Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Mayfield, K. K., Eisenhauer, A., Santiago Ramos, D. P., Higgins, J. A., Horner, T. J., Auro, M., Magna, T., Moosdorf, N., Charette, M. A., Gonneea, M. E., Brady, C. E., Komar, N., Peucker-Ehrenbrink, B., & Paytan, A. Groundwater discharge impacts marine isotope budgets of Li, Mg, Ca, Sr, and Ba. Nature Communications, 12(1), (2021): 148-020-20248-3, doi:10.1038/s41467-020-20248-3.

Description: 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.

Description: We thank A. Beck, H. Dulai, I. Santos, C. Benitez-Nelson, W. Moore, A. Martin, and H. Windom for sample access. We also thank A. Kolevica, A. Heuser, H. Pryer, J. Middleton, R. Franks, F. Lon, N. Slater, and O. Šebek for their laboratory and analytical assistance. This material is based upon research supported by the National Science Foundation Graduate Research Fellowship Program and an internship provided through the U.S. Geological Survey Graduate Research Internship Program (GRIP). This research was also supported by grants from: the German Academic Exchange Service (DAAD), Northern California chapter of the Achievement Rewards for College Scientists Foundation, International Association of GeoChemistry, Geological Society of America, Northern California Geological Society, Myers Trust, Friends of Long Marine Lab, and UC MEXUS (to K.K.M.). We acknowledge funding from EU-ITN Horizon project 643084 (to A.E. and T.M.) and NSF grant Award Number 1259440 (to A.P.). We also acknowledge funding from NSF grant award number OCE-1736949 (to T.J.H.). Any use of trade, firm or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Description: Anthropogenic impacts are perturbing the global nitrogen cycle via warming effects and pollutant sources such as chemical fertilizers and burning of fossil fuels. Understanding controls on past nitrogen inventories might improve predictions for future global biogeochemical cycling. Here we show the quantitative reconstruction of deglacial bottom water nitrate concentrations from intermediate depths of the Peruvian upwelling region, using foraminiferal pore density. Deglacial nitrate concentrations correlate strongly with downcore δ13C, consistent with modern water column observations in the intermediate Pacific, facilitating the use of δ13C records as a paleo-nitrate-proxy at intermediate depths and suggesting that the carbon and nitrogen cycles were closely coupled throughout the last deglaciation in the Peruvian upwelling region. Combining the pore density and intermediate Pacific δ13C records shows an elevated nitrate inventory of 〉10% during the Last Glacial Maximum relative to the Holocene, consistent with a δ13C-based and δ15N-based 3D ocean biogeochemical model and previous box modeling studies.

Description: Mid- to late-Holocene sea-level records from low-latitude regions serve as an important baseline of natural variability in sea level and global ice volume prior to the Anthropocene. Here, we reconstruct a high-resolution sea-level curve encompassing the last 6000 years based on a comprehensive study of coral microatolls, which are sensitive low-tide recorders. Our curve is based on microatolls from several islands in a single region and comprises a total of 82 sea-level index points. Assuming thermosteric contributions are negligible on millennial time scales, our results constrain global ice melting to be 1.5–2.5 m (sea-level equivalent) since ~5500 years before present. The reconstructed curve includes isolated rapid events of several decimetres within a few centuries, one of which is most likely related to loss from the Antarctic ice sheet mass around 5000 years before present. In contrast, the occurrence of large and flat microatolls indicates periods of significant sea-level stability lasting up to ~300 years.

Description: Weddell Sea-derived Antarctic Bottom Water (AABW) is one of the most important deep water masses in the Southern Hemisphere occupying large portions of the deep Southern Ocean (SO) today. While substantial changes in SO-overturning circulation were previously suggested, the state of Weddell Sea AABW export during glacial climates remains poorly understood. Here we report seawater-derived Nd and Pb isotope records that provide evidence for the absence of Weddell Sea-derived AABW in the Atlantic sector of the SO during the last two glacial maxima. Increasing delivery of Antarctic Pb to regions outside the Weddell Sea traced SO frontal displacements during both glacial terminations. The export of Weddell Sea-derived AABW resumed late during glacial terminations, coinciding with the last major atmospheric CO2 rise in the transition to the Holocene and the Eemian. Our new records lend strong support for a previously inferred AABW overturning stagnation event during the peak Eemian interglacial.

Description: The Permian/Triassic boundary approximately 251.9 million years ago marked the most severe environmental crisis identified in the geological record, which dictated the onwards course for the evolution of life. Magmatism from Siberian Traps is thought to have played an important role, but the causational trigger and its feedbacks are yet to be fully understood. Here we present a new boron-isotope-derived seawater pH record from fossil brachiopod shells deposited on the Tethys shelf that demonstrates a substantial decline in seawater pH coeval with the onset of the mass extinction in the latest Permian. Combined with carbon isotope data, our results are integrated in a geochemical model that resolves the carbon cycle dynamics as well as the ocean redox conditions and nitrogen isotope turnover. We find that the initial ocean acidification was intimately linked to a large pulse of carbon degassing from the Siberian sill intrusions. We unravel the consequences of the greenhouse effect on the marine environment, and show how elevated sea surface temperatures, export production and nutrient input driven by increased rates of chemical weathering gave rise to widespread deoxygenation and sporadic sulfide poisoning of the oceans in the earliest Triassic. Our findings enable us to assemble a consistent biogeochemical reconstruction of the mechanisms that resulted in the largest Phanerozoic mass extinction.

