Results

Table 16.2 shows calculated solute fluxes for Bench Glacier compared with measured solute fluxes from Bench Glacier. The model considerably overpredicts cation fluxes from carbonate dissolution, probably because of the effect of saturation on dissolution rate. Subglacial waters are often saturated with respect to calcite, which reduces to zero the solute flux from this reaction. For sili-

Effective erosion rate (mm/yr)

Effective erosion rate (mm/yr)

Figure 16.1 Annual silica flux as a function of the annual suspended sediment yield from active glaciers. Haut Glacier d'Arolla is an 11.7 km2 glacier in Switzerland, on metamorphic and igneous rocks (Sharp et al., 1995). Kennicott Glacier is a 420 km2 glacier in Alaska, on volcanics, limestone and greenstone rocks, and Bench Glacier is a 7.5 km2 glacier in Alaska on metasedimentary rock (Anderson et al., 2003). Broggerbreen is a 32 km2 polar (cold-based) glacier in Svalbard on carbonate-rich sedimentary rocks (Hodson et al., 2000).

cate weathering reactions, the best model fit is for t = 1yr. The measured silica fluxes are matched, whereas predicted cation fluxes are lower than measured fluxes by a factor of about five. The low value of the best-fitting t supports the assumption that little fine-grained sediment is stored subglacially. Underpredic-tion of the cation flux from silicate weathering in the model arises entirely from underprediction of K. Very high K concentrations commonly observed in glacier runoff have been attributed to physical shearing of biotite grains during glacial abrasion (Blum, 1997). Mechanical release of cations is not characterized by the dissolution rate constants, r, used in this model.

Table 16.2 Glacier chemical denudation rates: calculated and measured

Composition

Glacier erosion rate

(mmyr-1)

t (yr)

Cation flux from calcite (tkm2yr-1)

Cation flux from silicates (tkm2yr-1)

SiO2 flux (tkm2yr-1)

Metagreywacke

1.5

1

1330

0.99

2.41

(Bench Glacier)

10

2110

0.40

0.97

45% plag, 20% kspar,

100

3340

0.16

0.39

13% hbld, 2% bio, 20%

1000

5300

0.07

0.16

qtz, 0.1% calcite

10,000

8400

0.03

0.06

Bench

1.5

22.4

5.6

2.70

Glacier—measured

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