Coccolithophorid-based geochemical paleoproxies

Abstract

Coccolithophores are the only marine organisms that provide indicators of past climatic and oceanographic conditions from both the organic (molecular fossils or biomarkers) and inorganic (calcium carbonate) remains in sediments. The under-saturation ratio of alkenone biomarkers (U37 K) provides information about past sea surface temperatures and is gaining widespread use as a paleotemperature proxy, particularly in the Quaternary sediment record. The carbon isotopic fractionation in alkenone biomarkers (ealkenone) should allow reconstruction of past dissolved and atmospheric CO2 concentrations if independent proxies are able to consistently constrain the nutrient or growth rate influence on isotopic fractionation. The Sr/Ca ratio of coccolith carbonate is the most developed proxy from the elemental chemistry of coccoliths. Recent culture and field studies suggest that the Sr/Ca ratio has potential as an indicator of nutrient-stimulated coccolithophorid growth rates. In contrast, while the Mg/Ca ratio of coccoliths is probably controlled by temperature, formidable challenges in removing noncarbonate sources of Mg from coccolith fractions will probably preclude use of coccolith Mg/Ca for paleother-mometry. Similar challenges in cleaning will probably also preclude use of coccolith Cd, Ba, V, or U. Stable isotopic measurements in coccolith-dominated bulk carbonate have been widely used to infer temperature changes and changes in the carbon cycle in the Mesozoic and Early Cenozoic, despite an array of nonequilib-rium or “vital effects” in different species. In addition to paleoceanographic applications, continued study of the stable isotopic fractionation of coccoliths in culture may also elucidate mechanisms of carbon acquisition in different coccolithophorid species. As is the case for all paleoceanographic proxies, continued calibration studies are required to further improve our understanding of coccolithophorid-based proxy systems and increase confidence in their application.