Biogeochemical Impacts of Dust on the Global Carbon Cycle - Mineral Dust: A Key Player in the Earth System - page 370

associated organic carbon to the deep ocean, although recent studies suggest this
ballast effect is of limited quantitative importance (e.g. Le Moigne et al. 2012 and
references therein).
In ocean areas where nitrogen supply limits primary production, nitrogen fixation
may become competitively advantageous, despite its high energy requirements
(Mahaffey et al. 2005 ). Enzymes for nitrogen fixation have a relatively high iron
content, and it is now evident that rates of nitrogen fixation in the oceans may be
regulated at least in part by iron supply (Fig. 14.7 ) (Moore et al. 2004 ). Perhaps
the most striking example of this is seen in data collected on north to south
transects in the Atlantic Ocean passing from the high dust deposition regions in
the tropical North Atlantic to the much lower dust deposition regions of the tropical
South Atlantic (Fig. 14.8 ). Much higher dissolved iron concentrations are found
in waters underlying the Saharan dust plume (Fig. 14.2 ), and these are associated
with higher rates of nitrogen fixation. These higher rates of nitrogen fixation are
also associated with exceptionally low levels of surface water phosphate, and this
is believed to be due to phosphorus utilisation either by nitrogen-fixing organisms
or another phytoplankton, for which nitrogen limitations are relieved by nitrogen
release from nitrogen-fixing organisms (Moore et al. 2009 ). The waters of the
eastern Mediterranean are severely phosphorus depleted, and so even under the very
high dust deposition regime of this region, nitrogen fixation and primary production
rates become phosphorus limited and very low (Krom et al. 2010 ).
Over the last 20 years, it has become clear that in addition to light and the
macronutrients N, P and Si, iron also limits primary production over large areas of
the oceans. Figure 14.7 shows the results from a recent modelling study calculating
the limiting nutrient for ocean primary productivity in different regions of the
oceans. Overall Moore and Braucher ( 2008 ) estimate that 30 % of global oceanic
N fixation is limited by iron supply from dust. These model results suggest that
in different areas of the oceans, either nitrogen or iron supply is generally the
limiting factor for productivity (Moore et al. 2004 ), consistent with experimental
evidence (Moore et al. 2013 ). There is little evidence of short-term limitation of
ocean productivity by phosphorus supply, although this may be the ultimate long-
term limitation on ocean productivity (Tyrrell 1999 ; Moore et al. 2004 ). Using a
model it has been estimated that atmospheric dust deposition, and the associated iron
supply, drives 5 % of ocean productivity and 18 % of the export of carbon to deep
waters (Moore and Braucher 2008 ). Much of ocean productivity is driven by the
internal cycling of nutrients within the euphotic zone, so the export to deep waters
is of particular interest because it can potentially remove CO 2 from the atmosphere
for periods of hundreds of years and hence influence climate. Export production
is in this context comparable to the long-term carbon sequestration in terrestrial
ecosystems. Moore and Braucher use their model to estimate that the dust supply to
the oceans is responsible for the export to deep ocean waters of about 1 GtC year 1 .
This model suggests that the dust supply is particularly important for sustaining
nitrogen fixation, although the model also suggests that in terms of ocean carbon
cycling, the biggest impact of dust is directly on primary production rather than via
nitrogen fixation.