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Miller et al. [2015, https://doi.org/10.1002/2015JE004825] described the result of experiments testing the potential of Tenax TA, a polymer used on Sample Analysis at Mars (SAM), as a source of chlorinated benzene. Miller et al. [2015] conclude that the amount of chlorobenzene produced is low and that Tenax TA cannot be the source of the chlorobenzene observed on Mars by SAM. Miller et al. [2015] did not provide the identification of two unknown compounds produced during these pyrolysis experiments, though their abundance is orders of magnitude higher than that of chlorobenzene. Here, we tentatively identify these compounds based on the mass spectra provided by Miller et al. [2015], the most abundant of which is a chlorinated monomer of Tenax TA. This chlorinated monomer is likely to accumulate in the hydrocarbon Tenax trap and in the transfer line between the trap and the mass spectrometer. Further breakdown of these compounds could lead the high background of chlorobenzene observed on Mars.

The study of lipid molecular fossils by traditional biomarker analysis requires bulk sample crushing, followed by solvent extraction, and then the analysis of the extract by gas chromatography-mass spectrometry (GC-MS). This traditional analysis mixes all organic compounds in the sample regardless of their origins, with a loss of information on the spatial distribution of organic molecules within the sample. These shortcomings can be overcome using the chemical mapping of intact samples. Spectroscopic techniques such as UV fluorescence or Raman spectroscopy, laser ablation inductively coupled plasma mass spectrometry, and time-of-flight secondary ion mass spectrometry (ToF-SIMS) are among those elemental and molecular mapping techniques. This study employed femtosecond (fs) laser ablation combined with single-photon ionization, a method called fs-laser desorption postionization mass spectrometry (fs-LDPI-MS). A pulsed ∼75 fs, 800 nm laser was used to ablate the geological sample, which was then photoionized after a few microseconds by a pulsed 7.9 eV vacuum ultraviolet laser. An organic carbon-rich geological sample was used for this study to map hydrocarbon biomarkers in sediments that were previously studied by GC-MS. The petrography of this sample was examined by optical and fluorescence microscopy. It is demonstrated here that fs-LDPI-MS combined with petrography for multimodal imaging can expose buried compounds within the sample via in situ layer removal. When used in conjunction with traditional organic geochemical analysis, this method has the potential to determine the spatial distribution of organic biomarkers in geological material. Finally, fs-LDPI-MS imaging data are compared with ToF-SIMS imaging that is commonly used for such studies.

Lake Vida, located in a closed basin in the McMurdo Dry Valleys, East Antarctica, permanently encapsulates an interstitial anoxic, aphotic, cold (−13 °C), brine ecosystem within 27+ m of ice. Metabolically active, but cold-limited, slow-growing bacteria were detected in the brine. Lake Vida brine is derived from the evaporation of a body of water that occupied the same basin prior to ∼2800 years ago. The characteristics of this body of water changed over time and, at one point, likely resembled other modern well-studied perennial ice-covered lakes of the Dry Valleys. We characterized the dichloromethane-extractable fraction of the environmental metabolome of Lake Vida brine in order to constrain current and ancient biogeochemical processes. Analysis of the dichloromethane-extract of Lake Vida brine by gas chromatography-mass spectrometry and comprehensive multidimensional gas chromatography-time of flight-mass spectrometry reveals the presence of legacy compounds (i.e. diagenetic products of chlorophylls and carotenoids) deriving from photosynthetic algae and anaerobic, anoxygenic photosynthetic bacteria. This legacy component dilutes the environmental signal of metabolites deriving from the extant bacterial community. The persistence of legacy metabolites (paleometabolites), apparent in Lake Vida brine, is a result of the slow turnover rates of the extant bacterial population due to low metabolic activities caused by the cold limitation. Such paleometabolites may also be preserved in other cold-limited or nutrient-depleted slow-growing ecosystems. When analyzing ecosystems with low metabolic rates, the presence of legacy metabolites must first be addressed in order to confidently recognize and interpret the environmental metabolome of the extant ecosystem.

