Identifying Sources of Aeolian Mineral Dust: Present and Past - Mineral Dust: A Key Player in the Earth System - page 67

activities). Studies by Aleinikoff et al. ( 1999 , 2008 ) in the Great Plains region of
North America utilized this method in identifying sources of loess in Nebraska and
Colorado. Although loess in Nebraska lies downwind of the Nebraska Sand Hills
dune field, little or no loess appears to be derived from it, based on Pb-isotopic
compositions of K-feldspar. In fact, even the Platte River is an important source only
for those loess deposits that are very close to this fluvial source. The vast majority
of loess sediments in Nebraska are derived from silts of the White River Group, a
tertiary volcaniclastic siltstone that resulted from a major eruption 34 Ma (hence

its highly radiogenic composition).

3.4.6 Biologic Methods of Identifying Dust Sources

Our emphasis has been on physical properties of sediments or their mineralogical,
chemical, or isotopic compositions to identify dust sources. Biological materials can
also give important clues for mineral dust origins, because organic materials that are
in the form of fine-grained particles can be transported from the same sources as
mineral dust, can travel along the same pathways, can adhere to dust particles, and
can be deposited in the same geologic archives. Interestingly, one of the first written
observations of dust, linked to sources in Africa, was made by Charles Darwin, and
was based on biological materials. On his Beagle voyage in 1833, Darwin observed
dust aboard ship while in the vicinity of the Cape Verde Islands (Darwin 1846 ).
Examination of this dust by Darwin himself and by Christian Gottfried Ehrenberg
of Berlin showed that the samples contained abundant “infusoria” (obsolete term for
certain protozoans), but also phytoliths. Darwin ( 1846 ) speculated that the source of
the dust was Africa.
An interesting blend of both aquatic and terrestrial biological markers to identify
dust sources comes from a study of sediments in deep-sea cores in the eastern
Atlantic Ocean. In a core retrieved off northwestern Africa, Pokras and Mix ( 1985 )
generated an oxygen isotope stratigraphy (from foraminifera) of warm and cool
periods of the last interglacial/glacial cycle and determined the abundances of
Melosira and phytoliths in each depth interval. Melosira is a genus of freshwater
diatoms that includes many species that are abundant in all the large lakes of Africa.
Phytoliths are silica bodies produced in plants and are most abundant in grasses,
such as those that occur in the transitional zones between humid tropical forest and
desert regions of Africa. Pokras and Mix ( 1985 ) found that the greatest abundances
of Melosira occur in time periods that correspond either to glacial periods (marine
isotope stages [MIS] 6, 4 and 2) or cool interstadial periods (MIS 5.2 and 5.4). They
interpret these results to indicate that aeolian input of diatoms increased during cool
periods due to the desiccation of African lakes under arid conditions. In contrast,
during warm interglacial or interstadial periods (MIS 5.5, 5.3, 5.1, 3, and 1), more-
humid conditions prevailed, African lakes were extant, and aeolian diatom input
diminished. However, phytolith input increased during these times, due to northward