Molecular Signatures of Aptian Paleoclimate and Sea-Level Dynamics: Higher-Plant Aromatic Biomarkers in the Codó Formation, São Luís Basin

An organic geochemical study was conducted on thirty-two rock samples from the Aptian Codó Formation (São Luís Basin, northeastern Brazil), retrieved from the well 2-ANP-5-MA. Samples were crushed, Soxhlet-extracted, fractionated via open-column chromatography, and analyzed by GC-MS, focusing on the aromatic fraction to evaluate molecular signatures reflective of past paleoclimate, global sea level, and terrestrial plant input, providing key criteria to differentiate potential petroleum source rocks in the region. Molecular composition, pseudo-van Krevelen, and HI vs. T_max diagrams indicate thermally immature samples positioned at the onset of the oil generation window. The ~60 m stratigraphic section is divided into two distinct geochemical units: Group 1: exhibits strong organic enrichment, with Total Organic Carbon (TOC) ≥ 2.0 wt% and Total Sulfur (S) > 0.5 wt%; Group 2: dominated by Type III kerogen, with TOC ≤ 1.0 wt% and S < 0.5 wt%. Although sampling resolution (0.3–3.7 m) is locally constrained across the ~60 m section by sandstone and evaporite interbeds, it is sufficiently high to capture high-frequency local paleoclimatic changes recorded in sedimentary organic matter (OM). Paleoclimatic variations were evaluated using mutual relationships among retene (Ret), cadalene (Cad), and 6-isopropyl-1-isohexyl-2-methylnaphthalene (ip-iHMN). These compounds define the Higher Plant Parameter [HPP = Ret/(Ret + Cad)] and Plant Fingerprint (PF, relative distribution of Ret, Cad, and ip-iHMN). Cad is associated with plants growing under drier conditions, whereas Ret is linked to conifers thriving in humid regimes. Although the precise origin of ip-iHMN remains uncertain, aromatization-rearrangement reactions of land-plant terpenoids (and potentially bryophytes) have been proposed. Across the section, HPP values range from 0.35 to 0.99. Group 1 displays fluctuating HPP values (0.50–0.80) with a nearly constant ip-iHMN fraction (~0.05), indicating a humid-seasonal paleoclimate. Group 2 exhibits consistently high HPP values (>0.90) with ip-iHMN ranging from 0.0 to 0.10, recording a shift toward a stably warm and humid paleoclimate. The HPP profile shows remarkable alignment with third-order cycles in global sea level, suggesting that atmospheric CO₂ concentrations and greenhouse dynamics—likely induced by rift-related volcanism—exerted a major control on higher-plant distributions. Drier episodes reduced highland conifer forest input, leaving Cad predominant due to contributions from coastal flora. In contrast, warm, humid greenhouse intervals stimulated conifer expansion, causing Ret dominance. This demonstrates that higher-plant aromatic biomarkers serve as effective proxies for tracking paleoclimatic variations, correlating sedimentary sequences, and enhancing oil–source rock correlations in Brazilian basins.