The isoprenoid lycopane occurs in the membranes of cultured archaea, but in environmental samples it has traditionally been interpreted to derive from phytoplankton. In the deposits of the Muschelkalk Sea, a Triassic marginal sea, lycopane contents increased during a freshening trend at the Anisian-Ladinian boundary. Based on peak abundances of lycopane at pycnoclines, we hypothesize an episodic occurrence of haloclines in the Muschelkalk Sea, with freshwater runoff mostly affecting surface waters. Freshwater dissolved inorganic carbon is typically 13C-depleted. An up-section trend of decreasing delta 13C values of the biomarkers of phytoplankton (phytane) and heterotrophic consumers (2,6,10,14,18-pentamethylicosane; regular PMI) from -33%o to -41%o is therefore in accord with freshening. Even lower delta 13C values of lycopane and the co-occurring archaeal lipid biphytane (average: -48%o and -47%o, respectively) agree with archaea as source organisms of lycopane in the Muschelkalk Sea. We put forward a novel application of a common isoprenoid biomarker, linking the occurrence of 13C-depleted lycopane to marine freshening events.

Lake Faiyum provides a comprehensive Holocene sediment record, offering more insight into Nile flood deposition than fluvial sediments in the Nile Valley. Geochemical analysis of lake sediments reveals changes in climate, productivity, organic matter source and composition, salinity, and redox conditions. We integrated lipid biomarker and elemental data from core F1-08 taken at the southern margin of the lake. Our findings indicate that the Early Holocene (ca. >10-8.2 cal. ka BP) was characterized by a humid climate with high lake-levels. This is supported by low Ti/Al and Zr/Al ratios, low terrestrial/aquatic n-alkane ratios (TARs), and a significant contribution of freshwater algal homologues (low TOC/TN ratio). Salinity was low, indicated by low Sr/Ba and S/TOC ratios. Nutrient-rich Nile discharge fostered productivity and oxygen depletion, promoting organic carbon preservation. During the period from ca. 8.2 to 6.2 cal. ka BP (within the Middle Holocene), the lake experienced a slight increase in salinity, which caused a change from freshwater to brackish water conditions accompanied by a slight lake-level drop and delta-like wet land, indicated by increased Sr/Ba and S/TOC ratios. Brackish conditions in the lake coincide with the appearance and sharp increase of various botryococcenes, caused by elevated algal productivity including Botryococcus braunii and typified by low TOC/TN ratios. The Middle to Late Holocene (ca. 6.2 cal. ka BP to present) witnessed gradual aridification and declining lake-levels, organic productivity, and preservation. Evidence includes high Ti/Al and Zr/Al ratios, low TOC contents, and reduced contribution of freshwater algal homologues (higher TOC/TN) and a sharp decrease of botryococcenes. Salinity gradually increased, reflected by higher Sr/Ba and S/TOC ratios. An increase in Mo enrichment since ca. 4.2 cal. ka BP, without a corresponding rise in TOC contents, may be due to human-induced hydrological changes or transport to the lake through wind and limited rainfall. Elevated Ti levels at that time may be linked to increased aridity in the Ethiopian Highlands, potentially impacting Nile floods ca. 4.2 cal. ka BP and contributing to the decline of the Old Kingdom in Egypt.

Carbon is removed from Earth’s surface and may be stored within carbonate minerals over long periods of time. The formation of authigenic carbonate in marine sediments accounts for much of this sequestered carbon, whereby the rate of sequestration depends on mineral precipitation rates. Among the catalyzing agents of carbonate precipitation are biofilms and microbial mats, which are ubiquitous in Earth surface environments. Microbial carbonates are abundant at methane seeps where they form by the sulfate-driven anaerobic oxidation of methane (SD-AOM), mediated by anaerobic archaea and sulfate-reducing bacteria. We investigated a 5 m long core composed almost entirely of two microbially-derived carbonate cements from an active methane seep in the South China Sea, offshore Taiwan. Phase-specific U/Th dating, lipid biomarker analyses, and calcium isotope data suggest that one of these phases is a direct product of biofilm mineralization, typified by high precipitation rates. This study is the first to estimate the accretion rate of individual carbonate phases in microbial limestones, and provides first-order constraints on the catalytic effect of microbial activity on carbonate precipitation. This has implications on the rate of global carbon burial, which may be significantly increased by the influence of biofilms and microbial mats on carbonate precipitation.

