Cl/Ca ratio of coccoliths as a salinity proxy

Proxies for seawater salinity are still in development and include the chloride content of porewater inclusions, the Na/Ca ratio of foraminifera as well as the δD of organic biomarkers. Here, we suggest that the incorporation of chloride ions into the skeletons of calcifying plankton could serve as a salinity proxy. We have conducted a pilot study of single coccoliths from a Miocene chalk, which showed that Cl/Ca ratios of coccoliths can be determined using nano-scale secondary ion mass spectrometry (nanoSIMS). It remains to be established whether the chloride content of coccoliths shows a direct relationship with salinity and to which extent the incorporation of chloride into the coccoliths may be biologically controlled by the coccolithophores.