SEAFLOOR ENVIRONMENTAL CHANGES OFF THE COSTA RICAN MARGIN BETWEEN 90,000 AND 290,000 YEARS AGO INFERRED FROM BENTHIC FORAMINIFERAL ECOLOGY

The Pacific margin of Costa Rica has a dynamic geologic history likely to have caused significant changes in seafloor conditions. Examination of the >150µm fraction of Integrated Ocean Drilling Program Leg 344 core samples representing about 90,000 to 290,000 ybp of Costa Rican marine history revealed appreciable changes in total abundances and species percentages of benthic foraminifera. Foraminiferal abundances increased from 290,000 to 210,000 ybp with a pronounced increase at 205,300 ybp where the abundances were over 450 individuals per gram of sediment. By 91,800 ybp abundances returned to values similar to those prior to 205,000 ybp (from 200-250 individuals per gram). Foraminiferal abundances have been associated with changes in food (phytodetrital) availability. The primary source of phytodetritus to the deep sea is primarily from surface water productivity. Dominant infaunal taxa (living within sediments) of the assemblages examined include Uvigerina peregrina, Bolivina spp., Epistominella sp., and Angulogerina sp. Cibicidoides wuellerstorfi was among the dominant epifaunal species (living on or above surface sediments). Changes in species dominance and other foraminiferal assemblage characteristics may reflect changes in paleoenvironmental conditions. At 290,000 and 205,000 ybp, as much as 81.6% of Bolivina spp. suggests conditions favored this taxon. Modern analog relationships suggest that many of the dominant taxa observed in this study are tolerant of oxygen-poor conditions. However, most Bolivina species are known to be capable of living in habitats with very low oxygen availability compared to other species. Preliminary results from 6 samples, suggest an increase in food availability from 290,000 to 210,000 ybp, and a reduction in oxygen availability at 290,000 and 205,000 ybp. Benthic foraminiferal assemblages at this location reflect changes in surface water productivity and oxygen availability that may have been caused by changes in seafloor tectonics. Using information from fossil foraminifera and modern analogs the history of paleoenvironmental conditions of this dynamic environment can begin to be reconstructed.