Carbonate Versus Silicate Weathering Fluxes of Sr From a High Himalayan Watershed

Abstract

The Himalayan uplift has been implicated as a major factor in the global climatic cooling of the past 40 m.y. because of enhanced silicate weathering and consequent atmospheric CO2 drawdown (Raymo and Ruddiman, 1992). Supporting evidence includes the dramatic increase in marine 87Sr/86Sr ratios at 40 Ma, which appears to be related to Sr inputs from major rivers draining the Himalaya (Palmer and Edmond, 1992). Regional patterns of 87Sr/86Sr ratios in tributaries of the Ganges-Brahmaputra and Indus River systems as well as bedrock 87Sr/86Sr ratios suggest that the major source of the highly radiogenic Sr in Himalayan rivers is weathering of silicate minerals in the High Himalayan Crystalline Series (HHCS) (Krishnaswami et al., 1992; Harris, 1995). Here we report on a geochemical investigation of the ~200 km 2 Raikhot watershed in the western Himalaya on the north side of the Nanga Parbat massif in northern Pakistan, with the aim of elucidating the systematics of Sr release from individual minerals in the HHCS bedrock. The bedrock of the area is predominantly highgrade quartzofeldspathic biotite gneiss and schist with minor anatectic biotite granite and a small amount (-1%) of calc-silicate schist. The bedrock is generally representative of the HHCS, which extends over 2000 km across the length of the Himalaya and dominates the geology of the steep southern slopes that are subjected to rapid erosion during the monsoon season (Harris, 1995). Samples for analysis were taken of rainwater, snow, clear streams, Raikhot River water (which has suspended glacial flour), Raikhot riverbed sand (believed to be representative of unweathered bedrock from the watershed), bedrock from outcrops, and glacial boulders (Blum et al., 1998). Rock samples containing calcite were leached for 1 h in 4 N acetic acid to dissolve carbonate, and silicate rock samples were totally digested for elemental and Sr isotope analysis. Concentrations were measured by ICP-OES and ICP-MS; 878r/86Sr ratios were measured by TIMS. The most abundant minerals in the bedrock of the watershed are quartz, plagioclase, K-feldspar, and biotite; calcite is present in only minor abundance but is important due to its high reactivity. Plagioclase and K-feldspar are assumed to weather to kaolinite, and biotite to vermiculite (or hydrobiotite). A massbalance calculation was performed to quantify the proportions of each of the bedrock minerals that weathered to yield each water composition. This calculation has the following six steps based on the stoichiometry of the weathering reactions. (1) Atmospheric correction. (2) Attributing of Na to the weathering of albite to kaolinite. (3) Attributing of Ca in proportion to the Ca/Na ratio of plagioclase to the weathering of anorthite to kaolinite. (4) Attributing any remaining Si to the weathering of orthoclase to kaolinite. (5) Attributing the remaining K to the weathering of biotite to vermiculite. (6) Attributing excess Ca and Mg to carbonate dissolution. For the Raikhot River samples, the above calculation yields an estimate of the relative amounts of weathering of minerals as follows: 14% plagioclase, 2% orthoclase, 11% biotite, and 73% carbonate. This estimate corresponds to a riverine HCO3 flux that is 18% derived from silicate and 82% derived from carbonate weathering reactions. Analyses of the carbonate and silicate fractions of the riverbed sand indicate that only 1.0 wt.% of the sediment load is carbonate. The calculated percentages of weathering of bedrock minerals for the average quartzofeldspathic gneiss and granite bedrock stream-waters were: 24% plagioclase, <1% orthoclase, 12% biotite, and 64% carbonate. High proportions of riCO;are also derived from carbonate (68-78%) in the stream waters known to drain exclusively quartzofeldspathic gneiss and granite bedrock. The Ca/(1000Sr) ratios of silicate rocks (gneiss and granite) range from 0.07 to 0.4, whereas marble layers range from 1 to 2. 87Sr/86Sr ratios of the silicate rocks (and all their constituent minerals) range between 0.82 and 0.89, whereas marble layers