NOAA/WDS Paleoclimatology - Central Equatorial Pacific Dust Flux, Productivity and d15N Data for 0-27ka

The “major” nutrients for phytoplankton growth (nitrogen, phosphorus, and silicon) are supplied to the surface waters of the Equatorial Pacific (EP) by wind-driven upwelling along the equator. Their consumption by phytoplankton is thought to be limited in part by the low concentrations of the critical micro-nutrient iron. Greater atmospheric dust deposition may have fertilized the EP with iron during the Last Glacial Period (LGP, 17-27,000 years ago). Here we present meridional transects of dust (232Th), phytoplankton productivity (opal, 231Pa/230Th, excess Ba), and the degree of nitrate consumption (foraminifera-bound d15N) from the Central EP for Holocene and LGP time slices. Despite two- to three-fold greater dust deposition in the Central EP, productivity was the same or lower during the LGP compared to the Holocene, and the degree of nitrate consumption was unchanged. These biogeochemical findings suggest that the relatively greater ice age dust fluxes were inadequate to drive significant iron fertilization in the Central EP, which may be because the absolute rate of dust deposition in the LGP, despite being greater than the Holocene rate, was still very low. The lower productivity coupled with unchanged nitrate consumption suggests lower subsurface major nutrient concentrations in the Central EP during the LGP. As these nutrients are today dominantly sourced from the Subantarctic Zone of the Southern Ocean, the Central EP data are consistent with more complete nutrient consumption in the Subantarctic, apparently because of iron fertilization in that region, where absolute dust fluxes were much higher. Thus, ice age iron fertilization ultimately worked to lower, not raise, Equatorial Pacific productivity.