Trace metals in sediments of the Var Canyon turbidite system at the DYFAMED station in the Ligurian Sea

The magnitude and the chronology of anthropogenic impregnation by Hg and other trace metals of environmental concern (V, Cr, Ni, Cu, Zn, Ag, Cd and Pb, including its stable isotopes) in the sediments are determined at the DYFAMED station, a site in the Ligurian Sea (Northwestern Mediterranean) chosen for its supposed open-sea characteristics. The DYFAMED site (VD) is located on the right levee of the Var Canyon turbidite system, at the end of the Middle Valley. In order to trace the influence of the gravity current coming from the canyon on trace metal distribution in the sediment, we studied an additional sediment core (VA) from a terrace of the Var Canyon, and material collected in sediment traps at the both sites at 20 m above sea bottom. The patterns of Hg and other trace element distribution profiles are interpreted using stable Pb isotope ratios as proxies for its sources, taking into account the sedimentary context (turbidites, redox conditions, and sedimentation rates). Major element distributions, coupled with the stratigraphic examination of the sediment cores point out the high heterogeneity of the deposits at VA, and major turbiditic events at both sites. At the DYFAMED site, we observed direct anthropogenic influence in the upper sediment layer (<2 cm), while on the Var Canyon site (VA), the anthropization concerns the whole sedimentary column sampled (19 cm). Turbiditic events superimpose their specific signature on trace metal distributions. According to the 210Pbxs-derived sedimentation rate at the DYFAMED site (0.4 mm yr-1), the Hg-enriched layer of the top core corresponds to the sediment accumulation of the last 50 years, which is the period of the highest increase in Hg deposition on a global scale. With the hypothesis of the absence of significant post-depositional redistribution of Hg, the Hg/C-org ratio changes between the surface and below are used to estimate the anthropogenic contribution to the Hg flux accumulated in the sediment. The Hg enrichment, from pre-industrial to the present time is calculated to be around 60%, consistent with estimations of global Hg models. However, based on the chemical composition of the trapped material collected in sediment traps, we calculated that epibenthic mobilization of Hg would reach 73%. Conversely, the Cd/C-org ratio decreases in the upper 5 cm, which may reflect the recent decrease of atmospheric Cd inputs or losses due to diagenetic processes.本研究于地中海西北部利古里亚海的DYFAMED站位开展，该站位因具备典型外海特征而被选定，旨在明确沉积物中汞（Hg）及其他环境关注痕量金属（trace metals）的人为输入强度与年代序列，这类痕量金属包括钒V、铬Cr、镍Ni、铜Cu、锌Zn、银Ag、镉Cd、铅Pb，以及其稳定同位素（stable isotope）。DYFAMED站位（VD）位于瓦尔峡谷浊积系统的右堤，处于中谷末端。为厘清峡谷重力流对沉积物中痕量金属分布的影响，本研究额外采集了瓦尔峡谷一处阶地的沉积物岩芯（VA），并在两个站位海底以上20米处收集了沉积物捕集器（sediment traps）样品。本研究以铅稳定同位素比值（Pb stable isotope ratios）作为物源代用指标，结合沉积背景（浊积岩、氧化还原条件与沉积速率），解析汞与其他痕量元素的分布剖面特征。主量元素分布特征结合沉积物岩芯的地层学分析结果表明，VA站位的沉积物具有极强的非均质性，且两个站位均记录了重大浊积事件（turbiditic event）。DYFAMED站位的表层沉积物（<2 cm）中存在直接的人为影响，而瓦尔峡谷站位（VA）的19 cm完整沉积柱均受到人为活动改造。浊积事件会在痕量金属分布上叠加其独特的地球化学特征。基于DYFAMED站位的过剩铅-210（210Pbxs）所得沉积速率（0.4 mm yr⁻¹），表层岩芯的汞富集层对应过去50年的沉积物堆积量，而这一时期正是全球汞沉降量增幅最大的阶段。假设汞在沉积后未发生显著再分配，通过对比表层与下层沉积物的汞/总有机碳（Hg/Corg）比值变化，可估算人为活动对沉积物中累积汞通量的贡献占比。从前工业时代至今的汞富集比例约为60%，这与全球汞循环模型的估算结果一致。然而，基于沉积物捕集器收集的捕获物化学成分，本研究估算得到汞的底栖迁移（epibenthic mobilization）比例可达73%。与之相反，表层5 cm沉积物中的镉/总有机碳（Cd/Corg）比值呈下降趋势，这可能反映了近期大气镉输入量的减少，或是成岩作用（diagenetic processes）导致的镉流失。