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; FeHR (iron oxides soluble in dithionite), FePR (iron soluble in boiling HCl but not in dithionite) and FeU (total iron less that soluble in boiling HCl). River particulates show a close association between FeHR 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, continentalscale
average FeHR/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 FeHR with FeT in glacial
particulates, and instead both FePR and FeU 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 FeHR,
FePR and FeT but enriched in FeU. 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 FeHR, FePR, FeU and FeT.
This budget suggests that the differences between riverine particulates and marine
sediments can be explained by; (i) preferentially removing FeHR 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 FeHRdepleted
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 FeHR-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 FeHR-depleted glacial particulates when sealevel is low (large ice
masses, enhanced glacial erosion). Further important controls on the steady state for FeHR operate through the formation of euxinic sediments and ironstones, which also
provide sealevel-dependent sinks for FeHR-enriched sediment
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