COMPARING RATES AND PROCESSES OF SOIL DEVELOPMENT BETWEEN SUBTROPICAL AND ARID REGIONS USING WELL-DATED CHRONOSEQUENCES

The soil chronosequence, commonly established using soils formed on a series of alluvial deposits of different ages, provides a framework in which to evaluate rates of abiotic and biotic processes of soil development. One the biggest challenges in using the soil chronosequence approach is establishing meaningful geochronology; however, recent advances in luminescence (OSL) and terrestrial cosmogenic isotopes (TCN) has increased our ability to date alluvial deposits. We have established two well dated soil chronosequences (using OSL, TCN) in southern Baja California, Mexico (BC) and in the Mojave Desert (MD), southern California. Both sequences have formed in alluvium primarily derived from granitic rock, and have fan or terrace surfaces with general age ranges of ~0.5ka, ~2-5 ka, ~8-16ka, ~27-40 ka, ~50-65ka, and ~80-125ka. The BC sequence soils have formed under a modern sub-tropical climate (~350 mm ppt/yr) and the MD under an arid climate (~125-150 mm/yr), with annual precipitation roughly double during past intervals of wetter climate. Soil development occurs relatively faster in the BC soils; however, differences are not as great as might be expected given the considerable greater effective moisture for the BC soils. Comparison of major soil features between the BC and MD sequences show some similarity: soils on surfaces dated at (1) 8-16 ka have 8.75 YR hues, Bt horizons, and a maximum depth of B horizon development (MXB) of ~120 to 170 cm; (2) 27-40 ka have 7.5YR hues, Bt or Btk horizons, and a MXB of 130-230 cm; (3) 50-60 ka have 7.5-5YR hues, Bt or Bt/Bkm horizons, and a max B horizon depth of 225-250 cm; (4) 80-110 ka have 7.5 YR-5YR hues, Btk or Bt/Bkm horizons, and a max B depth of 250-300 cm. Soil rubification, iron-oxides, soil structure, and total soil depth increase with time in the BC soils faster relative to MD soils due to greater effective moisture, which mostly occurs during summer and late fall (MD moisture primarily winter-spring). Soil carbonate and accumulation of silt and clay increase faster in MD soils relative to BC soils due to incorporation of desert dust. Overall, results indicate that deep, well-developed soils can form in both arid and subtropical settings within 50-100 kyr.