LOESS MAGNETISM

Loess is a wind-blown Quaternary silt deposit that blankets vast tract s of land and in places reaches thicknesses in excess of 300 m. Over t he last decade it has emerged that certain loess sections have recorde d the polarity history of the geomagnetic field and now provide essent ially continuous magnetostratigraphic archives covering the last 2-3 m .y. Indeed, it is the chronology provided by the magnetic polarity sig nature itself that was largely responsible for establishing the timing of the initiation of loess accumulation, particularly in the celebrat ed Chinese loess plateau, where a starting date close to the Gauss-Mat uyama chron boundary (2.6 Ma) is now firmly established. This coincide s with a widely documented global climatic shift and accelerated uplif t of the Tibetan plateau. Many loess sections contain fossil soils (pa leosols) that bear witness to warmer and wetter climatic conditions co rresponding to interglacial periods in contrast to the cold, arid envi ronments in which pristine loess accumulated and which correspond to g lacial intervals. The resulting sequences of alternating loess and pal eosols also manifest themselves magnetically, in this case in terms of susceptibility changes, entirely distinct from the remanence characte ristics, which encode the geomagnetic polarity. The susceptibility tim e series obtained from localities in Alaska and China correlate remark ably well with the oceanic oxygen isotope signal and yield spectral po wer estimates in agreement with those predicted by the astronomical (M ilankovitch) theory of ice ages. Comparison of susceptibility patterns with corresponding profiles of Be-10 concentration in loess allows ma jor changes in rainfall to be estimated. In China, for example, data s panning the last 130 kyr (corresponding to oxygen isotope stages 1-5) indicate that paleoprecipitation was almost halved (from similar to 54 0 to similar to 310 mm yr(-1)) as the warm interglacial during which p aleosol S-1 formed gave way to the following glacial interval in which loess layer L(1) accumulated. It has also been found that increased a mounts of continent-derived dust delivered to the deep ocean correlate with loess formation and thereby permit certain broad features of atm ospheric circulation (paleowinds) to be worked out. Debate continues o ver the actual mechanism by which magnetic susceptibility becomes a cl imate proxy. The current consensus is that some form of in situ proces s must be responsible, at least in hart. Detailed laboratory investiga tions, both on whole samples and on magnetic extracts, indicate that t he enhancement observed in midlatitude weathered loess and paleosols i s largely due to a magnetically ''soft'' mineral which is either magne tite (Fe3O4) or maghemite (gamma-Fe2O3). Experimental evidence is accu mulating that tiny (<100 nm) ferromagnetic particles probably generate d by the activity of magnetotactic bacteria in the soil are responsibl e.