Biogeosciences
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2008
10.5194/bg-5-631-2008
http://www.biogeosciences.net/5/631/2008/
http://www.biogeosciences.net/5/631/2008/bg-5-631-2008.html
http://www.biogeosciences.net/5/631/2008/bg-5-631-2008.pdf
631
656
2008-05-05
Sedimentary and mineral dust sources of dissolved iron to the world ocean
J. K. Moore
jkmoore@uci.edu
O. Braucher
University of California, Irvine, Department of Earth System Science, Irvine, CA 92697-3100, USA
now at: Humboldt State University, Arcata, CA, 95521, USA
Analysis of a global compilation of dissolved-iron observations provides
insights into the processes controlling iron distributions and some
constraints for ocean biogeochemical models. The distribution of dissolved
iron appears consistent with the conceptual model developed for Th isotopes,
whereby particle scavenging is a two-step process of scavenging mainly by
colloidal and small particulates, followed by aggregation and removal on
larger sinking particles. Much of the dissolved iron (<0.4 μm) is
present as small colloids (>~0.02 μm) and, thus, is subject to
aggregation and scavenging removal. This implies distinct scavenging regimes
for dissolved iron consistent with the observations: 1) a high scavenging
regime – where dissolved-iron concentrations exceed the concentrations of
strongly binding organic ligands; and 2) a moderate scavenging regime –
where dissolved iron is bound to both colloidal and soluble ligands. Within
the moderate scavenging regime, biological uptake and particle scavenging
decrease surface iron concentrations to low levels (<0.2 nM) over a wide
range of low to moderate iron input levels. Removal rates are also highly
nonlinear in areas with higher iron inputs. Thus, observed surface-iron
concentrations exhibit a bi-modal distribution and are a poor proxy for iron
input rates. Our results suggest that there is substantial removal of
dissolved iron from subsurface waters (where iron concentrations are often
well below 0.6 nM), most likely due to aggregation and removal on sinking
particles of Fe bound to organic colloids.
<br><br>
We use the observational database to improve simulation of the iron cycle
within a global-scale, Biogeochemical Elemental Cycling (BEC) ocean model.
Modifications to the model include: 1) an improved particle scavenging
parameterization, based on the sinking mass flux of particulate organic
material, biogenic silica, calcium carbonate, and mineral dust particles; 2)
desorption of dissolved iron from sinking particles; and 3) an improved
sedimentary source for dissolved iron. Most scavenged iron (90%) is put
on sinking particles to remineralize deeper in the water column. The
model-observation differences are reduced with these modifications. The
improved BEC model is used to examine the relative contributions of mineral
dust and marine sediments in driving dissolved-iron distributions and marine
biogeochemistry. Mineral dust and sedimentary sources of iron contribute
roughly equally, on average, to dissolved iron concentrations. The
sedimentary source from the continental margins has a strong impact on
open-ocean iron concentrations, particularly in the North Pacific. Plumes of
elevated dissolved-iron concentrations develop at depth in the Southern
Ocean, extending from source regions in the SW Atlantic and around New
Zealand. The lower particle flux and weaker scavenging in the Southern Ocean
allows the continental iron source to be advected far from sources. Both the
margin sediment and mineral dust Fe sources substantially influence
global-scale primary production, export production, and nitrogen fixation,
with a stronger role for the dust source. Ocean biogeochemical models that
do not include the sedimentary source for dissolved iron, will overestimate
the impact of dust deposition variations on the marine carbon cycle.
Available iron observations place some strong constraints on ocean
biogeochemical models. Model results should be evaluated against both
surface and subsurface Fe observations in the waters that supply dissolved
iron to the euphotic zone.
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