Soil development over millennial timescales - a comparison of soil chronosequences of different climates and lithologies

This paper reports soil development over time in different climates, on time-scalesranging from a few thousand to several hundred thousand years. Changes in soil propertiesover time, underlying soil-forming processes and their rates are presented. The paper is basedon six soil chronosequences, i.e. sequences of soils of different age that are supposed to havedeveloped under the similar conditions with regard to climate, vegetation and other livingorganisms, relief and parent material. The six soil chronosequences are from humid-temperate,Mediterranean and semi-arid climates. They are compared with regard to soil thicknessincrease, changes in soil pH, formation of pedogenic iron oxides (expressed as Fed/Fet ratios),clay formation, dust influx (both reflected in clay/silt ratios), and silicate weathering andleaching of base cations (expressed as (Ca+Mg+K+Na)/Al molar ratios) over time. Thiscomparison reveals that the increase of solum thickness with time can be best described bylogarithmic equations in all three types of climates. Fed/Fet ratios (proportion of pedogenic ironFed compared to total iron Fet) reflects the transformation of iron in primary minerals intopedogenic iron. This ratio usually increases with time, except for regions, where the influx ofdust (having low Fed/Fet ratios) prevails over the process of pedogenic iron oxide formation,which is the case in the Patagonian chronosequences. Dust influx has also a substantialinfluence on the time courses of clay/silt ratios and on element indices of silicate weathering.Using the example of a 730 ka soil chronosequence from southern Italy, the fact that soils oflong chronosequences inevitably experienced major environmental changes is demonstrated,and, consequentially a modified definition of requirements for soil chronosequences issuggested. Moreover, pedogenic thresholds, feedback systems and progressive versusregressive processes identified in the soil chronosequences are discussed.