ABSTRACT
Ostracods are small bivalved crustaceans. They secrete shells made of low-Mg calcite, which are commonly preserved in Holocene sediments. This chapter examines the contribution of ostracods to Holocene environmental reconstruction, with particular emphasis on non-marine and marginal-marine environments. Three specific areas in which ostracods have contributed to Holocene research are examined, namely palaeosalinity and palaeo-solute reconstruction, hydrological habitat type and lake-level change, and finally palaeotemperature reconstruction. Salinity and solute composition are important aquatic variables. In hydrologically-closed lakes, salinity and solute composition change as a result of variations in effective precipitation. In coastal regions, salinity and solute composition of waterbodies may change with varying inputs of marine and non-marine water, such as may occur as a result of sea-level change. Ostracods respond to such variations in water composition by changes in the abundance and presence of different species, through ecophenotypic changes and by changes in shell chemistry. Many ostracod species are closely associated with a specific habitat type and this association can be used in environmental reconstruction. For example in lakes, individual species are often found in specific water depths, although this is often a response to the presence or absence of a particular substrate type or aquatic plants, rather than a depth control per se. The ostracod–depth relationship can be used to reconstruct lake-level changes, which may be a response to changing climate or hydrology. Ostracods may also be used to track variations in temperature. Individual species may have specific water temperature preferences; moreover, the uptake of the trace element magnesium into the ostracod shell is positively correlated with water temperature. The oxygen isotope composition of benthic ostracods in deep, hardwater lakes in humid-temperate regions may also provide information about past air temperature.
