Controls on seawater 231Pa, 230Th and 232Th concentrations along the flow paths of deep waters in the Southwest Atlantic

Measurements of dissolved Th-230, Pa-231 and Th-232 were made for twelve full-depth profiles along a Southwest Atlantic section during GEOTRACES cruise GA02S. Sampling captures all the main Atlantic deep water masses along their meridional flow paths and allows insight into the control on Th and Pa in a setting where waters are flowing in opposing directions, with direct relevance to understanding the use of Pa-231/Th-230 as an ocean-circulation proxy. Water-column Th-230 increases linearly with depth, in line with expected reversible scavenging models. Pa-231 increases from the surface to similar to 1200-1500 m, but is invariant or decreases with greater depth, deviating from the behavior expected for reversible scavenging. Dissolved Pa-231/Th-230 ratios display a mid-water-column maximum at similar to 1000-2000 m which is broadly coincident with Upper Circumpolar Deep Water. Below 2000 m, nuclide distributions and ratios exhibit no dependence on water mass, nor any indication of progressive change within a water mass, challenging the use of Pa-231/Th-230 as a past circulation tracer in the South Atlantic. Calculation of horizontal transport of Th-230 and Pa-231 by ocean circulation indicates a net southward export out of the Atlantic of 19% of the Pa-231 and 3% of the Th-230 produced in that ocean. This removal is all from the North Atlantic while, in the South Atlantic, removal to sediment equals production. Simple one-dimensional modeling can simulate Th-230 profiles but not the mid-water-column maximum observed in Pa-231 profiles, suggesting an additional source of Pa-231 (perhaps lateral transport from the margin) or removal at depth due to bottom scavenging. Near seafloor minima in concentrations indicates bottom scavenging of Th-230 and (231)pa, which is enhanced in the presence of nepheloid layers, particularly for 231Pa. This additional scavenging fractionates Th-230 and Pa-231 and, in the presence of nepheloid layers, may lead to an increase in sedimentary Pa-231/Th-230 ratios. Th-232 concentrations were paired with Th-230-derived residence times in the upper 250 m of the water column to test the application of Th as a tracer of dust deposition. Maxima in Th-232 indicate high dust input from the African and possibly South American continents

Drying in the Middle East during Northern Hemisphere cold events of the early glacial period

Few paleoclimate records exist to assess the central Middle East’s response to natural forcing beyond the instrumental record. Here we present a multi-proxy stalagmite-based climate reconstruction from Iran’s semi-arid northeast that spans 100-70 thousand years before present(ka). During severe cold (stadial) events in the North Atlantic at ≈88, 77, and 73 ka, stalagmite trace-element data indicate anomalously dry periods at this location. Stadial event increases in the stalagmite oxygen isotopes mirror those in a published Iranian stalagmite 800 km to the west. A global climate model simulates drying across the Middle East region in response to stadial event forcing, in agreement with oxygen isotope enrichments in both Iranian records, caused by a smaller fractional loss of moisture on the trajectory upstream. The paleoproxies and model experiments are consistent in indicating a drier Middle East climate during the cold North Atlantic stadials

Reply to Morel : cadmium as a micronutrient and macrotoxin in the oceans

Author Posting. © The Author(s), 2013. This is the author's version of the work. It is posted here by permission of National Academy of Sciences for personal use, not for redistribution. The definitive version was published in Proceedings of the National Academy of Sciences of the United States of America 110 (2013): E1878, doi:10.1073/pnas.1305068110.We thank François Morel for his interest in our study. Morel states that our conclusions are based on the approximate match between the Cd-isotope composition of cultured bacteria and the fractionation of Cd isotopes seen in seawater (1). This match is only a minor component of our argument, and we welcome the opportunity to reiterate our case

Rare earth elements (REEs) in the tropical South Atlantic and quantitative deconvolution of their non-conservative behavior

This study presents new concentration measurements of dissolved rare earth elements (dREEs) along a full-depth east–west section across the tropical South Atlantic (∼12°S), and uses these data to investigate the oceanic cycling of the REEs. Enrichment of dREEs, associated with the redox cycling of Fe–Mn oxides, is observed in the oxygen minimum zone (OMZ) off the African shelf. For deeper-waters, a multi-parameter mixing model was developed to deconvolve the relative importance of physical transport (i.e., water mass mixing) from biogeochemical controls on the dREE distribution in the deep Atlantic. This approach enables chemical processes involved in REE cycling, not apparent from the measurements alone, to be distinguished and quantified. Results show that the measured dREE concentrations below ∼1000 m are dominantly controlled (>75%) by preformed REE concentrations resulting from water mass mixing. This result indicates that the linear correlation between dREEs and dissolved Si observed in Atlantic deep waters results from the dominantly conservative behavior of these tracers, rather than from similar chemical processes influencing both dREEs and Si. Minor addition of dREEs (∼10% of dNd and ∼5% of dYb) is observed in the deep (>∼4000 m) Brazil Basin, resulting from either remineralization of particles in-situ or along the flow path. Greater addition of dREEs (up to 25% for dNd and 20% for dYb) is found at ∼1500 m and below ∼4000 m in the Angola Basin near the African continental margin. Cerium anomalies suggest that different sources are responsible for these dREE addition plumes. The 1500 m excess is most likely attributed to dREE release from Fe oxides, whereas the 4000 m excess may be due to remineralization of calcite. Higher particulate fluxes and a more sluggish ocean circulation in the Angola Basin may explain why the dREE excesses in this basin are significantly higher than that observed in the Brazil Basin. Hydrothermal venting over the mid-Atlantic ridge acts as a regional net sink for light REEs, but has little influence on the net budget of heavy REEs. The combination of dense REE measurements with water mass deconvolution is shown to provide quantitative assessment of the relative roles of physical and biogeochemical processes in the oceanic cycling of REEs

