THE LOW-TEMPERATURE GEOCHEMICAL CYCLE OF IRON: FROM CONTINENTAL FLUXES TO MARINE SEDIMENT DEPOSITION

摘要

Suspended sediments from 34 major rivers (geographically widespread) and 36 glacial meltwater streams have been examined for their variations in different operationally-defined iron fractions; Fe_(HR) (iron oxides soluble in dithionite), Fe_(PR) (iron soluble in boiling HCl but not in dithionite) and Fe_U (total iron less that soluble in boiling HCl. River particulates show a close association between Fe_(HR) and total iron (FeT), reflecting the effects of chemical weathering which derive oxide iron from, and retain it in close association with, total iron. Consistent with this, continental-scale average Fe_(HR)/FeT ratios vary with runoff ratios (average river runoff per unit area/average precipitation per unit area). By contrast, the diminished effects of chemical weathering produce no recognizable association of Fe_(HR) with FeT in glacial particulates, and instead both Fe_(PR) and Fe_U, are closely correlated with FeT, reflecting essentially pristine mineralogy. A comparison of the globally-averaged compositions of riverine particulates and marine sediments reveals that the latter are depleted in Fe_(HR), Fe_(PR) and FeT but enriched in Fe_U. The river and glacial particulate data are combined with estimates of authigenic, hydrothermal, atmospheric and coastal erosive iron fluxes from the literature to produce a global budget for Fe_(HR), Fe_(PR), Fe_U and FeT. This budget suggests that the differences between riverine particulates and marine sediments can be explained by; (ⅰ) preferentially removing Fe_(HR) from the riverine particulate flux by deposition into inner shore reservoirs such as floodplains, salt marshes and estuaries; and (ⅱ) mixing the resulting riverine particulates with Fe_(HR)-depleted glacial particulates. Preliminary measurements of inner shore sediments are consistent with (ⅰ) above. Phanerozoic and modern normal marine sediments have similar iron speciation characteristics, which implies the existence of a long-term steady state for the iron cycle. This steady state could be maintained by a glacioeustatic feedback, where Fe_(HR)-enriched riverine particulates are either more effectively trapped when sealevel is high (small ice masses, diminished glacial erosion), or are mixed with greater masses of Fe_(HR)-depleted glacial particulates when sealevel is low (large ice masses, enhanced glacial erosion). Further important controls on the steady state for Fe_(HR) operate through the formation of euxinic sediments and ironstones, which also provide sealevel-dependent sinks for Fe_(HR)-enriched sediment.