Development of paleoenvironmental proxies and their application to the Holocene climate record of the Cariaco Basin, Venezuela.

摘要

The Cariaco Basin is located on the northern continental shelf of Venezuela in a region characterized by intense upwelling due to the seasonal migration of the ITCZ. Hydrographic observations indicate that coastal upwelling occurs during the late winter-early spring at which time sea surface temperatures are lowest (∼22°C) and primary productivity and planktonic foraminiferal fluxes are high (4000–8000 shells m−2 day −1). A three-year sediment trap experiment was conducted in the Cariaco Basin (January 1997–December 1999) to study variations in flux, assemblage composition and shell chemistry for comparison with the climatology and hydrography of the region. Nine species of planktonic foraminifera constitute >85% of the assemblage: Orbulina universa, Globigerinoides ruber (pink and white varieties), Globigerina bulloides, Globigerina quinqueloba, Neogloboquadrina dutertrei, Globorotalia crassaformis, Globorotalia menardii , and Globigerinita glutinata. Globigerina bulloides is dominant during upwelling, however, the peak annual flux of all species occurs during this period. Temperature estimates from paleotemperature equations indicate that the δ18O record of G. ruber (pink) is most suitable for estimating mean annual SSTs while G. bulloides provides information regarding conditions during upwelling.; Oxygen isotope records of G. ruber (pink), G. bulloides and N. dutertrei from gravity core CAR7-1 were used to construct a high-resolution climate record for the Caribbean region for the last 6,000 years. Our results indicate major decreases in sea surface temperature and/or increases in salinity in the basin at least four times in the last 4,000 years. These events are coincident with high stands of Lake Titicaca, Bolivia/Peru suggesting a similar forcing mechanism. The onset of arid conditions in the circum-Caribbean region (∼3,600–3,200 cal yrs. B.P.) corresponds to an increase in the δ18O of G. ruber (pink), decreases in Δδ18 O, and the migration of the Globorotalia crassaformis into the basin. Taken together these data indicate colder SSTs and a shallow thermocline, possibly due to increased upwelling. Dry conditions in the circum-Caribbean tropics and wetter conditions in the Altiplano and the Amazon basin during the mid to late Holocene are consistent with a southward displacement of the ITCZ which would have resulted in decreased precipitation and increased trade wind intensity in the Cariaco Basin.