Occurrence of Inoceramus in the South Atlantic and Oxygen Isotopic Paleotemperatures in Hole 530A

Inoceramus occurs in every DSDP hole that penetrated Cretaceous sediments in the South Atlantic Ocean, and specimen occurrence has been mapped in detail for each core. Oxygen and carbon isotope measurements were completed on 18 Inoceramus specimens from Hole 53OA. Textural evidence of diagenesis is accompanied by depletion in O. Paleotemperature results were obtained from 11 well-preserved specimens. Bottom water temperatures in the Angola Basin decreased from 23°C during the Coniacian to 13°C near the end of the Campanian. INTRODUCTION A major objective at Site 530 was to determine the paleoceanographic evolution of the Angola Basin, particularly during the Cretaceous episodes of black shale deposition. Determination of paleotemperatures is an integral part of reconstructing the paleoenvironmental history. Most paleotemperature investigations have stressed isotopic analyses of foraminifers from deep-sea cores. Unfortunately, there is a notable lack of well-preserved foraminifers from Cretaceous sediments, despite the fact that many Deep Sea Drilling Project (DSDP) holes have penetrated Cretaceous sediments. As in sections elsewhere, there were no suitably preserved specimens of foraminifers found in the Cretaceous sediments in Hole 53OA. However, there were abundant Inoceramus specimens, ranging in age from Maestrichtian to Coniacian, many of which were clearly well preserved. The majority of quantitative paleotemperature estimates from the Cretaceous have been derived from oxygen isotopic analyses of belemnites and Inoceramus from outcrops of epicontinental sea deposits (e.g., Lowenstam and Epstein, 1954). The major advantage of analyses of these macrofossils is that the coarse crystalline structure is likely to preserve the isotopic signal. The major disadvantage is that the epicontinental-sea setting may be subject to more variable environmental conditions, such as fresh-water runoff, making the data difficult to interpret. Subaerial exposure may also result in substantial alteration. Because Inoceramus specimens preserved in deep sea cores do not have the same drawbacks as those in exposed continental sections, they have potential to expand greatly our knowledge of Cretaceous paleotemperatures. We have mapped the distribution of Inoceramus fragments in all DSDP cores which penetrated Cretaceous Hay, W. W. , Sibuet, J . -C, et al., Init. Repts. DSDP, 75: Washington (U.S. Govt. Printing Office). 2 Present address: National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307. sediments in the South Atlantic Ocean. Knowledge of the distribution of Inoceramus will aid in understanding their paleoecology as well as describe the potential source of isotopic data. Oxygen and carbon isotope ratios were measured on Inoceramus fragments in various states of preservation in order to interpret diagenetic trends and establish textural criteria for selection of samples for paleotemperature analysis. This study resulted in paleotemperature data from 11 well-preserved Inoceramus fragments from Hole 53OA, ranging in age from Coniacian to Campanian. These data confirm earlier studies of warm bottom water temperatures during the Cretaceous (see Savin, 1977 for view). Changes in bottom water sources during this time influenced the character of sedimentation in the Angola Basin. These changes were probably controlled by tectonic developments and eustatic sea level fluctuations in the early South Atlantic. OCCURRENCE OF INOCERAMUS IN SOUTH ATLANTIC CORES Inoceramus is an extinct epibenthic bivalve. The coarsely crystalline ostracum, which consists of vertically oriented calcite prisms, is preserved in deep-sea sediments (Fig. 1). Inoceramus is known to occur in a wide variety of paleoenvironments, within a large range of paleodepths and associated with a wide variety of sediment types (e.g., Thiede and Dinkelman, 1977). Studies of Inoceramus in the western interior seaway of North America (Kauffman, 1967) indicate that the organisms were adapted to soft mud bottoms. Detailed mapping of the distribution of Inoceramus in DSDP cores will aid in understanding the paleoecology of this extinct organism. Rather than relying on descriptions of the occurrence of Inoceramus available in DSDP reports, which in some cases are rather general, all DSDP cores which penetrated Cretaceous sediments in the South Atlantic were examined specifically for Inoceramus. However, this does not insure a quantitative description of the organising occurrence because visual examination is limited to the two faces of the split core.