CLIMATE-ISOTOPE RELATIONSHIPS IN A MODERN VERTISOL CLIMOSEQUENCE, COASTAL TEXAS

Studies that address the isotopic development of soils within well-constrained climo- or chrono-sequences elucidate the relationship between climate, time, and the isotopic signatures of pedogenic minerals; these relationships form the basis of isotope proxies for climate, paleo-pCO2, and paleoecology. Results from a modern Vertisol climosequence along the Texas Coastal Prairie (present MAP ~60-150 cm/yr; age ~35kyr) support the potential use of such proxies, provided the results are considered in the context of the dominant physical and chemical pedogenic processes. The Texan Vertisols contain a pedogenic discontinuity between an active, upper soil domain and less active, lower domain. Pedogenic processes dominating the upper domain at the wet end of the climosequence are high biotic activity/productivity and extensive leaching. At the dry end, soils show evidence for aeolian influx and little chemical alteration of parent material. Except at MAP extremes, all of the soils show similar d¹³C SOM - depth trends, with more negative values in deeper portions of the profiles. These are interpreted to record a shift in the predominant flora from C3 (~80% C3) to C4 (~80% C4) over the past 35kyr. In all but the driest soils, fractionations between SOM and pedogenic carbonate (PC) indicate a close approach to equilibrium (D¹³C ~14-16 ‰). Relatively low soil productivity and low MAP appear to hinder isotopic equilibration of PC and SOM in soils formed under relatively dry conditions. In contrast, high productivity and relatively rapid PC leaching/recrystallization in the wettest soils produce an isotopic record that is more strongly influenced by recent climate conditions; d¹³C SOM values in these wet soils show the greatest influence of C4 flora. The d¹⁸O PC values, except in the driest sites, are consistent with PC precipitated in equilibrium with modern meteoric waters, and suggest that d¹⁸O values may be reset throughout pedogenesis, recording only relatively recent water compositions. Thus, in general, isotopic compositions along the Vertisol climosequence record similar histories of climate/ecological change over the past ~35ka, particularly in d¹³C SOM-depth trends. In detail, different rates of PC precipitation/mobility and soil productivity, controlled largely by climate (MAP), affect isotopic compositions in the upper soil domains.