Trace-elements in ostracods as proxies of environmental parameters in shallow marine waters

Abstract

The Earth’s climate plays a major role in the organization of different ecosystems and habitats. Furthermore, it determines not only the abundance and diversity of species but also human socio-economic activities. Past climatic conditions allow us to better understand the present and future climate as well as their driving mechanisms. Oceans are a key player in the Earth’s climate, as incoming solar energy is absorbed and redistributed through the circulation of warm waters towards the poles. Atmospheric gases such as carbon dioxide (CO2) are also buffered by the oceans. The study of environmental marine properties, such as temperature and salinity, provide valuable information about ocean circulation, hydrological patterns and climate evolution. The environmental reconstruction of ocean parameters can be quantitatively performed through proxies, such as trace-elements to calcium ratio (E/Ca) of ostracod shells. Ostracods are small crustaceans found in different aquatic ecosystems including rivers, lakes and oceans. Several factors may interfere and modify the original biogenic chemical composition of the shell produced during calcification, such as poor preservation due to partial dissolution of the shell or inadequate cleaning protocols. It is also unclear if different ostracod stages (i.e., juveniles or adults) can be used in paleoreconstructions. Moreover, most research has focused on the study of Mg and Sr while the incorporation mechanisms and paleoenvironmental implications of other elements such as Mn, Zn and Ba are poorly understood. Here, I investigate trace-elements in ostracod shells of shallow marine specimens through different chemical techniques, including Electron Probe Micro-Analysis (EPMA), Inductively Coupled Plasma Mass Spectrometry (ICP-MS), Flow-Through Time-Resolved Analysis (FT TRA) and Scanning Electron Microscope (SEM) images. Two chemically unstudied ostracod species are investigated, Sinocytheridea impressa and Neomonoceratina delicata, which are abundant in shallow marine waters from Asian marine ecosystems. This thesis shows that E/Ca ratios of ostracod shells depends on the ostracod stage, cleaning protocol and shell preservation. For example, Mg/Ca ratios are lower in juvenile ostracods and show more variability in unclean shells. Fe/Ca and Mn/Ca increase in poorly preserved shells, while Sr/Ca ratios are mostly constant in cleaned and uncleaned shells considering different ostracod stages and preservations. This research shows an optimal preservation of E/Ca ratios of adult shells with a calcified epicuticle cleaned in 5% sodium hypochlorite. The Mg/Ca ratio of S. impressa and N. delicata specimens can be used as a paleothermometer of shallow marine waters. In addition, Mn/Ca, Zn/Ca and Ba/Ca ratios are likely controlled by water concentrations, which in turn, are controlled by several factors such as metal desorption from particulate matter and organic matter diagenesis. Thus, Mn/Ca, Zn/Ca and Ba/Ca can be used to reconstruct ocean parameters that account for these processes such as salinity, suspended solids, pH and total carbon. I present the development of multiple linear models that predict ostracod ratios based on physicochemical ocean and sediment variables. The combination of several multiple linear models allows the deconvolution of each variable, which subsequently allows for the simultaneous reconstructions of several ocean parameters.