title: No iron fertilization in the equatorial Pacific Ocean during the last ice age.
abstract: The equatorial Pacific Ocean is one of the major high-nutrient, low-chlorophyll regions in the global ocean. In such regions, the consumption of the available macro-nutrients such as nitrate and phosphate is thought to be limited in part by the low abundance of the critical micro-nutrient iron. Greater atmospheric dust deposition could have fertilized the equatorial Pacific with iron during the last ice age--the Last Glacial Period (LGP)--but the effect of increased ice-age dust fluxes on primary productivity in the equatorial Pacific remains uncertain. Here we present meridional transects of dust (derived from the (232)Th proxy), phytoplankton productivity (using opal, (231)Pa/(230)Th and excess Ba), and the degree of nitrate consumption (using foraminifera-bound δ(15)N) from six cores in the central equatorial Pacific for the Holocene (0-10,000 years ago) and the LGP (17,000-27,000 years ago). We find that, although dust deposition in the central equatorial Pacific was two to three times greater in the LGP than in the Holocene, productivity was the same or lower, and the degree of nitrate consumption was the same. These biogeochemical findings suggest that the relatively greater ice-age dust fluxes were not large enough to provide substantial iron fertilization to the central equatorial Pacific. This may have been because the absolute rate of dust deposition in the LGP (although greater than the Holocene rate) was very low. The lower productivity coupled with unchanged nitrate consumption suggests that the subsurface major nutrient concentrations were lower in the central equatorial Pacific during the LGP. As these nutrients are today dominantly sourced from the Subantarctic Zone of the Southern Ocean, we propose that the central equatorial Pacific data are consistent with more nutrient consumption in the Subantarctic Zone, possibly owing to iron fertilization as a result of higher absolute dust fluxes in this region. Thus, ice-age iron fertilization in the Subantarctic Zone would have ultimately worked to lower, not raise, equatorial Pacific productivity.
authors.affiliation: Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA.|Department of Earth and Environmental Sciences, Columbia University, New York, New York 10027, USA.
authors.forename: K M
authors.initials: KM
authors.lastname: Costa
authors.affiliation: Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA.|Department of Earth and Environmental Sciences, Columbia University, New York, New York 10027, USA.
authors.forename: J F
authors.initials: JF
authors.lastname: McManus
authors.affiliation: Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA.|Department of Earth and Environmental Sciences, Columbia University, New York, New York 10027, USA.
authors.forename: R F
authors.initials: RF
authors.lastname: Anderson
authors.affiliation: Department of Geosciences, National Taiwan University, Taipei 106, Taiwan.
authors.forename: H
authors.initials: H
authors.lastname: Ren
authors.affiliation: Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA.
authors.forename: D M
authors.initials: DM
authors.lastname: Sigman
authors.affiliation: Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA.|Department of Earth and Environmental Sciences, Columbia University, New York, New York 10027, USA.
authors.forename: G
authors.initials: G
authors.lastname: Winckler
authors.affiliation: Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA.
authors.forename: M Q
authors.initials: MQ
authors.lastname: Fleisher
authors.affiliation: Department of Geology and Geophysics, Texas A&M University, College Station, Texas 77843, USA.
authors.forename: F
authors.initials: F
authors.lastname: Marcantonio
authors.affiliation: Ocean Sciences Department, University of California, Santa Cruz, California 95064, USA.
authors.forename: A C
authors.initials: AC
authors.lastname: Ravelo
mesh_terms: D001272:Atmosphere / Q000737:chemistry; D004391:Dust / Q000032:analysis; D056908:Foraminifera / Q000378:metabolism; D049690:History, Ancient; D046448:Ice Cover; D007501:Iron / Q000032:analysis* / Q000737:chemistry*; D009566:Nitrates / Q000378:metabolism; D010140:Pacific Ocean; D010839:Phytoplankton / Q000378:metabolism; D012623:Seawater / Q000737:chemistry*
chemical_list: D004391:Dust; D009566:Nitrates; D007501:Iron