Some of the carbon removed from Earth’s surface is stored within authigenic carbonate in marine sediments. Methane seeps are crucial sites of global marine carbon cycling sustaining microbial activity, enabling carbonate formation and the transfer of methane-derived carbon to the geosphere. Carbon sequestration rates depend on carbonate precipitation rates, which can be accelerated by mat-forming microorganisms that are ubiquitous at methane seeps and other Earth surface environments today. We investigate a 5-m-long drill core from an active methane seep at 1350 m water depth in the South China Sea with an exceptional abundance of pink and clear aragonite cement derived from the sulfate-driven anaerobic oxidation of methane, yet both cements precipitated under different conditions. Phase-specific 230Th/U-based ages, lipid biomarker compositions, and calcium isotope data suggest that pink aragonite is a product of in situ biofilm mineralization. First estimated precipitation rates of these individual cements in the seep carbonates range from 0.04 cm/ka for clear aragonite to 1.0 cm/ka for pink aragonite, suggesting an up to 25-fold increase in precipitation rates associated with biofilm mineralization. These results provide first kinetic constraints for future quantitative carbon cycle models, emphasizing the role of biofilms in accelerating carbon sequestration in marine authigenic carbonates. Carbonate precipitation by microbial mats in active hydrocarbon seeps can accelerate carbon sequestration in these settings by up to 25 times, according to analysis of a drill core from the South China Sea.

The quantitative reconstruction of paleoclimatic and paleoenvironmental conditions in regions and in time periods characterized by recurrent and significant fluctuations is challenging. An example of strong paleoenvironmental change occurred in the Mediterranean Basin across the Miocene – Pliocene boundary (5.33 Ma), marked by the restoration of normal marine conditions after the 'Lago-Mare' terminal phase of the Messinian salinity crisis. Environmental conditions during the Lago-Mare phase are still uncertain due to the controversial body fossil record, consisting of freshwater to brackish assemblages (ostracods, dinocysts, mollusks, and foraminifera), as well as marine microfossils (otoliths of marine fishes, calcareous plankton).However, two scenarios were suggested to describe this transition: 1) a catastrophic and sudden sea level rise causing the drastic change from freshwater to marine deep environments; 2) a gradual sea level rise, characterized by a fast to gradual transition from brackish to marine environments.To quantify the changing conditions during the Miocene–Pliocene transition, we used a multivariate statistical approach to interpret a large array of terrestrial and aquatic molecular-based indices, in a sedimentary succession of the Northern Mediterranean (Maccarone section, Central Italy). The statistical procedure was specifically developed to address the complexities emerging from the heterogeneous dataset.An illustrative example suggests that using the TEX86, UK37’, and MBT´5ME paleothermometers, we obtained different values and trends in the changing Mediterranean during the study interval. While the study acknowledges the validity of UK37’ as a paleothermometer in variable environments, it highlights that TEX86and MBT´5ME are sometimes compromised by other sources, such as reworked sediment or other organisms that produce the same lipid inventory. In these cases, these proxies provide information about environmental processes rather different than temperatures.Cluster analysis supports a stepwise evolution during the Miocene-Pliocene transition, besides redundancy analysis (RDA) indicates that the water column structure changed from stratified (Tetrahymanol) during the Messinian to mixed during the Zanclean. A second gradual change is instead related to terrestrial vegetation modifications, indicating a gradual coastal environment reconfiguration after a marine transgression with the distancing of the costal line and a reduction of wetland aquatic plants signal. Finally, molecular fossils are also influenced by cyclical changes, not related to the Messinian salinity crisis demise but linked to astronomical-driven climatic cycles.

Carbonate concretions accompanied by elemental sulfur are found in many upper Miocene marine successions across the Mediterranean area (e.g. SE-Spain, Sicily, Apennine, Cyprus). Most of these rocks are characterized by molds of evaporitic minerals (mostly gypsum) suggesting an early (syngenetic) or late (epigenetic) diagenetic origin. In contrast to these findings, a case study from the Ripa dello Zolfo area in northern Italy lacks evidence of carbonate and sulfur replacement of preexisting sulfate minerals. An integrated approach including sedimentological, petrographical, stable isotope (carbon, oxygen, and multiple sulfur isotopes), and lipid biomarker analyses was used for the study of three main lithofacies: a) laminated lithofacies representing aphotic carbonate stromatolites enclosing fossils of filamentous sulfide-oxidizing bacteria; b) brecciated lithofacies deriving from the brecciation of carbonate stromatolites by mud injections; c) sulfur-bearing lithofacies deriving from the precipitation of thin laminae of elemental sulfur at or close to the sediment-water interface. The δ13C and δ18O values of authigenic carbonate minerals and δ13C of lipid biomarkers indicate that the initial formation of the laminated lithofacies was favored by organoclastic sulfate reduction in the shallow subsurface close to the sediment-water interface, producing sulfide that sustained dense microbial mats of sulfide-oxidizing bacteria at the seafloor. Calcification of the mats and consequent formation of stromatolites were possibly favored by nitrate-driven sulfide oxidation at the seafloor. The subsequent brecciation of the stromatolites was apparently the consequence of sulfate-driven anaerobic oxidation of methane (SD-AOM) in an underlying sulfate-methane transition zone (SMTZ). Focused fluid flow from a deeper zone was not only causing the brecciation of the stromatolites, but also delivered bicarbonate ions for the subsequent precipitation of additional, 13C-depleted calcite (δ13C values as low as -52‰). Along with bicarbonate, also hydrogen sulfide was produced by SD-AOM within an SMTZ in a zone below the stromatolites and was transported upwards. The oxidation of hydrogen sulfide at or close to the seafloor promoted the formation of elemental sulfur characterized by δ34S and Δ33S values close to coeval seawater sulfate. This study highlights that a multi-proxy approach has great potential for the reconstruction of spatially and temporarily separated biogeochemical processes in the shallow subsurface or at the seafloor (i.e., anaerobic oxidation of methane, sulfate reduction, sulfide oxidation) – processes that may induce the syngenetic formation of authigenic carbonate and sulfur deposits in marine sediments.

