Paloceanographic Changes in the Eastern Equatorial Pacific During the Late Pleistocene: Preliminary Results of Core MD02-2529 From the IMAGES VIII MONA Cruise

We applied principal component analysis and the modern analog method to study the relationship between species in planktic foraminiferal assemblages in the Late Pleistocene (upper 4 m) of core MD02-2529. Four main factors responsible for 54% of the total variability were extracted by principle component analysis. The first factor (dominated by G. bulloides and G. glutinata) mainly records mixed layer conditions whereas the second factor (dominated by N. dutertrei) is associated with thermocline variability. The interpretation of the other factors is as yet obscure. Visual correlation of each factor with the relative abundance of whole planktic foraminiferal tests clearly demonstrates that the distribution of factor loadings throughout the upper 4 m of the core is not significantly affected by dissolution. This is confirmed by comparison with sedimentary carbonate content inferred from diffuse spectral reflectance. Although the foraminiferal factors are uncorrelated with the sedimentary carbonate content, whole planktic foraminiferal tests are positively correlated with carbonate content, while the >150 um carbonate fraction is inversely related to carbonate content. Factor 1 decreases in importance from 4 m to the top of the core. In contrast, Factor 2 increases in important around 3.6 m at a transition associated with a sharp decrease in sediment carbonate content. Using the Brown University database of coretop foraminiferal assemblages and modern seasonal and mean annual sea-surface temperatures, we applied the modern analog method to reconstruct paleotemperature variations in core MD02-2529. Values of the sample dissimilarity index generally range between 0.15-0.25 throughout the record, indicating the presence of reasonable analogs. The results demonstrate rather small but consistent temporal variations of seasonal and mean annual temperature. Whereas SST variations are generally within 2° C (between 26 and 28° C) in the upper part of the record, lower winter and summer values are found during and below the transition at 3.6 m. These lower paleotemperatures are associated with higher values of Factor 1 and appear to indicate a cooling of the upper photic zone and enhanced productivity. The transition near 3.6 m depth in core is associated with an increase in Factor 2 and probably indicates a shoaling of the thermocline across a stadial to interstadial transition. The suggestion that the thermocline likely shoals at stadial/interstadial boundaries is consistent with observed variations in the S(H) diversity index. Further work on the core will provide improved age control and test whether the observed faunal patterns are persistent features of climate transitions in this part of the Eastern Equatorial Pacific.