Silicate versus carbonate weathering in the Himalaya: a comparison of the Arun and Seti River watersheds

Abstract

We studied the water chemistry of two large and geologically differing Himalayan watersheds in order to maximize the contrast between silicate versus carbonate weathering effects on river chemistry. Our previous research involved the Seti River of westernmost Nepal, geologically typical of many rivers in Nepal in draining mixed carbonate/silicate lithologies, including abundant carbonate rocks of the Lesser Himalayan Sequence. For comparison, the Arun River was chosen for study because south of the Himalayan front it drains almost exclusively Greater and Lesser Himalayan silicate rocks. Despite this dominance of silicate rocks, carbonate weathering—probably of metamorphic calcite in Great Himalayan paragneisses—is clearly in evidence in many Arun watersheds. Weathering of silicate rocks exposed all along the Arun south of the range front has a small impact on mainstem river chemistry. The mainstems of both the Seti and Arun systems are dominated by weathering of carbonate rocks, although the contribution of silicate weathering is more visible in Arun mainstem chemistry. The carbonate weathering source to the Arun mainstem is probably both limestone in the Tethyan Sequence widely exposed in northern headwaters of the system, and metamorphic calcite within the Greater Himalayan Sequence. A number of small watersheds along the Arun and Seti appear to be carbonate-free. They probably provide the best constraints to date on the Ca/Na and Mg/Na ratios of waters draining Himalayan silicate rocks, two critical parameters for calculation of CO 2 consumption by silicate weathering in the Himalaya. The observed Ca/Na and Mg/Na ratios would produce slightly higher estimates of silicate weathering fluxes than previous studies. The geologic contrasts between the Seti and the Arun produce large differences in the 87Sr/ 86Sr ratio of each mainstem, unlike the major element chemistry. The Seti mainstem displays much higher 87Sr/ 86Sr ratios than the Arun mainstem, the opposite of the expected relationship since radiogenic silicate rocks of the Greater and Lesser Himalaya are so widely exposed along the Arun. 87Sr/ 86Sr ratios of the Arun mainstem never exceed 0.734 and show little downstream change as the mainstem passes through silicate rocks of the Greater and Lesser Himalaya. 87Sr/ 86Sr ratios of the Seti mainstem increase sharply from 0.725 to 0.785 when the river enters the belt of metacarbonate rocks of the northern Lesser Himalayan Sequence, a pattern also displayed by other Himalayan rivers such as the Kali Gandaki and Bhotse Khola. Metacarbonate rocks, including those of the Lesser Himalaya, are a major source of radiogenic Sr in modern Himalayan Rivers and probably have been key players in elevating marine 87Sr/ 86Sr ratios since the Early Miocene.