Abstract:
ODP Site 1179 is at 5586.5 m water depth in the mid-latitude western North Pacific Ocean. The site is >1 km below the CCD, so the mean CaCO (sub 3) concentration is <1%. Total palynomorph concentrations in Late Miocene to Recent sediments are very low, as is typical of the Pacific Ocean, averaging <400 palynomorphs per milliliter. The most common dinocysts are Impagidinium aculeatum and Operculodinium centrocarpum, each found in nearly 2/3 of the samples analyzed. Pinus pollen was found in most of the samples, and is especially common in Pleistocene sediments. Two short intervals contained relatively abundant, quite well preserved calcareous planktonic microfossils, so that CaCO (sub 3) comprised up to 7.7 and 6.9% of the sediments. Biostratigraphic and paleomagnetic data provide ages of approximately 2.5 Ma and approximately 900 ka for these intervals, respectively. Both protoperidinioid dinocysts and pollen grains are relatively abundant in these planktonic foram-rich sediments, resulting in total concentrations nearly an order of magnitude greater than the mean value for the Late Miocene to Recent. Most of the cysts (up to 91%) in these samples are the smooth round brown cysts produced by the dinoflagellate genus Protoperidinium, which are highly susceptible to oxidation. This suggests that a combination of increased sea surface productivity and rapid burial allowed calcareous microfossils to be preserved. Protoperidinioid cysts are rare to absent in non-calcareous sediments, although some of these samples contain relatively high pollen concentrations. Two other intervals, dated approximately 3.4 and 1.64 Ma, are associated with even higher CaCO (sub 3) concentrations-17.7 and 20.7%, respectively. In these sediments, the CaCO (sub 3) is present as calcite crystals. Total palynomorph concentrations are also high in these samples, more than double the mean value for the Late Miocene to Recent, but the bulk of this is comprised of gonyaulacoid dinocysts. In fact, a single species, Operculodinium centrocarpum, makes up approximately 76% of the total palynological assemblage and 86% of the dinocyst assemblage in the middle Pliocene sample, suggesting a bloom of the dinoflagellate Protoceratium reticulatum. The presence of CaCO (sub 3) thus appears to have resulted from high productivity at the sea surface, but low rates of sediment accumulation resulted in oxidation of the palynological assemblage and dissolution of the calcareous microfossils, allowing the CaCO (sub 3) to recrystallize as calcite crystals. The significant global cooling and expansion of Northern Hemisphere ice recorded for the times represented by CaCO (sub 3) peaks suggests a link with the global climate system. Enhanced productivity in the Pacific Ocean may have driven late Cenozoic global cooling by sequestering high concentrations of CO (sub 2) in the equatorial to mid latitude Pacific basin.