400,000-HISTORY OF TERRIGENOUS ACCUMULATION IN THE CARIBBEAN SEA INFERRED FROM ELEMENTAL METAL ABUNDANCES OF ODP 999 SEDIMENTS

Paleoclimate records for tropical South America and the Caribbean region indicate that these regions have experienced changes in water balance likely associated with shifts in the latitudinal position of the Inter-Tropical Convergence Zone (ITCZ). The close correspondence between changes in local insolation and the inferred changes in water balance suggests a direct linkage between local insolation and shifts of the ITCZ. This direct linkage, however, has mainly been found in paleoclimate records extending back only to the last glacial interval. In contrast, the few tropical paleoclimate records that cover several interglacial/glacial intervals suggest that extra-tropical forcing plays a significant role in modulating tropical water balance. Because the seasonal passage of the ITCZ controls the detrital load in the Magdalena River and its accumulation in the Caribbean Sea, we present here geochemical and mineralogical results for the terrigenous fraction of ODP 999 sediments from the Colombia Basin, Caribbean Sea. The studied interval encompasses the last 400 kyr, thereby providing information of detrital accumulation during three complete glacial/interglacial intervals.

Illite and chlorite are the main constituents of the clay fraction of the studied sediments, and their relative abundances are similar to those found in the fine sediment load of the Magdalena River. These data verify that this river is the source of the terrigenous fraction of ODP 999. Time-series of sedimentary Al/Ti, K/Ti, and Fe/Ti ratios show a strong 100-kyr cyclicity, suggesting either (1) rainfall-induced changes in the type of riverine siliciclastic material, or (2) cyclic episodes of shelf erosion induced by changes in sea level. The former hypothesis would point to an extra-tropical forcing, while the later one would point to sedimentary processes largely decoupled from climate. Although mass-balance calculations support the former hypothesis, sea-level changes cannot completely be ruled out as a possible explanation.