Barium and cadmium stable isotopes as tracers for carbon export, remineralization strength and post-deposition diagenesis in the Peruvian oxygen minimum zone

• Applicant: Ruifang Xie, Ph.D.
• Subject Area: Oceanography ; Mineralogy, Petrology and Geochemistry
• Term: from 2019 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 432469432

• Final Report Year: 2022

Final Report Abstract

In this funded project, seawater, marine particulates, bulk sediment and porewater Ba isotope ratios were systematically analyzed to investigate the mechanism involved to fractionate Ba isotopes during biogenic and diagenetic barite formation. Water column data from the Peruvian OMZ indicated that particulate Ba could record surface water Ba isotope ratios, which, if properly reserved, may in turn provide valuable insights into past ocean export production. Conversely, sedimentary downcore Ba concentration and isotope ratios under low oxygen environment showed a strong barite front that induce large variations in porewater Ba isotopes. The observation that 40% increase in bulk sediment Ba content across the diagenetic barite front did not alter TD d138Ba demonstrated that other mineral phases rather than barite dominate bulk sediment d138Ba signatures. The significance of this finding challenges the proposal by others that bulk sediment d138Ba could reliably trace export production in the past ocean. The PI has terminated the funded project. However, he is still actively involved in ongoing work with how low oxygen waters impact Cd and Ba isotope biogeochemistry, how fluxes of POC may alter water column particulate Cd and Ba isotopes, and how sedimentary Cd and Ba records may or may not reliably trace upper water column processes. Two proposals were submitted this year to the Chinese National Science Foundation and Ministry of Science and Technology to address these questions. Through these projects, the PI will integrate findings from both the open ocean and major OMZs of the modern ocean to provide comprehensive insights into marine biogeochemical cycling of Cd and Ba isotopes.

Publications

• Effects of 238 U variability and physical transport on water column 234 Th downward fluxes in the coastal upwelling system off Peru. Biogeosciences, 17(19), 4919-4936.
  Xie, Ruifang C.; Le Moigne Frédéric, A. C.; Rapp, Insa; Lüdke, Jan; Gasser, Beat; Dengler, Marcus; Liebetrau, Volker & Achterberg, Eric P.