Caves are important sites of fossil preservation, especially for Quaternary vertebrates. Taphonomic processes operating in caves are not well understood and have never been experimentally examined. This study focuses on the potential role of bat guano, which impacts environmental biogeochemistry and serves as the base of the food chain in cave ecosystems. The presence or absence of guano is expected to be a major control of preservation potential in caves. Bats first appear in the fossil record in the early Eocene so bat guano likely influenced cave preservation only during the Cenozoic. This is a probable megabias of the cave fossil record. Microcosm experiments were used to determine the impact of guano presence and composition, moisture, temperature, and time on preservation potential of small mammal bones, leaves, and crickets. Guano came from insectivorous and frugivorous bats. The guano of insectivorous bats has an acidic pH, while the guano of frugivorous bats is close to neutral. Lab stud...

Lake Bonney is a perennially ice-covered lake in the McMurdo Dry Valleys (MDVs) that has long been studied in order to provide constraints on the paleoclimate of West Antarctica. The lake is divided into two lobes, West Lake Bonney (WLB) and East Lake Bonney (ELB) that are separated by a narrow ridge. The two lobes currently receive surface melt water during austral summers from glacier-fed ephemeral streams and this meltwater enters the lake via a narrow ring, or moat, of liquid water that forms around the lake during summer. The West Lobe also receives water from direct input of melt water from Taylor glacier and saline water from irregular subglacial discharge. Here, we combine previously published He data from Lake Bonney with new Kr and Xe concentration data to examine the signatures of water recharge via the seasonal moat and these data are used to constrain a model for He, Kr and Xe transport within both WLB and ELB over about the last 5000–6000 yrs. A detailed numerical simulation is presented that combines diffusive transport of noble gases within the stratified water column of Lake Bonney, along with ice ablation at the top of the ice cover, partitioning of noble gases between water and ice, plus exchange of noble gases between WLB and ELB. Results strongly suggest that open moats have only operated for about 2–3 centuries within the last millennium. These results are corroborated by the high concentration of He, especially within WLB, which points to a history of ice cover with no open moats operating for both lobes for at least about 5 millennia. In addition, the distribution of He, Kr and Xe suggest that a significant rise of the water level of Lake Bonney associated with a warmer period may have been interrupted by a roughly 4–5 century long cold period during which the moats were not large enough to allow air saturated water into the lake, with this cold period ending about one century ago. In addition, during this cold period, there is evidence for a thickening of the ice cover. In the last century Lake Bonney levels have been rising, pointing either to more extended austral summers or elevated summer temperatures, a rise that is compatible with the heavy noble gas record in the upper portion of the lake. A preliminary analysis that includes all stable noble gases heavier than He, suggests that glacial melt water (GMW) directly from Taylor Glacier below the ice cover may have been a significant input into the deepest parts of WLB, but not ELB.

Iron redox cycling in metal-rich, hypersaline, anoxic brines plays a central role in the biogeochemical evolution of life on Earth, and similar brines with the potential to harbor life are thought to exist elsewhere in the solar system. To investigate iron biogeochemical cycling in a terrestrial analog we determined the iron redox chemistry and isotopic signatures in the cryoencapsulated liquid brines found in frozen Lake Vida, East Antarctica. We used both in situ voltammetry and the spectrophotometric ferrozine method to determine iron speciation in Lake Vida brine (LVBr). Our results show that iron speciation in the anoxic LVBr was, unexpectedly, not free Fe(II). Iron isotope analysis revealed highly depleted values of −2.5‰ for the ferric iron of LVBr that are similar to iron isotopic signatures of Fe(II) produced by dissimilatory iron reduction. The presence of Fe(III) in LVBr therefore indicates dynamic iron redox cycling beyond iron reduction. Furthermore, extremely low δ18O–SO42− values (−9.7‰) support microbial iron-sulfur cycling reactions. In combination with evidence for chemodenitrification resulting in iron oxidation, we conclude that coupled abiotic and biotic redox reactions are driving the iron cycle in Lake Vida brine. Our findings challenge the current state of knowledge of anoxic brine chemistry and may serve as an analogue for icy brines found in the outer reaches of the solar system.