The microbially mediated replacement of sulfate-bearing evaporites by authigenic carbonate and native sulfur under anoxic conditions is poorly understood. Sulfur-bearing carbonates from the Monte Palco ridge (Sicily) replacing Messinian gypsum were therefore studied to better characterize the involved microorganisms. The lack of (1) sedimentary bedding, (2) lamination, and (3) significant water-column-derived lipid biomarkers in the secondary carbonates implies replacement after gypsum deposition (epigenesis). Allochthonous clasts from the older Calcare di Base and the younger Trubi Formation within these carbonates further evidence epigenetic formation. The sulfur-bearing carbonates are significantly 13C-depleted (δ13C as low as -51‰), identifying methane as a major carbon source. The 18O-enrichment of the carbonates (δ18O as high as 5.4‰) probably reflects precipitation from 18O-enriched fluids transported along adjacent faults or precipitation in a closed system with very little water. Native sulfur with variable 34S-enrichment (δ34S as high as 18.9‰), a relatively small maximum offset (12.3‰) between the sulfate source (gypsum) and native sulfur, and high δ34S values of carbonate-associated sulfate (as high as 61.1‰) suggest a high conversion to native sulfur in a (semi-)closed system, with insignificant sulfate removal. Anaerobic methanotrophic archaea (ANME) apparently affiliated with the ANME-1 clade mediated secondary mineral formation as evidenced by the biomarker inventory, which contains abundant 13C-depleted isoprenoids including sn3-hydroxyarchaeol as the sole hydroxyarchaeol isomer and glycerol dibiphytanyl glycerol tetraethers (GDGTs). A series of various, tentatively identified 13C-depleted non-isoprenoidal dialkyl glycerol diethers (DAGEs), 10me-C16 fatty acid, hydroxy C16 fatty acids, and cyclopropyl-C17:0ω7,8 fatty acid agree with sulfate-reducing bacteria participating in the anaerobic oxidation of methane. Specific conditions during gypsum replacement, unlike those at marine methane seeps, are reflected by the occurrence of 13C-depleted lipids such as lycopane, 9me-C17 fatty acid, and novel DAGEs. As a response to a confined environment probably characterized by high sulfate concentrations, sulfidic conditions, and elevated salinity, ANMEs and sulfate-reducing bacteria apparently adapted their membrane compositions to cope with such stressors.

Predictions of tropical cyclone (TC) frequencies are hampered by insufficient knowledge of their natural variability in the past. A 30-m-long sediment core from the Great Blue Hole, a marine sinkhole offshore Belize, provides the longest available, continuous, and annually resolved TC-frequency record. This record expands our understanding, derived from instrumental monitoring (73 years), historical documentations (173 years), and paleotempestological records (2000 years), to the past 5700 years. A total of 694 event layers were identified. They display a distinct regional trend of increasing storminess in the southwestern Caribbean, which follows an orbitally driven shift in the Intertropical Convergence Zone. Superimposed short-term variations match Holocene climate intervals and originate from solar irradiance-controlled sea-surface temperature anomalies and climate phenomena modes. A 21st-century extrapolation suggests an unprecedented increase in TC frequency, attributable to the Industrial Age warming.

