Seawater Temperature Estimates in Paleoceanography

Abstract

Temperature, together with salinity, determines the density of seawater, and therefore, it strongly influences its circulation and sinking characteristics. In turn, the important role oceanic circulation and deep‐water formation play in past, present, and future climatic change lend a special significance to the estimate of past seawater temperature. As the past values of this important climatic variable in the distant, geological past are impossible to measure directly, a variety of indirect methods, so‐called “proxies,” have been developed. These methods are usually based on the fossil faunal/floral composition of sediment core samples, as a close link exists between species occurrences/abundances and environmental variables. Semiquantitative (e.g., faunal provinces indexes) and quantitative (e.g., transfer functions, neural networks) techniques are therefore applied to reconstruct past sea surface water variables, including temperature. Additionally, several geochemical methods have been recently developed to estimate past ocean temperatures by measuring chemical elements (Mg/Ca, Sr/Ca) or organic compounds (alkenones), which are either incorporated in the organism shell, or produced during its life cycle.