Stable isotope and trace element composition of foraminiferal calcite - from incorporation to dissolution

Abstract

The ocean is the most important reservoir for the greenhouse gas CO2 and in order to understand the natural variability in the atmospheric concentration, we have to understand the storage capacity of the ocean over past timescales. To reconstruct the physicochemical conditions of the past, measurable recorders ( proxies ) have to be applied in order to estimate conditions which can no longer be observed. With regard to marine carbonate chemistry, a number of biogeochemical proxy-relationships have been proposed and established. Many of them rely on the chemical composition of calcareous skeletal remains such as foraminifera shells. However, problems in application and reliability of these proxies may arise from uncertainties in the incorporation behaviour and their potential instability in the geological archive. While physiological processes of the foraminifera and associated organisms are known to alter a number of proxy relationships, several proxy records are known or suspected to be modified by partial dissolution in the sediment. To investigate whether boron isotopes (pH-proxy) and Ba/Ca (proxy for the oceanwide distribution of alkalinity) in foraminiferal shells are affected by biological activity, culture experiments with living animals have been carried out. While symbiont photosynthesis causes the d11B/pH-record to deviate from the surrounding seawater-pH, Ba/Ca proved to be unaffected by changing alkalinity.Laboratory experiments were designed to investigate the effect of carbonate dissolution on foraminiferal shell chemistry. Comparison of various chemical proxies demonstrates that the dissolution behaviour is poorly understood. Similarly, evidence from culture experiments suggests that the use of foraminiferal shell weight to determine bottomwater-[CO32-] is much more complicated than so far assumed.The dissertation provides detailed descriptions of theexperimental methods and discusses the meaning of the results for paleoreconstructions.