construct time lines for stratigraphic correlation requires synchronous changes in carbon isotope ratios (d 13 C) to be preserved in carbonatedominated strata. Such changes are commonly interpreted to reflect primary secular variation in ocean chemistry. However, negative d 13 C anomalies developed in Pliocene-Pleistocene carbonate platforms following glacioeustatic sea-level fall due to remineralization of terrestrial biomass during meteoric diagenesis. These anomalies are similar in structure and magnitude to some Neoproterozoic d 13 C records, opening the possibility that the Neoproterozoic d 13 C anomalies have a meteoric origin derived from a large terrestrial biosphere. Here we test the hypothesis that a large terrestrial biosphere existed prior to the Silurian-Devonian land-plant radiation by examining d 13 C records of subaerial exposure surfaces formed in a shallow-water carbonate platform during the Ordovician-Silurian icehouse. The exposure surfaces include an unconformity at the Ordovician-Silurian boundary with terra rossa and dissolution-collapse breccia, and a lower Silurian quartz sand layer feeding a 50-m-deep system of karst pipes. There is no evidence for d 13 C depletion beneath either exposure surface. Strontium concentrations in the rocks are low (10-120 ppm) and covary with d 18 O; oxygen isotope ratios, however, do not positively correlate with d 13 C. Our results suggest that there was no significant terrestrial biosphere during Ordovician-Silurian time, and by extension, that Neoproterozoic negative carbon isotope anomalies cannot be explained by meteoric diagenesis.