Planktonic foraminifera transfer function approach to Red Sea paleoceanography

This study examines the potential of using the assemblage composition of planktonic foraminifera in the Red Sea as a quantitative proxy for paleoclimate reconstructions. To this end, a new surface sediment sample dataset was generated to evaluate the environmental control on foraminifera assemblages and to serve as the calibration dataset for the subsequent development of planktonic foraminifera transfer functions. Gradient analysis was employed to investigate the effect of temperature, salinity, primary productivity, water column stratification and the properties of the oxygen minimum zone on the foraminifera fauna of the Red Sea. Because of the high mutual correlation among all environmental parameters along the basin axis, no unambiguous identification of the controlling parameter could be achieved by statistical analyses of the calibration dataset alone. A comparison with published studies on the ecology of planktonic foraminifera, however, lead to the working hypothesis that primary productivity determines the distinct gradient in the foraminifera community in the Red Sea. Transfer functions were then generated by four mathematical methods to characterise the relationship between foraminifera assemblages and chlorophyll a concentration as a proxy for productivity. The transfer functions appear highly efficient in capturing the modern productivity gradient in the Red Sea, with an average root mean squared error of prediction of 0.124 mg/m3, corresponding to 10.3 % of the observed gradient. The working hypothesis was then tested and on two newly generated Holocene faunal records from the central and northern Red Sea. Apart from the impact of sea level on the hydrography of the Red Sea, these records showed a strong influence of the dominant regional climatic system, the Indian monsoon. The reconstructed chlorophyll a concentrations indicate low productivity for the early Holocene, which can be ascribed to lowered sea level and a strong SW monsoon. An earlier postglacial recovery of the fauna in the northern Red Sea than in the central Red Sea time is explained by a local additional freshwater source. During the Mid-Holocene, environmental conditions in the Red Sea shifted gradually towards higher productivity, reaching a maximum between 3.5 and 2 ka BP. This distinct event can be explained by a strengthened NE monsoon and generally higher aridity, leading to dominant winter circulation mode in the basin. After 2 ka BP, the modern circulation and climatic conditions became established. The developed transfer functions were also applied on existing Red Sea faunal records covering several glacial and interglacial periods. While non-analogue conditions to present day for fauna and environment were to be expected during the glacials, it became apparent that also for large intervals of interglacial periods, the analogy of the fauna and/or the analogy of the relationship between the fauna and the environment was not given. As a consequence, the applicability of the developed transfer functions is limited to portions of the interglacial intervals with high and stable sea level and atmospheric forcing similar to that during the Holocene.