The Great Blue Hole is a prominent flooded karst sinkhole, located in the lagoon of Lighthouse Reef atoll off the coast of Belize. Short cores recovered from varved bottom sediments have been used in previous studies as climate and cyclone archives covering as much as the past ca 1.7 ka BP. A new 30 m long sediment core, encompassing the entire Holocene and the latest Pleistocene, allows a reconstruction of the development of this geoscientifically significant site in the light of the postglacial and Holocene sea‐level rise. The sedimentary succession in the core is tripartite. The lowermost sedimentary unit A (30.0–28.6 m) comprises grey‐brown to black organic‐rich carbonate sediments, which contain freshwater snails ( Pyrgophorus ), tropical forest pollen (Myrtaceae) and a low‐diversity dinocyst assemblage. The intermediate unit B (28.6–24.65 m) is a dark greyish‐green to greyish‐brown, weakly laminated carbonate silt, which comprises marine fossils, that is, euryhaline foraminifera ( Elphidium ) and Halimeda (codiacean algal) platelets. Unit B contains abundant red mangrove ( Rhizophora ) pollen and a dinocyst assemblage indicating high productivity in surface waters. The uppermost unit C (24.65–0 m) is an annually layered buff and light green carbonate silt with abundant marine fossils. A total of 574 intercalated, coarser‐grained and lighter‐coloured event (storm) beds usually rich in Halimeda and coral fragments were identified in unit C. Fully marine conditions, including trophic seasonality, are also indicated by the dinocyst spectrum. The pollen spectrum derives from a variety of trees (largely pine, oak, podocarp), shrubs and herbs. The sedimentary succession represents the transition from an initial terrestrial cenote phase on a vegetated limestone island (unit A: 12.5–7.2 ka BP), via a subsequent restricted marine phase on an initially flooded carbonate platform with mangrove swamps (unit B: 7.2–5.7 ka BP), to a fully marine phase in an open, well‐circulated atoll lagoon (unit C: 5.7–0 ka BP).

Large and deep marine evaporitic basins (salt giants) are common in the geological record, but the reconstruction of the palaeoenvironment is challenging. The sedimentary products consist of clastic evaporites (mass transport and turbiditic deposits), commonly interbedded with evaporitic cumulates or non-evaporitic fine-grained deposits representing the background hemipelagic sedimentation. The study of the hemipelagic component can provide crucial palaeoenvironmental information on these basins. This study examines the non-evaporitic, hemipelagic sediments interbedded with gypsum turbidites from the Belice Basin (Italy), a deep onshore basin of the Late Miocene Mediterranean salt giant. The studied succession belongs to the Resedimented Lower Gypsum unit, formed during the second phase of the Messinian Salinity Crisis (5.60-5.55 Ma) and consists of laminated diatomaceous and organic-rich shales, dolomitic and aragonitic mudstones. Sedimentological, petrographic, and geochemical analyses indicate that the hemipelagites accumulated in a rather deep basin, with high primary productivity in superficial waters. The palaeoenvironmental conditions in the water column and at the seafloor were governed by the balance between the inflows of freshwater and marine water. Intervals of reduced continental runoff and enhanced marine ingression, which induced water column mixing and seafloor oxygenation, are recorded by the diatomaceous shales. The other lithologies record prolonged stratification of the water column, induced by riverine runoff and basin restriction. Seafloor anoxia and the input of terrestrial and marine organic matter favoured the formation of dolomite and pyrite following bacterial sulphate reduction. This study provides insights into the chemical, physical, and biological conditions in a deep basin during the Messinian Salinity Crisis.

The Lower Cretaceous (Barremian) Kuhnpasset sedimentary sequence of northeast Greenland contains many carbonate deposits in different stratigraphic positions in the area of Wollaston Forland, several of which have previously been identified as having a hydrocarbon seep origin based on their macrofossil content and morphology. Most of the seep carbonates contain complex internal microfabrics, including abundant yellow calcite and banded and botryoidal cement with S13C values as low as-54 %o. These S13C values are characteristic of carbonate microfabrics with biogenic methane as the major carbon source and carbonate authigenesis caused by sulfate-driven anaerobic oxidation of methane (SD-AOM). Such 13C-depletion confirms a methane seep origin of most of the Kuhnpasset carbonate deposits. Molecular fossils from the seep carbonates prove methanotrophic archaea were involved in the SD-AOM consortia, based on 13C-depleted 2,6,10,15,19-pentamethylicosane (PMI; S13C:-122 to-113 %o) and the co-eluting crocetane and phytane (-124 to-105 %o). The carbon source for the Kuhnpasset seeps was biogenic methane with a S13C value of approximately-70 %o, as calculated from compound-specific S13C values of archaeal lipid biomarkers. The S13C values of the biomarkers of sulfate-reducing bacteria, the synthrophic partners of archaea in SD-AOM, including isoand anteiso-C15/17 fatty acids are less 13C-depleted (-107 to-87 %o) than the archaeal biomarkers-a pattern known from modern seep environments. Preservation of bacterial fatty acids is rare for Mesozoic seep deposits, revealing excellent biomarker preservation and low thermal maturity of the Kuhnpasset seep deposits. The common isoprenoid hydrocarbon biomarkers of SD-AOM in the Kuhnpasset seep carbonates are accompanied by 13C-depleted isoprenoic acids (phytanoic acid:-112 to-101 %o); these compounds are attributed to the early degradation of glycerol diether membrane lipids such as archaeol or sn2-hydroxyarchaeol, which are not preserved in the samples. Similarly, regular PMI-acid (-120 to-107 %o) probably represents a derivative of extended archaeol or extended hydroxyarchaeol. The molecular fossil inventories in the Kuhnpasset seep carbonates and the predominance of early diagenetic banded and botryoidal cement are characteristic of the dominance of ANME-2 consortia adapted to high methane flux with the sulfate-methane transition zone (SMTZ) positioned at shallow sediment depth. Abundant wood fragments and well-preserved leafy conifer shoots enclosed in the Kuhnpasset seep deposits indicate a shallow marine paleoenvironment of deposition close to the former shoreline.

