Constraining the formation of authigenic carbonates in a seepage‐affected cold‐water coral mound by lipid biomarkers

Abstract

Cold‐water coral (CWC) mounds are build‐ups comprised of coral‐dominated intervals alternating with a mixed carbonate‐siliciclastic matrix. At some locations, CWC mounds are influenced by methane seepage, but the impact of methane on CWC mounds is poorly understood. To constrain the potential impact of methane on CWC mound growth, lipid biomarker investigations were combined with mineralogical and petrographic analyses to investigate the anaerobic oxidation of methane (AOM) and authigenic carbonate formation in sediment from a seep‐affected CWC mound in the Gulf of Cadiz. The occurrence of AOM was confirmed by characteristic lipids found within a semi‐lithified zone (SLZ) consisting of authigenic aragonite, high‐magnesium calcite and calcium‐excess dolomite. The formation of high‐Mg calcite is attributed to AOM, acting as a lithifying agent. Aragonite is only a minor phase. Ca‐excess dolomite in the SLZ and upper parts may be formed by organoclastic sulphate reduction, favouring precipitation by increased alkalinity. The AOM biomarkers in the SLZ include isoprenoid‐based archaeal membrane lipids, such as abundant glycerol dibiphytanyl glycerol tetraethers (GDGTs) dominated by GDGT‐2. The δ13C values of GDGT‐2, measured as ether‐cleaved monocyclic biphytanes, are as low as −100‰ versus V‐PDB. Further, bacterial dialkyl glycerol diethers with two anteiso‐C15 alkyl chains and δ13C values of −81‰ are interpreted as biomarkers of sulphate‐reducing bacteria. The lipid biomarker signatures and mineralogical patterns suggest that anaerobic methane‐oxidizing archaea of the ANME‐1 group thrived in the subsurface at times of slow and diffusive methane seepage. Petrographic analyses revealed that the SLZ was exhumed at some point (e.g. signs of bioerosion of the semi‐lithified sediment), providing a hard substrate for CWC larval settlement. In addition, this work reveals that AOM‐induced semi‐lithification likely played a role in mound stabilization. Lipid biomarker analysis proves to be a powerful tool to disentangle early diagenetic processes induced by microbial metabolisms.

Occurrence and Genesis of Cold‐Seep Authigenic Carbonates from the South‐Eastern Mediterranean Sea

“…Expulsion of hydrocarbon‐enriched fluids at the sea floor are a common feature along continental margins around the world, forming authigenic seep carbonates (Feenstra et al, 2020; Feng et al, 2014a; Himmler et al, 2015; Krause et al, 2014; Liebetrau et al, 2010; Suess, 2014). Sources of these fluids can be either organic‐rich sediments or deeper gas reservoirs (Gorman et al, 2002; Whiticar, 1999).…”

Distribution and geological controls of the seabed fluid flow system, the central‐western Bohai Sea: A general overview

“…overpressured groundwater, hydrocarbon, hydrogen sulfide, carbon dioxide (CO 2 ), brine and mixed with fine‐grained sediments) migrating in the subsurface and leaking into the ocean water column, which produces a series of physical, chemical and biological effects near the seafloor including reshaping seabed morphology, affecting mineral precipitation and sustaining chemosynthetic biological communities (e.g. Andresen, 2012; Feenstra et al, 2020; Judd & Hovland, 2007; Løseth et al, 2009; Serié et al, 2017; Suess, 2014). Shallow gas is a geohazard to offshore operations and seeps are indicative of extensive hydrocarbon reserves at greater depths (e.g.…”