The cycling of trace elements in near-surface sediments from the Benguela Upwelling System off Namibia: water column signals and early diagenesis

• Applicant: Dr. Philipp Böning
• Subject Area: Geology
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 502242241

Marine zones of intense oxygen deficiency are the oxygen minimum zones (OMZs) situated in upwelling areas of the productive eastern boundary currents. Modern OMZ environments are increasingly used as models to reconstruct marine anoxia in Earth history, in addition to the stagnant anoxic basin model (e.g., the Black Sea). OMZ sediments are indicated by enrichment of trace elements (TEs) involved in bio- and redox cycling (e.g., Cd, Mo, Re, U, V) as well as Fe species. Sediments of the Peruvian upwelling system have been intensely studied with respect to TE and Fe cycling, in contrast to sediments from the Benguela Upwelling System (BUS) off Namibia. Both systems are highly productive and hotspots of TE accumulation but the BUS is characterized by more dust input and a more variable OMZ, the effect of which on TE accumulation patterns remains to be determined. Hence, the goal of the proposed study is a comprehensive assessment of the sources and accumulation mechanisms of several TEs from the BUS. In the study proposed here, I intend to analyze eleven near-surface sediment cores from the central and southern shelf (from 22°S to 25.5°S) and another five from a transect across the southern slope (at ~ 25.5°S) as well as water column suspended particles (from GoFlo bottles and sediment traps) for Ag, Ba, Cd, Co, Mo, Pb, Re, Tl, U, V, W, Zr, major elements (e.g., C, S, Al, Fe) and Fe isotopes. In addition, I intend to analyze the sediments for Hg contents and 210Pb activities. In the proposed study, I will ground-truth the reliability of widely used TE proxies for redox conditions (e.g., U, Re, Mo) and paleo-productivity (e.g., Ag, Cd, Ba) as well as Tl and W, the sedimentary cycling of which is largely unknown in continental margin sediments. I will use 210Pb activities as well as Hg and Pb (anthropogenic indicator) to infer sedimentation rates, and Zr to detect dust inputs and/or current overprints on the sediments. I will test (1) whether water column particles are authigenic sources for Ag, Cd, Mo, U, V, Tl, W in the sediments by comparing water column TE fluxes with shelf TE accumulation rates, (2) whether TE relationships (e.g., Ag, Ba, Cd, Re, Mo) on the shelf (within the OMZ) bear a typical signature, which can be differentiated from that of the slope (beneath the OMZ), (3) how early diagenetic apatite formation influences the accumulation of e.g., U (which questions the use of U as a primary redox proxy), (4) whether Co is truly an indicator of sulfidic conditions or only detrital-lithogenic in origin, and (5) whether the cycling of authigenic Fe is constrained in local hotspots in the BUS, in contrast to the large-scale systematic Fe cycling off Peru. The expected data will fill an important gap in continental margin data sets of global relevance to recognize environments within OMZs, beneath OMZs and stagnant anoxic basins as models for the extent of past marine anoxia.

• DFG Programme: Research Grants