Description: Hydrocarbon seeps are ubiquitous at gas-prone Cenozoic deltas such as the Nile Deep Sea Fan (NDSF2) where seepage into the bottom water has been observed at several mud volcanoes (MVs3) including North Alex MV (NAMV4). Here we investigated the sources of hydrocarbon gases and sedimentary organic matter together with biomarkers of microbial activity at four locations of NAMV to constrain how venting at the seafloor relates to the generation of hydrocarbon gases in deeper sediments. At the centre, high upward flux of hot (70 °C) hydrocarbon-rich fluids is indicated by an absence of biomarkers of Anaerobic Oxidation of Methane (AOM) and nearly constant methane (CH4) concentration depth-profile. The presence of lipids of incompatible thermal maturities points to mixing between early-mature petroleum and immature organic matter, indicating that shallow mud has been mobilized by the influx of deep-sourced hydrocarbon-rich fluids. Methane is enriched in the heavier isotopes, with values of δ13C∼−46.6‰VPDB and δD ∼−228‰VSMOW, and is associated with high amounts of heavier homologues (C2+) suggesting a co-genetic origin with the petroleum. On the contrary at the periphery, a lower but sustained CH4 flux is indicated by deeper sulphate–methane transition zones and the presence of 13C-depleted biomarkers of AOM, consistent with predominantly immature organic matter. Values of δ13C-CH4∼−60‰VPDB and decreased concentrations of 13C-enriched C2+ are typical of mixed microbial CH4 and biodegraded thermogenic gas from Plio-Pleistocene reservoirs of the region. The maturity of gas condensate migrated from pre-Miocene sources into Miocene reservoirs of the Western NDSF is higher than that of the gas vented at the centre of NAMV, supporting the hypothesis that it is rather released from the degradation of oil in Neogene reservoirs. Combined with the finding of hot pore water and petroleum at the centre, our results suggest that clay mineral dehydration of Neogene sediments, which takes place posterior to reservoir filling, may contribute to intense gas generation at high sedimentation rate deltas.

Description: Detailed knowledge of the extent of post-genetic modifications affecting shallow submarine hydrocarbons fueled from the deep subsurface is fundamental for evaluating source and reservoir properties. We investigated gases from a submarine high-flux seepage site in the anoxic Eastern Black Sea in order to elucidate molecular and isotopic alterations of low-molecular-weight hydrocarbons (LMWHC) associated with upward migration through the sediment and precipitation of shallow gas hydrates. For this, near-surface sediment pressure cores and free gas venting from the seafloor were collected using autoclave technology at the Batumi seep area at 845 m water depth within the gas hydrate stability zone. Vent gas, gas from pressure core degassing, and from hydrate dissociation were strongly dominated by methane (〉 99.85 mol.% of ∑[C1–C4, CO2]). Molecular ratios of LMWHC (C1/[C2 + C3] 〉 1000) and stable isotopic compositions of methane (δ13C = − 53.5‰ V-PDB; D/H around − 175‰ SMOW) indicated predominant microbial methane formation. C1/C2+ ratios and stable isotopic compositions of LMWHC distinguished three gas types prevailing in the seepage area. Vent gas discharged into bottom waters was depleted in methane by 〉 0.03 mol.% (∑[C1–C4, CO2]) relative to the other gas types and the virtual lack of 14C–CH4 indicated a negligible input of methane from degradation of fresh organic matter. Of all gas types analyzed, vent gas was least affected by molecular fractionation, thus, its origin from the deep subsurface rather than from decomposing hydrates in near-surface sediments is likely. As a result of the anaerobic oxidation of methane, LMWHC in pressure cores in top sediments included smaller methane fractions [0.03 mol.% ∑(C1–C4, CO2)] than gas released from pressure cores of more deeply buried sediments, where the fraction of methane was maximal due to its preferential incorporation in hydrate lattices. No indications for stable carbon isotopic fractionations of methane during hydrate crystallization from vent gas were found. Enrichments of 14C–CH4 (1.4 pMC) in short cores relative to lower abundances (max. 0.6 pMC) in gas from long cores and gas hydrates substantiates recent methanogenesis utilizing modern organic matter deposited in top sediments of this high-flux hydrocarbon seep area.