The cold (−13.4°C), cryoencapsulated, anoxic, interstitial brine of the >27 m thick ice of Lake Vida (Victoria Valley, Antarctica) contains 49 µg · L−1 of perchlorate and 11 µg · L−1 of chlorate. Lake Vida brine (LVBr) may provide an analog for potential oxychlorine-rich subsurface brine on Mars. LVBr volatiles were analyzed by solid-phase microextraction (SPME) gas chromatography–mass spectrometry (GC-MS) with two different SPME fibers. With the exception of volatile organic sulfur compounds, most other volatiles observed were artifacts produced in the GC injector when the thermal decomposition products of oxychlorines reacted with reduced carbon derived from LVBr and the SPME fiber phases. Analysis of MilliQ water with perchlorate (40 µg · L−1) showed low level of organic artifacts, reflecting carbon limitation. In order to observe sample-derived organic compounds, both in analog samples and on Mars, the molar abundance of reduced carbon in a sample must exceed those of O2 and Cl2 produced during decomposition of oxychlorines. This suggests that the abundance of compounds observed by the Sample Analysis at Mars (SAM) instruments in Sheepbed samples (CB-3, CB5, and CB6) may be controlled by an increase in the reduced-carbon/oxychlorine ratio of these samples. To increase chances of in situ detection of Martian organics during pyrolysis-GC-MS, we propose that the derivatization agents stored on SAM may be used as an external source of reduced carbon, increasing artificially the reduced-carbon to perchlorate ratio during pyrolysis, allowing the expression of more abundant and perhaps more diverse Martian organic matter.

Lake Vida (LV) is located in the McMurdo Dry Valleys (Victoria Valley, East Antarctica) and has no inflows, outflows, or connectivity to the atmosphere due to a thick (16 m), turbid ice surface and cold (

Lake Bonney (LB), located in Taylor valley, Antarctica, is a perennially ice-covered lake with two lobes, West Lake Bonney (WLB) and East Lake Bonney (ELB), which are separated by a narrow ridge. Numerous studies have attempted to reconstruct the evolution of LB because of its sensitivity to climatic variations and the lack of reliable millennial-scale continental records of climate in this region of Antarctica. However, these studies are limited by the availability of accurate lacustrine chronologies. Here, we attempt to better constrain the chronology of LB and thus, the evolution of past regional climate by estimating water residence times based on He, Ne and Ar concentrations and isotopic ratios in both WLB and ELB. 3 He and 4 He excesses up to two and three orders of magnitude and 35–150 times the atmospheric values are observed for WLB and ELB samples, respectively. In comparison, while measured 40 Ar/ 36 Ar ratios are atmospheric (∼295.5) in ELB, WLB samples display 40 Ar/ 36 Ar ratios of up to ∼315 reflecting addition of radiogenic 40 Ar. Both 4 He and 40 Ar excesses clearly identify the addition of subglacial discharge (SGD) from underneath Taylor Glacier into WLB at depths of 25 m and 35 m. He isotopic ratios suggest that He excesses are predominantly crustal (>93%) in origin with small mantle contributions ( 4 He and 40 Ar excesses are used together with basement rock production rates of these isotopes to derive first-order approximations of water residence times for both lobes. Numerous factors capable of affecting water residence times are evaluated and corrected 4 He and 40 Ar water ages are used to place further constrains into the reconstruction of both WLB and ELB history. Combined 4 He and 40 Ar ages in WLB suggest maximum water residence times of ∼250 kyrs BP. These results support the presence of remnant water from proglacial lakes that existed during Marine Isotope Stage 7 (160–240 kyrs) in WLB, in agreement with previous studies. In comparison, 4 He ages in ELB are much younger ( 4 He ages also suggest that the ELB ice cover formed significantly earlier (∼1.5 kyrs BP) than previously reported. The timing of these hydrologic changes in ELB appears to correspond to regional and global climatic events that are recorded in both the Taylor Dome ice-core record as well as in other Dry Valley Lakes.

