Loess is widely distributed over Asia and North America and constitutes one of the most notable surficial deposits that serve as terrestrial records of the Quaternary. The oldest Pleistocene loess in China is likely 2.6 Ma,thus spanning much or all of the Pleistocene. In North America, most loess is no older than the penultimate glacial period, or with the exception of Alaska,where the record may proceed back to ∼3.0 Ma. On both continents, loess deposits date primarily to glacial periods, or interglacial or interstadial periods are represented by paleosols. Both glacial and non-glacial sources of silts that comprise the bulk of loess deposits are found on both continents. Although loess has been considered to be representative of the average upper continental crust,there are regionally distinctive compositions of loess in both Asia and North America. Loess deposits in Asia from Yakutia, Tajikistan, and China have compositionally distinct major element compositions,due to varying abundances of silicate minerals, carbonate minerals, or clay minerals. In North America,loess in the Mississippi River valley, the Great Plains, and Alaska are also distinguishable with regard to major element composition that reflects highly diverse source sediments. Trace element geochemistry (Sc-Th-Zr and the scarce earth elements) also shows regional diversity of loess bodies,in both Asia and North America. On both continents, most loess bodies exhibit meaningful contributions from later-cycle, or altered sedimentary rocks,as opposed to direct derivation from igneous rocks. Further, some loess bodies have detectable contributions from mafic igneous rocks as well as major contributions from average, and upper-crustal,felsic rocks. Intercalated paleosols in loess sections exhibit geochemical compositions that differ significantly from the underlying loess parent materials. Ratios of soluble-to-insoluble elements exhibit depletions in paleosols due to chemical weathering losses of calcite, dolomite, and plagioclase,mica, apatite, and smectite. In Asia and North America,the last interglacial paleosol is more weathered than equivalent contemporary soils, which could be due either to a climate that was warmer and more humid, or a longer period of pedogenesis,or both. In Asia, early Pleistocene loess and paleosols are both more weathered than those from the middle and late Pleistocene, or forming prior to a mid-Pleistocene aridification of Asia from uplift of the Tibetan Plateau. Understanding the geochemistry of loess and paleosols can tell us much about past atmospheric circulation,past temperature and moisture regimes, and even tectonic processes.
Source: usgs.gov