On the role of Lithogenic Particles for Iron Cycling in the Atlantic Ocean

Lithogenic material, carried by the atmosphere and deposited as dust,
is a major source of trace metals to the ocean. In the tropical
Atlantic, which receives at least half of the global deposition flux
to the oceans, dust is very likely to be the dominant source of iron
to biology, and its influence is clearly seen in the distribution of
dissolved iron. But iron is also hardly soluble in seawater and
scavenged onto particle surfaces. Dust deposition also is a source of
particles, and hence can act as a sink for dissolved iron.

In this study we quantify the double effect of dust both as an iron
source and sink in the Atlantic Ocean using a global biogeochemical
model which explicitly describes the aggregation and disaggregation of
particles, and their role in scavenging. We take advantage of the
growing data base of both dissolved and particulate trace element
concentration measurements obtained in GEOTRACES to test our model
predictions. We show that, with a comparatively simple aggregation
model, we are able to describe to first order the observed
distribution of lithogenic suspended particulate matter, with elevated
concentration of fine particles in the surface under the dust plume, a
minimum below the mixed layer, and increasing values below due to
disaggregation.

At the surface, the reduction of dissolved iron due to the presence of
lithogenic particles is limited mostly to the high dust-flux regions
directly downwind of the major deserts. At the same time, the
scavenging by dust particles clearly reduces dissolved iron in the
northerly sourced deep water masses of the Atlantic. Both effects
bring modelled iron distributions closer to observations. We discuss
the sensitivity of these results to the assumed rates of scavenging
and the presence of organic ligands.