Strontium Isotope Systematics of a Himalayan Glacial Chronosequence

Abstract

Chemical weathering of silicate minerals consumes atmospheric CO2, and therefore, changes in global silicate weathering rates may act as a mechanism to drive major fluctuations in the Earth's climate and long-term global carbon cycle. Recent studies suggest that steep increases in the Cenozoic marine Sr isotope record correlate with periods of global cooling and enhanced silicate weathering rates, initiated by the onset of glaciation in the uplifting Himalayan mountains (Raymo and Ruddiman, 1992). However, due to the Himalaya's unique orogenic history, strontium with high 87Sr/86Sr ratios may reside in easily weathered carbonate minerals that do not act as a net sip& for atmospheric CO2 when undergoing chemical weathering (Edmond, 1992; Blum et al., 1998). Thus, the dissolved flux of Sr from the Himalayan Mountains may not be an ideal proxy for silicate weathering and atmospheric CO2 consumption. By studying the major element and Sr isotope chemistry of soils developed on a Himalayan glacial chronosequence, we are investigating how the dissolved flux of ions derived from carbonate versus silicate mineral weathering varies as a function of time following the exposure of fresh rock surfaces to the agents of chemical weathering. Geologic setting and methods. The -200 km 2 Raikhot Valley, located at an average elevation of ~4000 m on the northern slope of the Nanga ParbatHarimosh Massif (NPHM) within the Pakistani Himalayas, contains both the ~50 km 2 Raikhot Glacier as well as a clearly visible glacial chronosequence. Four moraines, thought to be roughly Little Ice Age to neoglacial in age, are nested near the modem terminus of the glacier, while a fifth moraine, believed to be last glacial maximum in age, forms a bench about 200 m from the glacial terminus. The bedrock of the watershed, considered to be representative of the High Himalayan Crystalline Series (HHCS) which stretches for 2000 km across the length of the Himalayan Mountains, primarily consists of quartzofeldspathic biotite gneisses, biotite schists, leucogranite dikes, and granitic stocks, as well as minor amounts (~1%) of interbedded amphibolite and calc-silicate schists. In order of increasing relative age, the sample numbers 96PK1, 96PK2, 96PK3, and 96PK4 refer to soil profiles collected from the four young nested moraines, while the sample numbers 96PK5 and 96PK5a refer to 2 separate soil profiles developed on the fifth moraine. This report presents results obtained from the analysis of the major element (Ca, Mg, Na, K, and Si) concentrations, minor element (Sr, Ti, and Zr) concentrations, and the Sr isotope ratios (87Sr/86Sr) contained in the bulk and exchangeable fractions of these soils. Ca to Sr ratios. The analysis of Ca/(1000*Sr) ratios provides a useful mechanism for characterizing the mineral provenance of the bulk soil and exchangeable ions. The Ca/(1000*Sr) ratios of the bulk soil and exchangeable ions represent a mixture between the average calcium carbonate and silicate endmembers for the watershed, as determined by sequentially leaching and digesting modem riverbed sediment with 4N acetic acid and HF-HC104, respectively (Blum et al., 1998). The average Ca/(1000*Sr) ratio of the exchangeable fraction in profiles 96PK1, 96PK2, 96PK3, and 96PK4 appears similar to the calcium carbonate endmember for the watershed (1.4), while the Ca/(1000*Sr) ratios contained in the entire suite of bulk soils as well as the exchangeable fraction in the C-horizons of profiles 96PK5 and 96PK5a appears more similar to the silicate endmember (0.19). These observations suggest that even though calcium carbonate represents only -1% of the rock in the watershed, calcium carbonate is the primary source of Ca and Sr provided to the dissolved groundwater flux during the early stages of weathering. In addition, the Ca/(1000*Sr) ratios in the C-horizons of profiles 96PK5 and 96PK5a demonstrate that the calcium carbonate reservoir in the soils is nearly depleted within the -15,000 to 20,000 years during which the soils have developed on the moraines. However, a reversal toward carbonate-like Ca/(1000*Sr) ratios in the uppermost horizons of the older profiles indicates that the continual addition of fresh weatherable material,