Lithologic controls on osmium isotopes in the Rio Orinoco

The seawater Os-187/Os-188 ratio has been increasing over the last 50 Ma and at present is similar to 1.06 reflecting that about 80% of Os in the oceans is derived from the continents that provide Os with an average Os-187/Os-188 ratio of about 1.26. Whether the past variations in seawater Os isotopes record changes in the bulk Os flux with an average isotopic composition or the changes in the average Os-187/188Os ratio of the continental inputs or a combination of these two pathways is not clear. The fundamental importance of lithology in controlling continental weathering rates and dissolved ion yields has been well established for the major cations. It is dramatically manifested in the Sr isotopes of rivers that are affected by selective dissolution of carbonates. Accordingly, for Os isotopes it has been proposed that disproportionate contributions of Os with extremely high Os-187/Os-188 ratio come from weathering of organic-rich black shales. Here we examine the effect of the weathering of specific lithologies on osmium isotopic composition and concentration in water samples of the Orinoco River basin, a major river basin draining both black shales exposed in the Northern Andes and granites of the Precambrian Guyana Shield. The Northern Andes was uplifted similar to 17 Ma, which corresponds to the time of a dramatic rise in Os-187/Os-188 ratio. The osmium isotopic composition and concentrations vary considerably in the tributaries and the main stem of the Orinoco; the highest Os concentrations are observed in the rivers draining the Andes. However, waters draining the Andes are not highly radiogenic (Os-187/Os-188 < 1.2). A sample of black shale from the area is also not radiogenic. A surprising number of samples from tributaries draining the Shield are non-radiogenic. Os isotopic composition of Shield waters is often much lower than the bedload and underlying bedrock lithology. We explain this discrepancy in terms of the limited input of Os from the bedrock. The data reported here for the Orinoco and its tributaries do not support the hypothesis that increased weathering of highly radiogenic black shales is the underlying cause of the rising Os-187/Os-188 ratio of seawater during the last 17 million years. (c) 2006 Elsevier B.V. All rights reserved.