This study focuses on the ESAS with sediment samples from the Laptev Sea, Buor-Khaya Bay, Dmitry Laptev Strait and East Siberian Sea (Fig. 1). Comprehensive fieldwork was conducted in August–September 2008 as part of the International Siberian Shelf Studies 2008 expedition ISSS 08;. Surface sediments were recovered using a dual gravity corer or a van Veen grab sampler from H/V Yakob Smirnitskyi (ESAS) and TB-0012 (Buor-Khaya Bay). The sediment samples were transferred with stainless steel spatulas to polyethylene containers and frozen at -18 ∘C for transport and storage . Subsamples were taken and freeze-dried for subsequent total lipid extraction .

The sediments investigated in this study were grouped based on their location on the ESAS Fig. 1; Table S2;. Samples were grouped longitudinally, into the Buor-Khaya Bay and associated region offshore of the Lena River delta (the Laptev Sea), the Dmitry Laptev Strait (the narrow channel between the coastline at ∼ 140∘ E and the New Siberian Islands, splitting the ESAS up into two distinct areas – the Laptev Sea and the East Siberian Sea), the region offshore of the Indigirka River mouth and the region offshore of the Kolyma River mouth. The Indigirka and Kolyma River mouth offshore regions are generally equivalent to the western and eastern East Siberian Sea regions, respectively, as identified by . The ESAS samples have also been classified latitudinally, into the nearshore ESAS (<150 km from river outflows) and offshore ESAS (>150 km from river outflows).

In addition to surface sediment samples throughout the ESAS, this study also includes ICD samples from locations on the Siberian mainland, including the central Lena Delta, Cape Bykovsky, and the Kolyma and Indigirka river banks. The site on Kurungnakh Island (central Lena Delta; 72∘20′ N, 126∘17′ E) was drilled during the Russian–German LENA 2002 expedition in July 2002 and a 24 m long permafrost core from a low-centred ice-wedge polygon was recovered . In total 23 samples from depths 0.34 to 24.55 m (Table S3) were chosen for BHP analysis. An additional ice complex sample (CB IC 1.9; Table S3) was obtained from Cape Bykovsky. The Bykovsky Peninsula is located in the vicinity of the Lena Delta in an area of significant coastal erosion . Ice complex samples from the Kolyma region were obtained from the Chukochya River (CR; Fig. 1), which outflows in the Kolyma Gulf; the Omolon River (OR; Fig. 1), a tributary of the Kolyma River; and from Cherskii CH, Fig. 1; Table S3;. An additional profile from the Indigirka watershed is also included for comparison KY, Fig. 1;.

Data for the ISSS-08 sediments are taken from see Table S2. Total organic carbon (TOC) data for the Indigirka and Cherskii profiles are taken from (see Table S3). The Cape Bykovsky, Chukochya River and Omolon River permafrost ICD samples were prepared for carbon isotope analysis according to and analysed at the University of California, Davis, Stable Isotope Facility.

Sediment samples were extracted using a modified Bligh–Dyer method as described in more detail in and . Briefly, sediment (5 g) was ultrasonically extracted using a monophasic mixture of methanol/dichloromethane/phosphate buffer (0.05 M, pH 7.4; 2:1:0.8 v:v:v). The supernatant was separated by centrifugation and the remaining sediments re-extracted twice. The combined organic phases were evaporated to dryness. After extraction, the total lipid extracts (TLEs) were re-dissolved in dichloromethane/methanol (2:1) and separated into fractions with one-sixth of the TLE used for BHP analysis.

Freeze-dried and ground ICD samples were extracted using a modified Bligh–Dyer method () that was adapted from the method described in . Briefly, samples (∼ 3 g, dry weight) were ultrasonically extracted with a monophasic mixture of methanol/chloroform/water (10:5:4 v:v:v). Deviating from the method described in , the sonication steps were reduced to 30 min at 40 ∘C without the subsequent overnight shaking. The supernatant was removed after centrifugation (16 000 g, 10 min) and the remaining sediment was re-extracted twice. After phase separation via addition of water (5 mL) and chloroform (5 mL), the organic phases were combined, evaporated to dryness, and blown to dryness under N2. Extracts were then re-dissolved in 2:1 chloroform/methanol and one-third of the TLE was used for BHP analyses.

