Archaeal lipostratigraphy of the Scotian Slope shallow sediments, Atlantic Canada

Abstract. The Scotian Slope in the North Atlantic Ocean extends ~500 km along the coast of Nova Scotia, Canada, descending from 400 m to 5 km water depth. With a maximum sediment thickness of ~24 km, large portions of the deeper basin are affected by salt tectonism, which has greatly impacted the basin stratigraphy and locally facilitated hydrocarbon seepage to the ocean seafloor. The surface sediments along the slope may therefore be home to microbial communities, which respond to complex geochemical drivers that not only include communication with the overlying water column, but also potential advection of deeper basinal fluids. Archaea are fundamental components of these communities, and their lipids act as important indicators of environmental conditions and microbial interactions within marine sediments. This study evaluates the spatial abundance and diversity of archaeal lipids preserved in shallow buried Scotian Slope sediments to better understand deep marine archaea community dynamics. Seventy-four sediment samples from 32 gravity and piston cores reaching a maximum depth of 9 meters below seafloor (mbsf) were collected during three survey cruises across a large region of the Scotian Slope. The survey area extends across ~40,000 km², marking ~3° of latitudinal change over a water column depth that increases from ~1500 to 3500 m, of which one sampling site was a suspected cold seep environment. In total, 14 archaeal lipid classes comprising 42 unique compounds were detected. The lipid distributions reflect a high contribution of anaerobic methanotrophic (ANME) archaeal groups, such as ANME-1 to -3. Hierarchical cluster analysis (HCA) and principal components analysis (PCA) were used to show varying contributions of four lipid classes that included distinct assemblages of intact polar lipids (IPLs), which are largely sourced from living cells as well as core lipids (CLs), and their degradation products (CL-DPs) that collectively are sourced from different alteration stages following the death of the cell. From this, four stratigraphically distinct archaeal lipidomes, marking varying relative abundances of the lipid classes were observed in the upper 9 m of the surveyed slope sediments. One lipidome likely reflects archaeal communities impacted by a cold seep based on hydrocarbon head space gas analyses and high methane index and GDGT/Cren ratios. The other three lipidomes occur across overlapping sediment depth intervals in which the diversity and abundance of living, fossil, and degraded core lipids systematically change in what is likely depth. These changes likely mark systematic geochemically controlled, microbial community variations that are accompanying an increasing stockpile of diagenetic altered CLs. The three ambient sediment lipidomes appear to be highly conserved across the latitudinal extent of the study area marking a resolvable shallow sediment lipostratigraphy for the Scotian Slope.