Chemo- and bio-stratigraphic constraints on Cretaceous-Paleocene biotic turnover in the southern Tethys low-oxygen margin, Egypt

Abstract

The late Cretaceous-Danian (K-Pg) organic-rich sequence is a global archive that witnessed abrupt paleo-condition changes during a high sea-level. In Egypt, this sequence is present in the Duwi and Dakhla formations as an E-W belt from the Red Sea to the Western Desert. We analyzed elemental and isotopic distributions in an age-constrained core from the Quseir area to understand paleoclimatic and paleoenvironmental factors influencing its deposition. Calcareous nannofossil and the chemical index of alteration (CIA) confirm a global cooling trend during the late Cretaceous, with two warming episodes indicating early phases of the Deccan volcanism. The Danian stage experienced a warm climate and intense chemical weathering. Mo-U enrichment factors covariation suggest deposition under oxygen-deficient conditions with evident watermass restrictions. High-productivity-indicating taxa were more abundant in Danian strata than in late Cretaceous strata, consistent with organic carbon characteristics. The δ¹³C_Org-δ¹⁵N data show temporal heterogeneity due to organic matter type, redox conditions, preservation, stratigraphic condensation, and recycling of isotopically-light CO₂. The negative δ¹³C_Org shift around the K-Pg transition is globally correlative. Dynamic pyrite δ³⁴S distribution resulted from variation in riverine influx, degree of pore-water openness, sulfate reduction rate, and diagenetic disproportionation of sulfur intermediates. Generally, radiogenic ¹⁸⁷Os/¹⁸⁸Os values were observed that were seemingly influenced by continental weathering and subsequent runoff influx of ¹⁸⁷Os/¹⁸⁸Os under a greenhouse climate. A positive ¹⁸⁷Os/¹⁸⁸Os shift to 3.68 (¹⁸⁷Os/¹⁸⁸Os_i of 0.74) occurred at the onset of the Dakhla organic-rich spike. However, without a precise correlative stratigraphic framework, a global or regional seawater isotopic shift cannot be confirmed. We suggest that temporal variations in organic matter composition, redox, watermass conditions, and isotope fractionation of the K-Pg sequence in the study area were driven by discrepancies in climate, sea-level, and tectonic uplift. Further integrated approaches are needed to explore the geospheric drivers of this organic-rich sequence in time-equivalent sections.