Recent planktonic foraminiferal distribution patterns and their relation to hydrographic conditions of the Gulf of Tehuantepec, Mexican Pacific

Abstract

The distribution and abundance patterns of planktonic foraminifera in sediments and their relation to ocean dynamics were studied in the Gulf of Tehuantepec. Hydrographic and nutrient data outline the contrast between two extreme seasons in the gulf. The winter–early spring conditions, dominated by upwelling and a cyclonic eddy that fertilize the surface waters; and summer conditions with the typical stratified water column of the tropical Pacific. Foraminiferal abundances (number of individuals/g of sediment) are related to the ocean dynamics and productivity. Highest abundances are found in the areas influenced by upwelling and associated front and cold eddy and lower abundances are beneath the areas of influence of the warm-core, lower-productivity eddies. The effects of dissolution seem to be important at depths over 1000 m, but do not appear to affect critically the foraminiferal remains throughout most of the studied area, except near the phosphorite deposits of the Oaxaqueño bank. Dilution by terrigenous materials from riverine inputs, seafloor topography and strong transport under the upwelling jet are likely to modify the detailed pattern of foraminiferal abundance found, without blurring the major imprint of productivity driven by the upwelling. Three planktonic foraminiferal assemblages were found, reflecting the major oceanographic conditions in the gulf. The Globigerina bulloides assemblage dominates most of the gulf, indicating the overall high productivity of the upwelling-influenced surface waters. The dynamics of the winter–early spring upwelling and associated eddies leave a strong imprint in the planktonic foraminiferal assemblages in the sediments. The distribution of the Globigerinita glutinata assemblage is tied to the axis of upwelling, and the Globorotalia menardii–Neogloboquadrina dutertrei assemblage to the warm equatorial water cores. The distribution patterns of these assemblages should help identify Quaternary paleoenvironments influenced by upwelling and associated changes in productivity.