THE INFLUENCE OF DISSOLVED ORGANIC-CARBON AND ANAEROBIC CONDITIONS OF MINERAL WEATHERING

The role of dissolved organic carbon (DOC) in mineral weathering was investigated in laboratory experiments analyzing supernatant composition using treatments of DOC from peat or leaf leachates on clinochlore, microcline, and an iron-rich sediment. Weathering intensity, expressed as the sum of molar concentrations of major elements measured over a 400-h period, fell in the sequence Fe-sediment > clinochlore > microcline. The leaf leachates containing 50 and 300 mg DOC L-1 were stronger weathering agents than either the 50 mg DOC L-1 peat extract, water, or dilute HCI. Anaerobic conditions in the peat extract did not enhance weathering and, in some cases, appeared to retard weathering rates. Fe in the supernatant was primarily < 0.1 mum, and a large proportion of the total Fe concentration was unable to react with o-phenanthroline, suggesting either Fe3+ or Fe2+ bound tightly to organic or inorganic particles. Using the SOILCHEM thermodynamic speciation model, it was determined that the magnitude of weathering enhancement of cations in the supernatant of DOC-rich treatments could not be attributed to direct metal-DOC binding. The cations are likely held by a peptization mechanism, forming inorganic/organic colloids <0.1 mum.