Subaquatic Dust Deposits - Mineral Dust: A Key Player in the Earth System - page 447

Another marine archive with great potential for high-resolution dust records is
tropical corals. Although tropical corals are very efficient in removing pollutants
from their polyps, they seem to register dust that rains down on them in the He
isotopes (there are nine, of which only 3 He and 4 He are stable) that they precipitate
in their carbonates. Without comprehending exactly why and how corals do register
these isotopes, Mukhopadhyay and Kreycik ( 2008 ) observed a close co-occurrence
of 4 He in Porites corals from the Cape Verde Islands and the summer surface dust
concentration (SDC) record from Barbados (Prospero and Lamb 2003 ). As Porites
corals can grow several hundreds of years, they bear great potential for highly
resolved long-term palaeo-aridity records.
Next to the chemical composition, also the mineralogical composition of marine
sediments can be used to reconstruct amounts of dust being blown around. A classic
example is the mineral palygorskite, which is typical for Saharan dust (Schütz and
Sebert 1987 ) and of which the abundance in marine sediments can be used to
quantify Saharan dust in the marine sediment archive (e.g. Coudé-Gaussen 1989 ;
Scheuvens et al. 2013 ).
Related to the mineralogical proxies are various magnetic proxies, which are
based on the magnetic properties of various minerals and mineral coatings in wind-
blown dust. These coatings form under various environmental conditions with the
iron oxide minerals haematite and goethite as main indicators of wetter or drier
conditions (e.g. Bloemendal et al. 1988 ). As with all proxies, there are some pitfalls
with magnetic proxies, related to, e.g. diagenetic (post-depositional) alterations of
the magnetic minerals (Maher 2011 ) or occurrence of magnetic minerals produced
by magnetotactic bacteria (Blakemore 1975 ).

17.5 Wind-blown Dust in the Ocean as a Player

of Environmental Change

In the late 1990s, John Martin hypothesised that the ocean may play a crucial role
in global climate. This so-called Fe hypothesis relates to the Fe-limited parts of the
ocean (see Chap. 14 ) , where many nutrients are available yet primary productivity
is very low. Mineral dust may have played a role: as it carries Fe and other macro-
and micronutrients, it may have stimulated plankton growth, which in turn may
have led to lower atmospheric CO 2 concentrations. This hypothesis has been tested
extensively by many so-called fertilisation experiments (see de Baar et al. 2005 for
a synthesis of the results) which demonstrated that indeed primary productivity can
be stimulated using dissolved Fe. However, the net effect of these experiments in
terms of export of organic matter and hence actual CO 2 sequestration is still under
debate. Also, so far there is only one known example of observed ocean fertilisation
by mineral dust (Bishop et al. 2002 )thatindeedledtoanincreaseinorganicmatter
export from the surface ocean to the deep sea.
In a recent paper, Martinez-Garcia et al. ( 2011 ) demonstrated that there is indeed
a coupling between dust input into the Southern Ocean and global climate through