Quantitative reconstruction of paleoenvironmental and paleoclimatic changes that lack modern analogues is inherently challenging. A notable example of extreme paleoenvironmental transformation occurred in the Mediterranean Basin at the Messinian-Zanclean boundary (5.33 Ma), marked by the transition from the terminal Lago-Mare phase of the Messinian salinity crisis to the re-establishment of normal marine conditions starting with Zanclean. Despite its significance, the paleoenvironmental conditions of the Lago-Mare phase remain unresolved due to the conflicting fossil record, including both freshwater to brackish assemblages and marine microfossils. In this study, we applied a multivariate statistical approach to interpret a large dataset of terrestrial and aquatic biomarker indices from the Maccarone section in northern Italy. This analytical method was specifically tailored to address complexities associated with records originating from highly dynamic and contrasting, transitional environments (i.e., from lacustrine to marine) bringing the advantage of using statistical techniques to identify features that a traditional, visual, observation of complex array of proxies is unable to identify. Cluster analysis reveals a stepwise evolution of environmental conditions throughout the Messinian-Zanclean transition, while redundancy analysis indicates a sudden shift in the water column structure from stratified to well-mixed conditions at the Messinian-Zanclean boundary. Additionally, a gradual change is inferred in terrestrial vegetation, reflecting a progressive reconfiguration of coastal environments following the Zanclean marine transgression, with inland migration of the coastline and a decrease in wetland aquatic plant indicators.

Because of its fast growth, gypsum can rapidly entrap biogenic material and biomolecules, allowing for excellent preservation at geological time scale. The characterization of biota having thrived in the gypsum-mother brines can contribute to elucidate the paleoenvironmental conditions in the water column and at the seafloor during the formation of ancient salt giants; such reconstruction is challenged by the absence of modern analogues. A prominent example are the primary gypsum deposits that accumulated in the Mediterranean basin about 6 Ma ago, during the Messinian salinity crisis (MSC), when this basin turned into the youngest salt giant in Earth history following its partial isolation from the Atlantic Ocean. Two main types of gypsum are recognized: a) bottom-grown selenite, consisting of vertically-oriented twinned crystals and b) laminar gypsum cumulate, formed by the accumulation at the seafloor of tiny gypsum crystals nucleated in the water column, mixed with organic-rich material. Biosignatures in both types of gypsum were investigated through optical, electron and confocal laser scanning microscopy, Raman spectroscopy, and lipid biomarker analyses. Bottom grown gypsum is typified by abundant diatom remains that indicate deposition in a marine basin influenced by freshwater input. Other abundant components are filamentous microfossils corresponding to remains of sulfide oxidizing bacteria. These biogenic remains are commonly coated by microbial dolomite microcrystals and authigenic clays, which suggest that organic matter and biogenic silica underwent severe early diagenetic alteration. Planktic diatoms, calcareous nannofossils and early diagenetic dolomite are also recognized in cumulate gypsum deposits. Such composition suggests high primary productivity in the water column, inducing the export of organic matter to the seafloor and the formation of dolomite following bacterial sulfate reduction. The mechanisms promoting gypsum nucleation in the water column are instead still enigmatic. The late Miocene gypsum represents an excellent archive of biosignatures, including bacterial cells. The microbial assemblage indicates that gypsum formed in relatively deep, stratified marine basins with intermittent sulfidic bottom-water conditions, promoting intense microbially-mediated early diagenetic processes at the expenses of organic matter and biogenic silica. Such circumstances imply that the paucity of skeletal remains in the MSC sedimentary record may be the result of a taphonomic bias rather than of inhospitable hypersaline conditions induced by massive evaporation of the Mediterranean water mass.

