SOIL-FORMING RATES AND PROCESSES ON QUATERNARY MORAINES NEAR LAGO BUENOS AIRES, ARGENTINA

Thirty-four pedons on 4 moraine groups (12 individual moraines), with ages constrained to 16-24, 110-185, 200-760, and 760-1016 ka, are used to determine the rates and mechanisms of soil development in the Santa Cruz province of Argentina. The majority of the soils are classified as Calcic Haploxerolls.  They occur under short grass steppe with about 200 mm/yr precipitation and are forming in glacial sediment derived from igneous and metamorphic rocks of the Andean Cordillera. The dominant soil-forming processes are melanization (accumulation of organic matter) and calcification (accumulation of secondary carbonates). Dust is a major pedogenic factor in Patagonia. The organic carbon (OC) and carbonate composition of one dust sample collected over a one-month period are 4.3% and 7.2% respectively. An array of 6 dust collectors is currently deployed to broaden the spatial and temporal scope of this sampling.

Age related differences in soil development on moraines were tested with ANOVA. A-horizon OC quantities increase with age (1.3, 1.4, 2.4 and 2.4 kg/m² for the four groups) and are statistically different at the p=0.001 confidence level. This may be partially related to the slightly higher elevations of the older moraines, which therefore experience more precipitation and less evapotranspiration. The maximum carbonate morphology stage (II, III, III+, and III+), thickness of the calcic (Bk) horizons (46, 60, 57, and 62 cm), and mean profile development indices (89, 110, 116, and 110 index units.cm) follow increases of calcium carbonate accumulation with age (3.3, 16.6, 42.1 and 31.8 kg/m²). The younger 3 groups differ from one another at the p=0.003 and p=0.05 level, however the 2 oldest groups are statistically similar. Long-term rates of carbonate accumulation are similar for the youngest three moraine groups but are much lower for the oldest group (183, 118, and 140 as opposed to 40 g/m²/kyr; different at the p=0.003 level).

The implications of these data are: 1) soil OC appears to be stable in this environment and profile quantities of OC continue to increase over very long time periods, 2) rates of carbonate influx in this area are less than in other areas (e.g. arid southwest), and 3) carbonate influx has been constant for the last few glacial-interglacial cycles, but an equilibrium in total carbonate is approached after ~800 kyr.