Ca/Sr and Sr isotope systematics of a Himalayan glacial chronosequence: carbonate versus silicate weathering rates as a function of landscape surface age

Diatoms modify the relationship between dissolved silicon and bicarbonate in the impounded rivers

“…8), suggesting that, even in a catchment controlled by silicate weathering, HCO 3 -was still mainly derived from trace amounts of bedrock carbonate, this last being unevenly distributed in the river basin. This conclusion is consistent with other studies showing that 1% carbonate in the bedrock contributed to 82% HCO 3 -flux in the major Himalayan rivers (Blum et al, 1998;Jacobson et al, 2002). As for a catchment dominated by carbonate rock weathering, because DSi only originates from silicate rocks and these minor silicate rocks unevenly inlay carbonate rocks, a negative relationship between HCO 3 -and DSi is also expected.…”

Natural Weathering Rates of Silicate Minerals

“…Studies of chronosequences (e.g., Birkeland, 1984Birkeland, , 1990Bockheim, 1980;Harden, 1987;Jacobson et al, 2002) have quantified systematic changes in weathering properties of soils with ages ranging from 1 to 4000 ky. Mass-balance studies have characterized mineralogical changes (Mahaney and Halvorson, 1986;Mokma et al, 1973), chemical changes (Brimhall and Dietrich, 1987;Merritts et al, 1992;White et al, 1996White et al, , 2008, and development of etch pitting (Hall and Horn, 1993) as functions of soil age. An important conclusion of these chronosequence studies is that the rates of primary silicate weathering generally decrease with duration of weathering.…”

Records of Cenozoic Ocean Chemistry

“…It is well known that calcium carbonate weathers much more rapidly than silicate minerals and that estimates of silicate weathering fluxes based on strontium isotope data are complicated by this phenomenon (Palmer and Edmond, 1992; see also Jacobson et al (2002a) for a discussion). In general, though not always (see below), terrestrial weathering of carbonates increases strontium flux and lowers 87 Sr/ 86 Sr relative to an equivalent carbonate-free catchment.…”

“…Silicate weathering rate is only one of the many factors that influences the flux of 87 Sr to the ocean. Detailed comparisons of strontium isotope data and river solute chemistry in modern rivers demonstrate that other factors, particularly the influence of strontium released by rapidly weathering carbonates, can obscure the contribution from silicate weathering (Palmer and Edmond, 1989;Jacobson et al, 2002aJacobson et al, ,b, 2003. Similarly for osmium, silicateweathering contributions to the 187 Os flux are likely convoluted with those from weathering of organic-rich sediments.…”