Lake Vida, located in Victoria Valley, is one of the largest lakes in the McMurdo dry valleys and is known to contain hypersaline liquid brine sealed below 16 m of freshwater ice. For the first time, Lake Vida was drilled to a depth of 27 m. Below 21 m the ice is marked by well-sorted sand layers up to 20 cm thick within a matrix of salty ice. From ice chemistry, isotopic composition of δ18O and δ2H, and ground penetrating radar profiles, we conclude that the entire 27 m of ice formed from surface runoff and the sediment layers represent the accumulation of surface deposits. Radiocarbon and optically stimulated luminescence dating limit the maximum age of the lower ice to 6300 14C yr BP. As the ice cover ablated downwards during periods of low surface inflow, progressive accumulation of sediment layers insulated and preserved the ice and brine beneath, analogous to the processes that preserve shallow ground ice. The repetition of these sediment layers reveals hydrologic variability in Victoria Valley during the mid- to late Holocene. Lake Vida is an exemplar site for understanding the preservation of subsurface brine, ice, and sediment in a cold desert environment.

The permanent ice cover of Lake Vida (Antarctica) encapsulates an extreme cryogenic brine ecosystem (−13 °C; salinity, 200). This aphotic ecosystem is anoxic and consists of a slightly acidic (pH 6.2) sodium chloride-dominated brine. Expeditions in 2005 and 2010 were conducted to investigate the biogeochemistry of Lake Vida’s brine system. A phylogenetically diverse and metabolically active Bacteria dominated microbial assemblage was observed in the brine. These bacteria live under very high levels of reduced metals, ammonia, molecular hydrogen (H 2 ), and dissolved organic carbon, as well as high concentrations of oxidized species of nitrogen (i.e., supersaturated nitrous oxide and ∼1 mmol⋅L −1 nitrate) and sulfur (as sulfate). The existence of this system, with active biota, and a suite of reduced as well as oxidized compounds, is unusual given the millennial scale of its isolation from external sources of energy. The geochemistry of the brine suggests that abiotic brine-rock reactions may occur in this system and that the rich sources of dissolved electron acceptors prevent sulfate reduction and methanogenesis from being energetically favorable. The discovery of this ecosystem and the in situ biotic and abiotic processes occurring at low temperature provides a tractable system to study habitability of isolated terrestrial cryoenvironments (e.g., permafrost cryopegs and subglacial ecosystems), and is a potential analog for habitats on other icy worlds where water-rock reactions may cooccur with saline deposits and subsurface oceans.

Up to 2.3 m long sediment sequences were recovered from the deepest part of Lake Hoare in Taylor Valley, southern Victoria Land, Antarctica. Sedimentological, biogeochemical, and mineralogical analyses revealed a high spatial variability of these parameters in Lake Hoare. Five distinct lithological units were recognized. Radiocarbon dating of bulk organic carbon samples from the sediment sequences yielded apparently too old ages and significant age reversals, which prevented the establishment of reliable age-depth models. However, cross correlation of the sedimentary characteristics with those of sediment records from neighbouring Lake Fryxell indicates that the lowermost two units of the Lake Hoare sediment sequences were probably deposited during the final phase of proglacial Lake Washburn, which occupied Taylor Valley during the late Pleistocene and early Holocene. High amounts of angular gravel and the absence of fine-grained material imply a complete desiccation with subaerial conditions in the Lake Hoare basin in the middle of the Holocene. The late Holocene (< c. 3300 calendar yr bp) is characterized by the establishment of environmental conditions similar to those existing today. A late Holocene desiccation event, such as proposed in former studies, is not indicated in the sediment sequences recovered.

