Late Quaternary δ13C gradients and carbonate accumulation in the western equatorial Atlantic

We investigated glacial–interglacial changes in vertical δ13C gradients in the western equatorial Atlantic using the carbon isotopic composition of planktonic and benthonic foraminifera. Core top measurements show that the δ13C difference between shallow-dwelling G. sacculifer and thermocline-dwelling G. truncatulinoides is an indicator of the vertical nutrient gradient in the upper water column. In the western equatorial Atlantic, the δ13C differences between G. sacculifer and G. truncatulinoides are reduced during glacials and cold substages of interglacials, while the δ13C differences between G. truncatulinoides and the benthonic species C. wuellerstorfi are increased. This indicates that nutrients were depleted in the thermocline and enriched in deep waters during cold substages. Covariance between the δ13C records of G. truncatulinoides from the western equatorial Atlantic and C. wuellerstorfi from Caribbean intermediate water suggests that the upper part of the western equatorial Atlantic water column was largely composed of nutrient-poor central and intermediate waters of northern origin. This pattern might have been the result of a circulation mode in which subantarctic surface waters formed nutrient-rich deep waters rather than intermediate waters. Lower 230Th-normalized carbonate accumulation rates during cold substages imply that the decreased nutrient content of subsurface waters induced a lower primary productivity in the western equatorial Atlantic.