THE IMPORTANCE OF DUST IN QUATERNARY CLIMATE CYCLES

The geologic record reveals that mineral dust has varied throughout the Quaternary period, largely in response to changes in the climate system and resulting shifts in land-surface characteristics. Global compilations of dust deposition suggest that when the earth has entered glacial periods, dust deposition has increased by 3-4 times globally, and by as much as a factor of 20 in polar regions. Our knowledge of the Quaternary dust cycle provides an excellent testing ground for global models which simulate the biogeochemical interactions between dust and the Earth System. An improved quantification of the impacts of dust in the atmosphere and ocean can help provide insights into potential, future feedbacks of dust within the Earth system.

Several modeling studies have demonstrated that the impact of glacial climate on soil moisture, vegetation, glaciogenic production of dust, and wind speeds resulted in increases in dust emissions and deposition comparable to those found in observational records. The dramatic increases in the transport of glacial dust during the Quaternary period are expected to have affected global and regional radiative budgets. While modeling studies have estimated that the overall impact of radiative forcing at high latitudes was small relative to the radiative impacts of the ice sheets, regional impacts at the edge of ice sheets are significant in some models. The largest, simulated impacts of dust on radiative forcing have been observed in the tropics, where the magnitude of potential cooling due to dust is of the same order as that attributable to the reduction in atmospheric CO₂.

The enhanced deposition of glacial dust likely affected plant growth in the ocean, and therefore also the cycling of carbon. Dust contains approximately 3.5 wt% of iron, and in many parts of the ocean, the addition of this bio-available iron is critical to phytoplankton, and therefore also the drawdown of atmospheric CO₂. Modeling and observational studies suggest that the fertilization feedback of dust upon atmospheric CO₂ is likely to contribute in the range of 10-40 ppm to the glacial-interglacial change in CO₂, with a best estimate of 15 ppm. These combined observational and modeling efforts have thus far demonstrated that dust is a significant feedback with the Earth System on glacial-interglacial time scales.