The conventional geochemical view holds that the chitin and structural protein are not preserved in ancient fossils because they are readily degradable through microbial chitinolysis and proteolysis. Here we show a molecular signature of a relict chitin-protein complex preserved in a Pennsylvanian (310 Ma) scorpion cuticle and a Silurian (417 Ma) eurypterid cuticle via analysis with carbon, nitrogen, and oxygen X-ray absorption near edge structure (XANES) spectromicroscopy. High-resolution X-ray microscopy reveals the complex laminar variation in major biomolecule concentration across modern cuticle; XANES spectra highlight the presence of the characteristic functional groups of the chitin-protein complex. Modification of this complex is evident via changes in organic functional groups. Both fossil cuticles contain considerable aliphatic carbon relative to modern cuticle. However, the concentration of vestigial chitin-protein complex is high, 59% and 53% in the fossil scorpion and eurypterid, respectively. Preservation of a high-nitrogen-content chitin-protein residue in organic arthropod cuticle likely depends on condensation of cuticle-derived fatty acids onto a structurally modified chitin-protein molecular scaffold, thus preserving the remnant chitin-protein complex and cuticle from degradation by microorganisms.

A 270 cm long sediment sequence was recovered with a piston corer from east lobe Bonney, Taylor Valley, Antarctica, and characterized according to its sedimentological, mineralogical, and geochemical properties. It is the first record of such length recovered from east lobe Bonney. The sediment core is mainly composed of halite crystals of different sizes, water, and a relatively low and stable proportion of clastic particles. Although the sediment surface was probably disturbed by the coring process and absence or low contents of organic material or carbonates hampers the establishment of a robust chronology by radiocarbon dating, the core probably contains at least several hundred years of information about the history of the lake and the Bonney basin. Variations in halite crystal sizes and amount as well as variations in the composition of clastic material can be related to past lake level changes and evaporation cycles.

During a low sea level stand and wet climate phase at the end of the Mississippian, Lower Palaeozoic limestones at the northern edge of the Illinois Basin were karstified. The caves and fissures that formed were infilled subsequently with clastic sediments of Pennsylvanian age (late Bashkirian–early Moscovian (= Atokan, Duckmantian/Bolsovian, Westphalian B/C)). The earliest infills comprise organic-rich, almost coaly, sediments, while sands, silts and fine clays were deposited subsequently. The sediments are fossiliferous and contain abundant plant fragments, most notably cordaitalean and conifer remains. Many of the plants and megaspores are uncompressed and have undergone minimal thermal alteration. Charcoal occurs throughout the sedimentary infills but is most abundant in the later deposits, where conifer remains predominate. These conifers are currently amongst the oldest known from North America. The presence of cordaitalean remains in the earliest coaly infills is supported by the detection of specific fernane derivatives. The lipid extract of the cave fill sequence contains polyaromatic hydrocarbons (PAHs), such as pyrene, chrysene, benzo[ghi]perylene, and coronene, which distribution further attests to the importance of wildfire in preserving the plant remains.

Chromatographic and mass spectrometric analysis of surface sediments from perennially ice-covered Lake Fryxell (McMurdo Dry Valleys, East Antarctica) enabled tentative identification from mass spectral data of previously unreported pentaunsaturated ketones, octatriacontapentaen-2-one (C38:5Me), nonatriacontapentaen-3-one (C39:5Et) and tetradecapentaen-2-one (C40:5Me). In addition, the C37, C38 and C40 alkenones occur as di, tri and tetraunsaturated compounds, whereas the C39 alkenones only include the tetraunsaturated components. Analysis of environmental DNA extracted from the water column of Lake Fryxell produced two 18S rDNA sequences closely related to Isochrysis spp., Chrysotila lamellosa and haptophyte sequences previously recognized in sediment samples collected from a Greenland lake and from Ace Lake, Antarctica. Based on the alkenone distribution, coupled with phylogenetic data, we propose that haptophytes related to Isochrysis and/or C. lamellosa are the source organisms of alkenones in Lake Fryxell. The occurrence of pentaunsatured alkenones may represent a response to the year-round cold water temperature (ca. 1 °C average), likely combined with oxygen supersaturation and growth under light- and nutrient-limited conditions in the upper water column.

