Abstract:
The tropics play a critical role in controlling global climate as large amounts of energy and moisture are exported from low latitudes to temperate regions. Consequently it is important to understand tropical climate variations to assess possible global teleconnections. Here we investigate the possibility of using a marine core from the Caribbean Sea to assess changes in the hydrological balance of the region. For this purpose, we study the upper 20 m of ODP Site 999A from the Colombia Basin. Specimens of planktonic foraminifera (G. sacculifer and Neogloboquadrina dutertrei) were manually picked out of sediment samples, and their oxygen-isotopic composition was determined. The studied foraminiferal tests exhibit oxygen isotope values that fluctuate from +1.4 per mil to -0.8 per mil. Based on a comparison of the obtained isotopic values with the standard oxygen-isotope stages for the Quaternary, we conclude that the studied interval encompasses the last approximately 400,000 years. Additional chronostratigraphic control was provided by the relative abundance of Globorotalia menardii, G. inflata, and G. truncatulinoides. In parallel to the oxygen isotope analyses, magnesium/calcium (Mg/Ca) ratios were determined on foraminiferal tests to determine past changes in temperature and salinity of surface waters in the Caribbean Sea. Mg/Ca ratios range from 8.0 mmol/mol to 2.9 mmol/mol. Using published temperature calibrations, the obtained Mg/Ca ratios indicate that sea-surface temperatures have varied by up to 5˚ degrees C during the last 400,000 years. Because the oxygen-isotopic composition of foraminifera depends on that of seawater and temperature, the obtained Mg/Ca ratios can be used to assess the temperature-dependant isotope effect. Consequently the down-core residuals of oxygen isotope ratios of foraminifera reflect the variability in the oxygen isotopic composition of seawater through time. Residuals of isotope values for the studied core indicate significant changes in the isotopic composition of seawater that we correlate with the intensity of evaporation at the site. Since evaporation in the Caribbean Sea is closely coupled with the passage of the Inter-tropical Convergence Zone (ITCZ), we conclude that fluctuations in the intensity or zonality of the ITCZ exert a significant control on the amount of energy and moisture exported to mid-latitudes.