Remote vs. local control of changes in eastern equatorial Pacific bioproductivity from the Last Glacial Maximum to the Present

Data from 14 cores, and a transfer function based on benthic foraminifera, were used to map productivity gradients in the eastern equatorial Pacific (EEP) at the Last Glacial Maximum (LGM). These were compared to gradients in the modern ocean. The results support previous work indicating that, during the LGM, productivity was lower across the region in the South Equatorial Current (SEC) under the influence of Peru margin upwelling. Overall, productivity gradients are diminished during the LGM due to three changes: reduced productivity along the Peru margin and the SEC near the equator, increased productivity further south of the equator, and increased productivity in the Panama Basin area. These changes smooth gradients seen in the region today. This reduction of gradients in EEP productivity parallels observations for planktonic foraminiferal carbon isotopes and inferred nutrient concentrations in the thermocline. Reductions in productivity and thermocline nutrients in the SEC downstream of the Peru margin have been previously interpreted as the result of changes in the chemistry and/or upwelling of subantarctic Equatorial Undercurrent Current (EUC) water. Increases in productivity away from the equator may be associated to nitrogen enrichment of the global surface ocean during the LGM. Productivity and planktonic foraminiferal isotope records for the Panama Basin indicate a local process driving highly seasonal productivity for the glacial, perhaps related to changing upper water column stratification. The degree to which overall carbon export to the deep sea changed in the EEP during the LGM depends on the extent of the productivity increase away from the equator. If this increase occurred broadly in the lower latitude subtropical gyres then a marked LGM export increase would be possible despite productivity reductions along the equator.