The tail-to-tail linked C-40 isoprenoid hydrocarbon lycopane is a biomarker found in a wide variety of environments and in some extremophilic archaea but is also a possibly degradation product of a carotenoid (bacterioruberin) as well as plants (various lycopenes) and green algae (lycopadiene). Although potential producers are known, the source of lycopane in water-column and sedimentary environments is commonly ambiguous. In this study, the occurrence of C-13-depleted lycopane (delta C-13 = -114 to -106 parts per thousand) in authigenic, sulfur-bearing carbonates from Monte Palco (Sicily) is used to re-evaluate the environmental significance of lycopane. In the Monte Palco authigenic carbonates, lycopane is accompanied by other, similarly C-13-depleted apolar isoprenoids such as PMI, squalane, and sulfurized homologues, including a tail-to-tail linked C-35 isoprenoid, as well as typical lipids of anaerobic methanotrophic archaea (ANME) like glycerol dibiphytanyl glycerol tetraether (GDGT)-0, -2, archaeol, and sn3-hydroxyarchaeol. The delta C-13 values of lycopane reveal linear correlations with values of PMI and squalane (n = 7; r(2) = 0.71 and 0.82, respectively), probably reflecting a similar cellular function of the tail-to-tail linked isoprenoids in the archaeal source organisms. Lycopane is interpreted to represent a membrane intercalant, produced in response to extreme environmental conditions. The new observations resulting from the analysis of the Monte Palco authigenic carbonates - including the similarities in C-13-depletion of PMI, squalane, and lycopane but also with other archaeal membrane lipids like archaeol, sn3-hydroxyarchaeol, and GDGT-0 (measured as ether-cleaved biphytanes) - allowed us to re-interpret the potential source organism of lycopane in modern and ancient anoxic and euxinic basins (Cariaco Trench, Black Sea, late Cenomanian black shales of the Cape Verde Basin). These new data agree with unknown archaea as producers of lycopane as a response to environmental stress at chemoclines. The previous interpretation of lycopane as a biomarker of photoautotrophic bacteria in anoxic basins is challenged by our new findings. An alternative explanation for these occurrences is lycopane production by Marine Group II Euryarchaeota, archaea closely associated with primary producers. Lycopane production by archaea is probably more widespread than previously recognized. Future research should evaluate the potential of lycopane as a biomarker of archaeal adaptation to extreme environmental conditions.

We report a newly discovered hydrocarbon seep deposit from the Eocene bathyal flysch, exposed in the town of Buje in Istria, Croatia. Molecular fossils of methane-oxidizing prokaryotes and abundant banded botryoidal cements indicate strong fluid flux at this site. We systematically describe the fauna of this and another seep deposit previously reported from Buje. The faunal assemblages are composed of eight species, these being an unidentified solemyid protobranch bivalve, the nuculid Nucula bowerbanki?, the nuculanid Nuculana? sp., the mytilid Brachidontes? amanoi sp. nov., the two thyasirids Channelaxinus dinaricus sp. nov. and Thyasira histriaensis sp. nov., the lucinid bivalve Amanocina bujensis sp. nov., and a possible provannid gastropod. The two assemblages are of low diversity (4 and 5 species, respectively), and are dominated by chemosymbiotic species whose occurrence is largely restricted to seeps. Despite their spatial and stratigraphic proximity, the two deposits share only a single species, Channelaxinus dinaricus, probably due to different fluid flux regimes at both seeps. The Buje seep assemblages are among the very few Late Cretaceous to Palaeogene chemosynthesis-based faunal assemblages from the Tethys Ocean (the others being Late Cretaceous vent assemblages from Cyprus). From an evolutionary perspective, the Buje seep communities consist of genera with Mesozoic origins but lack Cenozoic novelties such as bathymodiolin mussels and vesicomyid clams, which are known from coeval deposits from the Pacific and dominate vents and seeps today. Thus, the Buje seep fauna support previous assertions that the Eocene Tethyan seep faunas preserved an ancient aspect, whereas evolutionary novelties arose in the Pacific.

