Abstract This paper mainly reviews our recent work on the biology and geochemistry of foraminifera with respect to their use as palaeoceanographic proxies. Our approach to proxy validation and development is described, primarily from a modeler's point of view. The approach is based on complementary steps in understanding the inorganic chemistry, inorganic isotope fractionation, and biological controls that determine palaeo-tracer signals in organisms used in climate reconstructions. Integration of laboratory experiments, field and culture studies, theoretical considerations and numerical modelling holds the key to the method's success. We describe effects of life-processes in foraminifera on stable carbon, oxygen, and boron isotopes as well as Mg incorporation into foraminiferal calcite shells. Stable boron isotopes will be used to illustrate our approach. We show that a mechanism-based understanding is often required before primary climate signals can be extracted from the geologic record because the signals can be heavily overprinted by secondary, non-climate related phenomena. Moreover, for some of the proxies, fundamental knowledge on the thermodynamic, inorganic basis is still lacking. One example is stable boron isotopes, a palaeo- p H proxy, for which the boron isotope fractionation between the dissolved boron compounds in seawater was not precisely known until recently. Attempts to overcome such hurdles are described and implications of our work for palaeoceanographic reconstructions are discussed.

ABSTRACT Fine-grained (<63 μm) carbonate from Plio-Pleistocene exploration wells, offshore Louisiana and Trinidad, has δ 13 CPDB signatures of about -50 to -10 ‰. In this paper we explore two hypotheses that appear to account for these negative δ 13 C anomalies. The first hypothesis is that the negative δ 13 C signatures observed in Green Canyon block, offshore Louisiana, are due to authigenic subsurface precipitation of fine-grained carbonate as a record of past migration pathways of hydrocarbons through subsurface Plio-Pleistocene sands. The second hypothesis is that the authigenic δ 13 C signatures record precipitation at or near the sediment-water interface and thus record the past locations of hydrocarbon seeps on the northcentral margin of the Gulf of Mexico. Preliminary geochemical and petrographic evidence is presented to examine these hypotheses. Within the limitations of not knowing past pore fluid chemistries, we also attempt to use the δ 18 O signatures of the <63 μ m carbonate to estimate the paleotemperatures, and perhaps paleodepths and timing, of emplacement of the negative δ 13 C signatures. The potential that this isotopically negative carbonate acts as a geochemical recorder or tracer of hydrocarbon movement makes documentation of this phenomenon important to offshore exploration efforts.