Biogeochemical Impacts of Dust on the Global Carbon Cycle - Mineral Dust: A Key Player in the Earth System - page 368

Table 14.2 Atmospheric
dust deposition to different
ocean basins and total global
dust emissions

Ocean basin Dust deposition Mt (10 12 g) year 1

North Pacific 72
South Pacific 29
North Atlantic 202
South Atlantic 17
Indian Ocean 118
Global emissions 1,790
Mahowald et al. ( 2005b )

there is therefore generally an adequate supply of iron for phytoplankton growth.
However, iron from these ocean margin sources is effectively trapped in coastal
waters and does not efficiently reach open ocean waters, and hence, in open ocean
regions, dust transport is therefore of particular interest as a supply of iron (Jickells
et al. 2005 ). The absolute requirement of phytoplankton photosynthesis for iron
when ocean concentrations are so low seems paradoxical, but probably reflects the
evolution of photosynthesis at a time when the oceans were more reducing and iron
concentrations much higher (Hunter and Boyd 2007 ).
Primary production is estimated to be limited by iron supply over about 30 % of
the world oceans (see later), and these areas largely coincide with regions of low dust
input. In many of these areas, the macronutrients such as nitrate are not efficiently
removed even in summer, and these regions are often referred to as high-nitrate low-
chlorophyll (HNLC) regions. In these regions, elegant oceanographic experiments
have demonstrated iron limitation of photosynthesis by showing that addition of
iron leads to increases in biomass on timescales of days (Boyd et al. 2007 ). Recent
studies suggest that some additional areas of the ocean such as the high-latitude
North Atlantic may become iron limited for part of the growing season after the
main spring bloom has removed most of the available dissolved iron (Nielsdottir
et al. 2009 ).
The global deposition of dust to the oceans is shown in Fig. 14.2 . This emphasises
the importance of desert sources, particularly the Saharan and Asian desert, and that
dust deposition to the oceans of the Northern Hemisphere is much higher than to
the Southern Hemisphere. Glacial deposits can also be dust sources (Schroth et al.
2011 ). This is further illustrated in Table 14.2 which also shows that about 24 % of
all dust produced reaches the oceans. The regions of low dust deposition coincide
with HNLC regions, implying a direct link between dust supply and HNLC status
as discussed above.
In addition to dust deposition, iron can also be supplied to the oceans through
the atmosphere, associated with anthropogenic emissions. Although these anthro-
pogenic sources are much smaller source of total iron than desert dust, they do
appear to be more soluble, and hence, this anthropogenic iron may be a quantita-
tively important source of iron downwind of large industrial emission regions (see
Chap. 4 , Sholkovitz et al. 2012 ) and possibly from emissions from ships (Ito 2013 ).
Volcanic emissions of dust are generally smaller than desert dust emissions, but
during eruptive episodes, these emissions can be large and can influence primary