Aliquots (one-sixth for sediments and one-third for terrestrial materials) of the TLE were loaded onto NH2 solid phase extraction (SPE) cartridges pre-conditioned with 6 mL of hexane (1 g / 6 mL; Isolute, Biotage, Sweden), in 200 µL chloroform and separated into two fractions (Fr): Fr. 1 (non-polar + acidic, 6 mL of diethylether/acetic acid (98:2, v:v)) and Fr. 2 (polar, 12 mL of methanol), which contained all BHPs except 32,35-anhydroBHT e.g.. After separation, the internal standard (5α-pregnane-3β,20β-diol) was added to Fr. 2 and dried under N2. This SPE method was adapted from a method commonly used in other studies of complex polar lipids from environmental samples e.g.. Fr. 2 was acetylated with pyridine/acetic anhydride (1:1, v:v; 500 µL) for 1 h at 50 ∘C and left at room temperature overnight. The samples were evaporated to dryness, re-dissolved in methanol/propan-2-ol (60:40, v:v) and filtered through a 0.2 µm PTFE syringe filter. For BHP analysis, the samples were dissolved in methanol/propan-2-ol (60:40, v:v; 500 µL). Sample injection volume was 10 µL.

BHPs were identified and measured using reverse-phase HPLC-APCI-MS as previously described in . Chromatographic separation was achieved under the conditions described in . BHP structures were identified based on previously published spectra . Semi-quantitative estimation of BHP concentrations was achieved by employing the characteristic base peak ion areas of individual BHPs in mass chromatograms (from SCAN 1) relative to the m/z 345 chromatogram base peak area of the acetylated 5α-pregnane-3β,20β-diol internal standard. Averaged relative response factors relative to the internal standard, determined from a suite of acetylated BHP standards, were used to adjust the BHP peak areas where N-containing compounds give an average response 12 times that of the standard and compounds without N 8 times that of the standard for further details see. The reproducibility of triplicate injections was 3–6 % RSD (standard error: ±1–4 µggOC-1) for BHT and 5–8 % RSD (standard error: ±1–2 µggOC-1) for adenosylhopane in the environmental samples, resulting in an absolute standard error of on average ±0.01 for R′soil (see Sect. 1, Eq. 2).

In total, 92 surface sediment samples throughout the ESAS in Buor-Khaya Bay, Dmitry Laptev Strait, and Kolyma River and Indigirka River mouth transects were analysed for their BHP composition (Table S2). Up to 16 individual BHPs were identified in the ESAS sediments, with the total concentration of BHPs ranging from 12 to 824 µggOC-1 (Table S2). However, their concentrations and distributions differed with distance to the mainland throughout the shelf. BHT (1f) was the most abundant single BHP, ranging from 9 to 313 µggOC-1 (Table S1, Figs. 2a and 3a). The relative proportion of BHT was lowest in Buor-Khaya Bay sediments close to the mainland (mean = 37 % of all detected BHPs) rising to 80 % (mean = 65 %) of all detected BHPs in the ESAS offshore sediments furthest away from the mainland. Highest BHT concentrations were measured closest to the mainland except in the region of the Indigirka outflow (samples YS-26 to 30; Table S2), where values were lower than in all other nearshore settings (Fig. 2a). However, mean concentrations were stable with increasing distance from the mainland (Fig. 3a) and considerable amounts of BHT (up to 77 µggOC-1) are still detectable at 293 km offshore.

