ALBERT

Beschreibung: We present here the first mercury speciation study in the water column of the Southern Ocean, using a high-resolution south-to-north section (27 stations from 65.50°S to 44.00°S) with up to 15 depths (0-4440 m) between Antarctica and Tasmania (Australia) along the 140°E meridian. In addition, in order to explore the role of sea ice in Hg cycling, a study of mercury speciation in the 'snow-sea ice-seawater' continuum was conducted at a coastal site, near the Australian Casey station (66.40°S; 101.14°E). In the open ocean waters, total Hg (Hg(T)) concentrations varied from 0.63 to 2.76 pmol/L with 'transient-type' vertical profiles and a latitudinal distribution suggesting an atmospheric mercury source south of the Southern Polar Front (SPF) and a surface removal north of the Subantartic Front (SAF). Slightly higher mean Hg(T) concentrations (1.35 ± 0.39 pmol/L) were measured in Antarctic Bottom Water (AABW) compared to Antarctic Intermediate water (AAIW) (1.15 ± 0.22 pmol/L). Labile Hg (Hg(R)) concentrations varied from 0.01 to 2.28 pmol/L, with a distribution showing that the Hg(T) enrichment south of the SPF consisted mainly of Hg(R) (67 ± 23%), whereas, in contrast, the percentage was half that in surface waters north of PFZ (33 ± 23%). Methylated mercury species (MeHg(T)) concentrations ranged from 0.02 to 0.86 pmol/L. All vertical MeHg(T) profiles exhibited roughly the same pattern, with low concentrations observed in the surface layer and increasing concentrations with depth up to an intermediate depth maximum. As for Hg(T), low mean MeHg(T) concentrations were associated with AAIW, and higher ones with AABW. The maximum of MeHg(T) concentration at each station was systematically observed within the oxygen minimum zone, with a statistically significant MeHg(T) vs Apparent Oxygen Utilization (AOU) relationship (p 〈0.001). The proportion of Hg(T) as methylated species was lower than 5% in the surface waters, around 50% in deep waters below 1000 m, reaching a maximum of 78% south of the SPF. At Casey coastal station Hg(T) and Hg(R) concentrations found in the 'snow-sea ice-seawater' continuum were one order of magnitude higher than those measured in open ocean waters. The distribution of Hg(T) there suggests an atmospheric Hg deposition with snow and a fractionation process during sea ice formation, which excludes Hg from the ice with a parallel Hg enrichment of brine, probably concurring with the Hg enrichment of AABW observed in the open ocean waters. Contrastingly, MeHg(T) concentrations in the sea ice environment were in the same range as in the open ocean waters, remaining below 0.45 pmol/L. The MeHg(T) vertical profile through the continuum suggests different sources, including atmosphere, seawater and methylation in basal ice. Whereas Hg(T) concentrations in the water samples collected between the Antarctic continent and Tasmania are comparable to recent measurements made in the other parts of the World Ocean (e.g., Soerensen et al., 2010; doi:10.1021/es903839n), the Hg species distribution suggests distinct features in the Southern Ocean Hg cycle: (i) a net atmospheric Hg deposition on surface water near the ice edge, (ii) the Hg enrichment in brine during sea ice formation, and (iii) a net methylation of Hg south of the SPF.

Beschreibung: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Hayes, C. T., Costa, K. M., Anderson, R. F., Calvo, E., Chase, Z., Demina, L. L., Dutay, J., German, C. R., Heimburger-Boavida, L., Jaccard, S. L., Jacobel, A., Kohfeld, K. E., Kravchishina, M. D., Lippold, J., Mekik, F., Missiaen, L., Pavia, F. J., Paytan, A., Pedrosa-Pamies, R., Petrova, M., V., Rahman, S., Robinson, L. F., Roy-Barman, M., Sanchez-Vidal, A., Shiller, A., Tagliabue, A., Tessin, A. C., van Hulten, M., & Zhang, J. Global ocean sediment composition and burial flux in the deep sea. Global Biogeochemical Cycles, 35(4), (2021): e2020GB006769, https://doi.org/10.1029/2020GB006769.

Beschreibung: Quantitative knowledge about the burial of sedimentary components at the seafloor has wide-ranging implications in ocean science, from global climate to continental weathering. The use of 230Th-normalized fluxes reduces uncertainties that many prior studies faced by accounting for the effects of sediment redistribution by bottom currents and minimizing the impact of age model uncertainty. Here we employ a recently compiled global data set of 230Th-normalized fluxes with an updated database of seafloor surface sediment composition to derive atlases of the deep-sea burial flux of calcium carbonate, biogenic opal, total organic carbon (TOC), nonbiogenic material, iron, mercury, and excess barium (Baxs). The spatial patterns of major component burial are mainly consistent with prior work, but the new quantitative estimates allow evaluations of deep-sea budgets. Our integrated deep-sea burial fluxes are 136 Tg C/yr CaCO3, 153 Tg Si/yr opal, 20Tg C/yr TOC, 220 Mg Hg/yr, and 2.6 Tg Baxs/yr. This opal flux is roughly a factor of 2 increase over previous estimates, with important implications for the global Si cycle. Sedimentary Fe fluxes reflect a mixture of sources including lithogenic material, hydrothermal inputs and authigenic phases. The fluxes of some commonly used paleo-productivity proxies (TOC, biogenic opal, and Baxs) are not well-correlated geographically with satellite-based productivity estimates. Our new compilation of sedimentary fluxes provides detailed regional and global information, which will help refine the understanding of sediment preservation.

Beschreibung: 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. The 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, and the authors would like to acknowledge all attendees of this meeting. The authors acknowledge the participants of the 68th cruise of RV Akademik Mstislav Keldysh for helping acquire samples. Christopher T. Hayes acknowledges support from US-NSF awards 1658445 and 1737023. Some data compilation on Arctic shelf seas was supported by the Russian Science Foundation, grant number 20-17-00157. This work was also supported through project CRESCENDO (grant no. 641816, European Commission). Zanna Chase acknowledges support from the Australian Research Council’s Discovery Projects funding scheme (project DP180102357). Christopher R. German acknowledges US-NSF awards 1235248 and 1234827. Some colorbars used in the figures were designed by Kristen Thyng et al. (2016) and Patrick Rafter.