Document Type

Article

Date of Original Version

1995

Department

Oceanography

Abstract

Many ocean regions important to the global carbon budget, including the equatorial Pacific Ocean, have low chlorophyll concentrations despite high levels of conventional nutrients. Iron may instead be the limiting nutrient, and elevated input of terrigenous Fe during windy glacial episodes has been hypothesized to stimulate oceanic productivity through time and thus regulate the oceanic and atmospheric CO2 balance. To test whether particulate Fe input is related to the accumulation of biogenic matter in one important low chlorophyll‐high nutrient area, that is, the equatorial Pacific Ocean, we present results from a suite of sediment cores that collectively record biogenic deposition through the last six glacial‐interglacial cycles (∼600,000 years). Our data set includes new chemical data on total Fe, terrigenous, and biogenic components in three cores as well as previously published mineralogic records of eolian input to the region. Chemical, spectral, and stratigraphic analysis indicates that (1) terrigenous input to the region shows no consistent pattern of either glacial or interglacial maxima, (2) the accumulation of particulate Fe is closely related to the accumulation of terrigenous matter (linear r2 = 0.81–0.98), (3) there are no coherent spectral relationships between Fe input and glacial periodicity (i.e., δ18O) in any of the orbital frequency bands, (4) the linear and cross‐spectral correlations between Fe or eolian input and CaCO3 concentration are most commonly the strongest observed relationships between Fe and any biogenic component, yet indicate a largely inverse pattern, with higher Fe being associated with low CaCO3, (5) there is no consistent linear r2 correlation or spectral coherence between the accumulation of Fe and that of CaCO3, Corg, or opal. Thus in total there is no relationship between terrigenous Fe input and sedimentary sequestering of carbon. Additionally, although we cannot specifically address the potential for changes in solubility of the terrigenous fraction that may be driven by a terrigenous compositional change, the Fe/Ti ratio (which monitors first‐order mineralogic changes) records only slight variations that also are linearly and spectrally unrelated to glacial periodicity, the bulk Fe flux, and the accumulation of any biogenic component. Finally, we find that the paleoceanographic flux of Fe is several order‐of‐magnitudes larger than modern observations of eolian Fe input, suggesting that the long‐term importance of Fe input by dust storms (which deliver Fe on the order of the sedimentary burial) may be underestimated. The removal of particulate terrigenous Fe from the recently discovered source within the Equatorial Undercurrent, however, remains unquantified and may also prove significant.

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