Molecular indicators of early stage diagenesis in the tropical coastal Bolgoda Lake, Sri Lanka

Bacterial reactions are one of the major processes that control nutrient cycles in sediments. The aim of this study was to trace the bacterial biomarkers (hopanes) to understand early diagenetic processes in tropical mangrove dominant brackish sediments over modern timescales. Hopane homologues were examined by gas chromatography-mass spectrometry (GC-MS). The results suggested that hopane concentrations were high in Bolgoda Lake surface sediments (average ± SD = 1336.5 ng/g TOC ± 748.7) compared to mangrove mud cores (676.4 ng/g TOC ± 268.5). Therefore, the variation of bacterial biomasses can be controlled by the available reactive organic matter type and depositional environments (sulfate concentration). Total hopane concentrations gradually declined with the depth in core samples. In addition, total hopane concentrations showed moderate and weak correlations with organic matter types in upper (0–60 cm) and lower (> 60 cm) sedimentary successions, respectively. These features in core samples suggest the formation of geohopanoids during diagenesis. The variations of C31 hopane isomerisation ratio suggest the minor influence of reworking bacterial biomarkers in the upper sedimentary succession. Therefore, C31 22S (αβ) hopane distribution can indicate soil acidity (chemical origin) in the mangrove dominant swamp.