Chemical weathering mechanisms and sources of solute

High proglacial chemical weathering rates at Finsterwalderbreen can be explained only by the chemical weathering of rock material in the proglacial zone. Increases in Ca2+, Mg2+ and HCO3-signify the importance of limestone and dolomite dissolution in contributing solute to meltwaters. The ultimate source of the SO42- is oxidation of sulphide minerals, although the dissolution

Figure 15.2 Temporal variation in glacial bulk meltwater discharge, and active-layer groundwater stage in Wells 1 and 3 during the 1999 melt season at Finsterwalderbreen.
Table 15.1 Glacial and proglacial solute fluxes and chemical weathering rates during the 1999 melt season

Species

Glacial solute

Proglacial solute

Proglacial/glacial

flux (kg)

flux (kg)

flux X100

Cl-

110,000

5200

0

hco3-

1,200,000

310,000

26

SO42-

680,000

250,000

47

Na+

83,000

7500

9

K+

29,000

2800

10

Mg2+

120,000

35,000

29

Ca2+

470,000

160,000

34

Si

14,000

-800

0

Total

2,700,000

770,000

29

Water

4.9 X 107

-5 X 106

-13

Chemical weathering

790

2,600

329

rate (meq Em-2yr-1)

of secondary sulphate salts is also important locally. In-channel weathering of suspended sediment contributes some Ca2+ and HCO3- to glacial meltwaters, but most proglacial solute derives from weathering of material in the active layer. Hydrochemical trends in active layer groundwaters can be explained by four geo-chemical processes: (i) sulphide oxidation, (ii) carbonate dissolution, (iii) precipitation of calcite and Mg-Ca sulphate salts and (iv) dissolution of the secondary calcite and sulphate salts (Fig. 15.3). At distance from the glacial bulk meltwater channel, there is little direct mixing of channel and active-layer groundwaters (although variations in stage are forced by channel discharge fluctuations). Here (Well 3), waters evolve by carbonate dissolution/sulphide oxidation, together with the dissolution of sulphate salts. The latter form by freeze concentration in winter and evaporative concentration in summer, and give rise to high Ca2+ and SO42- concentrations that decrease through the melt season. Close to the glacial channel (Wells 1 and 2), any salts formed on the proglacial surface are rapidly removed by glacial meltwaters during frequent channel flood. Hence, carbonate dissolution coupled to sulphide oxidation is the dominant geochemical weathering process, with trends of dilution being evident as sediments are flushed by dilute channel waters (Fig. 15.3).

vp

B

a 0\ a zk (

5, Progression through

a

n. melt season

a

a a

•VvO^

a

0

+

Bulk meltwater

i A

Well 1 (5 m from channel)

1 •

Well 2 (53 m from channel)

0

Well 3 (118 m from channel)

5000 10000 15000 20000

S042"

Figure 15.3 Association between SO42- and HCO3- for active layer groundwaters from Wells 1, 2 and 3 and bulk meltwaters sampled during the 1999 melt season in the Finsterwalbreen proglacial zone.

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