The Geologic Record of Dust Deposition

Dust flux in the Earth-atmosphere system has varied over both space and time. Dramatic shifts in rates of dust entrainment and deposition have occurred on glacial-period-to-interglacial-period timescales. For example, over much of the Earth's surface, dust deposition rates were much greater during the last glacial period than during the present interglacial period. Study of geologic records of dust composition, sources and deposition rates is important for understanding the role of dust in the overall planetary radiation balance, the role of dust in fertilization of organisms in the world's oceans, the role of dust in nutrient additions to the terrestrial biosphere and soils, and for paleoclimatic reconstructions. Geologic records of dust flux occur in a number of depositional archives for sediments: (1) large ice sheets, such as those on Greenland and Antarctica, (2) deep-ocean basins, (3) lakes, (4) loess-paleosol sections, and (5) soils. These archives have several distinct characteristics that make them highly suitable for understanding the dynamics of dust entrainment, transport, and deposition. First, sediment archives are often distributed over wide geographic areas, which permits reconstruction of spatial variation of dust flux. Second, a number of dating methods can be applied to sediment archives, which allows for identification of specific periods of greater or lesser dust flux. Third, eolian sediment particle size and composition can be determined so that dust source areas can be ascertained and dust transport pathways can be reconstructed. Geologic records of dust flux in the past ~20,000 years generally show good agreement with models of dust flux, but modeled rates on some continents still show underestimates compared to the geologic record, emphasizing the need for continuing work for both modelers and geologists.