Description: Hydrocarbon seeps are ubiquitous at gas-prone Cenozoic deltas such as the Nile Deep Sea Fan (NDSF2) where seepage into the bottom water has been observed at several mud volcanoes (MVs3) including North Alex MV (NAMV4). Here we investigated the sources of hydrocarbon gases and sedimentary organic matter together with biomarkers of microbial activity at four locations of NAMV to constrain how venting at the seafloor relates to the generation of hydrocarbon gases in deeper sediments. At the centre, high upward flux of hot (70 °C) hydrocarbon-rich fluids is indicated by an absence of biomarkers of Anaerobic Oxidation of Methane (AOM) and nearly constant methane (CH4) concentration depth-profile. The presence of lipids of incompatible thermal maturities points to mixing between early-mature petroleum and immature organic matter, indicating that shallow mud has been mobilized by the influx of deep-sourced hydrocarbon-rich fluids. Methane is enriched in the heavier isotopes, with values of δ13C∼−46.6‰VPDB and δD ∼−228‰VSMOW, and is associated with high amounts of heavier homologues (C2+) suggesting a co-genetic origin with the petroleum. On the contrary at the periphery, a lower but sustained CH4 flux is indicated by deeper sulphate–methane transition zones and the presence of 13C-depleted biomarkers of AOM, consistent with predominantly immature organic matter. Values of δ13C-CH4∼−60‰VPDB and decreased concentrations of 13C-enriched C2+ are typical of mixed microbial CH4 and biodegraded thermogenic gas from Plio-Pleistocene reservoirs of the region. The maturity of gas condensate migrated from pre-Miocene sources into Miocene reservoirs of the Western NDSF is higher than that of the gas vented at the centre of NAMV, supporting the hypothesis that it is rather released from the degradation of oil in Neogene reservoirs. Combined with the finding of hot pore water and petroleum at the centre, our results suggest that clay mineral dehydration of Neogene sediments, which takes place posterior to reservoir filling, may contribute to intense gas generation at high sedimentation rate deltas. Highlights ► Extensive seepage of biodegraded gas at the periphery of North Alex mud volcano. ► At the centre seepage of deeper-sourced hot water, oil and thermogenic gas. ► At the centre, degradation of reservoired-oil to gas is most likely. ► Multivariate statistics on biomarkers show oil degradation at the centre and AOM at the periphery. ► Shallow gas production is enhanced by hot water influx from actively dewatering clays.

Description: Collectively, marine sediments comprise the largest reservoir of methane on Earth. The flux of methane from the sea bed to the overlying water column is mitigated by the sulphate-dependent anaerobic oxidation of methane by marine microbes within a discrete sedimentary horizon termed the sulphate–methane transition zone. According to conventional isotope systematics, the biological consumption of methane leaves a residue of methane enriched in 13C (refs 1, 2, 3). However, in many instances the methane within sulphate–methane transition zones is depleted in 13C, consistent with the production of methane, and interpreted as evidence for the intertwined anaerobic oxidation and production of methane4, 5, 6. Here, we report results from experiments in which we incubated cultures of microbial methane consumers with methane and low levels of sulphate, and monitored the stable isotope composition of the methane and dissolved inorganic carbon pools over time. Residual methane became progressively enriched in 13C at sulphate concentrations above 0.5 mM, and progressively depleted in 13C below this threshold. We attribute the shift to 13C depletion during the anaerobic oxidation of methane at low sulphate concentrations to the microbially mediated carbon isotope equilibration between methane and carbon dioxide. We suggest that this isotopic effect could help to explain the 13C-depletion of methane in subseafloor sulphate–methane transition zones.

Description: Butanetriol and pentanetriol dialkyl glycerol tetraethers (BDGTs and PDGTs) are membrane lipids, recently discovered in sedimentary environments and in the methanogenic archaeon Methanomassiliicoccus luminyensis. They possess an unusual structure, which challenges fundamental assumptions in lipid biochemistry. Indeed, they bear a butanetriol or a pentanetriol backbone instead of a glycerol at one end of their core structure. In this study, we unambiguously located the additional methyl group of the BDGT compound on the C3 carbon of the lipid backbone via high-field nuclear magnetic resonance (NMR) experiments. We further systematically explored the abundance, distribution and isotopic composition of BDGTs and PDGTs as both intact polar and core lipid forms in marine sediments collected in contrasting environments of the Mediterranean Sea and Black Sea. High proportions of intact polar BDGTs and PDGTs in the deeper methane-laden sedimentary layers and relatively C-13-depleted BDGTs, especially in the Rhone Delta and in the Black Sea, are in agreement with a probable methanogenic source for these lipids. However, contributions from heterotrophic Archaea to BDGTs (and PDGTs) cannot be excluded, particularly in the eastern Mediterranean Sea, and contrasting BDGT and PDGT headgroup distribution patterns were observed between the different sites studied. This points to additional, non-methanogenic, archaeal sources for these lipids.

