Benthic foraminifera as bioindicators of gas seep intensity in the offshore zone of the Sinú fold belt

Abstract

Benthic foraminifera are valuable tools for understanding the dynamics of methane seepage worldwide. This study was conducted to characterize the filtration levels of surface samples from 18 stations within a filtration field on the outer continental shelf of the Colombian Caribbean margin. The characterization was based on variables of benthic foraminifera (wall types, spatial distribution of abundances, and associations of dominant species), shell modifications (overgrowth, dissolution, and fragmentation), multivariate statistics, and models explaining the relationships between foraminifera populations and species with levels of filtration activity. Four activity zones were identified. The assemblage of Quinqueloculina candeiana, Triloculina trigonula, Lagenammina difflugiformis, Criboelphidium poeyanum, and Criboelphidium sp. represented low activity; the assemblage of Lobatula ungeriana, Cibicidoides mundulus, and Cibicidoides pseudoungerianus represented moderate filtration; and the assemblage of Liebusella soldanii, Bigenerina irregularis, and Reophax agglutinatus represented moderate–high filtration, whereas high filtration was identified for the assemblage of Cibicidoides mundulus and other hyaline species. Moreover, the type of substrate, methane transport, and physiological adaptations such as symbiosis affected the abundances of these species in different filtration zones, indicating a preference for greater abundances of benthic foraminifera in zones of moderate activity. Furthermore, the δ¹³C values of three species, Criboelphidium poeyanum (−0.17 to −3.85 PDB), Quinqueloculina candeiana (0.02‰ to −1.18‰ PDB), and Lobatula ungeriana (1.99‰ to −3.03‰ PDB), reflected isotopic signals related to CO₂ plumes that preserved the effects of hydrocarbon oxidation and microbial gas. The response of the species was associated with their living depth. Finally, a redundancy analysis demonstrated that the benthic foraminifera populations examined in this study respond primarily to the type of substrate, salinity, and gas seepage.