The central Arabian Sea, a unique tropical basin, is profoundly impacted by monsoon wind reversal affecting its surface circulation and biogeochemistry. Phytoplankton blooms associated with high biological productivity and particle flux occur in the northern part of the central Arabian Sea due to summer-monsoon-induced open-ocean upwelling and winter convection. The core oxygen minimum zone (OMZ) at intermediate water depths is another important feature of the northern central Arabian Sea and fades southward. In this study, we attempt to interlink how these factors collectively impact phytodetrital export to the sediment. Short sediment core-top (1 cm) samples representing the recent particle flux signatures were analysed from five locations (21 to 11 degrees N; 64 degrees E) in the central Arabian Sea. Previously, we used core-top (0-0.5 cm) samples and observed a trend between diatom frustule abundance and diversity with bulk sedimentary parameters indicating a spatial variability in phytodetrital export to the sediment. To verify this observation further, lipid biomarkers of key phytoplankton groups and a sea surface temperature (SST) proxy have been analysed in addition to diatom frustules. The C37 alkenone-based SST proxy indicated cooler SST (27.6 +/- 0.25 degrees C) in the north (21-15 degrees N) mostly due to upwelling (summer) and convective mixing (winter). Warmer SSTs (+0.4 degrees C) are measured in the south, which usually remains nutrient-poor. This trend was consistent with satellite-derived average SST values (2017-2020). Lipid biomarker analysis suggests that dinoflagellates were likely to be the highest contributor, as indicated by dinosterol and its degradative product dinostanol, followed by brassicasterol and C37 alkenone, likely representing diatoms and coccolithophores, respectively. The north, which largely experiences periodic phytoplankton blooms and is influenced by the thick OMZ, revealed the highest contents of organic matter, diatom frustules (diversity and abundance), dominated by large, thickly silicified cells (e.g. Coscinodiscus and Rhizosolenia) and phytoplankton lipid biomarkers, as well as lower contents of zooplankton biomarkers (cholesterol and cholestanol). In contrast, relatively smaller chain-forming centric (e.g. Thalassiosira) and pennate (e.g. Pseudo-nitzschia, Nitzschia, Thalassionema) diatom frustules along with lower phytoplankton lipid biomarker contents were found in the south, where zooplankton biomarkers and silicious radiolarians were more abundant. The possible impacts of the OMZ on particle flux related to the phytoplankton community, including zooplankton grazing and other factors, have been discussed.

Carbonate concretions accompanied by elemental sulfur were found in an earlyMessinian (Late Miocene) marine succession of NW Italy. The rocks were studied with an integrated approach including sedimentological, petrographical, stable isotope (carbon, oxygen, and multiple sulfur isotopes), and lipid biomarker analyses. Unlike other examples from Messinian strata of the Mediterranean area, the studied carbonate and sulfur concretions did not derive from the diagenetic replacement of sulfate minerals. Three lithofacies were distinguished: a) laminated lithofacies representing aphotic carbonate stromatolites enclosing fossils of filamentous sulfide-oxidizing bacteria; b) brecciated lithofacies deriving from the brecciation of carbonate stromatolites by mud injections; c) sulfur-bearing lithofacies deriving from the precipitation of thin laminae of elemental sulfur at or close to the sediment-water interface. The carbon and oxygen stable isotope composition of authigenic carbonate minerals and lipid biomarkers indicate that the initial formation of the laminated lithofacies was favored by organoclastic sulfate reduction in the shallow subsurface close to the sediment-water interface, producing sulfide that sustained dense microbial mats of sulfide-oxidizing bacteria at the sea floor. Calcification of the mats and consequent formation of stromatolites were possibly favored by nitrate-driven sulfide oxidation at the sea floor. The subsequent brecciation of the stromatolites was apparently the consequence of sulfate-driven anaerobic oxidation of methane (SD-AOM) in an underlying sulfate-methane transition zone (SMTZ). Focused fluid flow from below, possibly resulting from destabilization of gas hydrates, was not only responsible for the brecciation of the stromatolites, but also for the delivery of bicarbonate ions and the consequent precipitation of additional, 13C-depleted calcite (δ13C values as low as −52‰). Along with bicarbonate, also hydrogen sulfide was produced by SD-AOM at the SMTZ and was transported upwards. The oxidation of hydrogen sulfide at or close to the sea floor promoted the formation of elemental sulfur characterized by higher δ34S values and lower Δ33S values than coeval seawater sulfate.

Fine-grained authigenic magnetite has been recognized increasingly in iron-rich marine environments affected by methane seepage and is a major sedimentary magnetization source. However, it is unknown whether this magnetite forms via microbial or abiotic processes. We report here abundant fine magnetite crystals, in close association with goethite, within coarse-grained sediments from two adjacent methane seepage sites in the South China Sea. The magnetite- and goethite-rich horizons have sharply increased Zr/Ti, Zr/Rb, Ti/Al, and Fe/Al ratios, probably reflecting deposition by turbidity currents. Deeper intervals have elevated pyrite content, positive delta S-34 excursions of chromium reducible sulfur, and low magnetic susceptibilities, which is consistent with past sulfate-driven anaerobic oxidation of methane in environments with dynamically variable seepage intensity. In magnetically extracted aggregates (>63 mu m), magnetite particles are mainly clustered euhedral crystals with 0.2-0.8 mu m sizes, which will likely impact sedimentary magnetic signals. The fine, euhedral crystalline nature of the magnetite suggests formation in sulfide-free, ferrous iron-rich sedimentary environments. Based on 16S rRNA gene sequences, anaerobic methanotrophic archaea coincide with pyrite rich horizons. In contrast, two co-occurring methanogenic archaea groups of the Methanomicrobia class (mainly Methanosarcina and Methanocella) are particularly abundant in turbidites but have low abundance in all other horizons. Increased Methanomicrobia abundances suggest that this class of archaea may be involved in microbial iron reduction in turbidites with abundant goethite as a reactive iron source, and that they apparently trigger magnetite formation. Our findings provide new clues to microbial magnetite formation in iron-rich marine sediments.

