Chemical composition of glacial runoff

The chemical composition of glacial runoff from ice sheets, ice caps and glaciers around the world is shown in Table 14.1 (after Brown, 2002; Tranter, 2003), which also includes the composition of global mean river water for comparative purposes. Sea salt is a variable component of glacial runoff, and the dominant non-sea-

Figure 14.1 Scatterplot of crustal calcium flux and calcium concentration versus discharge at Manitsoq Glacier, southwest Greenland.

Table 14.2 Specific runoff and cationic denudation rates for glaciers in different regions (after Hodson et al., 2000)

0 5 10 15 20 25 30 35 Discharge (cumecs)

Figure 14.1 Scatterplot of crustal calcium flux and calcium concentration versus discharge at Manitsoq Glacier, southwest Greenland.

salt ions are Ca2+-HCO3--SO42--Mg2+. The concentration of glacial runoff is usually inverse to discharge, so that a rough rule of thumb is that low discharge waters are concentrated, whereas high discharge waters are dilute. The concentration of low discharge waters approaches ca. 1meqL-1 (of positive charge) in temperate, lower latitude glacial runoff, and ca. 3 meq L-1 at higher latitudes, presumably as a consequence of freezing effects. Low discharge waters do not usually make a significant contribution to the annual solute flux (Sharp et al., 1995; Tranter, 2003). Paradoxically, the concentration of the dilute, high discharge waters is more important, because high discharge transports significantly more solute than low discharge (see Fig. 14.1). This is because the increase in discharge over the ablation season may be of the order of one to threee orders of magnitude, whereas the dilution of the solutes is less, being up to an order of magnitude less.

The sum of cation equivalents in glacial runoff ranges from ca. 10 to 3500|leqL-1. Glacial runoff is usually more dilute than global mean river water, and usually contains more K+ and less Si for a given specific runoff (Anderson et al., 1997). The Ca2+ :Si and HCO3- :SO42- ratios of glacial meltwaters are high and low respectively, when compared with the principal world river water, making glacial runoff an end member of global riverine water (Tranter, 2003). This is because glaciers preferentially weather carbonates and sulphides from the bedrock.

Raiswell (1984) showed that the base cation composition of glacial meltwaters does not reflect that of the lithology of the bedrock. The predominant cation is always Ca2+, even on acid igneous and metamorphic bedrocks. This is because the dissolution kinetics of Ca2+ from trace carbonates, which are ubiquitous in most bedrocks, and from aluminosilicates are more rapid than those of monovalent ions. Hence, Ca2+ may be a relatively minor base cation in the bedrock, but becomes the dominant base cation in solution (White et al., 2001).

Holland (1978) showed that specific annual discharge is the most significant control upon chemical erosion in temperate catchments, and the same is true in glacierized basins (Anderson et al., 1997; Hodson et al., 2000). The lithology of the catchment

Region

Svalbard European Alps North America Iceland Asia

Continental average

Specific runoff Cationic denudation rate

Specific runoff Cationic denudation rate

Region

Svalbard European Alps North America Iceland Asia

Continental average

(myr ')

(Smeq+ m 2yr ')

0.4-1.5

190-560

1.4-2.3

450-690

0.7-7.7

94-1600

1.8-2.1

650-1100

1.1-3.5

460-1600

0.31

390

is an important secondary control on chemical erosion rates, with carbonate-rich and basaltic lithologies exhibiting the highest chemical weathering rates. Hodson et al. (2000) show that there is year on year variability in specific runoff, and therefore on cationic denudation rate. Currently, there are more studies of chemical erosion rates in the glacierized basins of Svalbard than in other region (Table 14.2). Crustally derived solute fluxes from 10 basins are equivalent to a mean cationic denudation rate of 350 £meq+ m-2yr-1 (range: 160-560 £meq+ m-2yr-1), which lies within the global range of 94-1650 £meq+ m-2yr-1for the other 15 glacier basins in the Northern Hemisphere (Hodson et al., 2000). The mean value for Svalbard is close to the continental average of 390 £meq+ m-2yr-1 (Livingstone, 1963).

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