Tracking drainage basin evolution, continental tectonics, and climate change: Implications from osmium isotopes of lacustrine systems

Abstract

Organic matter in the lacustrine Eocene Green River Formation in two cores separated by 16 km in the Uinta Basin record similar centimeter-scale initial 187Os/188Os (denoted as Osi) stratigraphic profiles. This demonstrates that the Os isotopic composition of lacustrine organic matter is spatially homogeneous and in this case documents variation on the time scale of thousands of years. The Osi stratigraphy developed here indicates temporal change in the Os isotopic composition of lake water which is regulated by the Os isotopic composition of inflowing chemical weathering products from the surrounding drainage basin. Although observed Osi variability is just above the level of analytical uncertainty, it can be modeled with less than a 6% change in water input from radiogenic to nonradiogenic sources. This highlights the sensitivity of this isotopic tracer. Changes in Os isotope composition may reflect changes in relative discharge of rivers with different Os isotope composition, changes in the footprint of the drainage area to include or exclude different rock types, or unroofing. Thus, this study suggests that Os isotope stratigraphy in lacustrine settings can be used to link basin stratigraphy to changes in the drainage basin which are driven by climatic, tectonic, and geomorphic processes. The similarity in Os isotope stratigraphy between the two cores documented here highlights its utility for detailed chronostratigraphic correlation within and between lacustrine basins. A conservative sedimentation rate suggests that Osi in these lacustrine deposits changed at a rate of ~0.01 per kiloyear, two orders of magnitude faster than has been reported in marine organic-rich sedimentary rocks (Ravizza, 2007). Furthermore, linear regression of 187Re/188Os and 187Os/188Os ratios of all samples yields a Model 3 depositional age determination of 48.3 ± 2.8 Ma (2σ, MSWD = 8.2) which statistically indistinguishable from a weighted mean laser fusion 40Ar/39Ar sanidine age of 49.58 ± 0.32 Ma (Smith and Carroll, 2015) of a temporally adjacent volcanic ash.