Description: The fibrous calcite layer of modern brachiopod shells is a hybrid composite material and forms a substantial part of the hard tissue. We investigated how cells of the outer mantle epithelium (OME) secrete calcite material and generate the characteristic fibre morphology and composite microstructure of the shell. We employed AFM, FE-SEM, and TEM imaging of embedded/etched, chemically fixed/decalcified and high-pressure frozen/freeze substituted samples. Calcite fibres are secreted by outer mantle epithelium (OME) cells. Biometric analysis of TEM micrographs indicates that about 50% of these cells are attached via hemidesmosomes to an extracellular organic membrane present at the proximal, convex surface of the fibres. At these sites, mineral secretion is not active. Instead, ion transport from OME cells to developing fibres occurs at regions of closest contact between cells and fibres, however only at sites where the extracellular membrane at the proximal fibre surface is not developed yet. Fibre formation requires the cooperation of several adjacent OME cells. It is a spatially and temporally changing process comprising of detachment of OME cells from the extracellular organic membrane, mineral secretion at detachment sites, termination of secretion with formation of the extracellular organic membrane, and attachment of cells via hemidesmosomes to this membrane.

Description: A proposal is made to standardise the reporting of Ca isotope data to the δ44Ca/40Ca notation (or δ44Ca/42Ca) and to adopt NIST SRM 915a as the reference standard.

Description: We report delta Ca-44/40((SRM 915a)) values for eight fused MPI-DING glasses and the respective original powders, six USGS igneous rock reference materials, the U-Th disequilibria reference material TML, IAEA-CO1 (Carrara marble) and several igneous rocks (komatiites and carbonatites). Sample selection was guided by three considerations: (1) to address the need for information values on reference materials that are widely available in support of interlaboratory comparison studies; (2) support the development of in situ laser ablation and ion microprobe techniques, which require isotopically homogenous reference samples for ablation; and (3) provide Ca isotope values on a wider range of igneous and metamorphic rock types than is currently available in the scientific literature. Calcium isotope ratios were measured by thermal ionisation mass spectrometry in two laboratories (IFM-GEOMAR and Saskatchewan Isotope Laboratory) using Ca-43/Ca-48- and Ca-42/Ca-43-double spike techniques and reported relative to the calcium carbonate reference material NIST SRM 915a. The measurement uncertainty in both laboratories was better than 0.2 parts per thousand at the 95% confidence level. The impact of different preparation methods on the delta Ca-44/40((SRM 915a)) values was found to be negligible. Except for ML3-B, the original powders and the respective MPI-DING glasses showed identical delta Ca-44/40((SRM 915a)) values; therefore, possible variations in the Ca isotope compositions resulting from the fusion process are excluded. Individual analyses of different glass fragments indicated that the glasses are well homogenised on the mm scale with respect to Ca. The range of delta Ca-44/40((SRM 915a)) values in the igneous rocks studied was larger than previously observed, mostly owing to the inclusion of ultramafic rocks from ophiolite sections. In particular, the dunite DTS-1 (1.49 +/- 0.06 parts per thousand) and the peridotite PCC-1 (1.14 +/- 0.07 parts per thousand) are enriched in Ca-44 relative to volcanic rocks (0.8 +/- 0.1 parts per thousand). The Carrara marble (1.32 +/- 0.06 parts per thousand) was also found to be enriched in Ca-44 relative to the values of assumed precursor carbonates (〈 0.8 parts per thousand). These findings suggest that the isotopes of Ca are susceptible to fractionation at high temperatures by, as yet, unidentified igneous and metamorphic processes.

Description: The calcium isotopic composition of NIST SRM 915b and 1486 provided by the National Institute of Standards and Technology was analysed. The δ44/40Ca values of the two reference materials relative to NIST SRM 915a were: NIST SRM 915b =+0.72 ± 0.04‰ and NIST SRM 1486 =−1.01 ± 0.02‰. NIST SRM 1486 did not require any chemical separation prior to measurement. La composition isotopique du calcium de NIST SRM 915b et 1486, fournis par l'Institut National des Standards et de la Technologie (NIST), a été analysée. Les valeurs du δ44/40Ca obtenues sur ces deux matériaux de référence, relativement au NIST SRM 915a sont: NIST SRM 915b =+0.72 ± 0.04‰ et NIST SRM 1486 =−1.01 ± 0.02‰. Le NIST SRM 1486 n'a nécessité aucune séparation chimique avant analyse.