Paleoclimatic and Paleomagnetic Variability over the Past 40,000 Years as Recorded by Sediments at ODP Site 1002 in the Cariaco Basin

The Cariaco Basin, an anoxic basin off the coast of Venezuela, acts as a depocenter for sediments of marine and terrestrial origin. Sedimentation rates for the upper 170 m of sediment recovered during ODP Leg 165 at Site 1002 average 35 cm/k.y. and are fairly constant over the entire 600-k.y.-long interval cored. The high sedimentation rates and anoxic conditions have resulted in a very favorable setting for studying short-term changes in the paleoenvironment as recorded by geochemical and rock-magnetic variations. Paleomagnetic field variability is also recorded, although the sediments are less than ideal owing to their relatively weak magnetizations. Prior geochemical studies of Cariaco Basin sediments have shown that variations in Ti and Fe content correlate with changes in precipitation. These results are being augmented by XRF determinations of a wider range of elements, obtained using the Advanced Light Source at the Lawrence Berkeley National Laboratory. In the laminated intervals, the light and dark lamina that comprise a varve correspond with the drier winter-spring season and wetter summer-fall season, respectively. These changes are interpreted mainly as resulting from changes in the position of the Intertropical Convergence Zone (ITCZ), which currently passes over or near the Cariaco Basin during the summer-fall seasons and migrates well south of the basin in the winter-spring seasons. Several of the rock magnetic parameters that are sensitive to magnetic (Fe) concentration have much larger variations than do the geochemical data, with increases by more than an order of magnitude in several discrete intervals indicating large changes in the amount of Fe entering the basin. Although these changes are ideal for monitoring how the marine and terrigenous sources have abruptly changed over time, they are not favorable for constructing paleosecular variation or relative paleointensity records. Even so, the mean paleomagnetic inclination corresponds to the expected value, but the variation about the mean is much larger than expected, which we interpret as noise related to the variable and weak magnetizations.