The nature of varved sedimentation in the Cariaco Basin, Venezuela, and its palaeoclimatic significance

A laminated sediment record of the last 12 600 radiocarbon years from the anoxic Cariaco Basin provides a rare opportunity to study interannual to millenial-scale climatic change in a marine setting. Sedimentological and radiometric analyses of laminae couplets in the basin sediments indicate that they are annually deposited varves. The varves consist of light and dark laminae which are deposited as the result of the Intertropical Convergence Zone's annual north-south migration over the tropical North Atlantic, and the impact of this migration on regional upwelling and rainfall patterns. Light laminae contain planktonic fossils (predominantly diatoms) and are an indicator of productivity over the basin during the winter-spring upwelling season. Dark laminae contain terrigenous mineral grains and are an indicator of runoff from northern South America during the late summer-fall rainy season. Varve measurements developed using X-ray images and thin sections indicate that a continuous 14 000 calendar year long varve chronology may be generated. Preliminary measurements of thickness of individual light and dark laminae for the period 12.6 to 9 ka BP show similar palaeoclimate signals. Thickness of dark laminae, interpreted as measuring changes in regional runoff, increased rapidly at 10.8 ka BP and decreased rapidly at 9.8 ka BP, an interval coincident with the Younger Dryas cold period. Thickness of light (diatom-rich) laminae, indicative of productivity and possibly upwelling intensity, also show an increase at 10.8 ka BP, and a rapid decrease at 9.8 ka BP, simultaneous with the changes in the dark laminae. Current work is focusing on building long multiple-core varve chronologies and using these for palaeoclimatic reconstructions and geochronological research.