Calcium Dynamics in a Marine Terrace Chronosequence: New Insights From Calcium Stable Isotopes

Understanding the origins, distribution, transport and cycling mechanisms of calcium during progressive mineral weathering and soil development is critical to determining the status of this important ecosystem nutrient. In previous studies of Ca dynamics at the watershed scale, strontium isotopes have proven useful for identifying mineral sources of calcium and for distinguishing mineral weathering from atmospheric sources of calcium, but are inherently unable to provide information about Ca cycling during processes such as biologic uptake and ion exchange. In order to gain a more direct understanding of Ca dynamics during mineral weathering, soil development and ecosystem evolution, we have initiated a study of the Ca stable isotope systematics of a chronosequence of marine terrace deposits north of Santa Cruz, California. The five terraces that comprise the chronosequence range in age from 65 ka to 226 ka, and are made up of reworked granitoid minerals derived largely from the Miocene Santa Margarita sandstone and Santa Cruz mudstone. In soil profiles collected from each terrace, Ca is concentrated mainly in plagioclase feldspar in the deepest samples and on soil exchange sites near the surface, but has been largely depleted due to weathering from intermediate-depth samples. δ⁴⁴Ca values of deep porewaters from each terrace are uniform and identical to that of Ca in plagioclase (~-0.6‰ ), consistent with a purely weathering source. δ⁴⁴Ca values of shallow porewaters systematically increase with increasing terrace age from values similar to that of Ca in grasses collected from each site (~-2.0‰ ) toward that of Ca in seawater and local precipitation (~0.0‰ ). In contrast, δ⁴⁴Ca values of intermediate depth porewaters decrease with increasing terrace age, in opposite sense to the variation observed for the shallow porewaters. We attribute this contrasting variability to the progressive development of a clay fraction over the soil profile that promotes preferential retention of relatively heavy Ca in the shallow soils and allows downward transport of relatively light Ca to intermediate depths. Our results reveal that both biotic and abiotic processes are responsible for determining the Ca stable isotope distribution in these soils.