Glacial–interglacial surface water variations in the Caribbean Sea during the last 300 ky based on calcareous nannofossil analysis

Abstract

Quantitative and qualitative analyses of nannofossil assemblages from ODP Site 999 in the Caribbean Sea were performed to reveal changes in upper seawater structures and implications for climatic changes during the last 300,000 years. Overall, there is an inverse relationship in stratigraphic fluctuations between the absolute abundance (AA) of a lower photic taxon, Florisphaera profunda, and δ 18O; this fluctuation pattern is considered to represent climatic changes in the Subtropical Under Water (SUW) in the Caribbean. Cross-spectral analysis demonstrates that the SUW was characterized by nutrient-rich water during interglacials; changes in orbitally driven gyre circulation system strongly affected fluctuations in the SUW in the Caribbean. Conversely, the surface water was enriched in nutrients during glacials as the result of increased terrigenous materials and shallowing of the thermocline based on changes of the N ratio (abundance of reticulofenestrids relative to F. profunda) and results of cross-spectral analysis between them and δ 18O. The relationship between the thermocline variation and the ice volume fluctuation suggests that changes in the subtropical gyre circulation system in the North Atlantic have had stronger influences on the Caribbean thermocline variations and there may be a little contribution of climatic fluctuations that originated in the Southern Ocean to the Caribbean Sea. A large floral turnover in nannofossil assemblages between oxygen isotope stages 6 and 5 was observed in Hole 999A, which corresponds to Termination II. On the basis of floral compositions, photosynthetic primary productions had been more active than today before Termination II.