The use of paleoceanographic proxies in carbonate periplatform settings-opportunities and pitfalls

In a sediment core from the leeward side of the Great Bahama Bank (Ocean Drilling Program ODP-Leg 166, 1006A-24H, early Pliocene) several paleoceanographic proxies were compared with carbonate mineralogy. The proxies measured on the planktonic foraminifera species Globigerinoides sacculifer are Mg/Ca and Sr/Ca elemental ratios, and δ18O values. These proxies are widely used in pelagic sediments to derive parameters of water masses, such as paleo-temperature and -salinity. To produce quantitative estimates it is necessary to verify that the primary shell mineralogy of the foraminifera was not altered diagenetically. Diagenetic alteration can be rapid in periplatform settings relative to pelagic deep-sea sediments, since the meta-stable carbonate phases provide an additional driving force for recrystallisation. We show that the Sr/Ca ratio of foraminiferal calcite can be used to assess the degree of diagenetic alteration of their shell chemistry. Using this method, we demonstrate that the primary signal in the oxygen-isotope ratios is preserved in the studied core. In the Mg/Ca record, the relative change of foraminiferal Mg/Ca ratios seems to be preserved, but absolute values are influenced by diagenesis. The second set of proxies comprises δ13C and C/N ratios of organic carbon. These are used in pelagic sediments in order to assess the origin of organic carbon (terrestrial or marine phytoplankton). It is demonstrated here that a simple two-end mixing model cannot explain the observed co-occurrence of relatively high δ13Corg and C/N values. As the δ13Corg values co-vary with the aragonite content, we attribute this signature to varying contribution of organic carbon from benthic algae, which are also the major aragonite producers on the platform top.