Barium isotopes in mid-ocean ridge hydrothermal vent fluids : a source of isotopically heavy Ba to the ocean

Funding: These field and related experimental studies were supported through US NSF grants: 0549547, 0751771, 0813861, 0961188 and 1736679 (WES).Mid-ocean ridge (MOR) hydrothermal vent fluids are enriched with dissolved barium, but due to barite (BaSO4) precipitation during mixing between Ba-bearing vent fluids and SO4-bearing seawater, the magnitude of hydrothermal Ba input to the ocean remains uncertain. Deep-ocean Ba isotopes show evidence for non-conservative behavior, which might be explained by input of isotopically heavy hydrothermal Ba. In this study we present the first Ba isotope data in mid-ocean ridge hydrothermal vent fluids and particles from systems on the Mid-Atlantic Ridge (Rainbow 36°N and TAG 26°N), the East Pacific Rise (EPR9–10°N and 13°N) and the Juan de Fuca Ridge (MEF and ASHES). The vent fluids display a wide range of dissolved Ba concentrations from 0.43 to 97.9 μmol/kg and δ138/134Ba values from −0.26 to +0.91‰, but are modified relative to initial composition due to precipitation of barite. Calculated endmember vent fluid δ138/134Ba values, prior to barite precipitation, are between −0.17 and +0.09‰, consistent with the values observed in oceanic basalts and pelagic sediments. Water-rock interaction at depth in the oceanic crust appears to occur without Ba isotope fractionation. During subsequent venting and mixing with seawater, barite precipitation preferentially removes isotopically light Ba from vent fluids with a fractionation factor of Δ138/134Bahyd-barite-fluid = −0.35 ± 0.10‰ (2SE, n = 2). Based on knowledge of barite saturation and isotope fractionation during precipitation, the effective hydrothermal Ba component that mixes with seawater after barite precipitation has completed can be calculated: δ138/134Bahyd = +1.7 ± 0.7‰ (2SD). This value is isotopically heavier than deep ocean waters and may explain the observed non-conservative of Ba isotopes in deep waters. These new constraints on hydrothermal Ba compositions enable the hydrothermal input of Ba to Atlantic deep waters to be assessed at ≈3–9% of the observed Ba. Barium isotopes might be used as a tracer to reconstruct the history of hydrothermal Ba inputs and seawater SO4 concentrations in the past.PostprintPeer reviewe

Improved determination of marine sedimentation rates using 230Thxs

Measurements of excess 230Th (230Thxs) have proved to be a useful tool in constraining changes in sedimentation rate, and improving our understanding of the fluxes of other components into marine sediments. To obtain the initial activity of 230Thxs (230Thxs0) in sediment: the total measured 230Th must be corrected for the presence of 230Th associated with detrital minerals, for ingrowth from uranium-bearing authigenic phases and then also corrected for the decay of 230Thxs since deposition. We describe a number of improvements in the way these corrections are applied to obtain more accurate determinations of 230Thxs0. We present a new method for the determination of a local estimate for the detrital 238U/232Th activity ratio; suggest more appropriate values for the isotopic composition of authigenic uranium; and question the assumption of secular equilibrium in detrital material. We also present a new, freely-available MATLAB® script called ‘XSage’ that can calculate 230Thxs0, from user-supplied datasets of uranium and thorium isotope activities from sedimentary samples following the theoretical approach described. ‘XSage’ can determine variations in sedimentation rate between stratigraphic horizons of known age and thus produce high-resolution age models. Using a Monte Carlo approach, the program calculates uncertainties for these age models and on the durations of intervals between tie-points. An example of the application of the XSage program using a previously published record is provided

A simplified isotope dilution approach for the U–Pb dating of speleogenic and other low-232Th carbonates by multi-collector ICP-MS

We describe a new method for the measurement of ratios by isotope dilution multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) for the dating of geologically young clean carbonates, particularly speleothems. The method is intended for materials containing little or no initial 232Th. We illustrate and validate the method with four examples ranging from 0.57 to 20 Ma. The new method is capable of applying the 235U–207Pb and 238U–234U–206Pb chronometers, common Pb and quantifiable residual disequilibrium permitting. These provide an alternative to the more widely used 238U–206Pb chronometer, which can be highly inaccurate for samples that are < ca. 20 million years old, owing to uncertainties in the excess initial 234U (hence, excess radiogenic 206Pb) commonly observed in speleothems

