Description: Author Posting. © The Author(s), 2015. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters, 430 (2015): 511-522, doi:10.1016/j.epsl.2015.07.027.

Description: The marine biogeochemical cycle of Ba is thought to be controlled by particulate BaSO4 (barite) precipitation associated with the microbial oxidation of organic carbon and its subsequent dissolution in the BaSO4-undersaturated water column. Despite many of these processes being largely unique to Ba cycling, concentrations of Ba and Si in seawater exhibit a strong linear correlation. The reasons for this correlation are ambiguous, as are the depth ranges corresponding to the most active BaSO4 cycling and the intermediate sources of Ba to particulate BaSO4. Stable isotopic analyses of dissolved Ba in seawater should help address these issues, as Ba-isotopic compositions are predicted to be sensitive to the physical and biogeochemical process that cycle Ba. We report a new methodology for the determination of dissolved Ba-isotopic compositions in seawater and results from a 4, 500 m depth profile in the South Atlantic at 39.99 S, 0.92 E that exhibit oceanographically-consistent variation with depth. These data reveal that water masses obtain their [Ba] and Ba-isotopic signatures when at or near the surface, which relates to the cycling of marine BaSO4. The shallow origin of these signatures requires that the substantial Ba-isotopic variations in the bathypelagic zone were inherited from when those deep waters were last ventilated. Indeed, the water column below 600 m is well explained by conservative mixing of water masses with distinct [Ba] and Ba-isotopic compositions. This leads us to conclude that large scale oceanic circulation is important for sustaining the similar oceanographic distributions of Ba and Si in the South Atlantic, and possibly elsewhere. These data demonstrate that the processes of organic carbon oxidation, BaSO4 cycling, and Ba-isotopic fractionation in seawater are closely coupled, such that Ba-isotopic analyses harbor great potential as a tracer of the carbon cycle in the modern and paleo-oceans.

Description: T.J.H. acknowledges support from Makoto A. Saito (Gordon and Betty Moore Foundation; Project # 3782) and the Postdoctoral Scholar Program at the Woods Hole Oceanographic Institution, with funding provided by the Doherty Foundation. Development of Ba-isotopic protocols at NIRVANA was made possible with funding from The Andrew W. Mellon Foundation Endowed Fund for Innovative Research (T.J.H. and S.G.N.).

Description: © The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 213 (2017): 291-307, doi:10.1016/j.gca.2017.06.041.

Description: Thallium (Tl) isotopes are a new and potentially powerful paleoredox proxy that may track bottom water oxygen conditions based on the global burial flux of manganese oxides. Thallium has a residence time of ~20 thousand years, which is longer than the ocean mixing time, and it has been inferred that modern oxic seawater is conservative with respect to both concentration and isotopes. Marine sources of Tl have nearly identical isotopic values. Therefore, the Tl sinks, adsorption onto manganese oxides and low temperature oceanic crust alteration (the dominant seawater output), are the primary controls of the seawater isotopic composition. For relatively short-term, ~million years, redox events it is reasonable to assume that the dominant mechanism that alters the Tl isotopic composition of seawater is associated with manganese oxide burial because large variability in low temperature ocean crust alteration is controlled by long-term, multi-million years, average ocean crust production rates. This study presents new Tl isotope data for an open ocean transect in the South Atlantic, and depth transects for two euxinic basins (anoxic and free sulfide in the water column), the Cariaco Basin and Black Sea. The Tl isotopic signature of open ocean seawater in the South Atlantic was found to be homogeneous with ε205Tl = -6.0 ± 0.3 (± 2 SD, n = 41) while oxic waters from Cariaco and the Black Sea are -5.6 and -2.2, respectively. Combined with existing data from the Pacific and Arctic Oceans, our Atlantic data establish the conservatism of Tl isotopes in the global ocean. In contrast, partially- and predominantly-restricted basins reveal Tl isotope differences that vary between open-ocean (-6) and continental material (-2) ε205Tl, scaling with the degree of restriction. Regardless of the differences between basins, Tl is quantitatively removed from their euxinic waters below the chemocline. The burial of Tl in euxinic sediments is estimated to be an order of magnitude less than each of the modern ocean outputs and imparts no isotopic fractionation. Thallium removal into pyrite appears to be associated with a small negative fractionation between -1 and -3 ε205Tl, which renders Tl-depleted waters below the chemocline enriched in isotopically-heavy Tl. Due to the quantitative removal of Tl from euxinic seawater, Tl isotope analyses of the authigenic fraction of underlying euxinic sediments from both the Black Sea and Cariaco Basin capture the Tl isotope value of the oxic portion of their respective water column with no net isotope fractionation. Since the Tl isotope composition of seawater is largely dictated by the relative fraction of Mn-oxide burial versus oceanic crust alteration, we contend that the Tl isotope composition of authigenic Tl in black shales, deposited under euxinic conditions but well-connected to the open ocean, can be utilized to reconstruct the Tl isotope composition of seawater, and thus to reconstruct the global history of Mn-oxide burial.

