GDGT source dynamics across the Sergipe–Alagoas margin

The balance between terrestrial and marine organic matter (OM) inputs remains poorly constrained along the Sergipe-Alagoas (SE-AL) margin, a relevant oil and gas province in Northeast Brazil. This narrow shelf receives fluvial input primarily from the regulated São Francisco River (SFR), whose discharge has declined by ~40% since the 1970s due to damming, alongside smaller rivers (Sergipe, Vaza-Barris, Piauí-Real). We investigate glycerol dialkyl glycerol tetraethers (GDGTs) in surface sediments collected along eight shore-perpendicular transects spanning the shelf (10-50 m) and slope (400-3,000 m) to (i) assess terrestrial versus marine OM contributions, (ii) evaluate marine in-situ brGDGT production, and (iii) test the reliability of TEX86 and related proxies for future paleoceanographic reconstructions on this margin. GDGTs were extracted, purified by silica chromatography, and analyzed by HPLC-APCI-MS in selected ion monitoring mode. Branched (br) and isoprenoid (iso) GDGT distributions were used to calculate the BIT index, #ringstetra, isomer ratio (IR), Ring Index, TEX86-derived sea surface temperature (SST), and Methane Index (MI). BIT values ranged from 0.071 to 0.62, with a higher shelf median (0.34 ± 0.13) than slope (0.28 ± 0.11), reflecting a modest terrestrial imprint that attenuates rapidly offshore, consistent with efficient dispersal of the weakened SFR plume by the South Equatorial Current and trade-wind-driven waves. Transects near the SFR and Japaratuba River retained elevated BIT and brGDGT concentrations even at slope depths, indicating episodic offshore export of fluvial material. Conversely, IR increased seaward (0.53 to 0.92) and #ringstetra was highest on the lower slope, signaling intensifying marine in-situ brGDGT production offshore that can inflate BIT-based terrestrial estimates. Hierarchical cluster analysis of brGDGT compositions distinguished four groups reflecting this shelf-to-slope gradient, plus a distinct southern shallow-shelf cluster with high #ringstetra, possibly linked to localized upwelling near the Japaratuba Canyon. TEX86-derived SSTs (21.6–33.4°C, mean 26.1 ± 1.8°C) were broadly consistent with regional sea-surface temperatures, though anomalously warm shelf values coincided with high-BIT stations. Low GDGT-0/crenarchaeol ratios (<0.8) and MI values (<0.4) at nearly all stations indicate negligible methanogenic/methanotrophic influence on the isoGDGT pool. These results place the SE-AL margin in an intermediate position between strongly terrestrially-dominated systems, such as the Amazon shelf, and settings where in-situ production dominates, such as the Campos Basin mid-shelf, highlighting the need for combined BIT/IR/#ringstetra approaches when applying GDGT-based proxies to reconstruct spatial and temporal organic matter dynamics along tropical passive margins.