CALCIUM-SILICATE WEATHERING AT COWEETA HYDROLOGIC LABORATORY, NORTH CAROLINA: THE USE OF SAPROLITE BULK CHEMISTRY TO GAIN INSIGHT INTO ELEMENTAL TRANSFERS FROM REGOLITH TO NATURAL WATERS

Solute analyses of many streams draining small watersheds underlain by crystalline silicate bedrock often exhibit Ca/Na ratios higher than predicted from the congruent dissolution of plagioclase feldspar. This phenomenon has been term the “calcium problem” in the literature. Explanations for the calcium problem include: (1) An additional, unrecognized, source of calcium, often calcite; (2) an unidentified sodium sink; (3) plagioclase dissolution is incongruent with respect to calcium and sodium; or (4) calcium is released from cation exchange sites. At the Coweeta Hydrologic Laboratory in western North Carolina, a different calcium problem has been recognized. In this instance, Ca/Na ratios in the stream waters of undisturbed watersheds at Coweeta are generally either comparable to that of the plagioclase or lower. However, the need to incorporate an additional Ca-mineral was identified by applying isovolumetric methods to bulk chemistry data for the regolith solids (bedrock and saprolite), and performing mass balance calculations using saprolite bulk chemistry and bulk density measurements. Previously invoked Ca-silicates (plagioclase and garnet), weathering stoichiometrically and in reasonable modal abundances, cannot alone explain the entire calcium depletion observed in bulk chemical data from thoroughly weathered saprolite (relative to the parent material from which the saprolite weathered). This difference between the observed calcium inventory in the bulk solids and the calcium inventory predicted from the weathering of previously recognized Ca-bearing silicates at Coweeta may be explained by the rapid weathering of small quantities of an additional Ca-mineral. Several candidates exist including apatite, but a variety of evidence favors the Ca-silicate, allanite, which has not previously been recognized as weathering in Coweeta regolith. This study demonstrates that an accessory calcium-rich mineral in a weathering profile may not only substantially influence the bulk chemistry of the regolith, but also that the chemistry of the regolith may reveal important information on elemental transfers (in particular calcium) from weathering profiles to ground waters, and ultimately to stream waters. Furthermore, such information may not be recognizable from solute chemistry alone.