Paleoceanographic reconstruction of the late Cretaceous oil shale of the Negev, Israel: Integration of geochemical, and stable isotope records of the organic matter

The Levantine high productivity system was an extensive coastal upwelling that operated in the Late Cretaceous along the SE Tethyan margin. This study focuses on the top Phosphate Member of the Mishash Formation and the Oil Shale Member (OSM) of the Ghareb Formation (latest Campanian-early Maastrichtian), which represent the last phase of this high productivity system in the Negev, Israel. Bulk organic matter (TOC), δ ¹³C _org, δ ¹⁵N _org, C/N and pristane/phytane (Pr/Ph) ratios, were studied for reconstruction of seafloor and water column depositional environments. Our records indicate a gradual decrease with time in surface water productivity in the OSM and a marked weakening at the overlying Marl Member. High C/N ratio along with relatively low δ ¹⁵N _org (4‰ to 6‰) and δ ¹³C _org (-29‰ to -28‰) probably reflect significant diagenetic preferential loss of nitrogen-rich organic compounds enriched with ¹⁵N and ¹³C isotopes (e.g. proteins). This along with the low Pr/Ph values (0.11-0.7), indicate oxygen depleted bottom water (anoxia-dysoxia) during the deposition of the top Phosphate Member and the OSM. The moderate gradual upward increase in δ ¹⁵N _org, and in Pr/Ph values and the decrease of TOC and C/N values from the top Phosphate Member through the OSM indicate transition from anoxic (Phosphate-lower OSM) to dysoxic (middle-upper OSM) conditions. This environmental trend is consistent with co-occurring foraminiferal assemblages in the studied succession and implies that the benthic species in the Negev sequence were adapted to persistent minimum oxygen conditions by performing complete denitrification as recently found in many modern benthic foraminifera.