Unlike other lakes in the McMurdo Dry Valleys, Antarctica, Lake Vida has a thick (~ 19 m) ice cover sealing a liquid brine body of unusually high salinity (~ 245 g/L) from the atmosphere. To constrain the conditions under which the atypical Lake Vida ice cover formed and evolved, 19 ice samples were collected down to a depth of ~ 14 m, together with three brine samples trapped in the ice at ~ 16 m for analysis of helium, neon, argon, krypton, and xenon concentrations. The broad pattern of noble gas concentrations for Lake Vida samples is fundamentally different from that of air saturated water (ASW) at 0 °C and an elevation of 340 m for salinities of 0 (ice) and 245 g/L (brine). Overall, ice samples are enriched in He and depleted in Ne with saturation relative to ASW averages of 1.38 and 0.82, respectively, and strongly depleted in Ar, Kr, and Xe with relative saturations of 0.10, 0.06, and 0.05, respectively. By contrast, brine samples are generally depleted in He and Ne (relative saturation averages of 0.33 and 0.27, respectively) but enriched in Ar, Kr, and Xe, with relative saturation averages of 1.45, 3.15, and 8.86, respectively. A three-phase freezing partitioning model generating brine, ice and bubble concentrations for all stable noble gases was tested and compared with our data. Measured brine values are best reproduced for a salinity value of 175 g/L, a pressure of 1.1 atm, and a bubble volume of 20 cm 3 kg − 1 . Sensitivity tests for ice + bubble samples show an ideal fit for bubble volumes of ~ 1–2 cm 3 kg − 1 . Our results show that the conditions under which ice and brine formed and evolved at Lake Vida are significantly different from other ice-covered lakes in the area. Our brine data suggest that Lake Vida may be transitioning from a wet to a dry-based lake, while the ice + bubble data suggest at least partial re-equilibration of residual liquid with the atmosphere as ice forms at the top of Lake Vida ice cover.

A new fault-associated paleokarst and cave fill has been discovered in north-central Illinois, emplaced in Ordovician limestones. The paleokarst preserves many original solution features, such as oriented grooves, pendants, and half tubes. Many of the ancient cave passages have rounded bottoms and flat roofs. Together these suggest that the original elliptical, phreatic cave passages grew upward by paragenesis, in which the floor of the cave is protected from dissolution by the presence of sediment, while the ceiling of the cave grows upward by dissolution. The fill is dated as Moscovian (Middle Pennsylvanian) based on palynological data and can be correlated with the Tradewater Formation. The fills are composed of a fining-upward sequence of relatively unindurated clastic sediments that contain well-preserved plant fossils, most notably voltzialean conifer and cordaite remains, representative of vegetation living in well-drained areas. Many of the macrofossils are fragmentary but charcoalified and, along with the megaspores, are uncompressed and preserve exceptional morphological and anatomical data. The presence of abundant charcoal in the fills, as well as diagnostic polycyclic aromatic hydrocarbons, indicates significant wildfire activity in this area during this interval.

