Abrupt Late Pleistocene Changes in Northern South American River Run-Off

Abstract:

Paleoenvironmental studies as well as climate models demonstrate that fluvial run-off and moisture availability in the hinterland of the Caribbean react highly sensitively to climatic variations. Deglacial (Late Pleistocene) records document pronounced dry and wet spells over tropical South America which are mainly caused by shifts of the Intertropical Convergence Zone (ITCZ) coupled with South American monsoonal activity.

Here we present a high-resolution foraminiferal Ba/Ca and δ¹⁸O_seawater record from a core located within the Orinoco outflow area, that give insights into abrupt changes of the hydrology of the Orinoco catchment area and, furthermore, enables us to reconstruct circulation patterns within the Caribbean during deglacial times. Our data, obtained from the surface-dwelling foraminifera Globigerinoides ruber (pink variety), show a distinct increase in Ba/Ca ratios during the Heinrich 1 (H1) interval, as well as during the Dansgaard-Oeschger cycles up to 36 kyr. Based on the multi-proxy evidence we largely attribute the Ba/Ca increase during H1 to enhanced Amazon river run-off, while Orinoco river discharge appears not to be significantly elevated. During Dansgaard-Oeschger cycles, the causal mechanism for enhanced Ba/Ca ratios is an insolation-driven shift of the ITCZ and/or enhanced South American summer monsoon activity. Interestingly, the H1 Ba/Ca shows strong similarities in shape and timing to published Ba/Ca data from Florida Straits.This leads us to the assumption that the South American run-off signal is transported directly into the Atlantic Ocean via Yukatan Strait and Florida Strait and therefore alter the salinity budget in the North Atlantic. The results point to immediate high to low latitude feedbacks which might help to re-inforce the weakening of the overturning circulation during Heinrich Events and Dansgaard-Oeschger cycles.