Bacteriohopanepolyols and glycerol dialkyl glycerol tetraethers record Holocene redox regime shifts in a marine inlet in eastern Prydz Bay, Antarctica

Abstract

Prydz Bay is located at the terminus of one of the largest marine-terminating glacial systems of East Antarctica. Consequently, its sedimentary record may hold information on the response of marine biogeochemical cycling to changes in Antarctic palaeoceanography and palaeoclimate. Bacteriohopanepolyols (BHPs) and glycerol dialkyl glycerol tetraethers (GDGTs) offer great potential for tracing such changes, as these membrane lipids have been linked to distinct biogeochemical processes and environmental niches. Here, we present the BHP and GDGT inventory of a Holocene sediment record, recovered from a marine basin in eastern Prydz Bay. BHP and GDGT lipidomics were complemented with (organic) geochemical tools to reconstruct palaeoenvironmental conditions. GDGT-based indices suggest terrestrial organic matter influx was insignificant. Our results show elevated nucleoside-BHPs in early Holocene sediments from ∼9.1 to 10.7 cal ka BP. Their presence is interpreted to reflect autochthonous production under hypoxic-anoxic water column conditions, as inferred from BHT-x ratios of ≥0.2. The record is characterized by a large diversity of rare and novel unsaturated BHPs and BHP isomers. Their occurrence is potentially related to homeoviscous adaptations of the membranes of predominantly aerobic bacteria to cold conditions (i.e., reconstructed sea surface temperatures are −2.4 to −0.8 °C based on ${TEX}_{86}^{OH}$). Increased abundance and diversity of these BHPs between ∼5.5–7.5 and ∼8.9–9.8 cal ka BP is likely associated with an oxygenation of the basin at this time. During the late Holocene (<3.0 cal ka BP), the water column experienced photic zone euxinia, high rates of nitrogen loss and methanogenesis, as indicated by the presence of isorenieratene, enriched δ¹⁵N values and high GDGT-0/cren ratios, respectively. BHPs in these sediments likely derive from two different ecological niches: i) the uppermost oxygenated to suboxic waters where aerobic methane oxidation occurred and ii) the deeper sulfidic waters. More reducing conditions in the late Holocene coincide with increased brGDGT cyclization and methylation, and is proposed to indicate a shift in the source bacterial niche from sediments to the water column. This study shows that GDGT and BHP distributions can be linked to distinct redox regime shifts within the basin, which appear regulated by changes in sea level and sea ice in Prydz Bay. Our findings highlight the potential use of these lipids as tracers for biogeochemical cycling in marine polar regions.