The influence of the inter-tropical convergence zone on the Orinoco River and Late Quaternary tropical planktonic foraminiferal assemblages

Abstract

Although deep-water oil and gas exploration offshore eastern Trinidad is becoming prevalent, the use of ecostratigraphy around eastern Trinidad are limited mostly to neritic water depths. Changes in planktonic foraminiferal assemblages of Core BGT086, offshore eastern Trinidad, are used here to indicate biozonal events useful for deep-water (>1000 m water depth) exploration. Decreased proportional abundances of Globigerinella obesa and Globigerina bulloides mark the southward migration of the inter-tropical convergence zone (ITCZ) during the Younger Dryas (YD). A decrease in species richness and Globigerinoides ruberwhite proportional abundance is consistent with rising temperature during the YD. During Biozone Z2, nutrient-preferring species (G. bulloides, Neogloboquadrina dutertrei) declined in proportional abundance, while G. ruberpink increased in proportional abundance, reflecting the arrival of the ITCZ in the area. Decreases in mean diversity (measured using the Shannon Function, H) and evenness (equitability index, E), and an increase in mean dominance (max pi) support the claim that the ITCZ arrived in the area after the YD. A sudden increase in Globorotalia menardii proportional abundance marked the start of Biozone Z1, at which time there was a change from a stressed community to a less stressed one. Boundaries of SHE abundance biozones coincided with noticeably enhanced assemblage changes, being consistent with the southward migration of the ITCZ and the onset of Biozone Z1. The mean value of the pairwise assemblage turnover index (ATIs) is here used as a proxy for what we term 'assemblage stability'. Comparisons of mean planktonic foraminiferal ATIs from Core BGT086 and six other nearby tropical piston cores (Cores 1, 2 and 3 off NW Tobago, and En20-2, En20-10 and En20-16 from the Leeward Islands, Lesser Antilles) showed no statistical difference in assemblage stabilities within similar planktonic foraminiferal biozones. Kruskal-Wallis tests determined a significant difference in median ATIs values when all seven cores' ATIs were used, but the difference was not significant when data above the YD boundary (i.e., Biozone Z) were used. The lowered nutrient influx from the Orinoco River due to the arrival of the ITCZ in the area after the YD would have triggered a stable environment for planktonic foraminiferal assemblages. This implies that caution must be used to ensure data from the same biozone is used when analyzing assemblage stabilities of planktonic foraminifera.