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Paleoclimatologists are using wind-borne dust deposits in China to study the last 2.6 million years of continental climate change.
by Subir K. Banerjee and Mike Jackson, Institute for Rock Magnetism, University of Minnesota, Minneapolis, Minn.
Changes in the Earth's climate are recorded in the physical properties of sediments that accumulate in the ocean and on land. Sediment is deposited more or less continuously on the ocean floor, and by measuring the oxygen isotope stratigraphy of the layers of sediment, scientists can determine past climate conditions of the oceans. But on land, sediment is deposited less continuously and less uniformly around the globe. As a result, little is known about the climate history of most of the continents, except for Antarctica and Greenland, whose past climates are recorded in the long glacial ice cores that scientists drill. Determining what past climate was like in the temperate and tropical regions of the continents has posed a difficult problem for scientists until recently. But now they are turning to the magnetic properties of thick (100–300 meter) deposits of wind-borne dust, called loess, from China to provide new insight into continental climate history. These deposits vie with the ice cores in providing a continuous record of climate over the last 2.6 m.y. Although the loess paleoclimate records are not as detailed as those from the ice cores, they are the best source of continental paleoclimate information for non-arctic regions.
The paleosols and loesses can generally be distinguished visually, allowing scientists to recognize and count the number of glacial-interglacial cycles. Magnetic studies make it possible to be more quantitative: sediment ages can be determined by means of the magnetic polarity timescale, and the magnitude of climate changes can be related to the magnitude of variation in magnetic properties. Magnetic measurements are easily and rapidly made in the laboratory or in the field, and they are highly accurate. In the last decade, many scientists who study past continental climate change have begun to investigate the magnetic properties of Chinese loess/paleosol sequences, in addition to more traditional methods that consider the sediment's major-element, trace-element, or isotopic geochemistry.
Paleosols deposited between glacial periods are about 200 times more magnetic than loess deposited during glacials. The increased magnetic susceptibility values may result (in principle) from either a higher concentration of magnetic iron-bearing minerals in the interglacial wind-borne dust, or formation of such minerals from preexisting nonmagnetic or magnetic materials, as a result of chemical changes during soil formation. In the Chinese loess, we have found soil formation to be the principal factor controlling susceptibility, as described in the next section. In either case, magnetic measurements provide a new measure of climate change from glacial to interglacial epochs. Calibration of the sediment ages against the magnetic polarity timescale shows that susceptibility in the loess of central China varies at the same milankovitch frequencies determined for ocean sediments and high-latitude ice cores.
In modern soils on the loess plateau, the abundance of superparamagnetic material is strongly correlated with annual rainfall. The superparamagnetic grains are a by-product of chemical reactions related to soil formation, enhanced by moisture. Thus variations in magnetic susceptibility in the paleosol layers can be used to estimate paleorainfall across the loess plateau, using modern rainfall data for calibration. This technique has yielded some unexpected results. For example, the western part of the Chinese loess plateau is now arid, but 6,000–9,000 years ago it had 60–100% more rainfall than today. On the other hand, rainfall at the eastern sites has remained steady for the last 8,000 years.
Furthermore, we tested the local versus global components of climate change by comparing two sites, Baicaoyuan and Xifeng, which are only 200 kilometers apart, but on either side of a rainshadow, the nearly north-south Liupan mountains in the western loess plateau. Overall, the intensity of soil formation as measured by magnetic proxies shows that for most of the last 130,000 years, soil formation was indeed weaker in arid Baicaoyuan than in humid Xifeng. During early Holocene, however, soil formed at both sites at the same rate. The eastern plateau, it seems, has always experienced the same high rainfall during the current and previous interglacials. Although more work remains to be done in using magnetic methods to study past climate, it is clear that the high sensitivity and rapid measurement time for magnetic parameters are a great boon to obtaining regionally averaged continental paleoclimate proxies through magnetism. The hard work of separating wind-controlled effects from soil formation effects is underway in many countries. It is hoped that it will lead to more accurate measurements of paleorainfall intensities as well as past wind strengths.
Source: Eos, January 2, 1996, p. 3.
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