SEASONAL AND INTERANNUAL VARIATIONS IN PLANKTONIC FORAMINIFERAL FLUX AND ASSEMBLAGE COMPOSITION IN THE CARIACO BASIN, VENEZUELA

Abstract

A three-year sediment trap experiment to study the production and flux of planktonic foraminifera was conducted in the Cariaco Basin, Venezuela, between January, 1997, and December, 1999. The results allow direct comparison between planktonic foraminiferal flux and assemblage composition, and the climatology and hydrography of the region on seasonal and interannual time scales. 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 Inter-Tropical Convergence Zone (ITCZ) and associated variations in tradewind intensity. Hydrographic observations indicate that coastal upwelling occurs during the late winter-spring at which time sea surface temperatures are lowest (~22°C) and primary productivity is high. Immediately following maximum primary production the total flux of planktonic foraminifera increases from less than 100 shells m −2 day −1 to 4000–8000 shells m −2 day −1 . Approximately half of the total annual flux is produced during this period (February–May). Nine species, or varieties, of planktonic foraminifera constitute >85% of the assemblages: Orbulina universa, Globigerinoides ruber (pink and white varieties), Globigerina bulloides, Globigerina quinqueloba, Neogloboquadrina dutertrei, Globorotalia crassaformis, Globorotalia menardii, and Globigerinita glutinata. These species are present year-round; however, their flux and contribution to the population vary seasonally and interannually. The peak annual flux of all nine species occurs during spring upwelling. However, Globigerina bulloides is dominant during this period, with values reaching ~4000 shells m − 2 d − 1 , or 50 to 75% of the total flux during upwelling. Sea surface temperatures were estimated for both sediment trap and core-top assemblages using the Modern Analog Technique (MAT) and compared with conductivity-temperature-depth (CTD) data to evaluate the accuracy of this method for reconstructing paleotemperatures in Cariaco Basin. The assemblage temperature estimates are within ~1–3°C of the measured temperatures.