Periodic Reporting for period 1 - SedTraceFlux (The critical role of sedimentary trace element fluxes in ocean biogeochemistry)
Período documentado: 2021-09-01 hasta 2023-08-31
Consequently, there is a growing effort to incorporate marine trace elements into state-of-the-art global models, aiming to enhance our ability to predict future changes in ocean chemistry and climate. However, these efforts have faced challenges due to our limited understanding of the sources and sinks of dissolved trace elements in the ocean. Recent findings specifically indicate that sedimentary source and sink fluxes may play significantly larger roles in shaping the distributions of marine trace elements than previously assumed. Yet, our knowledge of these fluxes and the relevant sediment biogeochemical processes remains insufficient, resulting in substantial uncertainties in the modern and paleo-applications of trace elements in the field of ocean biogeochemistry. The project aims to characterize the sedimentary processes and fluxes of these trace elements and isotopes, develop new modeling tools to study their biogeochemical cycling in marine sediments and understand the role of sedimentary processes in their global ocean cycles. The project is focused on the rare earth elements and transition metals.
The project has concluded that the sedimentary fluxes of trace elements and isotopes are regulated by diagenetic processes including organic matter remineralization, redox cycling, authigenesis, marine silicate weathering and reverse weathering; the sedimentary fluxes contribute significantly to the elemental and isotopic budgets of trace elements and isotopes; including sedimentary processes and fluxes in ocean models is needed to fully explain the distributions of trace elements and isotopes in the ocean.
The project has modeled trace element speciation and diagenesis on the continental margin and abyssal seafloor, and the impact on the sedimentary fluxes. The speciation model considered both inorganic and organic ligands, as well as adsorption/desorption onto sediment particles including Fe-Mn oxides. A complex biogeochemical reaction network was included. The results showed that in continental margin sediments, Fe-Mn reduction and authigenic phosphate formation are the main drivers of rare earth element biogeochemistry and thus control their pore water profiles and fluxes. However, the pore water and authigenic Nd isotope data require input from silicate-sourced Nd, which is the result of the weathering of volcanic materials coupled with reverse weathering. In oxic sediment, in contrast, trace element cycling is mainly driven by organic matter remineralization and adsorption onto Fe-Mn oxide.
The project has tested and developed the procedure of measuring transition metals, Fe and Nd isotopes from the same pore water or sediment samples. The project has measured the pore water and sediment Fe, Nd, and transition metal concentration and isotope composition from a variety of environmental settings. This project has produced a large collection of pore water trace element isotope data.
In collaboration with colleagues, this project constrained the impacts of ocean circulation and internal cycling on the distribution of Ni concentration and isotope composition using ocean biogeochemical models. The results showed that marine Ni cycling is strongly tied to the phytoplankton community composition. The project has also created an ocean model of Nd cycling, including benthic flux and processes. The model demonstrates that to successfully simulate the distributions of both Nd concentration and isotope composition, sedimentary flux is needed in addition to the top-down processes.
Five peer-reviewed research articles have been published as a result of SedTraceFlux. An open-source software package SedTrace has been produced, and a website has been created to describe this software and other SedTraceFlux results. The results have also been presented at international conferences.