Biogeochemical Impacts of Dust on the Global Carbon Cycle - Mineral Dust: A Key Player in the Earth System - page 369

production, as elegantly demonstrated by the impact of dust deposition from the
eruption of the Kasatochi Volcano in the Aleutian arc in phytoplankton productivity
in the Gulf of Alaska (Hamme et al. 2010 ).
Dust particles deposited to the oceans remain in the surface waters for a period
of tens of days before sinking into the deep ocean usually associated with larger
particles such as biological aggregates and faeces (Croot et al. 2004 ; Chester and
Jickells 2012 ). These particles sink to the ocean floor on timescales of many tens
of days (Chester and Jickells 2012 ). The residence time of dissolved iron in surface
waters is a little longer (a few years) and may vary systematically and inversely
with dust loading (Ussher et al. 2013 ). Thus, the timescales for interactions between
dust and the surface water pelagic ocean biogeochemical system are of the order
of 100 days to a few years, which can be compared to the timescales for soil
process measured in thousands or even millions of years. Some iron will dissolve
immediately from the dust (Chap. 4 ) , although this is often only a few percent of
the total, and more may dissolve during active cycling within the euphotic zone and
possibly the deeper ocean biogeochemical community (e.g. Frew et al. 2006 ). Iron
is regenerated at depth in the oceans along with N, P and Si as biological material is
degraded, although the subsequent relatively rapid removal of iron from the oceans
to sediments compared to N and P means that mixing of deep water into the surface
will generally lead to water that is deficient in iron relative to N and P compared to
phytoplankton requirements. Thus, while supply of iron from deeper waters is very
important in sustaining ocean primary production, external inputs from atmospheric
deposition or other sources are required to allow phytoplankton to utilise all of the
available N and P. Hence, in region of low external iron inputs, primary production
is iron limited and unable to utilise all of the available nitrogen and phosphorus,
leading to the creation of the HNLC waters.
Iron supply to the oceans not only influences primary production; it also influ-
ences the phytoplankton ecosystem structure (Boyd and Ellwood 2010 ). Diatoms
are an important group of phytoplankton characterised by their siliceous skeletons.
Diatoms have a high iron requirement compared to other phytoplankton species,
and addition of iron in HNLC waters, where N, P and Si are present at sufficient
concentrations, favours their growth particularly relative to other species. This shift
to larger diatoms compared to smaller phytoplankton species has implications for
grazers and the higher food chain, and also for the export of carbon to deep waters,
since larger diatoms may sink faster from the surface waters and hence export carbon
to deep waters more efficiently (de Baar et al. 2005 ). This argument then suggests
that increasing dust supply to the oceans increases both photosynthesis and the
proportion of the fixed carbon exported to deep waters, although recent evidence
suggests this conceptual model of carbon export may be too simplistic and that
fluxes may depend on plankton community structure in a complex way (Buesseler
and Boyd 2009 ). It has also been argued that the dust itself may contribute to the
efficient sinking of material within the ocean by adding “ballast” to the sinking
material and increasing its sinking rate. This hypothesis would therefore also imply
that dust influences both the primary production and the subsequent export of the