OXYGEN-ISOTOPE COMPOSITIONS OF SINISTRAL NEOGLOBOQUADRINA-PACHYDERMA TESTS IN SURFACE SEDIMENTS - NORTH-ATLANTIC OCEAN

Neogloboquadrina pachyderma (Left-coiled) is a planktonic foraminifer indicator species for polar waters. It is present in both the glacial and interglacial sediments of the high-latitude oceans, and therefore is frequently used as a signal-carrier for constructing late Pleistocene-Holocene planktonic deltaO-18 records. In this study the deltaO-18 compositions of N. pachyderma (L) tests from deep-sea surface sediments are compared with surface water deltaO-18 values and summer sea surface temperatures (SST) in the North Atlantic Ocean, the Norwegian-Greenland Sea, and the labrador Sea. We document that for a temperature range of 2-8-degrees-C, the planktonic deltaO-18 values follow paleotemperature equations with an offset of less than 0.4 parts per thousand, and that above 8-degrees-C the planktonic deltaO-18 compositions become almost independent of SST and remain nearly constant. This pattern suggests that N. pachyderma (L) precipitates its shell nearly at isotopic equilibrium with surface waters colder than 8-degrees-C. and that in warmer surface waters N. pachyderma (L) changes its habitat depth or its growth season to secrete its shell at an ambient temperature around 7-9-degrees-C.