CLIMATICALLY CONTROLLED VARIATIONS IN REDOX CHEMISTRY OF THE CARIACO BASIN, VENEZUELA: COMPARING RESULTS FROM THE OCEAN DRILLING PROGRAM AND SEDIMENT TRAPS

The sedimentary sequence in the Cariaco Basin is a premier high-resolution paleoceanographic marine record. In addition to its tropical location and high sedimentation rate of ~30 cm/kyr, the basin currently is anoxic. Shallow banks surrounding the basin affect circulation and become exposed during sea level lows, leaving only two narrow sills sufficiently deep to remain submerged. During glacially low sea levels and restricted circulation, sea water delivered from the open Caribbean is primarily from the mixed layer, which is rich in dissolved oxygen but depleted in nutrients. This results in a decrease in the particulate organic flux and in an oxygenated water column. During interglacials, higher sea levels deliver nutrient-rich waters that are seasonally upwelled and lead to high productivity. The resulting increased oxygen demand exceeds oxygen replenishment in the stratified basin and leads to anoxia. Changes in the relative input of local fluvial sources also contribute to climatic variations in biogenic and terrigenous sedimentation.

Sedimentary distributions of the redox-sensitive metals Mo, V, Mn, and Fe show that varying bottom water oxygenation also responds to climatically-driven sea level change. During 100 kyr and 41 kyr cycles anoxic enrichments of Mo and V occur during highly productive interglacials. During oxic glacials, sediments are less depleted or enriched in Mn and Fe. During 23 kyr and 19 kyr cycles these redox patterns are not observed, indicating that the basin responds differently to the higher frequency climatic variations. These redox changes occur in concert with low values of Al/Ti during glacials, reflecting an increase in eolian, Saharan-derived rutile, and of K/Al, reflecting the importance of local tropical fluvial sources relatively rich in kaolinite.

The paleoceanographic results are compared with ongoing studies of a pair of sediment traps located just above and below the redoxcline. Relative ratios of Al and Ti are similar in both traps, confirming their refractory nature. The redox behavior of Mn and other redox sensitive metals is clearly evident, and helps constrain end-member values for delivery of metals to the seafloor. Water column uptake and release of the REEs is also apparent.