Description: Highlights: • Constraining sources of core and intact archaeal lipids with stable C isotopic ratios. • No evidence for sedimentary sources of IPL crenarchaeol. • Evidence of sedimentary production of IPL caldarchaeol and BDGT-0. • Higher organic matter content promotes higher activity of sedimentary archaea. • Archaeol is a sensitive indicator of sedimentary archaea. Archaea occupy an important niche in the global carbon cycle and their lipids are widely used as indicators of environmental conditions in both paleoenvironmental and modern biogeochemical studies. The principal sources of archaeal lipids in marine sediments are benthic archaea, fossil remnants of planktonic archaea, and allochthonous sources such as soils. However, the relative contributions of these sources to the sedimentary lipid pool have not been comprehensively constrained, complicating a mechanistic understanding of archaeal lipid proxies. In order to provide insights into the relative contributions of these sources and identify signals derived from sedimentary activity, we performed a systematic survey of stable carbon isotopic compositions (delta C-13) of both core and intact archaeal lipids via analyses of their phytanyl (Phy) and biphytanyl (BP) moieties in diverse marine sediments. The sample set consisted of 44 sediment horizons from the Mediterranean and adjacent basins and represented diverse sources of organic matter and depositional conditions. Complementary geochemical data enabled the comparison of lipid distributions and carbon isotopic signatures with prevailing redox conditions. The delta C-13 of tricyclic BP (BPcren) from the core and intact forms of crenarchaeol ranged from -19.1 to -28.6% and -18.1 to -27.4%, respectively. delta C-13 values of core and intact BPcren did not differ, suggesting that intact crenarchaeol is either a fossil relic from planktonic archaea or a product of lipid recycling by benthic archaea, as opposed to being synthesized de novo by sedimentary archaea. delta C-13 values of BP0 derived from core and intact forms of glycerol and butanetriol dibiphytanyl glycerol tetraethers (GDGTs and BGDTs, respectively), but predominantly from caldarchaeol (GDGT-0), ranged from -19.4 to -32.0% and -20.9 to -37.0%, respectively. In contrast to BPcren, intact-lipid derived BP0 was often C-13-depleted relative to its core counterpart, consistent with in situ production by sedimentary archaea. This relative depletion was most pronounced in sulfate reduction zones, likely due to heterotrophic activity. Core and intact archaeol exhibited the largest ranges in delta C-13 values, from -21.6 to -42.1% and -22.7 to -58.9%, respectively. This strong C-13-depletion relative to both total organic carbon and dissolved inorganic carbon is consistent with mixtures of functional sources of sedimentary chemolithoautotrophic, methanotrophic, methanogenic and heterotrophic archaea. Based on the substantial C-13-depletion of BPcren and BP0 in samples in the vicinity of the Rhone River delta relative to a distal marine reference site, we infer that the terrestrial soil contribution of archaeal lipids to these sediments is as high as 43%. Hence, terrestrial input of archaeal lipids, including their intact forms, can be substantial and suggests caution when using existing molecular proxies aimed at constraining riverine input. In summary, our comparative isotopic analysis of sedimentary core versus intact archaeal lipids improves the apportionment of their diverse sources and confidence in distinguishing in situ lipid production by sedimentary archaea.

Description: The Last Interglacial (~129,000–116,000 years ago) is the most recent geologic period with a warmer-than-present climate. Proxy-based temperature reconstructions from this interval can help contextualize natural climate variability in our currently warming world, especially if they can define changes on decadal timescales. Here, we established a ~4.800-year-long record of sea surface temperature (SST) variability from the eastern Mediterranean Sea at 1–4-year resolution by applying mass spectrometry imaging of long-chain alkenones to a finely laminated organic-matter-rich sapropel deposited during the Last Interglacial. We observe the highest amplitude of decadal variability in the early stage of sapropel deposition, plausibly due to reduced vertical mixing of the highly stratified water column. With the subsequent reorganization of oceanographic conditions in the later stage of sapropel deposition, when SST forcing resembled the modern situation, we observe that the maximum amplitude of reconstructed decadal variability did not exceed the range of the recent period of warming climate. The more gradual, centennial SST trends reveal that the maximal centennial scale SST increase in our Last Interglacial record is below the projected temperature warming in the twenty-first century.

