Chemical weathering of phosphate and germanium in glacial meltwater streams: Effects of subglacial pyrite oxidation

Abstract

Data from three glacial meltwater streams draining Mt. Tronador in the southern Argentine Andes (72"W, 4 1"s) show that subglacial pyrite oxidation and the subsequent precipitation of iron oxides strongly influence dissolved phosphate concentrations but do not appear to affect dissolved germanium concentrations. Total-Fe (TFe) and total-P (TP) concentrations are high in all three glacial streams, but the ratios of soluble reactive phosphate (SRP) to TP are very different, The Upper Manso Stream that drains the Manso Glacier has relatively low SRP (0.01-0.23 PM), representing a small fraction of its TP (O. l-5%). The SRP of the two streams draining smaller side glaciers (0.124.69 PM) represents a much larger portion of their TP (20-32%). TFe, dissolved sulfate, and 634S data suggest that pyrite oxidation is the most likely weathering reaction contributing sulfate to the Upper Manso Stream but not to the two smaller glacialfed streams, in which atmospheric inputs can account for all of the sulfate. Dissolved Ge appears to show no preferential sorption relative to dissolved silica, in contrast to the speculation that Ge preferentially sorbs on fresh iron hydroxyoxide surfaces.Rock-water interactions are an important part of the geochemical cycles of the earth, ultimately controlling the compositions of rivers, streams, and lakes-both dissolved and suspended solids-and thus the fluxes of chemicals to the sea. Sorption-desorption reactions with surfaces of suspended solids strongly affect the dissolved composition of waters, and the riverine fluxes of most elements are dominated by the suspended solids load (Martin and Meybeck 1979).Two driving forces of rock-water reactions are acid hydrolysis reactions and the formation of fresh solid surfaces for sorption reactions. Pyrite oxidation provides both. Its oxidation produces sulfuric acid, and iron hydroxyoxides are precipitated as a by-product. Whereas a AcknowledgmentsWe thank the following for their work, encouragement, and help:

Modelled glacial and non-glacial HCO3−, Si and Ge fluxes since the LGM: little potential for impact on atmospheric CO2 concentrations and a potential proxy of continental chemical erosion, the marine Ge/Si ratio

“…The potential for chemical weathering of glaciogenic sediment in proglacial environments also suggests that our estimate of the glacial Si flux is also conservative. Chillrud et al (1994) derived the Ge/Si ratio of a glacial meltwater stream of the Argentine Andes mountains, and found that these data agree well with those of global river data (Mortlock and Froelich, 1987). The highest Ge/Si ratio (1.35 pmol/Amol) measured by Chillrud et al (1994) derives from a water sample which had a Si concentration of f 36 Amol/l (see also Froelich et al, 1992).…”

“…The Ge flux is calculated directly from the Si flux and hence the variation of the Ge flux is similar to that of the Si flux over the last deglaciation. Since glacial systems have higher Ge/Si ratios than those of non-glacial systems (Chillrud et al, 1994), the incorporation of the former increases the Ge flux by a maximum of f 10% (at 10 ka BP). No significant change in Si flux between the present day and the LGM is contrary to previous suggestions that Si fluxes were double at the LGM (Froelich et al, 1992).…”

“…Chillrud et al (1994) derived the Ge/Si ratio of a glacial meltwater stream of the Argentine Andes mountains, and found that these data agree well with those of global river data (Mortlock and Froelich, 1987). The highest Ge/Si ratio (1.35 pmol/Amol) measured by Chillrud et al (1994) derives from a water sample which had a Si concentration of f 36 Amol/l (see also Froelich et al, 1992). On the basis of the regression line given by Froelich et al (1992) for the meltwater stream studied by Chillrud et al (1994), a glacial Ge/Si ratio of 6.6 pmol/Amol is calculated for a Si concentration of 5.9 Amol/l.…”

“…The highest Ge/Si ratio (1.35 pmol/Amol) measured by Chillrud et al (1994) derives from a water sample which had a Si concentration of f 36 Amol/l (see also Froelich et al, 1992). On the basis of the regression line given by Froelich et al (1992) for the meltwater stream studied by Chillrud et al (1994), a glacial Ge/Si ratio of 6.6 pmol/Amol is calculated for a Si concentration of 5.9 Amol/l. This glacial Ge/Si ratio is used to derive the glacial Ge flux from the modelled Si flux and will give the best estimate of changes in the glacial Ge flux at the LGM and during the last deglaciation.…”

“…Froelich et al (1992) suggested that the variations in (Ge/Si) seawater are driven by large glacial to interglacial changes in continental weathering intensity, resulting in a two-fold increase in the amount of dissolved silica and a 20% reduction in the (Ge/Si) riv ratio. Chillrud et al (1994) have published the only glacial meltwater Ge/Si ratio to date. A maximum ratio of 1.35 pmol/Amol is given, which is more than double that of the global average riverine Ge/Si of 0.6 pmol/Amol (Froelich et al, 1992).…”