Pedogenic modification of loess: significance for palaeoclimatic reconstructions

This review considers the role of pedogenic processes in modifying wind-blo wn dust (loess), concentrating particularly on the ways that resulting prop erties may be interpreted as indicators of past climatic conditions and cha nges. Emphasis is placed on the sequences of palaeosols developed within lo ess deposits that are frequently regarded as some of the best terrestrial e quivalents of marine-sediment records of long-term global climatic change. A palaeosol is generally interpreted in terms of the broad pedogenic proces ses and environments assumed to be currently responsible for that type of s oil forming at the present surface. Even the very presence of a palaeosol m ay have palaeoclimatic significance, however, in that it is often taken to indicate a period of relative land surface stability and warmer and/or mois ter conditions between cold and/or and phases of loess accumulation. In rea lity, it may be more useful to consider many loess-palaeosol sequences in t erms of changing balances between pedogenesis and loess accumulation over g eological time. In most regions, it seems that the balance swings towards p edogenesis during interglacials or interstadials when sediment supply and t ransport are limited and the climate is warmer and/or wetter. Where accumul ation rates are still appreciable during these 'soil-forming intervals', ho wever, the soils and palaeosols may be accretionary with surface build-up k eeping pace with pedogenesis. Welding may also occur where covering sedimen ts are insufficiently thick to isolate an underlying palaeosol from the eff ects of pedogenesis active at a new land surface. Further complications occ ur due to reworking of palaeosols and syndepositional pedogenic alteration of loess units. Generally, such pedocomplexes can only be deciphered if the different pedogenic, geomorphic and sedimentary processes are identified a nd ordered within a pedosedimentary reconstruction. A recent trend has been to treat some loess-palaeosol sequences as quasi-continuous time series, p articularly when comparing depth functions of climatic-proxy properties suc h as magnetic susceptibility and grain size with marine and ice-core isotop e curves. (C) 2001 Elsevier Science B.V. All rights reserved.