In addition to the ubiquitous and abundant BHT (1f), a suite of other polyhydroxylated BHPs related to BHT were detected, including the BHT isomer (1f'), which has been linked to production by pelagic anaerobic organisms performing anaerobic ammonium oxidation annamox;. The C-2 methylated homologue 2-MeBHT (2f), unsaturated BHT (Δ6-1f) and bacteriohopane-30,31,32,33,34,35-hexol (1g) were also common, especially in Buor-Khaya Bay, although at much lower concentration than BHT (Table S2). Soil marker BHPs identified included high proportions of adenosylhopane (1a), followed by adenosylhopane type 2 (1b) and adenosylhopane type 3 (1b'). The soil marker type 2 and 3 compounds are related to adenosylhopane but have different and as yet uncharacterised terminal groups compared to adenosylhopane as identified by LC-MSn Table S1;. The C-2 methylated soil markers (2a, 2b, 2b'; Table S1) were present intermittently and always at lower concentration than the corresponding non-methylated structures. Generally, the concentrations of all non-methylated soil markers are highest in samples closer to the coast (0–100 km) and decrease with distance from the river outflows (Figs. 2b and 3b; Table S2) showing similar trends.

Sediments of Buor-Khaya Bay and Dmitry Laptev Strait were characterised by high amounts of adenosylhopane, with a mean average of 64 µggOC-1 (range 7–137 µggOC-1; Table S2) and total non-methylated soil markers accounted for up to 66 % of total BHPs although the mean proportion was lower at 36 %. Sediments collected from the ESAS offshore region contain noticeably lower soil marker BHPs both in absolute and relative concentrations with non-methylated soil markers ranging from 0 to 62 µggOC-1 (mean 11 % of all detected BHPs; Table S2; Figs. 2b and 3b). Methylated compounds were frequently below the detection limit (Table S2) as previously reported in Arctic and sub-Arctic River mouth surface sediment transects .

Total concentrations of non-methylated soil markers in ESAS sediments with a range of 0–218 µggOC-1 are similar to sediments from the Yenisei River system, including the Yenisei River mouth, gulf, outflow and nearby Kara Sea 9–508 µggOC-1;. The highest values occurred in the Yenisei River mouth sediments and also in Khalmyer Bay (329–435 µggOC-1), a nearby area of coastal erosion, although not directly drained by the Yenisei River itself. However, in the Yenisei mouth, Khalmyer Bay, Buor-Khaya Bay and Dmitry Laptev Strait, non-methylated soil markers had very similar mean relative abundance as a proportion of total BHPs of 30–40 %. This reflects a significant contribution of these terrestrial compounds to the total BHP assemblage in Arctic settings see also and indicates a strong terrestrial signal in the Arctic Shelf sediments. The proportion of total soil markers in sediments from the outflow of other non-Arctic rivers has been shown to be somewhat lower (e.g. <20 % from the estuary and seaward of the Yangtze River, , 2011; <8 % Congo River estuary, , 2015), thus emphasising the importance of obtaining values for local endmembers. The higher abundance of these highly functionalised compounds in Arctic sediments may be a result of better preservation under the cold-temperature conditions of this region. Additionally, temperature may also influence the microbiological community, leading to deliberate accumulation of adenosylhopane and/or limited biosynthetic transformation of the BHP precursor in this extreme environment see.

Other BHPs identified include three structures with amine functional groups at the C-35 position. The concentration of aminotriol (1e; Table S1) varied from 0 to 53 µggOC-1 (mean 6.4 % of all analysed BHPs) throughout the ESAS (Table S2). Aminotetrol (1d) was generally less abundant across the ESAS (0–13 µggOC-1) and aminopentol (1c) was identified in 37 of the 47 Buor-Khaya Bay sediments and only occasionally in other areas (Table S2). Aminotetrol and aminopentol in particular have been linked to aerobic methane oxidising bacterial sources and have been proposed as markers for terrestrial methane oxidation in continental wetlands, which is then subsequently recorded in marine (e.g. deep-sea fan) sediments from the Republic of Congo . More recently, identified aminopentol in sediments of the Yenisei River outflow and tentatively proposed that it might indicate decomposition of sub-sea permafrost with associated methane release and subsequent microbial oxidation. Here in Buor-Khaya Bay, concentrations of aminopentol (1c) ranged from 0 to 9.5 µggOC-1, considerably lower than values reported in 0–48 µggOC-1. It is therefore possible that the aminopentol signature is fluvially transported from areas of active aerobic methane oxidation within the catchment (i.e. polygonal tundra, wetlands, thermokarst lakes) or, alternatively, indicates oxidation of methane released from sub-sea permafrost deposits known to be common across the ESAS including Buor-Khaya Bay .

