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No iron fertilization in the equatorial Pacific Ocean during the last ice age

Nature volume 529, pages 519–522 (2016)Cite this article

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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 iron1. Greater atmospheric dust deposition2 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 uncertain3,4,5,6. Here we present meridional transects of dust (derived from the 232Th proxy), phytoplankton productivity (using opal, 231Pa/230Th and excess Ba), and the degree of nitrate consumption (using foraminifera-bound δ15N) 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 region7,8. Thus, ice-age iron fertilization in the Subantarctic Zone would have ultimately worked to lower, not raise, equatorial Pacific productivity.

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Figure 1: 230Th-normalized dust flux (see Methods), opal flux, 231Pa/230Th, excess Ba flux, and FB-δ15N.
Figure 2: Dust flux and 231Pa/230Th across the equatorial Pacific.
Figure 3: Changes in nutrient dynamics between the Holocene and the LGP.

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Acknowledgements

We thank M. Soon at the University of British Columbia for assistance in running the opal analyses and M. A. Weigand and S. Oleynik for assistance with the FB-δ15N analyses. Funding was provided in part by NSF award AGS 15-02889 (to J.F.McM., G.W. and F.M.), NSF award OCE-1060947 (to D.M.S.) and the Grand Challenges Program of Princeton University (to D.M.S.). This project also benefited from previous support from the NSF (grant numbers OCE-1003374, OCE-1159053, OCE-1158886) and the Comer Science and Education Foundation (to J.F.McM.) and travel funding from MOST, Taiwan (grant number 103-2116-M-002-032-MY2 to H.R.).

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Authors and Affiliations

  1. Lamont-Doherty Earth Observatory of Columbia University, Palisades, 10964, New York, USA

    K. M. Costa, J. F. McManus, R. F. Anderson, G. Winckler & M. Q. Fleisher

  2. Department of Earth and Environmental Sciences, Columbia University, New York, 10027, New York, USA

    K. M. Costa, J. F. McManus, R. F. Anderson & G. Winckler

  3. Department of Geosciences, National Taiwan University, Taipei, Taiwan, 106

    H. Ren

  4. Department of Geosciences, Princeton University, Princeton, 08544, New Jersey, USA

    D. M. Sigman

  5. Department of Geology and Geophysics, Texas A&M University, College Station, Texas, 77843, USA

    F. Marcantonio

  6. Ocean Sciences Department, University of California, Santa Cruz, California, 95064, USA

    A. C. Ravelo

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  1. K. M. Costa
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Contributions

K.M.C., J.F.McM., and R.F.A. designed the study. K.M.C performed the core sampling, U-Th-Pa chemistry, and instrumental analyses, with technical assistance from M.Q.F. Nitrogen isotope analyses were carried out by H.R. (for the Holocene) and K.M.C. with the assistance of H.R. (for the LGP). F.M. performed the barium measurements. All authors contributed to interpretation of the data. K.M.C. wrote the manuscript, and all authors provided comments and revisions.

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Extended data figures and tables

Extended Data Figure 1 Line Islands core locations.

Core sites are identified by their multicore numbers. The respective piston core numbers as well as latitude, longitude and depth are provided in Extended Data Table 1. The bathymetric map was generated using GeoMapApp64.

Extended Data Figure 2 Radiocarbon-based age models for all six cores.

Core chronologies were established with four radiocarbon dates on G. ruber: 0 cm and 8 cm depth in the multicores (MC), and two depths (>8 cm) in the Big Bertha piston cores (BB) bracketing the δ18O maximum inferred to represent Marine Isotope Stage 2 (ref. 31). Age models were established via linear interpolation between radiocarbon dates (ka, thousands of years ago), which are provided in Extended Data Table 2.

Extended Data Figure 3 Time series for 231Pa/230Th data within the LGP time slice.

The relatively constant values for each core argue against any systematic bias from bioturbation of transient features, such as a deglacial productivity peak.

Extended Data Figure 4 Concentrations of opal, excess Ba and CaCO3.