Description: An understanding of how the coupled cycles of carbon, iron and sulfur in sediments respond to environmental change throughout Earth history requires the reconstruction of biogeochemical processes over a range of spatial and temporal scales. In this study, sediment cores from the southwestern Black Sea were analyzed to gain insight into past changes in biogeochemical processes with particular focus on the cycling of dissolved organic carbon (DOC). The sediment consists of Late Pleistocene deposits of iron oxide-rich and organic-poor lacustrine sediments, a Holocene sapropel layer deposited after the inflow of saline Mediterranean seawater about 9300 yr BP, and overlying recent marine sediments. The porewaters displayed high concentrations of DOC, acetate, dissolved iron and an extended depth interval over which sulfate and methane were both present. The historical fluctuations of the fluxes of carbon, sulfur and iron species at the seafloor that led to these present-day geochemical profiles, and which cannot be easily interpreted from the measured data alone, were hindcasted with a reaction-transport model. The model suggests that the inflow of Mediterranean seawater impacted the rain rate and reactivity of organic matter reaching the sediments, which shifted the sedimentary redox regimes throughout the Holocene that now are reflected on different lithology units. Organic matter in the sapropel layer is apparently the main source of modern-day accumulations of DOC and acetate, both of which probably sustained subsurface microbial activity throughout the post-glacial period. The ratio between DOC and dissolved inorganic carbon (DIC) flux to the bottom water decreased from ∼40% before the inflow of Mediterranean water to ∼2% at the present day. We suggest that the coexistence of methanogenesis and sulfate reduction was associated with sulfate-reducing bacteria and methanogens sharing common substrates of acetate and lactate and utilizing non-competitive substrates such as methylated compounds in the sapropel layer and in the bottom of modern marine deposits. Intense sulfur and iron cycling mainly took place in the organic-poor freshwater deposits, today characterized by high concentrations of dissolved iron and methane. In contrast to previous studies in similar environments, anaerobic oxidation of methane coupled to the reduction of ferric iron was negligible. The results have broad implications for coastal environments that are currently experiencing deoxygenation and seawater intrusion and also for understanding the role of DOC in the sedimentary carbon cycle.

Description: The Black Sea is a permanently anoxic, marine basin serving as model system for the deposition of organic-rich sediments in a highly stratified ocean. In such systems, archaeal lipids are widely used as paleoceanographic and biogeochemical proxies; however, the diverse planktonic and benthic sources as well as their potentially distinct diagenetic fate may complicate their application. To track the flux of archaeal lipids and to constrain their sources and turnover, we quantitatively examined the distributions and stable carbon isotopic compositions (delta 13C) of intact polar lipids (IPLs) and core lipids (CLs) from the upper oxic water column into the underlying sediments, reaching deposits from the last glacial. The distribution of IPLs responded more sensitively to the geochemical zonation than the CLs, with the latter being governed by the deposition from the chemocline. The isotopic composition of archaeal lipids indicates CLs and IPLs in the deep anoxic water column have negligible influence on the sedimentary pool. Archaeol substitutes tetraether lipids as the most abundant IPL in the deep anoxic water column and the lacustrine methanic zone. Its elevated IPL/CL ratios and negative delta 13C values indicate active methane metabolism. Sedimentary CL- and IPL-crenarchaeol were exclusively derived from the water column, as indicated by non-variable delta 13C values that are identical to those in the chemocline and by the low BIT (branched isoprenoid tetraether index). By contrast, in situ production accounts on average for 22% of the sedimentary IPL-GDGT-0 (glycerol dibiphytanyl glycerol tetraether) based on isotopic mass balance using the fermentation product lactate as an endmember for the dissolved substrate pool. Despite the structural similarity, glycosidic crenarchaeol appears to be more recalcitrant in comparison to its non-cycloalkylated counterpart GDGT-0, as indicated by its consistently higher IPL/CL ratio in sediments. The higher TEX86, CCaT, and GDGT-2/-3 values in glacial sediments could plausibly result from selective turnover of archaeal lipids and/or an archaeal ecology shift during the transition from the glacial lacustrine to the Holocene marine setting. Our in-depth molecular-isotopic examination of archaeal core and intact polar lipids provided new constraints on the sources and fate of archaeal lipids and their applicability in paleoceanographic and biogeochemical studies.