The ESAS surface sediments generally had only low levels of composite BHPs (i.e. BHPs with more complex moiety at the C-35 position such as a sugar or amino-sugar; e.g. , 1993). The only exceptions were BHT cyclitol ether (1h), which was found in concentrations ranging from 0 to 49 µggOC-1, with a mean of 7.7 µggOC-1 (0–10 % of all analysed BHPs), and BHT glucosamine (1i), which was even less common (Table S2). Both of these structures have a wide range of known sources, so they cannot be assigned to any specific group of source organisms see review in.

Bulk carbon isotope values (δ13C) are commonly used as a proxy for marine vs. terrestrial influence on sedimentary OC composition as terrestrial plants using the C3 synthesis pathway typically have more depleted values than OC produced via marine primary productivity . Here, we compared bulk carbon isotopes to the BHP-based R′soil proxy (Eq. 2). As expected, R′soil values were higher closer to the coast and reduced gradually with increasing distance offshore (Figs. 2c, 3c), including the region off the Indigirka which had somewhat lower absolute concentrations of the individual BHPs compared to the more eastern and western extents of the ESAS region (Fig. 2a, b). The highest values occurred in Buor-Khaya Bay (maximum R′soil=0.80; Table S2); however, the mean values for Buor-Khaya Bay and Dmitry Laptev Strait sediments were very similar at 0.49 and 0.52, respectively (Fig. 4). The ESAS nearshore sediments had an average only slightly lower at 0.41, whilst the average for the offshore sediments was 0.14 (range 0.42 to 0.00; Fig. 4, Table S2).

A clear negative linear relationship was observed between R′soil and bulk δ13C (r2=-0.73, p<0.001; Fig. 5a) across the ESAS in agreement with a pilot study of soil microbial biomarkers in ESAS surface sediments from the Kolyma River mouth offshore transect where strong linear correlations were observed between the R′soil proxy and distance from river mouth (r2=0.97) and bulk δ13C (r2=0.96; , 2015). Although bulk δ13C in ESAS surface sediments did display a linear relationship with distance offshore , previous comparison of bulk δ13C and BIT values from the ESAS surface sediments displayed a strongly non-linear correlation Fig. 5c;. This was shown to result from a rapid reduction in concentration of brGDGTs in near-coastal sediments (<150 km from shoreline), causing a drop in BIT values from ∼ 1 to ∼ 0.25 with little variation in bulk δ13C followed by an enrichment in bulk carbon isotopes towards more marine values concomitant with a slower decline in BIT values towards 0 further than 150 km offshore . A similar non-linear relationship is observed here between R′soil and BIT (Fig. 5b), as also previously demonstrated for the surface sediment offshore transect off the Kolyma River mouth , indicating that similar processes are operating across the entire ESAS.

The simple linear correlation between R′soil and bulk carbon isotope values is intriguing as it suggests that, unlike the brGDGTs, which in this region are proposed to be primarily derived from fluvial transport , the R′soil proxy provides a more integrated signature of different terrestrial sources including ICD and fluvially transported topsoil-permafrost or riverine-produced material. Therefore, soil marker BHPs and brGDGTs, despite being nominally derived from similar sources, i.e. terrestrial microbial membrane lipids, appear in fact to be representing different aspects of terrestrial OC export.

