Stable carbon and oxygen isotope ratios of planktonic foraminifera in upper Miocene to Holocene sediments of ODP Leg 105 sites

Oxygen and carbon isotopic compositions of planktonic foraminifers Neogloboquadrina pachyderma and N. atlantica were determined at approximately 150-cm intervals through late Pliocene to Holocene sediments in Hole 647A and late Miocene to Holocene sediments in Hole 646B. The isotopic record shows an overall enrichment of about 4.0 per mil in d18O and 1.0 per mil in d13C from the base to top of Hole 646B. In general, this enrichment probably represents the progressive climatic cooling that started in the middle and late Miocene, continued throughout the Pliocene, and led to the Pleistocene glaciations. The d18O values in sediments from approximately 2.66 to 2.42 Ma show a considerable enrichment (~2.5 per mil) and little change in the d13C values, which probably reflects significant long-term ice storage in the Northern Hemisphere. We interpreted enrichments of ~1.5 per mil in d18O and ~1.0 per mil in d13C at -3.2 Ma, associated with minor ice-rafted material, as substantial cooling of surface waters in the Labrador Sea. Near the upper part of magnetochron 10 in Hole 646B, we observed an approximately 2 to 3 per mil enrichment in the d18O, which is associated with a much smaller enrichment of ~1.0 per mil in the d13C. This correlates with the right-to-left coiling change in N. atlantica. We do not completely understand the cause of the large shift in isotopic record