CaCO3 is the dominant sedimentary component, and systematic changes in its concentration dilute the concentrations of minor sedimentary components such as opal and excess Ba (Baxs).

Extended Data Figure 5 Lithogenic correction for Ba excess flux calculation.

Samples from the Holocene (0–10,000 years ago) are orange; samples from the LGP (17,000–27,000 years ago) are blue. In the top panel, excess Ba concentrations were calculated by subtracting the lithogenic Ba fraction from the total Ba concentration using a lithogenic Ba/Th ratio of 51.4, based on the average elemental concentrations in upper continental crust43. The lithogenic corrections are small, <2% for the Holocene and <6% for the LGP. Excess Ba concentrations were then multiplied by the total mass flux (230Th-normalized) in order to generate the excess Ba flux. The bottom panel shows a comparison of excess Ba fluxes calculated using different lithogenic Ba/Th ratios, ranging from 10 to 100, normalized to the fluxes determined using a Ba/Th ratio of 51.4. Ratios less than 51.4 result in slightly higher excess Ba fluxes, while ratios greater than 51.4 result in slightly lower excess Ba fluxes. Overall, the excess Ba fluxes are insensitive to the Ba/Th ratio chosen, with deviations only over a range of ±10%. ppm, parts per million; ppb, parts per billion.

Extended Data Figure 6 230Th-normalized opal flux, excess Ba flux, and 231Pa/230Th.

Samples from the Holocene (0–10,000 years ago) are orange; samples from the LGP (17,000–27,000 years ago) are blue. 231Pa/230Th is positively correlated with the opal flux (r2 = 0.90, P < 0.001) and excess Ba flux (r2 = 0.85, P < 0.01). Excess Ba flux and opal flux are also positively correlated (r2 = 0.63, P < 0.01). For opal flux, the correlation with 231Pa/230Th is especially strong during the LGP (opal flux r2 = 0.98, P < 0.001). The relationship (that is, the slope) between 231Pa/230Th and opal flux may be altered by changes in preservation, which affects opal but not 231Pa/230Th. Poor opal preservation (for example, if the diatom frustules were less silicified) would elevate the 231Pa/230Th relative to the sedimentary opal flux, thus steepening the slope. However, the relationship between 231Pa/230Th and opal flux is temporally invariant, with slopes of 7.89 ± 1.24 × 10−3 in the Holocene, 8.09 ± 1.31 × 10−3 in the LGP, and 7.60 ± 0.53 × 10−3 overall. We interpret these results to indicate that there was no important change in opal preservation between the LGP and Holocene and, therefore, that frustule silicification (potentially related to iron stress32,33,34), similarly remained unchanged.

Extended Data Figure 7 Absolute and relative change for each productivity proxy versus the Holocene value of that proxy.

The absolute change (Holocene minus LGP) in productivity proxy is shown in the top panels; the relative change is shown in the bottom panels. Excess Ba flux is shown in green, opal flux in purple, and 231Pa/230Th in red. The greatest change in productivity occurred at the sites with the highest Holocene productivity values. The core that shows a negative change in productivity is the most northerly core (7° N), which is outside the high-nutrient, low-chlorophyll equatorial upwelling zone, and thus displays different glacial–interglacial nutrient dynamics. The relative change in productivity (Holocene to the LGP) is fairly constant across the five cores within the equatorial upwelling zone at 11% for excess Ba flux, 24% for opal flux, and 8% for 231Pa/230Th. The inter-proxy difference may reflect a nonlinear scaling of productivity with 231Pa/230Th, because these radionuclides are scavenged to some extent by all particle phases. In practice, this difference suggests that 231Pa/230Th may provide a conservative estimate for changes in productivity, with true productivity changes potentially at much higher amplitude. Error bars are 2σ and indicate analytical precision.

Extended Data Table 1 Locations of the Line Islands cores
Full size table
Extended Data Table 2 Radiocarbon ages for age model generation
Full size table

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Costa, K., McManus, J., Anderson, R. et al. No iron fertilization in the equatorial Pacific Ocean during the last ice age. Nature 529, 519–522 (2016). https://doi.org/10.1038/nature16453

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