Himalayan uplift and osmium isotopes in oceans and rivers

Previous studies have shown that ¹⁸⁷Os/¹⁸⁸Os in seawater has become increasingly radiogenic over the last 40 Ma in a manner analogous to strontium. This rapid rise in the marine ¹⁸⁷Os/¹⁸⁸Os over the last 17 Ma has been attributed to an increase in the bulk silicate weathering rates resulting from the rise of the Himalayas and/or selective weathering and erosion of highly radiogenic organic rich ancient sediments. The key test of this hypothesis is the ¹⁸⁷Os/¹⁸⁸Os and the total osmium concentration of the Himalayan rivers. We report the concentration and isotopic composition of osmium in the Ganges, the Brahmaputra, and the Indus rivers. The ¹⁸⁷Os/¹⁸⁸Os of the Ganges close to its source (at Kaudiyal, 30°05′N, 78°50′E) is 2.65 and [Os] = 45 fM/kg. A second sample of the lower reaches of the Ganges at Patna (25°30′N, 85°10′E) gives ¹⁸⁷Os/¹⁸⁸Os =1.59 and [Os] = 171 fM/kg. The ¹⁸⁷Os/¹⁸⁸Os of the Brahmaputra at Guwahati (26°10′N, 91°58′E) is 1.07 and [Os] = 52 fM/kg. A sample of the Indus (Besham, 34°55′N, 72°51′E) has a ¹⁸⁷Os/¹⁸⁸Os of 1.2 and [Os] = 59 fM/kg. We infer that the Himalayas do not provide either a high flow of osmium or a highly radiogenic osmium component to the oceans. The overall trend for osmium and strontium could be explained by a regularly increasing input of global continental weathering sources but the Himalayas themselves appear not to be the dominant source.