The late Pleistocene history of seawater temperature and salinity variability in the western tropical Pacific warm pool is reconstructed from oxygen isotope (delta18O) and magnesium/calcium composition of planktonic foraminifera. Differentiating the calcite delta18O record into components of temperature and local water delta18O reveals a dominant salinity signal that varied in accord with Dansgaard/Oeschger cycles over Greenland. Salinities were higher at times of high-latitude cooling and were lower during interstadials. The pattern and magnitude of the salinity variations imply shifts in the tropical Pacific ocean/atmosphere system analogous to modern El Niño-Southern Oscillation (ENSO). El Niño conditions correlate with stadials at high latitudes, whereas La Niña conditions correlate with interstadials. Millennial-scale shifts in atmospheric convection away from the western tropical Pacific may explain many paleo-observations, including lower atmospheric CO2, N2O, and CH4 during stadials and patterns of extratropical ocean variability that have tropical source functions that are negatively correlated with El Niño.

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N of oceanic nitrate is controlled primarily by the fraction of total denitrification that occurs in the water column. Therefore a deglacial peak in the ratio of water column-to-sediment denitrification, caused by either a strong feedback between water column denitrification and the N reservoir or by an increase in sediment denitrification due to sea level rise, can explain the observed deglacial 14 N maximum further requires that the combined negative feedbacks from N 2 fixation and denitrification are relatively strong, and N losses are relatively small. Our results suggest that the glacial oceanic N inventory was at most 30% greater than today's and probably less than 10% greater.

During the CARIACO time series program, microbial standing stocks, bacterial production, and acetate turnover were consistently elevated in the redox transition zone (RTZ) of the Cariaco Basin, the depth interval (ϳ240-450 m) of steepest gradient in oxidation-reduction potential. Anomalously high fluxes of particulate carbon were captured in sediment traps below this zone (455 m) in 16 of 71 observations. Here we present new evidence that bacterial chemoautotrophy, fueled by reduced sulfur species, supports an active secondary microbial food web in the RTZ and is potentially a large midwater source of labile, chemically unique, sedimenting biogenic debris to the basin's interior. Dissolved inorganic carbon assimilation (27-159 mmol C m Ϫ2 d Ϫ1 ) in this zone was equivalent to 10%-333% of contemporaneous primary production, depending on the season. However, vertical diffusion rates to the RTZ of electron donors and electron acceptors were inadequate to support this production. Therefore, significant lateral intrusions of oxic waters, mixing processes, or intensive cycling of C, S, N, Mn, and Fe across the RTZ are necessary to balance electron equivalents. Chemoautotrophic production appears to be decoupled temporally from short-term surface processes, such as seasonal upwelling and blooms, and potentially is more responsive to longterm changes in surface productivity and deep-water ventilation on interannual to decadal timescales. Findings suggest that midwater production of organic carbon may contribute a unique signature to the basin's sediment record, thereby altering its paleoclimatological interpretation.