Loess magnetism

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Abstract
Loess is a wind‐blown Quaternary silt deposit that blankets vast tracts of land and in places reaches thicknesses in excess of 300 m. Over the last decade it has emerged that certain loess sections have recorded the polarity history of the geomagnetic field and now provide essentially continuous magnetostratigraphic archives covering the last 2–3 m.y. Indeed, it is the chronology provided by the magnetic polarity signature itself that was largely responsible for establishing the timing of the initiation of loess accumulation, particularly in the celebrated Chinese loess plateau, where a starting date close to the Gauss‐Matuyama chron boundary (2.6 Ma) is now firmly established. This coincides with a widely documented global climatic shift and accelerated uplift of the Tibetan plateau. Many loess sections contain fossil soils (paleosols) that bear witness to warmer and wetter climatic conditions corresponding to interglacial periods in contrast to the cold, arid environments in which pristine loess accumulated and which correspond to glacial intervals. The resulting sequences of alternating loess and paleosols also manifest themselves magnetically, in this case in terms of susceptibility changes, entirely distinct from the remanence characteristics, which encode the geomagnetic polarity. The susceptibility time series obtained from localities in Alaska and China correlate remarkably well with the oceanic oxygen isotope signal and yield spectral power estimates in agreement with those predicted by the astronomical (Milankovitch) theory of ice ages. Comparison of susceptibility patterns with corresponding profiles of 10Be concentration in loess allows major changes in rainfall to be estimated. In China, for example, data spanning the last 130 kyr (corresponding to oxygen isotope stages 1–5) indicate that paleoprecipitation was almost halved (from ∼540 to ∼310 mm yr−1) as the warm interglacial during which paleosol S1 formed gave way to the following glacial interval in which loess layer L1 accumulated. It has also been found that increased amounts of continent‐derived dust delivered to the deep ocean correlate with loess formation and thereby permit certain broad features of atmospheric circulation (paleowinds) to be worked out. Debate continues over the actual mechanism by which magnetic susceptibility becomes a climate proxy. The current consensus is that some form of in situ process must be responsible, at least in part. Detailed laboratory investigations, both on whole samples and on magnetic extracts, indicate that the enhancement observed in midlatitude weathered loess and paleosols is largely due to a magnetically “soft” mineral which is either magnetite (Fe3O4) or maghemite (γ‐Fe2O3). Experimental evidence is accumulating that tiny (<100 nm) ferromagnetic particles probably generated by the activity of magnetotactic bacteria in the soil are responsible.