Chemical weathering of a marine terrace chronosequence, Santa Cruz, California. Part II: Solute profiles, gradients and the comparisons of contemporary and long-term weathering rates Art F. White a, * , Marjorie S. Schulz a , David A. Stonestrom a , Davison V. Vivit a , John Fitzpatrick a , Tom D. Bullen a , Kate Maher b , Alex E. Blum c a U.S. Geological Survey, Menlo Park, CA 94025, USA b Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, USA c U.S. Geological Survey, Boulder 80303, CO, USA Received 30 April 2008; accepted in revised form 28 January 2009; available online 14 February 2009 Abstract The spatial and temporal changes in hydrology and pore water elemental and 87 Sr/ 86 Sr compositions are used to determine contemporary weathering rates in a 65- to 226-kyr-old soil chronosequence formed from granitic sediments deposited on mar- ine terraces along coastal California. Soil moisture, tension and saturation exhibit large seasonal variations in shallow soils in response to a Mediterranean climate. These climate effects are dampened in underlying argillic horizons that progressively developed in older soils, and reached steady-state conditions in unsaturated horizons extending to depths in excess of 15 m. Hydraulic fluxes (q h ), based on Cl mass balances, vary from 0.06 to 0.22 m yr 1 , resulting in fluid residence times in the terraces of 10–24 yrs. As expected for a coastal environment, the order of cation abundances in soil pore waters is comparable to sea water, i.e., Na > Mg > Ca > K > Sr, while the anion sequence Cl > NO 3 > HCO 3 > SO 4 reflects modifying effects of nutrient cycling in the grassland vegetation. Net Cl-corrected solute Na, K and Si increase with depth, denoting inputs from feldspar weathering. Solute 87 Sr/ 86 Sr ratios exhibit progressive mixing of sea water-dominated precipitation with inputs from less radiogenic pla- gioclase. While net Sr and Ca concentrations are anomalously high in shallow soils due to biological cycling, they decline with depth to low and/or negative net concentrations. Ca/Mg, Sr/Mg and 87 Sr/ 86 Sr solute and exchange ratios are similar in all the terraces, denoting active exchange equilibration with selectivities close to unity for both detrital smectite and secondary kao- linite. Large differences in the magnitudes of the pore waters and exchange reservoirs result in short-term buffering of the sol- ute Ca, Sr, and Mg. Such buffering over geologic time scales can not be sustained due to declining inputs from residual plagioclase and smectite, implying periodic resetting of the exchange reservoir such as by past vegetational changes and/or climate. Pore waters approach thermodynamic saturation with respect to albite at depth in the younger terraces, indicating that weathering rates ultimately become transport-limited and dependent on hydrologic flux. Contemporary rates R solute are esti- mated from linear Na and Si pore weathering gradients b solute such that R solute ¼ q h b solute bS v where S v is the volumetric surface area and b is the stoichiometric coefficient. Plagioclase weathering rates (0.38–2.8  10 15 mol m 2 s 1 ) are comparable to those based on 87 Sr/ 86 Sr mass balances and solid-state Na and Ca gradients using analogous gradient approximations. In addition, contemporary solute gradients, under transport-limited conditions, approximate long-term solid-state gradients when normalized against the mass of protolith plagioclase and its corresponding 0016-7037/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.gca.2009.01.029 * Corresponding author. Fax: +1 650 329 4538. E-mail address: afwhite@usgs.gov (A.F. White). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 73 (2009) 2769–2803