arising from Matthew R. Loveley et al. Nature Geoscience https://www.nature.com/articles/ngeo3024 (2017)

Loveley and co-authors1 present 232Th and biogenic barium (Baxs) fluxes from eastern equatorial Pacific (EEP) site MV1014-02-17JC (17JC). They argue that millennial increases in dust flux enhanced productivity via Fe fertilization and increased efficiency of the biological pump. Here, we show that this conclusion is unwarranted, and present an alternative explanation that is consistent with data from nearby Ocean Drilling Program (ODP) Site 1240. Our understanding of the carbon cycle further suggests that Fe fertilization in the EEP cannot influence \(p_{{\mathrm {CO}}_2}\) (ref. 2), necessitating a careful examination of claims to the contrary.

At the centre of the conclusion reached by Loveley et al.1 is their suggestion of a correlation between the flux of 232Th, a dust proxy3,4,5, and the flux of Baxs, an export productivity proxy5,6. However, the coefficient of multiple correlation (R2) between these two parameters is 0.0007, with correlations of 0.006 for stadial events and 0.0005 for the residual record (Fig. 1a) (with corrresponding P values of 0.59, 0.43 and 0.70). No function, nonlinear or otherwise, can provide a statistically significant relationship between these variables, rendering any variations in the production or preservation of Baxs immaterial. Furthermore, several productivity pulses precede dust flux increases, and millennial increases in dust and productivity occur independently of one another or of any stadial event. Thus, there is no relationship between the flux of 232Th and Baxs at 17JC, and we conclude that there is no dust fertilization of the EEP on millennial or orbital timescales. This result matches previous reconstructions from 11 equatorial Pacific sites (including two from the EEP) that show no correlation between dust flux and productivity3,5 or nutrient utilization7.

Fig. 1: Statistical assessment of factors influencing productivity.
Fig. 1

a, Correlation between 232Th and Baxs fluxes at site 17JC (0.18° S, 85.87° W; 2.8 km depth) (R2 = 0.0007, P = 0.59) (circles), during Heinrich events 0–8 (R2 = 0.006, P = 0.43) (grey circles), non-stadial periods (R2 = 0.0005, P = 0.70) (white circles), and at ODP Site 1240 (0.02° N, 86.46° W; 2.9 km depth) (R2 = 0.24, P < 0.01) (blue diamonds). b, Correlation between 232Th and Fe fluxes at ODP Site 1240 (R2 = 0.38, P < 0.01) (blue diamonds). c, Correlation between Fe and Baxs fluxes at ODP Site 1240 (R2 = 0.51, P < 0.01) (blue diamonds).

New data from nearby ODP Site 1240 show a weak correlation between records of 232Th and Fe flux (Fig. 1b), suggesting that dust did not enhance productivity because it is not the primary control on Fe availability in the EEP. This is consistent with evidence3,8,9 that upwelling of the Equatorial Undercurrent delivers an order of magnitude more dissolved Fe than aeolian dust. Indeed, the flux of Baxs is more strongly correlated with Fe flux than it is with 232Th (Fig. 1c), suggesting that upwelling may be related to both the Fe flux and productivity. Upwelling, in turn, probably responds to changes in wind strength, a potential common influence on the fluxes of both total Fe and aeolian 232Th. Thus, Fig. 1 suggests that increases in EEP productivity are related to the upwelling of nutrient and Fe-rich subsurface waters, not aeolian dust.

Although export production in the EEP does vary with climate, these changes do not alter the net efficiency of the biological pump. At present, the EEP is a net source of CO2 to the atmosphere due to a reduction in gas solubility when upwelled seawater is warmed2,10. As equatorial waters diverge polewards, nutrients not immediately utilized are fully consumed off-axis2. Consequently, although the addition of Fe may stimulate productivity locally, it does so at the expense of nutrient consumption downstream. Thus, the biological carbon pump in the EEP operates at full efficiency when integrated regionally2. Data from 17JC provide neither evidence of aeolian Fe fertilization, nor of enhanced CO2 drawdown in the EEP during times of greater dust supply.

Data availability

Data supporting the results of this contribution are available through PANGAEA at https://doi.pangaea.de/10.1594/PANGAEA.897587.

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References

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    Loveley, M. R. et al. Millennial-scale iron fertilization of the eastern equatorial Pacific over the past 100,000 years. Nat. Geosci. 113, 760–764 (2017).

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    Sigman, D. M. & Haug, G. H. Treatise on Geochemistry Vol. 6 491–528 (Elsevier-Pergamon, Oxford, 2003).

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    Winckler, G., Anderson, R. F., Jaccard, S. L. & Marcantonio, F. Ocean dynamics, not dust, have controlled equatorial Pacific productivity over the past 500,000 years. Proc. Natl Acad. Sci. USA 113, 6119–6124 (2016).

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    Jacobel, A. W., McManus, J. F., Anderson, R. F. & Winckler, G. Climate-related response of dust flux to the central equatorial Pacific over the past 150 kyr. Earth Planet. Sci. Lett. 457, 160–172 (2017).

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    Costa, K. M. et al. No iron fertilization in the equatorial Pacific Ocean during the last ice age. Nature 529, 519–522 (2016).

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    Dymond, J., Seuss, E. & Lyle, M. Barium in deep-sea sediment: a geochemical proxy for paleoproductivity. Paleoceanography 7, 163–181 (1992).

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    Robinson, R. S., Martinez, P., Pena, L. D. & Cacho, I. Nitrogen isotopic evidence for deglacial changes in nutrient supply in the eastern equatorial Pacific. Paleoceanography 24, PA4213 (2009).

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    Rafter, P. A., Sigman, D. M. & Mackey, K. R. M. Recycled iron fuels new production in the eastern equatorial Pacific Ocean. Nat. Commun. 8, 1100 (2017).

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    Gordon, R. M., Coale, K. H. & Johnson, K. S. Iron distributions in the equatorial Pacific: implications for new production. Limnol. Oceanogr. 42, 419–431 (2003).

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    Feely, R. A., Wanninkhof, R., McGillis, W., Carr, M. E. & Cosca, C. E. Effects of wind speed and gas exchange parameterizations on the air–sea CO2 fluxes in the equatorial Pacific Ocean. J. Geophys. Res. 109, C08S03 (2004).

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Author notes

    • A. W. Jacobel

    Present address: Brown University, Providence, RI, USA

    • K. M. Costa

    Present address: Woods Hole Oceanographic Institution, Woods Hole, MA, USA

    • E. M. Shoenfelt

    Present address: Massachusetts Institute of Technology, Cambridge, MA, USA

Affiliations

  1. Lamont-Doherty Earth Observatory, Palisades, NY, USA

    • A. W. Jacobel
    • , R. F. Anderson
    • , G. Winckler
    • , K. M. Costa
    • , J. Gottschalk
    • , J. L. Middleton
    • , F. J. Pavia
    • , E. M. Shoenfelt
    •  & Y. Zhou
  2. Columbia University, New York, NY, USA

    • R. F. Anderson
    • , G. Winckler
    • , K. M. Costa
    • , F. J. Pavia
    • , E. M. Shoenfelt
    •  & Y. Zhou

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R.F.A., G.W. and A.W.J. designed the research, A.W.J. performed the analyses and all authors were involved in writing and revising the manuscript.

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