ONE MILLION YEARS OF SOIL DEVELOPMENT IN PATAGONIA: PRELIMINARY RESULTS FROM LAGO BUENOS AIRES, ARGENTINA (46°S)

Soils developing on a sequence of 17 moraines in the vicinity of Lago Buenos Aires, Argentina (46°S) provide an excellent means to address long-term pedogenesis in Patagonia. Ages of the moraines are constrained by stratigraphic correlation with three ⁴⁰Ar/³⁹Ar dated basalt flows (one moraine >1 Ma, six moraines between 1 Ma and 760 ka, four between 760 ka and 109 ka, and six <109 ka), as well as preliminary cosmogenic surface exposure ages (the six youngest moraines 13-21 ka, the next oldest 123-165 ka). Despite their antiquity, the moraines are remarkably well preserved, indicating a stable landscape and limited soil erosion over this time. Moraine structures are highly variable, including compact basal till (48/43/9 - percent sand/silt/clay), ice contact stratified gravels (90/7/3), and lake sediments (17/61/22).

Fourteen soils from seven moraines were described to a depth of ca. 80 cm and tentatively classified as Typic Haplocryids (3) and Typic Calcicryids (11). Many older soils have thick A horizons but do not classify as mollic epipedons because they are too dry. Pedogenesis in this area is dominated by calcification; younger soils on last glacial moraines have ~5% carbonate in the calcic horizons, whereas these horizons in older soils contain ~15% carbonate. There appears to be a limit to the concentration of carbonate in the older calcic horizons as similar concentrations occur in soils 700 k.y. different in age. Argilluviation does not seem to be an active pedogenic process in these soils. Clays are dominated by illite with lesser kaolinite and chlorite. The absence of smectite is unexpected considering the dominantly arid climate.

Our working hypothesis is that these soils experienced wetter climates during glacial periods, but conditions were not moist enough to cause noticeable clay translocation and accumulated carbonate during drier interglacials. A northward migration of the Antarctic Polar Front during glacial times and the consequent increase in the role of the polar easterlies in the area is one possible source of moisture. There are few paleoclimate records on the east side of the Andes; these soils may therefore provide important insight to regional climatic changes through a complete glacial cycle.