Last glacial paleoproductivity patterns in the eastern equatorial Pacific: benthic foraminifera records

Abstract

This study explores the export-productivity changes for the eastern equatorial region between 100 and 115°W during the Last Glacial with the aid of benthic foraminifera accumulation rates (BFAR) from the well-dated Pleiades (PLDS) core collection (Berger and Killingley, 1981) and compares these results with western equatorial Pacific ones. We present some new data for the calibration between BFAR and the organic carbon flux to the sea-floor and discuss some of the limitations of using absolute values for this type of reconstruction. Contrary to expectation, not all export productivity to the deep ocean under the equatorial Pacific responds with the same magnitude to changes in ocean circulation during the Last Glacial period. Two export-productivity patterns emerge from this reconstruction based on benthic foraminifera accumulation. Sites slightly North of the eastern equatorial Pacific (≈1°N) show a minor increase in the organic carbon flux to the sea-floor, up to a factor of 1.2, while the reconstructed BFAR values are very similar to western equatorial Pacific ones. One of the implications of this finding is a decreased east–west equatorial export-productivity gradient relative to the modern one. In contrast, sites south of the equator (≈3°S) show more than a double-fold increase in the accumulation rates of benthic foraminifera, equivalent to an increase by a factor of 1.6–1.8 in the organic carbon flux to the sea-floor. Although there is not enough data to decide unambiguously among the different mechanisms that may have forced this pattern of export productivity, we propose that while upwelling at the equator may have been stronger during the Last Glacial, its east–west gradient was significantly decreased with respect to the modern one. Either a southern migration of the equatorial divergence zone or a relative decrease in the nutrient concentration of the upwelled waters 1°N of the equator may explain the observed lower export-productivity gradient. To the south, stronger advection of coastal upwelled nutrient-rich waters of southern origin may have enhanced the production and export of organic carbon to the deep sea, which fueled the observed higher abundance in benthic foraminifera.