Past climate and redox conditions of organic-rich shale within the Late Jurassic Shuqra Formation from the Say’un–Masilah Basin, Yemen

This study integrates multi-scale organic geochemical and elemental composition analyses, along with microscopic observations of organic-rich shale (ORS) sediments of the Late Jurassic Shuqra Formation (SF) in the Say’un–Masilah Basin (SMB), and was designed to appraise past climate and redox conditions and their impact on organic carbon accumulation and preservation. The geochemical data reveal that most Shuqra shale samples incorporate total organic carbon (TOC) values exceeding 1 wt% and reaching up to 2.95 wt%, promoting the preservation of organic matter (OM) in an oxygen-depleted environment, mainly in dysoxic conditions. The interpretation is supported by relatively low pristane (Pr)/phytane (Ph) ratios in the range of 1.23–1.85, and elevated Vanadium (V) concentrations relative to Nickel (Ni) and chromium (Cr), reflected in relatively low V/Ni and V/Cr ratios up to 2.37 and 3.52, respectively. The presence of sulfide mineral (FeS) in framboidal pyrite form associated with OM is consistent with low-oxygenated conditions, as the sulphidation of iron forms it. However, the framboidal pyrite shows a range of sizes from more than 50 µ to less than 5µ, suggesting depositional conditions from oxic to anoxic (dysoxic). The biomarker markers provide evidence of mainly marine OM input, with low amounts of a terrigenous land plant during the deposition time of the Shuqra shale sediments. The dominance of marine-derived material is supported by the abundant occurrence of alginite macerals, particularly lamalginite and telalginite algae, as identified through reflected light microscopy. Additionally, paleoclimatic indicators indicate that warm and humid climates dominated during the accumulation period of the Shuqra shale sediments. These warm-water and humid climatic conditions promoted eutrophication, enhancing subaerial weathering and nutrient influx, which in turn increased marine primary productivity within the photic zone. As a result, the combination of bioproductivity and efficient preservation led to significant OM enrichment in the Shuqra shale facies throughout deposition.