recently demonstrated that soil marker BHPs can indeed be transported in suspended particulate matter (SPM) from the Yenisei River, a large river located west of the ESAS. In the Yenisei study, only a moderate correlation was observed between the δ13C values and R′soil (r2=0.44, p<0.01), suggesting they trace different pools of OM (bulk terrigenous OM versus bacterial OM). However, unlike in the current study, also found a strong linear correlation between R′soil and BIT (r2=0.82, p<0.05). The Yenisei River catchment and outflow have significantly different characteristics to those of the rivers entering the ESAS, specifically in terms of the extent of different permafrost regimes. Permafrost is classified as isolated, sporadic, discontinuous and continuous according to its spatial distribution e.g.. Continuous permafrost means that over 90 % of the area is frozen in contrast to discontinuous permafrost, where only 30–80 % of the area is underlain by permafrost. The Yenisei drains an area with a high proportion of discontinuous permafrost (55 %), whilst the proportion of continuous permafrost is lower 33 %;and references therein. The proportion of continuous permafrost in the eastern river catchments is higher, ranging from 79 % (Lena) to 100 % (Kolyma and Indigirka; ; and references therein). This may in turn affect the composition and preservation of terrestrial microbial markers. Indeed, previous work has indicated that OC from continuous permafrost areas is older but also less degraded and more biolabile than that from areas of discontinuous permafrost e.g.. Additionally, although there is some evidence of coastal erosion of ICD in this region, such as in the Khalmyer Bay area, this is not directly drained by the Yenisei River. Therefore, in this more westerly region it is likely that the primary source for both soil marker BHPs and brGDGTs is fluvial transport, hence the linear relationship between soil microbial biomarkers and bulk δ13C values . Whereas in the more easterly region we assume brGDGTs to be of fluvial origin and the soil marker BHPs to have an integrated signature of terrestrial sources including ICD and fluvially transported topsoil-permafrost or riverine-produced material.

Given the apparent discrepancy between the BHP and GDGT-derived signals, further consideration of the terrestrial endmembers is clearly required. Although extensive databases exist for different bulk isotopic endmembers for the Arctic region (e.g. ; ; and references therein), data on the soil microbial lipid derived proxy values are limited for the Siberian region (see Table 1 for summary). Recently, reported high BIT values for a range of materials from the Kolyma region (eastern ESAS), including thermokarst and floodplain lake sediments (BIT = 1), yedoma (and associated streams; BIT = 0.81–0.89) and SPM from the Kolyma River including samples collected during the spring freshet (BIT = 0.99–1). However, reported values for three ICD (Yedoma) samples from the same area ranging from 0.44 to 0.7. The lower values resulted from relatively high levels of crenarchaeol, which is unusual for terrestrial materials typical BIT values >0.8;, although this compound has been reported from several Thaumarchaeota isolated from soil . Data on BHPs from the (East Siberian) Arctic region are scarce as previous studies of terrestrial BHPs have primarily focused on temperate and more recently on tropical regions see review in. reported average R′soil values of 0.76 (range 0.70–0.84) for the same three ICD yedoma samples from the Kolyma region (CHYED-2; Table 1, Table S3). also reported the BHP composition in polygonal active layer deposits (to a maximum depth of 48 cm) from two locations in the Lena Delta, Samoylov Island and Kurungnakh Island. Calculating R′soil values from these data revealed a wide range of values from 0.18 to 0.79 and a mean average of 0.41 (Table 1). Although this simple average will likely not represent a spatially and depth resolved average for the region, it is still close to the mean values found in Buor-Khaya Bay and Dmitry Laptev Strait sediments (Table S2).

