Sources of strontium and calcium in desert soil and calcrete

The carbon-cycle significance of soil carbonate fluxes is subject to large uncertainties because it is not clear precisely how much calcium is derived from atmospheric sources compared with that from the chemical weathering o f silicate minerals. In the petrocalcic horizon (calcrete) of a Pleistocene soil from the USDA-SCS Desert Project area near Las Cruces, NM, approximat ely 1.5 g Ca/cm(3) has been added, with an associated expansion of the prof ile of similar to 200%. Strontium isotope values for the labile cations and carbonate from the A, B and K soil horizons have Sr-87/Sr-86 values that r ange From 0.7087 to 0.7093, similar to the values for easily soluble local dust and rain. The parent material, non-calcareous Camp Rice alluvial sedim ent, has a Sr-87/Sr-86 ratio of similar to 0.7165. Mixing calculations indi cate a minimum atmospheric contribution to soil carbonate calcium of simila r to 94%; the more likely scenarios indicate at least 98% of the Ca origina ted from atmospheric input. The variations in 87Sr/86Sr ratios of soil sili cate (0.7131 to 0.7173) are consistent with weathering of volcanogenic sedi ments and neoformation of clay minerals in the petrocalcic horizon. Moreove r, the Sr isotope data suggest that 50-70% of silicate in the uppermost 25 cm of the profile could be atmosphere-derived. The isotopic composition of labile strontium in the A horizon and the mass distribution of silicon and calcium indicate that the uppermost portion of the profile is the present z one for the release of cations due to silicate weathering. Steady-state mod els of the whole profile yield a Sr weathering flux ranging from similar to 200 to 400 mu g cm(-2) Ma(-1). The results indicate that both the present- day and long-term contribution of calcium from silicate weathering is less than 2% of that supplied from the atmosphere, and confirm that desert soil formation is not a significant sink for atmospheric carbon. (C) 1999 Elsevi er Science B.V. All rights reserved.