A Strontium Isotope Reconnaissance of a Marine Terrace Chronosequence in Central California, USA.

Long-term sampling locations have been established on a chronosequence of marine terraces north of Santa Cruz, CA. Investigation of these sites will allow a multi-disciplinary assessment of mineral weathering and soil formation processes. The five surfaces comprising the chronosequence (terraces 1-5) have been dated at 65ka, 92ka, 137ka, 139ka, and 226ka, respectively, by Perg et al, 2001 using cosmogenic radionuclides (CRN). Soil horizons have developed in the sediments covering the terraces, derived locally from the Miocene Santa Margarita sandstone, Santa Cruz mudstone, and the Cretaceous Ben Lomond Granodiorite. The terrace sites have been sampled and instrumented with suction water samplers and precipitation collectors. Bi-monthly collection of soil waters, surface waters, precipitation, and vegetation are on going. This component of the study will use Sr isotopes (87Sr/86Sr) to understand sources, cycling, and behavior of strontium and associated base cations. 87Sr/86Sr measurements have been made on a variety of materials from terrace sites 1-5, including precipitation, soil waters, ground and surface waters, vegetation, ammonium-acetate extractable soil Sr, and soil digests. Additionally, 87Sr/86Sr measurements have been made on local bedrock and beach sands. The measured values of 87Sr/86Sr in the samples range from 0.706 in deep soil water and soil exchange extracts to 0.710 in surface waters and soil digests. Isotopic values for eight precipitation samples at the terraces average 0.7091, reflecting the marine influence. Depth profiles of ammonium-acetate exchangeable soil Sr and soil waters from equivalent depths yield similar values and trends in 87Sr/86Sr (from 0.709 at the top toward 0.706 at 6 meter depth), suggesting that equilibrium is established between exchange sites and associated waters. Decreasing 87Sr/86Sr values with depth suggest a dominant influence of precipitation derived Sr on the cation exchange pool at the surface and emergence of a mineral weathering signal at depth. The Sr isotopic data in conjunction with cation chemistry will allow for formulation of Sr budgets on each of the terraces and ultimately, temporal comparisons of Sr behavior across the chronosequence.