To further evaluate potential endmember ranges, additional ICD samples from the Lena, Indigirka and Kolyma regions were investigated for BHPs (Table S3). We analysed 23 samples from a 25 m permafrost core from Kurungnakh Island see details in and an additional sample from Cape Bykovsky at 1.9 m depth (Table S3). BHT and a range of soil marker BHPs were present in all samples. BHT concentration ranged from 7 to 643 µggOC-1 (mean average 248 µggOC-1) in the Kurungnakh Island permafrost deposits. As in the ESAS sediments (Table S2), the soil marker BHPs included high proportions of adenosylhopane (1a), followed by adenosylhopane type 2 (1b) and adenosylhopane type 3 (1b') (mean average 250 µggOC-1; Table S3). As expected, observation of the methylated compounds was intermittent and then only at very low levels (Table S3), justifying their exclusion from R′soil . R′soil values ranged from 0.37 to 0.64 with a mean value of 0.50 (Table 1), whilst the Cape Bykovsky (CB) sample had an R′soil of 0.68 (Table 1). The low R′soil values in ICD from this region (0.34 to 0.80, mean 0.49; Figs. 3c, 4) are in excellent agreement with the mean and range of values found within the ISSS-08 Buor-Khaya Bay and Dmitry Laptev Strait sediments (0.49 and 0.52 respectively; Table S2; Fig. 5a). Although additional sources of BHPs from fluvial transport and from material transported via changes in hydrological conduits resulting from thermokarst erosion are also possible POC; e.g., there are currently no data on the BHP composition in this fraction from this region for comparison.

Bulk δ13C was not measured for the KUR core used in this study, but reported values between -23.1 and -24.6 ‰ for the OC fraction of selected permafrost sediments from Samoylov Island, which lies close to Kurungnakh Island in the Lena Delta see map in. These values from Samoylov Island are significantly enriched relative to the value for the Cape Bykovsky sample (-26.0 ‰; Table S3) and may be because these deposits are genetically different (Holocene fluvial sediments vs. Pleistocene ICD) or reflect input from aquatic plants in low-centre polygon ponds . However, input from peat, grasses, herbs and shrubs results in more negative values such as those reported by for a range of sites in the region including Kurungnakh Island and the Bykovsky Peninsula (range -30 to -25 ‰). The δ13C values for the ISSS-08 sediments from Buor-Khaya Bay (-25.3 to -26.6 ‰) and the Dmitry Laptev Strait (-26.9 to -27.4 ‰) therefore suggest a significant contribution of terrestrial material. reported an extensive compilation of circum-Arctic literature data with an average δ13C value of -26.3 ± 0.7 ‰ for ICD OC (coastal, inland and sub-sea; formed before inundation) and even more depleted values for topsoil permafrost with δ13C of -28.2 ± 2.0 ‰ (Table 1). By combining bulk 13C and 14C data, these authors estimated the proportion of sedimentary OM derived from ICD, topsoil and marine sources, with over two-thirds of the OM in Buor-Khaya Bay derived from terrestrial sources and even higher values in Dmitry Laptev Strait sediments. These terrestrial OM estimates can now be compared to estimates based on the BHP concentrations/compositions obtained using a similar approach with the results from the ICD samples as the terrestrial endmembers.

ICD samples from the Indigirka (n=3) and Kolyma River (n=8) regions had lower absolute concentrations of BHPs (BHT range 8.5 to 62 µggOC-1; non-methylated soil markers range from 43 to 123 µggOC-1) and generally higher R′soil values than those from the Lena region (mean 0.76, range 0.62–0.84; Tables 1, S3; Fig. 5a). Although there are some differences in the relative abundances of the non-methylated soil markers BHPs between the different regions (Table S4), in all cases BHP 1a was the most abundant, followed by BHP 1b and minor relative amounts of BHP 1b' (see Table S4). Correlation of concentrations of all three pairs of these non-methylated soil markers (BHP 1a vs. BHP 1b, BHP 1b vs. BHP 1b' and BHP 1a vs. BHP 1b') in all ICD samples gives an r2>0.6 and p value <0.001 for each pair of compounds. This shows that the distribution of the non-methylated soil markers in all ICD samples is comparable across the East Siberian Arctic region. Given the higher R′soil values for ICD in the eastern region, this suggests that although some removal of soil marker BHPs may have already occurred in the river estuaries before reaching the near-coastal shelf sediments, a significant proportion remains intact. Based on the range of ICD permafrost endmember data for the Kolyma and Indigirka samples, this would suggest that 58 to 92 % of the OC derived from terrestrial OM (e.g. % ICD and topsoil) at the start of the offshore Kolyma River mouth surface sediment transect (sample YS-34B, R′soil=0.57) and 49 to 77 % of the OC derived from terrestrial OM at the start of the offshore transect off the Indigirka River mouth (sample YS-30, R′soil=0.48) is of terrestrial origin. These values should be treated with caution given the limited number of samples involved, but are close to the average value reported for the ESAS surface sediments based on dual-carbon-isotope (δ13C and Δ14C) mixing models 75±14 % from terrestrial origin;.

