Tracing spatial and temporal variations in the sources of calcium in pedogenic carbonates in a semiarid environment

In arid environments, significant amounts of calcium (Ca) are stored in pedogenic carbonates that develop in soils. There are two sources for Ca in pedogenic carbonates in soils: local parent material and dust deposited on the surface. In this study, the strontium (Sr) isotopes (⁸⁷Sr/⁸⁶Sr ratios) were used as a tracer for Ca from these sources. We studied spatial and temporal variations in the sources of Ca in pedogenic carbonates in soils developed in six Quaternary basalt flows and a cinder deposit in the Zuni-Bandera volcanic field near Grants, NM. Young basalt is an excellent rock type on which to study soil development because it provides a chemically and isotopically uniform substrate. Age dating of the flows provides control for the beginning of soil development. Other rock types (with varying ⁸⁷Sr/⁸⁶Sr ratios) cropping out around the basalt provide an opportunity to evaluate spatial variations in sources of Ca and Sr. The ⁸⁷Sr/⁸⁶Sr ratio of pedogenic carbonates (average 0.7078) is similar to the average ratio of marine limestone cropping out in the region, and is distinctly different from that of basalt bedrock (average 0.7053), indicating that chemical alteration of dust is the principle source of Ca. There is a strong correlation between the ⁸⁷Sr/⁸⁶Sr ratio of pedogenic carbonate and the labile fraction of dust and A soil horizon samples. There is little correlation between the ⁸⁷Sr/⁸⁶Sr ratio of pedogenic carbonate and that of the residual (silicate) fraction of dust or A horizon samples. This indicates that the major source of Ca cycling through these soils is chemical weathering of carbonate minerals in dust and that only a minor component is from chemical weathering of silicate minerals, either in dust or the underlying basalt. Mixing calculations using Sr isotopes as a tracer for Ca support the qualitative observations. Some variability in the ⁸⁷Sr/⁸⁶Sr ratio of pedogenic carbonates was found on a local scale (several hundred meters to several kilometers), and can be attributed to changes in dust composition due to contributions from local bedrock. There was no correlation found between the age of a basalt flow and the ⁸⁷Sr/⁸⁶Sr ratio of the pedogenic carbonate developed on them, indicating that the composition of dust has not changed significantly over the last 0.7 Ma.