Rhenium–osmium isotope and platinum group element systematics of marine vs. non-marine organic-rich sediments and coals from Egypt

Sediments rich in organic matter are important archives of paleo-environmental conditions. These archives are often assumed to reflect fully marine conditions. However, the land-proximal environments where many of these archives were deposited require an understanding of the sources of matter. Here we investigate organic-rich sediments from Egypt deposited in fully marine, land-proximal and coastal plain environments. We focus on platinum group element inventories and Re–Os isotope systematics and combine them with organic carbon and nitrogen analyses as well as organic petrography to distinguish between terrestrial and marine sources of organic matter. We show that fully marine organic-rich sediments have more strongly fractionated platinum group element patterns than average continental crust that mimic that of seawater, with low Ir and high Os, Pt, Pd and Re concentrations, as well as high 187Re/188Os values. Rhenium and Os concentrations correlate well with organic carbon and nitrogen in marine organic-rich sediments. In contrast, platinum group element concentrations in terrestrial organic-rich sediments (coal) and land-proximal shales are similar to the upper continental crust. Such deposits generally do not show correlations of Re and Os with organic carbon and nitrogen concentrations. Complementing Re–Os isotope analyses with PGE analyses, analogous to coupled Nd isotope and REE analyses, aids in the interpretation of Re–Os isochron ages and Os isotope initial ratios. This is particularly helpful in interpreting geochemical data from sediments deposited in environments that may have been, even temporarily, isolated from the open ocean and thus record local rather than global geochemical signatures.