The importance of organic-rich shales to the geochemical cycles of rhenium and osmium

- We compile a geochemical database of Re, Os, and organic carbon
- We estimate the mean 187Re/188Os and 187Os/188Os values of organic-rich sediments
- Organic-rich sediments are limited to less than 1% of the upper continental crust
- These sediments contribute a disproportionate amount to the Re and Os budgets

The role that marine organic-rich sediments play in the surficial osmium and rhenium cycles has been assumed to be important due to the high Re/Os ratios and high concentrations of these elements in such reducing sediments compared to the average continental crust. The high Re/Os leads to rapidly increasing 187Os/188Os ratios. We determine how large a role these sediments play in the surficial osmium and rhenium budgets by compiling a geochemical database to quantify the mean 187Re/188Os and 187Os/188Os ratios of organic-rich sediments. We then utilize estimates of the abundance of organic-rich sediments in the geologic record of different ages to estimate an average 187Os/188Os for these sediments. We find that the calculated 187Os/188Os value of organic-rich sediments varies between 2.22 and 2.27 depending on the volume-age model used, with error estimates of 2.07 to 2.42 and 2.12 to 2.48, respectively. Despite the fact that these sediments constitute a relatively small proportion of the upper continental crust, they contribute a disproportionately large amount (> 50%) to the upper continental crustal rhenium and, to a lesser extent (5-10%), osmium inventories owing to their high concentrations of these elements.