Permafrost-related hiatuses in stalagmites: Evaluating the potential for reconstruction of carbon cycle dynamics

Permafrost is widely present throughout the Northern Hemisphere high latitudes, and stores large amounts of carbon in the form of frozen soil organic matter. The response of permafrost regions to anthropogenic climate change remains uncertain, in part because of a lack of information on their response to past changes in global climate. Here we test the use of stalagmites from two caves in Siberia as a novel, precisely dated, and highly localised archive of past permafrost carbon cycle dynamics. Stalagmite growth at these sites is controlled by the presence/absence of permafrost above the cave over glacial-interglacial time scales. We target the transition layer between two subsequent growth phases (interglacials) and the interval directly following growth resumption after the last glacial in three stalagmites, as this is where a geochemical imprint of thaw-related processes in the frozen zone between surface and cave would be recorded. We apply a multi-proxy approach including carbon isotopes (δ13C and 14C) and trace element concentrations, combined with petrographic analyses and high-resolution U-Th chronology. Our dataset indicates complex growth patterns and possible intervals of microbial colonisation of the stalagmite surface in the transition layers. High-resolution U-Th ages confirm that the transition layer is not a single, long growth hiatus, but rather a period of extremely slow or episodic growth phases, possibly during “skipped” interglacials. However, we find no conclusive evidence for a geochemical signature related to permafrost degradation and related local carbon cycle dynamics, which might be related to insufficient sensitivity of the archive for high-frequency processes and/or insufficient measurement resolution

230 Th normalization: new insights on an essential tool for quantifying sedimentary fluxes in the modern and quaternary ocean

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Costa, K. M., Hayes, C. T., Anderson, R. F., Pavia, F. J., Bausch, A., Deng, F., Dutay, J., Geibert, W., Heinze, C., Henderson, G., Hillaire-Marcel, C., Hoffmann, S., Jaccard, S. L., Jacobel, A. W., Kienast, S. S., Kipp, L., Lerner, P., Lippold, J., Lund, D., Marcantonio, F., McGee, D., McManus, J. F., Mekik, F., Middleton, J. L., Missiaen, L., Not, C., Pichat, S., Robinson, L. F., Rowland, G. H., Roy-Barman, M., Alessandro, Torfstein, A., Winckler, G., & Zhou, Y. 230 Th normalization: new insights on an essential tool for quantifying sedimentary fluxes in the modern and quaternary ocean. Paleoceanography and Paleoclimatology, 35(2), (2020): e2019PA003820, doi:10.1029/2019PA003820.230Th normalization is a valuable paleoceanographic tool for reconstructing high‐resolution sediment fluxes during the late Pleistocene (last ~500,000 years). As its application has expanded to ever more diverse marine environments, the nuances of 230Th systematics, with regard to particle type, particle size, lateral advective/diffusive redistribution, and other processes, have emerged. We synthesized over 1000 sedimentary records of 230Th from across the global ocean at two time slices, the late Holocene (0–5,000 years ago, or 0–5 ka) and the Last Glacial Maximum (18.5–23.5 ka), and investigated the spatial structure of 230Th‐normalized mass fluxes. On a global scale, sedimentary mass fluxes were significantly higher during the Last Glacial Maximum (1.79–2.17 g/cm2kyr, 95% confidence) relative to the Holocene (1.48–1.68 g/cm2kyr, 95% confidence). We then examined the potential confounding influences of boundary scavenging, nepheloid layers, hydrothermal scavenging, size‐dependent sediment fractionation, and carbonate dissolution on the efficacy of 230Th as a constant flux proxy. Anomalous 230Th behavior is sometimes observed proximal to hydrothermal ridges and in continental margins where high particle fluxes and steep continental slopes can lead to the combined effects of boundary scavenging and nepheloid interference. Notwithstanding these limitations, we found that 230Th normalization is a robust tool for determining sediment mass accumulation rates in the majority of pelagic marine settings (>1,000 m water depth).We thank Zanna Chase and one anonymous reviewer for valuable feedback. K. M. C. was supported by a Postdoctoral Scholarship at WHOI. L. M. acknowledges funding from the Australian Research Council grant DP180100048. The contribution of C. T. H., J. F. M., and R. F. A. were supported in part by the U.S. National Science Foundation (US‐NSF). G. H. R. was supported by the Natural Environment Research Council (grant NE/L002434/1). S. L. J. acknowledges support from the Swiss National Science Foundation (grants PP002P2_144811 and PP00P2_172915). This study was supported by the Past Global Changes (PAGES) project, which in turn received support from the Swiss Academy of Sciences and the US‐NSF. This work grew out of a 2018 workshop in Aix‐Marseille, France, funded by PAGES, GEOTRACES, SCOR, US‐NSF, Aix‐Marseille Université, and John Cantle Scientific. All data are publicly available as supporting information to this document and on the National Center for Environmental Information (NCEI) at https://www.ncdc.noaa.gov/paleo/study/28791