EFFECTS OF REGIONAL CLIMATE DIFFERENCES ON RATES OF SOIL DEVELOPMENT: INSIGHTS FROM WELL-DATED CHRONOSEQUENCES IN THE RIO GRANDE RIFT

Soil development in arid and semiarid regions of the southwestern United States is predominantly controlled by influx of eolian dust, yet our ability to quantify the rates of dust and carbonate incorporation into these calcic soils is limited by available age control. We described soil profiles on terraces in the Socorro area of the central New Mexico Rio Grande Rift valley and analyzed their silt, clay, and carbonate contents. These soils have well-established direct- or indirect-age control that we use to compute rates of dust and carbonate accumulation for durations of ~0.5-800 ka. We also compute the profile development index (PDI) for these soils and compare our chronofunction to PDIs from northern and southern New Mexico. Soils in the Socorro area are gravelly loamy sands with A/Av-Bw/Bk-Ck-C horizon sequences that are formed primarily in volcaniclastic alluvium. Principal components analysis identifies four dominant soil properties that explain variations in soil age; total texture, color lightening, dry consistence, and CaCO₃ stage morphology. These properties together explain 91% of the variation in our dataset, supporting models of soil development by incorporation of eolian dust. We find that the net silt-and-clay contents in B horizons of progressively older soils increases at rates similar to the profile-mass carbonate contents. Our power-law regressions for these properties yield slopes of 0.34 and 0.30, respectively. We find a similar slope of 0.32 for our power-law regression through the PDI data. A well-dated chronofunction from the Los Alamos area of northern New Mexico indicates that soil development occurs more rapidly in higher latitude regions of New Mexico than in the Socorro area and farther south. We interpret this trend in light of a regional climate gradient manifested by greater mean annual precipitation (~430 mm/yr vs. 230 mm/yr) and cooler mean annual temperatures at higher latitudes and/or slower rates of eolian dust accumulation into the soil profile. This setting would promote greater mobility of available silt and clay, and also drive faster rates of in-situ weathering compare with soils in central and southern New Mexico, as indicated by the presence of argillans in late-Pleistocene soils near Los Alamos.