Sulfate-driven anaerobic oxidation of methane (SD-AOM) is the key biogeochemical process at marine seeps, seafloor environments sustaining lush chemosynthesis-based life. While an extensive molecular record of SD-AOM has been established for Cenozoic and Mesozoic seeps, to date only one reported case of SD-AOM exists for the Paleozoic. To get new insight into the dominant biogeochemical processes at Paleozoic seeps, a detailed lipid biomarker study was conducted on post-glacial early Permian seep carbonates from Western Australia. The encountered biomarker inventory comprises two diagnostic isoprenoid hydrocarbons with low delta C-13 values: mixed phytane and crocetane (-124 to -110 parts per thousand) and 2,6,10,15,19-pentamethylicosane (PMI; -128 to -102 parts per thousand), compounds known to be produced by anaerobic methane-oxidizing archaea (ANME). Other known biomarkers of ANME like glycerol dibiphytanyl glycerol tetraethers (GDGTs) and sn2-hydroxyarchaeol are not preserved in the Permian seep deposits despite the low to moderate thermal maturity of the Paleozoic limestones. Still, degradation products of these compounds including biphytanes and phytane, respectively, yield delta C-13 values (biphytanes: -117 to -111 parts per thousand) typical of ANME lipids. The combined phytane/crocetane peaks show similar C-13 depletion as other ANME lipids, suggesting a derivation of the precursor lipids of phytane from ANME. Among the detected lipids, biomarkers of sulfate-reducing bacteria, the syntrophic partners of ANME in SD-AOM, include the C-13-depleted terminally branched fatty acids iso- and anteiso-C-15:0 and -C-17:0 as well as iso- and anteiso-alkanes with 15 and 17 carbons (delta C-13 values: -97 to -63 parts per thousand), the latter representing probable degradation products of fatty acid and bacterial mono- and diether precursors. ANME-derived lipids (phytane and PMI) are recognized as organic sulfur compounds (OSCs) in the free hydrocarbon fraction, comprising thiolanes, thianes, and thiophenes. The ANME-derived OSCs are accompanied by sulfurized alkanes with 16 and 18 carbons (delta C-13 values: -83 to -79 parts per thousand), tentatively interpreted to derive from unsaturated glycerol ester or ether lipids synthesized by seep-dwelling sulfate-reducing bacteria, while a derivation of these compounds from sulfide-oxidizing bacteria can neither be substantiated nor excluded. We suggest that OSCs formed in the shallow sedimentary subsurface during early diagenesis, reflecting fast entombment and preservation in authigenic carbonates. Rapid OSC formation was probably caused by (1) the presence of excess hydrogen sulfide, which derived from SD-AOM and (2) the scarcity of reactive iron. The studied Permian seep limestones of Western Australia expand our knowledge of the biogeochemical processes at Paleozoic seeps and provide a unique example of how early sulfurization of organic compounds may aid the preservation of biomarkers.

The invertebrate macrofauna of four methane-seep deposits of Miocene to Pleistocene age from Taiwan is taxonomically evaluated. The Neogene faunas, with one example each from the early and late Miocene and the Pliocene, consist exclusively of one or two large, infaunal lucinid bivalve genera per site, namely Meganodontia and Lucinoma. The former includes the large, charismatic Pliocene ‘Loripes’ goliath, which is here identified as belonging to the widespread, Neogene to Recent genus Meganodontia. The Pleistocene seep fauna is more diverse, includes extant species like the mytilid Gigantidas horikoshii and Meganodontia aff. acetabulum, unidentified species belonging to Lucinoma and the vesicomyid Isorropodon, and specimens resembling the enigmatic bivalve Sisonia frijellanae, described so far only from the late Miocene of the Philippines; the gastropods could not be identified below family level. The two Miocene seep faunules formed on the shelf of the Eurasian plate, the Pliocene faunule in an upper offshore setting in the Tainan foreland basin, whereas the Pleistocene site with its more diverse fauna formed closer to the shelf edge of the Tainan foreland basin. We suggest that water depth is the most likely driver of this difference in species diversity. The very negative δ13C signature of the seep carbonates of all fossil sites indicates the seepage of mostly biogenic rather than thermogenic methane, analogous to the extant seep sites around Taiwan. Biogeographically, the Miocene seep faunas show, with Meganodontia, links to seep faunas from lower to mid-latitudes worldwide (Caribbean Sea, Mediterranean Sea, Philippines, and New Zealand). From the Pliocene onward, biogeographic ties are more restricted to areas in the central Indo-West Pacific Ocean.