CYCLING OF CALCIUM BY DUST

An important pathway for the cycling of Ca (and other elements) is the transportation, deposition, and weathering of dust. In central Texas, the dust depostion rate is at least 28 g/m² per year which could account for 10% of the total dissolved solids in local waters if a portion of these solids are dissolved in precipitation. In order to investigate the role that dust plays in cycling elements, we have used the Sr isotopic composition of dust, soils, pedogenic minerals, rocks, shallow groundwater and surface waters as a proxy for the behavior of Ca in the exogenic cycle.

Dust in central Texas has a Sr isotopic composition that ranges between 0.708 to ca. 0.713 depending on the fraction of dust analyzed (the water and acetic-acid soluble fraction is less radiogenic and the acetic-acid insoluble fraction is more radiogenic). For many parts of central Texas, there are no rock sources that contain such radiogenic Sr ratios (measured values of central Texas rocks are 0.7074 +/- 0.0002), yet surface waters and ground waters display a range of Sr ratios between 0.7082 to 0.7091.

It therefore appears that a significant portion of the flux of Ca (and probably many other major and minor elements) in central Texas streams and groundwaters is derived from an external source rather than from conventionally understood local sources such as rain and bedrock. Central Texas streams apparently derive 50% of their Sr and between 10 to 25% of their calcium from the weathering of dust in soil horizons. Because the dust flux changes during climate perturbations, it is expected that major element cycling will be modified during these times also.