[1] In order to investigate the link between the methane dynamics in permafrost deposits and climate changes in the past, we studied the abundance, composition, and methane production of methanogenic communities in Late Pleistocene and Holocene sediments of the Siberian Arctic. We detected intervals of increased methane concentrations in Late Pleistocene and Holocene deposits along a 42 ka old permafrost sequence from Kurungnakh Island in the Lena Delta (northeast Siberia). Increased amounts of archaeal life markers (intact phospholipid ethers) and a high variety in genetic fingerprints detected by 16S ribosomal ribonucleic acid gene analyses of methanogenic archaea suggest presently living and presumably active methanogenic archaea in distinct layers predominantly in Holocene deposits, but also in deep frozen ground at 17 m depth. Potential methanogenic activity was confirmed by incubation experiments. By comparing methane concentrations, microbial incubation experiments, gene analysis of methanogens, and microbial life markers (intact phospholipid esters and ethers) to already partly degraded membrane lipids, such as archaeol and isoprenoid glycerol dialkyl glycerol tetraethers, we demonstrated that archaeol likely represents a signal of past methanogenic archaea. The archaeol signal was used to reconstruct the response of methanogenic communities to past temperature changes in the Siberian Arctic, and the data suggest higher methane emissions occurred during warm periods, particularly during an interval in the Late Pleistocene and during the Holocene. This new data on present and past methanogenic communities in the Siberian terrestrial permafrost imply that these microorganisms will respond to the predicted future temperature rise in the Arctic with increasing methane production, as demonstrated in previous warmer periods.

Abstract. Siberian permafrost contains a globally significant pool of organic carbon (OC) that is vulnerable to enhanced warming and subsequent release into the contemporary carbon cycle. OC release by both fluvial and coastal erosion has been reported in the region, but the behaviour of this material in the Arctic Ocean is insufficiently understood. The balance between OC deposition and degradation on the East Siberian Arctic Shelf (ESAS) influences the climate–carbon cycle feedback in this area. In this study we couple measurements of glycerol dialkyl glycerol tetraethers (GDGTs) with bulk geochemical observations to improve knowledge of the sources of OC to the ESAS, the behaviour of specific biomarkers on the shelf and the balance between delivery and removal of different carbon pools. Branched GDGT (brGDGT) concentrations were highest close to river mouths, yet low in "ice complex" permafrost deposits, supporting recent observations that brGDGTs are mostly delivered by fluvial erosion, and may be a tracer for this in complex sedimentary environments. BrGDGT concentrations and the branched and isoprenoidal tetraether (BIT) index reduced quickly offshore, demonstrating a rapid reduction in river influence. Stable carbon isotope ratios changed at a different rate to the BIT index, suggesting not only that OC on the shelf is sourced from fluvial erosion but also that erosion of coastal sediments delivers substantial quantities of OC to the Arctic Ocean. A model of OC export from fluvial, coastal and marine sources is able to recreate the biomarker and bulk observations and provide estimates for the influence of fluvial and coastal OC across the whole shelf. The model shows that coastal erosion delivers 43 % of the OC and 87 % of the mineral sediment to the ESAS, but that rivers deliver 72 % of brGDGTs, indicating that brGDGTs can be used as a proxy for river-derived sediment.

25We investigated the bacteriohopanepolyol (BHP) distribution in the Cobham Lignite 26 sequence (SE England) deposited across the Palaeocene-Eocene boundary including part 27 of the Palaeocene-Eocene Thermal Maximum (PETM) as shown previously by a negative 28 carbon isotope excursion (CIE). A variety of BHPs were identified, including the commonly 29 occurring and non-source specific biohopanoid bacteriohopanetetrol (BHT) and 32, anhydroBHT which was the most abundant polyfunctionalised geohopanoid in the majority of 31 samples. BHPs with a terminal amine functionality, diagnostic biomarkers for methanotrophic 32 bacteria were identified throughout the sequence, with similar distributions in both the lower 33 laminated and upper blocky lignite except that 35-aminobacteriohopanepentol (aminopentol) 34 indicative of Type I methanotrophs (gammaproteobacteria) was generally more abundant in 35 the upper section within the CIE. 36 37The diagenetic fate of these compounds is currently poorly constrained, however, we also 38 identified the recently reported N-containing transformation product anhydroaminotriol and 39 several tentatively assigned novel N-containing structures potentially containing ketone 40 functionalities. Although present throughout the section, there is a sharp peak in the 41 occurrence of these novel compounds correlated with the onset of the CIE and highly 42 isotopically depleted hopanes in the upper part of the laminated lignite, both also correlate 43 well with peak abundance of aminopentol. The significant abundance of these compounds 44 suggests that 35-amino BHPs have their own specific diagenetic pathway, potentially 45 providing an alternative method allowing methanotroph activity to be traced in older samples 46 even if the original biohopanoid markers are no longer present. 47At this time we cannot preclude the possibility that some or all of these BHPs have been 48produced by more recent subsurface activity, post deposition of the lignite to date; however, 49 that would not be expected to generate the observed stratigraphic variability and we suggest 50 that unprecedented observations of a range of highly functionalised biohopanoids in 51 samples of this age could significantly extend the window of their known occurrence. 52