Biogeochemical Impacts of Dust on the Global Carbon Cycle - Mineral Dust: A Key Player in the Earth System - page 373

and other global change pressures, although the impacts of such changes on ocean
productivity are very uncertain (Moore et al. 2013 ).

14.4 Conclusion

Atmospheric dust is transported from source regions (predominantly deserts) to
terrestrial and marine ecosystems. There are strong global gradients in that depo-
sition with highest deposition over the tropical North Atlantic and Central/South
America downwind of the Sahara desert as well as over the North Indian Ocean
and surrounding land area and the NW Pacific ocean and northwestern Asia. This
dust can contribute to the formation of soils themselves in some regions, and more
generally, this transport represents a net loss of organic carbon and nutrients from
these regions and a net import to the receiving regions. The regions of soil loss are
desert regions where primary production rates are low due to water limitation, so
the impacts of the soil and nutrient loss from these regions on the global carbon cycle
are small, although the impacts are locally important in the dust source regions. The
deposition of these nutrients has been demonstrated to have biogeochemical impacts
on receiving terrestrial and marine ecosystems, but the scale and the nature of
these impacts vary geographically. This geographical pattern of impacts inevitably
reflects, in part, the deposition pattern of dust and also the characteristics of the
receiving ecosystems.
In the case of terrestrial ecosystems, the main impact appears to arise from inputs
of phosphorus associated with dust, which can impact primary production both
directly and also via encouragement of nitrogen fixation. Overall about 20 % of
terrestrial ecosystems are P limited. This impact is greatest in tropical ecosystems
developed on highly weathered soils and/or on nutrient-poor underlying rock and
develops over timescales of thousands to millions of years as the soils evolve and
ultimately lose nutrients derived from weathering of the underlying rock.
In the case of marine ecosystems, the main impact arises from the iron associated
with dust. This iron input influences two important but distinctly different ocean
biogeochemical processes.
The first biogeochemical process arises in situations where the iron is used
to support photosynthesis. The supply of iron from dust and other ocean margin
sources is sufficient to meet phytoplankton photosynthetic needs over about 70 % of
the oceans. In the remaining regions of the oceans, HNLC systems develop where
iron supply limits primary production either on its own or as a co-limiting factor.
These essentially permanently iron-limited ocean regions are all regions remote
from dust sources including areas of the Southern Ocean, the North Pacific and
the tropical South East Pacific. Some other regions may become seasonally iron
limited. The iron limitation of the Southern Ocean may be particularly important
because of its size and because it is a region in which deep ocean waters form due to
cooling, allowing the unutilised nutrients to be transported deep into the oceans and
hence reducing overall ocean CO 2 uptake. Changes in productivity in this region,