Biogeochemical Impacts of Dust on the Global Carbon Cycle - Mineral Dust: A Key Player in the Earth System - page 354

Keywords Biogeochemistry ￿ Nutrients ￿ Iron ￿ Nitrogen ￿ Carbon
￿ Phosphorous ￿ Deposition ￿ Productivity ￿ Oceans ￿ Terrestrial environment ￿
Soil formation ￿ Marine environment ￿ Ocean

14.1 Introduction

The sources (Chap. 3 ) , transport (Chap. 7 ) and transformations (Chap. 4 ) of dust
have been considered in previous chapters of this topic. In this chapter on the
biogeochemical impacts of dust, we focus on its impacts on photosynthetic fixation
of carbon and the associated supply of key nutrients to sustain this fixation, which is
the basis of the planetary ecosystem, the carbon cycle and global biogeochemistry.
Mahowald et al. ( 2011 ) consider the overall impacts of aerosols (of which dust
represents an important but only locally dominant part) on climate in terms of
overall radiative forcing. She identifies the reasonably well-established “direct”
and “indirect” effects of aerosols on radiative forcing and also the biogeochemical
impacts from aerosol deposition on ocean productivity and terrestrial carbon
storage. Mahowald et al. ( 2011 ) estimate that the overall biogeochemical aerosol
effects are comparable to the aerosol direct effect and slightly smaller than the
aerosol indirect effect, although the magnitudes of all three are rather uncertain.
These biogeochemical impacts of dust are therefore substantial and are the subject
of this chapter. The focus is on dust specifically, although many of the effects
considered by Mahowald et al. ( 2011 ) apply to other components of the aerosol
such as fertilisation of primary production by atmospheric deposition of nitrogen.
Dust in the atmosphere also affects solar radiation (Chap. 11 ) , supply to ecosystems
(Chap. 8 ) and temperatures (Chap. 6 ) as discussed earlier in this topic, and both
light supply and temperature can potentially influence the biological carbon cycle.
Climate change is therefore expected to have substantial impacts on terrestrial
(Malhi et al. 1999 ; Mahowald et al. 2010 ; Cox et al. 2013 ) and marine (Moore et al.
2013 ) productivity and hence global biogeochemistry. The focus of this chapter is on
biogeochemical impacts associated primarily with nutrients supplied by dust, rather
than effects arising from changes in climate, although we recognise such changes
are potentially very important.
On a long-term basis, the amounts of carbon fixed by the terrestrial and marine
ecosystems (measured as annual carbon fluxes) are similar (Denman et al. 2007 ),
but the character of the photosynthetic carbon fixation is very different in the two
systems as discussed below. In the oceans primary production is dominated by
microscopic and short-lived (days) algae, while terrestrial productivity is dominated
by large plants, which are rooted in soil and can live for periods from weeks to
centuries.
Marine algae live within an ocean water column that is generally well oxygenated
with a pH

8. Ocean waters mix completely on timescales of about 1,000 years, but
the geochemical residence times of different key plant nutrients within the ocean
range from tens of years to millions of years (Moore et al. 2013 ). Sunlight only