An evaluation of cleaning methods, preservation and specimen stages on trace elements in modern shallow marine ostracod shells of Sinocytheridea impressa and their implications as proxies

Abstract

Trace elements of marine shells are important tools for reconstructing past ocean conditions, which are usually obtained through empirical calibrations with ocean parameters. For example, Mg/Ca ratios of ostracod shells have been linked to ocean temperature. However, some uncertainties usually arise from extraneous impacts, such as the selection of cleaning methods, specimen ontogenetic stage and shell preservation. Here, we present an evaluation of trace elements (Ca, Mg, Sr, Na, Al and Fe) in a shallow marine ostracod, Sinocytheridea impressa, and their potential utility as paleoceanographic proxies. We investigated single-shell variability of trace-element-to‑calcium ratios through Inductively Couple Plasma Mass Spectrometry (ICP-MS). In addition, the internal distribution of trace-elements was studied by Electron Probe Micro Analysis (EPMA) and Flow-Through Time-Resolved Analysis (FT-TRA). We conducted these analyses on ostracod specimens of different life stages and preservation conditions, subject to different cleaning methods, to resolve the suitability of this species as a new paleoenvironmental archive. Based on cleaning tests, we observe that clays stuck to the shells are the major contaminants in our specimens. We report an optimal cleaning procedure using sodium hypochlorite, which reduced the adherence of clays to the organism without significantly dissolving adult and juvenile calcite. We additionally find that the internal Mg distribution of S. impressa specimens is different for adults and the oldest stage of juveniles, namely A-1, with only the former having an enriched inner Mg layer. Therefore, precautions should be taken when combining life stages for trace-element analysis. While Sr and Mn are found to be mostly homogenous throughout all shells, the distribution of Na was found to be heterogeneous, which suggests that the incorporation of Na is controlled by a different mechanism than Mg, Sr and Mn. Finally, we report that transparent and opaque shells show similar Mg/Ca, Sr/Ca and Na/Ca ratios, unlike Mn/Ca and Fe/Ca, which suggests that opaque shells result from Mn- and Fe-enriched porewaters and Mn-enriched waters from the Pearl River. Our combined analysis suggests that Mg, Sr and Mn of adult S. impressa are incorporated into the carbonate matrix during calcification, and may therefore be suitable proxies of important environmental variables such as temperature and aqueous elemental concentrations.