Mixed layer characteristics in the equatorial Atlantic during the late Quaternary as deduced from planktonic foraminifera.

The oxygen isotope composition and the relative abundances of planktonic foraminifera are used to reconstruct mixed layer characteristics in the tropical Atlantic in late Quaternary deep sea sediments.Methods for the reconstruction of paleosalinities in the tropical Atlantic are investigated and evaluated in terms of their respective uncertainties. The methods are used in conjunction with surface sediments and the results are compared to present day salinities. Furthermore, they are applied to core GeoB 1523-1 in the western equatorial Atlantic.The quality of the salinity estimates derived from the oxygen isotope signature of planktonic foraminifera is constrained by the accuracy of the independent temperature estimate. Errors in the reconstructed salinities range between 0.5 and 1.2 depending on the slope of the regional oxygen-isotope salinity relationship. The findings suggest that sea surface salinity estimates from oxygen isotopes are still rather qualitative for tropical oceans.Additionally, two methods are investigated both of which employ foraminiferal abundance data. An Artifical Neural Network is used for the first time in order to reconstruct paleosalinities. Results are compared with those of a Modern Analog Technique. Both methods give reasonable salinity estimates for surface sediments, but application to downcore sediments yields unrealistic results and shows that the reconstructions are controlled by today's salinity-temperature relationship.