An investigation of the pedogenesis of soils developed in Quaternary alluvial deposits of eastern Crete

A pedologic investigation of an area of highly calcareous Late Quaternary alluvial sediments in northeastern Crete, Greece was undertaken to study the nature and distribution of the soils formed in these deposits and to evaluate the pedogenic processes responsible for their development. The specific objectives of this research were; (i) to describe and classify the soils, (ii) examine the nature and distribution of various forms of iron, manganese and silicon in these soils in order to better understand the weathering processes which had occurred, (iii) characterize the mineralogical properties of the soils and sediments to elucidate the processes responsible for the mineral assemblages identified and (iv) to develop a model of soil and landscape genesis for these alluvial soils. All four soils exhibited red-colored, clay-rich surface horizons. Calcic horizons had developed in all soils which resulted in paling of subsoil colors. Two pedons were classified as sandy-skeletal, carbonatic, thermic Calcixerollic Xerochrepts and two were sandy-skeletal, carbonatic, thermic Fluventic Xerochrepts. The major pedogenic processes identified were rubification, accumulation of clay and trace elements, notably Ti, and calcic horizon development. Morphological characteristics of the calcic horizons varied from Stage 1 to III. These features suggested an age of latest Pleistocene to early Holocene for these soils. Iron, Mn and Si contents were highest in surface horizons, decreasing with depth. Silicate- and dithionite-Fe as a fraction of total-Fe also suggested a maximum upper age limit of Late Pleistocene to early Holocene. Active iron ratios were low (0.05 to 0.15) which suggested rapid conversion of amorphous iron compounds to crystalline phases. Goethite was the dominant secondary iron phase. Manganese in these soils existed primarily as secondary oxides and organically-bound species. Hydroxylamine-HCl extracted an average of 58.8 % of the total Mn, while removing only 0.05 to 4 % of the total iron. This suggested that even in these highly calcareous soils hydroxylamine is still specific for Mn-compounds. Primary silicates were the major source of Si in these soils; however, dithionite. IV oxalate and hydroxylamine all extracted varying proportions of Si. These fractions were probably associated with the secondary Fe- and Mn- compounds. Dolomite, calcite, quartz, illite, chlorite, kaolinite, feldspars (K-spar and plagioclase) and goethite were identified in these soils. Coarse and fine silt fractions were dominated by dolomite, calcite, and quartz. Clay fractions contained chlorite and illite, with minor amounts of kaolinite, quartz, dolomite, calcite and goethite. The extent of clay mineral inheritance, limited expansion of the chlorite and the lack of a chlorite or mica weathering product (vermiculite), as well as the presence of carbonates in the clay fractions all suggested that these soils were relatively young. The following sequence of events was proposed for development of the sediments and soils of these alluvial deposits. A period of valley-filling followed by stabilization and stream incision, possibly in the Middle Pleistocene, led to the development of marine and fluvial terraces which now exist as conglomeratic inliers surrounded by younger sediments. Continued valley filling and landscape adjustment probably occurred throughout the Late Pleistocene, but the extent of this activity is unknown. One last period of alluviation probably occurred near the transition between the Pleistocene and Holocene, in response to climatic change. This event could have taken place between 8,300 and 9,000 BP based on evidence from southern Crete (Bottema, 1980). Weathering and soil development was probably rapid up to about 7,500 BP, when a shift toward a drier climate would have changed the rate of weathering. Calcic horizon development was probably initiated by about 8,000 BP in response to the shift toward drier conditions. Another period of increased weathering and clay accumulation likely took place between 7,500 and 7,300 BP, as pollen data for this period implied a re-expansion of forests in Crete (Bottema, 1980). Modem soil forming conditions have probably been controlling soil genesis in the area since 4,500 to 4,000 BP. Periods of extreme climate, combined with human population pressure, have likely resulted in brief episodes of landscape instability over the last 4000 to 5000 years