Rainfall and the amount of water available to leach ions from soil are among the most important features determining mineral weathering, secondary mineral synthesis and soil chemical properties. Along an and to humid climosequence on Kohala Mountain, Hawaii, we sampled 16 soil profiles and found that weathering and soil properties change in a nonlinear fashion with increased rainfall. The lavas are influenced by a strong rain shadow with mean annual precipitation (MAP) averaging 160 mm near the coast and rising to >3000 mm near the summit. A temperature decline from 24 to 15 degreesC with increasing elevation is matched by lower potential evapotranspiration (ET). A water balance model (monthly precipitation minus monthly ET) defines three broad climate zones along the sampling transect: an and zone with moisture deficit in every month, an intermediate zone with moisture deficit during low-rainfall summer months and moisture surplus during high-rainfall winter months, and a humid zone with moisture surplus during every month. The annualized water balance can be ratioed with the integrated porosity of the top meter of soil to provide a leaching index.The index reaches 1 (total filling of the pore space on an annual basis) at about 1400 mm MAP. Index values >1 imply intense leaching conditions because of pore water replacement. In these 170 ka soils, leaching losses of soluble base cations and Si are nearly complete at index values >1, whereas only 60% of Al has been lost. At index values <1 leaching losses are progressively lower with the lowest rainfall sites having lost 10-20% of the original base cations and Si and none of the Al. At all sites, the secondary clay mineral assemblage is dominated by metastable noncrystalline weathering products; humid soil profiles contain very few crystalline minerals whereas the and profiles contain halloysite, hematite, gibbsite and small amounts of carbonates. Soil surface exchange properties are influenced strongly by climatic conditions and show a dramatic threshold in base cation saturation, pH and effective cation exchange capacity (ECEC) at leaching index of 1 (1400 mm MAP). Soils with leaching index of <1 have high base cation saturation, near-neutral pH and high ECEC. At MAP >1400 mm, soil buffering capacity has been totally exhausted leading to low pH and low ECEC.The nonlinear decline in ECEC is irreversible under natural conditions; base cation depleted soils will remain so even if the climate shifts to drier conditions. In contrast, a climate shift to wetter conditions can drastically modify surface chemical properties existing in the drier soils as weathering depletes primary minerals, elements are lost to leaching, and surface chemistry is modified. There is a time-dimension implied in climate gradient studies; soils forming in recently rejuvenated landscapes contain more primary minerals and should experience loss of buffering capacity at higher rainfall. Loss of buffering capacity means that biological acidity will move more deeply into the vadose zone or into the aquatic system. The details of this transfer depend on present and past climate, and the age and erosional stability of landscapes. (C) 2003 Published by Elsevier B.V.

The Cimarron Bend of southwestern Kansas is characterized by loess, dunes, and sand sheets that reflect a complex eolian–alluvial history. Stratigraphic evidence indicates that at least two major dune and sand sheet reactivation episodes occurred. These follow intervals of alluviation, stability, and soil development in the Holocene. Radiocarbon ages from paleosol horizons bracket these periods and their δ13C values provide evidence of a shift from a cooler, wetter climate in the late Pleistocene and early Holocene dominated by C3 plants, to a warmer, drier middle Holocene period characterized by an increase in C4 plants. Stable isotopic results for late Holocene soils may reflect local rather than regional climatic conditions. Our results compare favorably with studies in the central and southern Great Plains. Together, these results imply that a regional change toward a warmer, drier climate occurred from the early to the middle Holocene on the southern Great Plains.

Alteration of paleosols following development and burial is common. Chemical, physical and mechanical processes can weld younger to older soil profiles and affect the accumulation, dissolution, and reprecipitation of mineral material in soils. Soil properties most affected include texture and porosity, and, the content and distribution of soluble salts, amorphous silica, gypsum, carbonate, and clay. Organic matter retention is highly dependent on post-burial biologically controlled oxidation reduction and pH conditions. Processes such as erosion and deposition can truncate profiles or bury them either rapidly or extremely slowly. Effects of these and other processes on soil properties must be examined with care in paleosols that have been buried even for a short interval. This study presents several examples of soil properties which should be evaluated carefully and possibly avoided as indicators in paleoenvironmental reconstruction, paleosol classification, and regional stratigraphic correlation.

The stable C isotope composition of organic matter and opal phytoliths in diverse ecosystems demonstrate that soils carry a C isotopic signature that reflects long-term inputs of above- and below-ground C3 or C4 biomass. The utility of these isotopic characterization data for paleoenvironmental study is based on a knowledge of dominant soil forming processes as well as geomorphic and climatic conditions. This paper reports on both the theory and applicability of isotopic characterization of soil organic C and opal phytoliths. The theoretical perspectives are discussed in light of broad environmental applications. Details of the sample collection and preparation are provided with insights into quantities and pretreatment required for stable C isotopic characterization. Two case studies from the central Great Plains region are presented. Paleoclimatic interpretations are discussed for a portion of the central Great Plains. These interpretations are based on the stable C isotope data recovered from Holocene paleosols. These data indicate higher proportions of C3 vegetation persisted during the early Holocene. The concordance in the C isotopic signatures of soil organic matter and phytoliths provide strong biological evidence of regionally cooler conditions. This C isotopic concordance also appears during the mid-Holocene; C isotope values indicate an increase in the proportion of C4 vegetation, which reflects regionally warmer climatic conditions than present. Isotopic discordance in soil organic C and phytoliths can indicate the degree of diagenesis resulting from pedological alteration. Geomorphic and pedologic evidence indicate that the C isotope discordance between soil organic matter and opal phytoliths is the result of local topographic variations and spatial heterogeneity associated with plant distribution.

Radiocarbon dates of paleosols in northeastern Colorado indicate distinct periods of stability and soil formation with intervening periods of instability resulting in soil truncation or burial, A combination of pedologic and geomorphic indicators were used to resolve the duration of, and prevailing climate during, these periods, Five sites, each having a paleosol, were examined using both traditional soil analyses and grain-size statistics, the latter to decipher the mode of parent material deposition. Twenty local stream, dune and bedrock deposits were analyzed using grain-size statistics to establish benchmarks for comparison with soils, Field investigation supported by grain-size frequency statistics indicate early Holocene, middle Holocene and contemporary soils all formed in alluvium. Organic C and phytolith data suggest the early and middle Holocene climatic conditions were more favorable for plant productivity than the present climate. Soil development in early and middle Holocene paleosols suggests wetter soil moisture regimes than present. Low parent material carbonate contents suggest an eolian source for the carbonate in the Bk and Btk horizons. The presence of paleosol Btk horizons suggests a decrease in precipitation at the end of soil-forming intervals followed by drought and subsequent soil burial.

The uppermost surficial stratigraphic unit south of the Arkansas River in the Kansas High Plains is a previously unidentified middle Holocene or younger loess. The unit fits both a loess-thickness and a particle- size model for loess distribution with increasing distance from source. A soil immediately below this unit is radiocarbon dated 6000 to 6700 years BP. The radiocarbon ages indicate that the loess unit is younger than the commonly reported ranges for Bignell Loess in the Great Plains and demonstrate a need for re-examining Holocene loess stratigraphy of the Great Plains. In some locations closest to the Arkansas valley source, this middle Holocene unit is overlain by, or interfingers with, dune sands. The presence of these aeolian sands indicates that, following a period of relative landscape stability and soil development, a shift toward a middle- Holocene climate in which aeolian processes dominated occurred about 6000 years ago on the High Plains of west-central Kansas.