Chemical weathering in the foreland of a retreating glacier

Details Chemical weathering in the foreland of a retreating glacier Chemical weathering in the foreland of a retreating glacier

Apr 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000gecoa..64.1173p&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 64, Issue 7, pp.1173-1189

Physics

28

Scientific paper

Chemical denudation rates and strontium isotope ratios in streams vary substantially and systematically in the foreland of the retreating Bench Glacier in south-central Alaska. To study weathering of young glacier sediments, we sampled 12 streams draining a chronosequence of till and moraine soils derived from Cretaceous metagraywacke-metapelite bedrock. Both sediment age and vegetation cover increase with distance from the glacier. Cation denudation rates decline with increasing distance from the glacier, whereas silica denudation rates increase. Carbonate dissolution and sulfide oxidation account for roughly 90% of the solute flux from the youngest sediments. Biotite alteration accounts for 5-11% of the solute flux; its peak contribution is found in the glacier outlet stream. Silicate weathering is the dominant reaction only in the oldest sediments. In a laboratory dissolution experiment using fresh glacial sediment, carbonate dissolution dominated the solute flux during the first 700 hours, paralleling the behavior of young sediments in the field. In contrast to trends in the field, the silica flux did not increase after the carbonate was exhausted from the reactor. A possible reason for this difference is that establishment of vegetation causes an increase in silicate weathering. The 87 Sr/ 86 Sr ratio in the glacier outlet stream is greater than that in proglacial streams and in bulk rock, due to a greater contribution of biotite weathering in the outlet than in proglacial streams. Strontium isotope ratios decline with sediment exposure age in the proglacial streams, and are consistent with a carbonate source. Because the dominant weathering reactions in the young sediments are of carbonate and sulfide rather than silicate minerals, weathering at the glacier margin is not an important long-term sink for atmospheric CO 2 .

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