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.

A Three-Dimensional Model of the Marine Nitrogen Cycle during the Last Glacial Maximum Constrained by Sedimentary Isotopes

“…Since atmospheric Fe deposition was mainly increased over the Southern Ocean, enhanced export production stores more nutrients in Antarctic Bottom Water that fills the deep ocean. This causes less preformed nutrients transport to the tropics via Subantarctic Mode Waters, thereby reducing productivity and water column N-loss there, generally consistent with the scenario suggested by Costa et al (2016).…”

“…These data, based on bulk organic nitrogen measurements, were corrected for a diagenetic effect according to Robinson et al (2012), who examined over 100 locations and found a significant correlation with water depth (∼1‰/km), while noting that oxygen exposure time is likely the main driver. We use the five additional foraminifera and diatom bound measurements included in Schmittner and Somes (2016), plus 6 more from recent studies in the subarctic Pacific and central equatorial Pacific (Ren et al, 2015;Costa et al, 2016), all of which were not corrected for diagenesis. Although combining different forms of δ 15 N records creates uncertainty, excluding the diatom and foraminifera bound measurements does not affect which simulations performed best in terms of the statistical model-data metrics so we include them into the main analysis to improve spatial coverage.…”

Barium Concentrations as a Proxy for Equatorial Pacific Productivity During the Past 140,000 Years

“…One might expect, therefore, that when dust fluxes are high, there may be increased efficiency of the export of CO2 to the ocean bottom (because of increased productivity), where it is isolated from the surface ocean/atmospheric system. Costa et al (2016) assert that the greater dust flux of the last glacial period was not large enough to generate significant iron fertilization and increased productivity. A similar conclusion was reached by Winckler et al (2016) who additionally suggested that iron, rather than being provided by increased dust fluxes, may be provided to the surface ocean as a result of diffusive and advective upwelling along the equator.…”

“…Costa et al (2016) assert that the greater dust flux of the last glacial period was not large enough to generate significant iron fertilization and increased productivity. A similar conclusion was reached by Winckler et al (2016) who additionally suggested that iron, rather than being provided by increased dust fluxes, may be provided to the surface ocean as a result of diffusive and advective upwelling along the equator. Thus, dust fertilization of the ocean may not contribute to the changing patterns of productivity in the Central Equatorial Pacific (CEP).…”

Coupled climate-carbon cycle simulation of the Last Glacial Maximum atmospheric CO<sub>2</sub> decrease using a large ensemble of modern plausible parameter sets

“…They may additionally be due to the model not simulating enhanced nutrient inventories in response to enhanced weathering (2005) suggesting that the export fluxes were higher during the LGM while Costa et al (2016) suggest that productivity was reduced around the centre of the region. Ocean productivity data for the South Pacific appears to still be rather limited.…”