Organic and Inorganic Geochemical Integration for Paleoenvironmental Characterization during OAE-2, Demerara Plateau, South American Equatorial Margin

The Oceanic Anoxic Event 2 (OAE-2), which occurred during the Cenomanian–Turonian transition (Cretaceous, ~ 93.5 Ma), is marked by episodes of marine anoxia in the Equatorial Atlantic Ocean that promoted the deposition and preservation of thick organic matter-rich black shale sequences on the Demerara Plateau, within the Guiana–Suriname Basin. In this context, this study aims to identify the depositional paleoenvironmental conditions of this region of the South American Equatorial Margin through the integrated application of organic and inorganic geochemical analyses. To achieve this objective, eleven samples from ODP Site 1258, located on the northwestern edge of the plateau, were selected to investigate geochemical contrasts before, during, and after OAE-2. Soluble organic matter (SOM) was extracted, subjected to porphyrin removal, and fractionated into saturated, aromatic, and polar compounds by liquid chromatography. Subsequently, the saturated fraction was analyzed by gas chromatography with flame ionization detection (GC-FID) to determine n-alkanes and isoprenoids. Porphyrins were identified by UV–Visible absorption spectrophotometry. The mineralogical composition of the shale samples was evaluated by X-ray fluorescence (XRF). As a result, n-alkane profiles, together with preliminary Tmax (394–401 °C), δ¹³Corg (−25.6 to −29.3‰), TOC (10.86–28.13 wt.%), and HI (488–625 mg HC/g TOC) data, indicate marine thermally immature organic matter deposited under conditions favorable for high organic matter accumulation and preservation. n-Alkanes exhibited a predominant distribution in the n-C18–n-C35 range. Accordingly, n-alkane concentrations ranged from 0.03 to 0.62 mg/mg SOM, reaching their highest values during OAE-2, which may reflect enhanced primary productivity. Porphyrin concentrations reached their maximum during the Turonian (0.8 mg/mg SOM), contrasting with the OAE-2 and Cenomanian intervals, which exhibited lower values. This pattern may reflect changes in primary productivity during the event, coupled with reduced metal availability under highly reducing conditions, which may have hindered porphyrin preservation. High SO₃ contents (~19 wt.%) may further support euxinic conditions during the peak of OAE-2. During OAE-2, lower CaO (wt.%) and higher SiO₂ (wt.%) may reflect reduced carbonate accumulation and/or enhanced siliciclastic input. Such a pattern could be associated with environmental changes linked to ocean anoxia and possible acidification. Overall, the distribution of inorganic proxies and organic geochemical parameters indicates that the black shales were deposited under highly oxygen-depleted conditions. These results contribute to understanding the OAE-2 in the Equatorial Atlantic, providing a basis for comparison with coeval records from other regions and supporting the broader reconstruction of the paleoenvironmental and geochemical conditions associated with this global event.