This difference in R′soil values in permafrost deposits/ICDs across the East Siberian Arctic region is likely due to two separate regional provinces, both underlain by mostly continuous permafrost, but with permafrost deposits of different ages and formation mechanisms . Further, samples in the western and far eastern region are subject to different environmental conditions (with colder and drier conditions in the far eastern region , potentially indicating better preservation of the structurally more complex soil marker BHPs relative to BHT (see Sect. 3.2). This agrees with previous studies which have also found better preservation of more highly functionalised and biolabile molecules in materials from the eastern region e.g.. reported that fine and ultrafine grain size fractions contain a high proportion of high-molecular-weight lipid markers which are preferably bound to the mineral matrix and that the reactivity of lipid biomarkers on the ESAS seems to be lower and inversely proportional to the number of functional groups (cutin acids > n-alkanolic acid > n-alkanols > n-alkanes). Even though the reactivity for different BHPs is currently unknown, this points towards a potential recalcitrance of highly functionalised BHP molecules on the ESAS. Furthermore, studies from soils have shown the potential for mineral–organic interactions, leading to increased resistance to degradation for aromatic compounds . Adenosylhopane, the most abundant single soil marker BHP on the ESAS, is the only BHP containing an aromatic moiety (adenine). Although found organic–mineral associations to be of minor importance in the polygonal tundra of the Lena Delta, organic–mineral interactions may still be among several factors explaining the high relative abundance of these compounds under certain conditions.

However, the current sample set discussed is limited given the enormous spatial scale and extremely heterogeneous nature of these environments and does not include, for example, material released from thermokarst environments, including thermokarst lake sediments, which can be an important source of OC and inorganic material . Furthermore, environmental parameters other than location (and inferred temperature) must also be considered as potentially affecting the overall BHP assemblage. For example, recently demonstrated using principal component analysis that increasing pH, over a range of 4.5 to 6.7, was positively correlated with soil marker BHP concentration in Lena Delta permafrost, which is in close proximity (<10 km) to the site of the KUR core studied here. These authors proposed that this might indicate source organisms do not need to further extend their BHP side chains to alter membrane architecture at near-neutral conditions. Furthermore, studies of temperate Sphagnum peat deposits, which typically have low pH values, show low abundance of soil marker compounds relative to total BHPs when compared to mineral soils, but have higher abundance of BHT resulting in very low R′soil values mean 0.4; calculated from data in. This could suggest that, in areas with a significant input from peat-derived material in some areas/layers of the Lena Delta ICD , lower R′soil values should be expected in agreement with observations in this study (Table S3). Indeed, pH has also been shown to play a dominant role in shaping bacterial communities with the capacity to produce hopanoids in an acidic peatland . Clearly a more comprehensive assessment of different terrestrial endmembers across the region is required, as are additional studies on primary environmental factors affecting BHP biosynthesis in culture. For example, to date no studies have investigated biosynthesis of adenosylhopane in psychrophilic/psychrotolerant organisms at different temperatures or at different growth stages, and studies of BHPs in association with pH adaptation have yet to measure adenosylhopane abundances e.g.. Given that adenosylhopane is the precursor for biosynthesis of all other side-chain extended BHPs , it is possible that, under conditions of stress (such as extreme temperature and nutrient limitation), production of adenosylhopane without further modification is sufficient and/or all that some organisms are capable of and further modification is metabolically unfavourable/unnecessary.