Weathering of disseminated calcite in granitoid rocks of the WEBB watersheds: implications for global Ca and C fluxes

U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA, 94025-3591, USA, afwhite@usgs.gov, 650-329-4519

Abstract

The proportions of carbonate-to-silicate weathering products in the world rivers have profound implications in global carbon balances and the long-term feedback mechanisms controlling atmospheric CO₂. Recently completed 2 year flow through column experiments, using crushed granitoid rocks from WEBB watersheds at Loch Vale, CO; Río Icacos, PR; and Panola, GA, showed initially high release rates of Ca and alkalinity and low ⁸⁷Sr/⁸⁶Sr ratios derived from weathering of disseminated calcite. The presence of up to 0.2 wt. percent calcite was confirmed by cathode luminescence and is related both with late-stage magmatic cooling and secondary hydrothermal processes. Initial column effluents were initially close to calcite saturation, becoming significantly undersaturated with time. Mass balance indicated that calcite was completely leached out of the column material after 2 years. This is consistent with the lack of measurable calcite in the naturally weathered granitoids in the watersheds.

The rapid weathering of disseminated calcite explains the excess Ca and alkalinity associated with the weathering of granitoid rocks in many watersheds glaciated within the last 15 Kyrs. Half of the Ca flux from the Loch Vale watershed is derived from disseminated calcite dissolution. In contrast, longer term surficial exposure of granitoids in the Río Icacos and Panola watersheds produces weathering fluxes with no excess Ca and alkalinity. The disproportionate rates of silicate and calcite weathering are demonstrated in weathering profiles through granitoids in geomorphically young and old watersheds. Exfoliated granite from recently glaciated watersheds exhibits a strong preferential loss of calcite relative to silicates whereas deep weathering profiles in the older watersheds show concurrent losses of both calcite and Ca- silicates. These findings imply that interglacial periods and episodes of major tectonic activity produce excess fluxes in Ca and alkalinity which may impact rates of atmospheric CO₂ draw down

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