Description: JDO and SGN would like to thank NSF and NASA for funding; JDO and TJH gratefully acknowledges support from the Agouron Institute Postdoctoral Fellowship Program.

Description: © The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Whitmore, L., Shiller, A., Horner, T., Xiang, Y., Auro, M., Bauch, D., Dehairs, F., Lam, P., Li, J., Maldonado, M., Mears, C., Newton, R., Pasqualini, A., Planquette, H., Rember, R., & Thomas, H. Strong margin influence on the Arctic Ocean Barium Cycle revealed by pan‐Arctic synthesis. Journal of Geophysical Research: Oceans, 127(4), (2022): e2021JC017417, https://doi.org/10.1029/2021jc017417.

Description: Early studies revealed relationships between barium (Ba), particulate organic carbon and silicate, suggesting applications for Ba as a paleoproductivity tracer and as a tracer of modern ocean circulation. But, what controls the distribution of barium (Ba) in the oceans? Here, we investigated the Arctic Ocean Ba cycle through a one-of-a-kind data set containing dissolved (dBa), particulate (pBa), and stable isotope Ba ratio (δ138Ba) data from four Arctic GEOTRACES expeditions conducted in 2015. We hypothesized that margins would be a substantial source of Ba to the Arctic Ocean water column. The dBa, pBa, and δ138Ba distributions all suggest significant modification of inflowing Pacific seawater over the shelves, and the dBa mass balance implies that ∼50% of the dBa inventory (upper 500 m of the Arctic water column) was supplied by nonconservative inputs. Calculated areal dBa fluxes are up to 10 μmol m−2 day−1 on the margin, which is comparable to fluxes described in other regions. Applying this approach to dBa data from the 1994 Arctic Ocean Survey yields similar results. The Canadian Arctic Archipelago did not appear to have a similar margin source; rather, the dBa distribution in this section is consistent with mixing of Arctic Ocean-derived waters and Baffin Bay-derived waters. Although we lack enough information to identify the specifics of the shelf sediment Ba source, we suspect that a sedimentary remineralization and terrigenous sources (e.g., submarine groundwater discharge or fluvial particles) are contributors.

Description: This research was supported by the National Science Foundation [OCE-1434312 (AMS), OCE-1436666 (RN), OCE-1535854 (PL), OCE-1736949, OCE-2023456 (TJH), and OCE-1829563 (R. Anderson for open access support)], Natural Sciences and Engineering Research Council of Canada (NSERC)-Climate Change and Atmospheric Research (CCAR) Program (MTM), and LEFE-CYBER EXPATE (HP). HT acknowledges support by the Canadian GEOTRACES via NSERC-CCAR and the German Academic Exchange Service (DAAD): MOPGA-GRI (Make Our Planet Great Again—Research Initiative) sponsored by BMBF (Federal German Ministry of Education and Research; Grant No. 57429828).