Silicate versus carbonate weathering in the Himalaya Mountains and the New Zealand Southern Alps

The research presented in this dissertation examines how the relative dominance of silicate versus carbonate weathering varies both spatially and temporally in two rapidly uplifting, young orogenic belts—the Himalaya Mountains and the New Zealand Southern Alps.; Silicate rocks in the Pakistan Himalaya contain small amounts of disseminated calcite precipitated by hydrothermal fluids during recent uplift of the region. Disseminated calcite has a wide range of Ca/Sr and 87Sr/ 86Sr ratios, with the latter similar to values measured within the silicate host rock. The rapid weathering of disseminated calcite with high Ca/Sr and 86Sr/86Sr ratios strongly influences stream water chemistry and exceeds contributions from silicate dissolution. Analyses of a glacial soil chronosequence show that carbonate weathering continues to dominate the dissolved flux of Ca2+ and Sr2+ from stable landforms for at least ∼55 kyr after the exposure of rock surfaces, even though carbonate represents only a minor proportion of fresh glacial till.; Findings from northern Pakistan were used as a model to evaluate silicate vs. carbonate weathering throughout the Himalaya. Himalayan streams are predominantly Ca2+-Mg2+-HCO₃− waters derived from carbonate dissolution. The dominance of carbonate weathering is less apparent when Ca/Sr and 86Sr/86Sr are considered. However, Ca/Sr ratios decrease as the degree of supersaturation with respect to calcite increases, suggesting preferential removal of Ca 2+ by calcite precipitation. By accounting for the nonconservative behavior of Ca2+ mass-balance calculations demonstrate that carbonate weathering dominates both the solute and Sr isotope geochemistry of Himalayan streams.; The New Zealand Southern Alps provided an opportunity to examine the effects of climate, tectonics, and bedrock mineralogy on chemical weathering. Similar to the Pakistan Himalaya, hydrothermal calcite weathering in the Southern Alps dominates the solute and Sr isotope geochemistry of stream waters. Streams underlain by highly metamorphosed bedrock in regions of active uplift have the highest abundance of carbonate-derived solutes. The results presented in this dissertation suggest that the Sr isotope equilibration between silicate and carbonate minerals during uplift and the relatively rapid rate of carbonate versus silicate weathering may be an important process linking orogenesis to patterns of river water chemistry.