Himalayan uplift and osmium isotopes in oceans and rivers

Previous studies have shown that Os-187/Os-188 in seawater:has become increasingly radiogenic over the last 40 Ma in a manner analogous to strontium. This rapid rise in the marine Os-187/Os-188 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 Os-187/Os-188 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 Os-187/Os-188 Of the Ganges close to its source (at Kaudiyal, 30 degrees 05'N, 78 degrees 50'E) is 2.65 and [Os] = 45 fM/kg. A second sample of the lower reaches of the Ganges at Patna (25 degrees 30'N, 85 degrees 10'E) gives Os-187/Os-188 =1.59 and [Os] = 171 fM/kg. The Os-187/Os-188 Of the: Brahmaputra at Guwahati (26 degrees 10'N, 91 degrees 58'E) is 1.07 and [Os] = 52 fM/kg. A sample of the Indus (Besham, 34 degrees 55'N, 72 degrees 51'E) has a Os-187/Os-188 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. Copyright (C) 1999 Elsevier Science Ltd.