Modern climate forcing of terrigenous deposition in the tropics (Cariaco Basin, Venezuela)

One approach to deciphering tropical Quaternary paleoclimate records has been to study the composition of terrigenous material in order to infer past changes in hydrologic and atmospheric conditions. Here we present the inorganic geochemistry (major and trace elements) of modern sediment traps and shelf sediments in the tropical Cariaco Basin, Venezuela, in order to characterize seasonal variation in deposition of the terrigenous component and to document linkages within the ocean-atmosphere-climate system. We show that variation in the chemistry of terrigenous input to the Basin is a sensitive monitor of the annual meridional migration of the Intertropical Convergence Zone (ITCZ). Additionally, seasonal terrigenous relative abundances are decoupled from terrigenous absolute fluxes, showing that abundances are not directly related to terrigenous delivery. Changes in mixing between local fluvial sediments, fluvial and/or eolian mafic material, and Saharan dust explains the observed seasonality. The modern seasonal variations are significantly smaller than those observed in the glacial-interglacial paleorecord, indicating that climate sensitivity at the two time scales is very different. A striking contrast in the behavior of Ti/Al between the longer glacial-interglacial time scale and the modern seasonality also suggests that long-term terrigenous deposition is not linked simply to 100-kyr shifting of the ITCZ. The modern data can be reconciled with the glacial-interglacial record by invoking generally drier conditions during glacial periods over tropical latitudes, combined with intermittent wet conditions caused by precessional forcing.