The response of benthic foraminifera to productivity cycles in the eastern equatorial Pacific: Faunal and geochemical constraints on glacial bottom water oxygen levels

Abstract

The Stage 2 glacial maximum in eastern equatorial Pacific Ocean sediments is characterized by a pronounced increase in the organic carbon content which has been attributed either to increased upwelling and higher production along the equatorial divergence during the last glacial period or to enhanced preservation as a consequence of a lower concentration of dissolved O2 in glacial bottom water. Several lines of evidence are presented, based on data from two gravity cores collected from the central Panama Basin, which support the former interpretation. First, both the abundance and the size of benthic foraminifera of the genus Uvigerina increase in Stage 2, coincident with the increase in organic carbon concentration, most probably because of the greater availability of organic detritus during the glacial period. The increase in foraminiferal size and associated greater requirement of oxygen for respiration argues against the probability that oxygen was substantially depleted in the upper centimeter where Uvigerina spp. are thought to live. Second, the number of meiofaunal faecal pellets increases markedly during the Stage 2 maximum, indicating a more populous and active infauna, which again argues against oxygen depletion. Third, there is no significant decrease in the iodine:organic carbon (I:Corg) ratio in the Stage 2 sediments. Because iodine is depleted relative to carbon in anoxic basins but enriched in association with organic matter in oxic environments, the absence of a depletion in the I:Corg ratio in Stage 2 is evidence that bottom water remained oxygen‐ replete during the last glacial. Fourth, molybdenum is not enriched in sediments deposited in Stage 2; the presence of anoxic or dysaerobic bottom water or shallow pore waters during that time would have been recorded by the addition of Mo to the sediments via coprecipitation with authigenic iron sulphides. The combined faunal and geochemical data are consistent with the notion that Panama Basin deep waters remained oxygen‐replete during the last glacial maximum. It is concluded that the late Stage 2 carbon enrichment in the eastern equatorial Pacific more probably reflects higher productivity rather than enhanced preservation.