GEOCHEMISTRY OF SOIL SALTS IN THE HYPER-ARID CORE OF THE ATACAMA DESERT, CHILE

Soils in the Atacama Desert contain thick (3-8m) accumulations of soluble salts of eolian origin, including nitrates, sulfates, chlorides, perchlorates, and phosphates. In 2005 we described and sampled three deep (~3m) soil profiles within the Baquedano nitrate region of the Atacama Desert to characterize the nature and concentration of saline salts with depth in soil profiles within the hyperarid core of the Atacama. Two soil profiles (Estoncia Ercilla and Rencoret N.W.) are located on old, stable alluvial fans, whereas the third soil profile (Valenzuela) is on a fluvial terrace. The age of these geomorphic landforms are not known, but are thought to be among the older landscape surfaces in the Atacama (5-15 Ma?). Each profile consists of an upper set of unconsolidated soil horizons extending ~20cm below the surface, and a set of very well-cemented lower soil horizons. At Estoncia Ercilla (EE) the soil contains ~37% soluble salt (6.38% NO₃, 5.7% SO₄, 4.35% Cl, 0.061% PO₄), with an average bulk density of 2.38 g/m³. At Rencoret N.W. (Rnw) the soil contains ~34% soluble salt (2.04% NO₃, 12.48% SO₄, 3.11% Cl, 0.071% PO₄), with an average bulk density of 2.15 g/m³. At Valenzuela (V) the soil contains ~26% soluble salt (2.36% NO₃, 12.09% SO₄, 2.21% Cl, 0.102% PO₄), with an average bulk density of 2.19 g/m³. Total NO₃ and SO₄ inventories for EE, Rnw, and V are 151, 44 and 51 kg NO₃ m^-3 and 135, 268, and 264 kg SO₄ m^-3, respectively. Total Cl for EE, Rnw, and V is 99, 52, and 58, kg Cl m^-3, and PO₄ inventories are 1.45, 1.54, and 2.24 kg PO₄ m^-3 respectively. Nitrate and phosphate are limiting nutrients that play a key role in regulating global biogeochemical systems. The age of these surfaces will be determined with cosmogenic nuclides in order to help constrain pre-industrial flux rates of these nutrients and to characterize the nature of soil development on older landscape surfaces in the Atacama.