LATE QUATERNARY DELTA C-13 GRADIENTS AND CARBONATE ACCUMULATION IN THE WESTERN EQUATORIAL ATLANTIC

We investigated glacial-interglacial changes in vertical delta(13)C gr adients in the western equatorial Atlantic using the carbon isotopic c omposition of planktonic and benthonic foraminifera. Core top measurem ents show that the delta(13)C difference between shallow-dwelling G. s acculifer and thermocline-dwelling G. truncatulinoides is an indicator of the vertical nutrient gradient in the upper water column. In the w estern equatorial Atlantic, the delta(13)C differences between G. sacc ulifer and G. truncatulinoides are reduced during glacials and cold su bstages of interglacials, while the delta(13)C differences between C. truncatulinoides and the benthonic species C. wuellerstorfi are increa sed. This indicates that nutrients were depleted in the thermocline an d enriched in deep waters during cold substages. Covariance between th e delta(13)C records of G. truncatulinoides from the western equatoria l Atlantic and C. wuellerstorfi from Caribbean intermediate water sugg ests that the upper part of the western equatorial Atlantic water colu mn was largely composed of nutrient-poor central and intermediate wate rs of northern origin. This pattern might have been the result of a ci rculation mode in which subantarctic surface waters formed nutrient-ri ch deep waters rather than intermediate waters. Lower Th-230-normalize d carbonate accumulation rates during cold substages imply that the de creased nutrient content of subsurface waters induced a lower primary productivity in the western equatorial Atlantic. (C) 1998 Elsevier Sci ence B.V.