Four megaspore genera: Cystosporites, Rotatisporites, Zonalessporites, and Valvisisporites were isolated from palaeokarst fill deposits of early Middle Pennsylvanian age, hosted within Ordovician limestones at Central Quarry in Central, Kendall County, Illinois, U.S.A. Five uncompacted megaspore species were identified and studied using scanning and transmission electron microscopy (SEM and TEM). The species Valvisisporites sculptus Bhardwaj is emended based on improved morphological and new ultrastructural data. The genus Pseudovalvisisporites Lachkar is recognized as a junior synonym of Valvisisporites. The ontogenetic development of aborted and functional Cystosporites diabolicus megaspores is investigated and the timing of sporopollenin deposition in both the exine and massa is reported. In Carboniferous megaspores, the importance of detailed investigation of the periapertural region is emphasized by the discovery of fusiform lamellate thickenings in Valvisisporites and Rotatisporites. The taxonomic and systematic importance of these features are considered in light of the otherwise highly conserved ultrastructure that is recognized in many lycopsid megaspores of this age.

In perennially ice-covered lakes of Taylor Valley, Antarctica, “legacy”, a carryover of past ecosystem events, has primarily been discussed in terms of nutrient and salinity concentrations and its effect on the current ecology of the lakes. In this study, we determine how residual pools of ancient carbon affect the modern carbon abundance and character in the water columns of Lakes Fryxell, Hoare, and Bonney. We measure the stable carbon isotopic compositions and concentrations of particulate organic carbon (POC) and dissolved inorganic carbon (DIC) in the water column of these lakes over four seasons (1999–2002). These data are presented and compared with all the previously published Taylor Valley lacustrine carbon stable isotopic data. Our results show that the carbon concentrations and isotopic compositions of the upper water columns of those lakes are controlled by modern processes, while the lower water columns are controlled to varying degrees by inherited carbon pools. The water column of the west lobe of Lake Bonney is dominated by exceptionally high concentrations of DIC (55,000–75,000 μmol l−1) reflecting the long period of ice-cover on this lake. The east lobe of Lake Bonney has highly enriched δ13CDIC values resulting from paleo-brine evaporation effects in its bottom waters, while its high DIC concentrations provide geochemical evidence that its middle depth waters are derived from West Lake Bonney during a hydrologically connected past. Although ancient carbon is present in both Lake Hoare and Lake Fryxell, the δ13CDIC values in bottom waters suggest dominance by modern primary productivity-related processes. Anaerobic methanogenesis and methanotrophy are also taking place in the lower water column of Lake Fryxell with enough methane, oxidized anaerobically, to contribute to the DIC pool. We also show how stream proximity and high flood years are only a minor influence on the carbon isotopic values of both POC and DIC. The Taylor Valley lake system is remarkably stable in both inter-lake and intra-lake carbon dynamics.

In January 2003, a helicopter crashed on the 5 m thick perennial ice cover of Lake Fryxell (McMurdo Dry Valleys, East Antarctica), spilling approximately 730 l of aviation diesel fuel (JP5-AN8 mixture). The molecular composition of the initial fuel was analyzed by solid phase microextraction (SPME) gas chromatography-mass spectrometry (GC-MS), then compared to the composition of the contaminated ice, water, and sediments collected a year after the spill. Evaporation is the major agent of diesel weathering in meltpool waters and in the ice. This process is facilitated by the light non-aqueous phase liquid properties of the aviation diesel and by the net upward movement of the ice as a result of ablation. In contrast, in sediment-bearing ice, biodegradation by both alkane- and aromatic-degraders was the prominent attenuation mechanism. The composition of the diesel contaminant in the ice was also affected by the differential solubility of its constituents, some ice containing water-washed diesel and some ice containing exclusively relatively soluble low molecular weight aromatic hydrocarbons such as alkylbenzene and naphthalene homologues. The extent of evaporation, water washing and biodegradation between sites and at different depths in the ice are evaluated on the basis of molecular ratios and the results of JP5-AN8 diesel evaporation experiment at 4 degrees C. Immediate spread of the aviation diesel was enhanced where the presence of aeolian sediments induced formations of meltpools. However, in absence of melt pools, slow spreading of the diesel is possible through the porous ice and the ice cover aquifer.

Ice-sealed lakes, potentially home to novel microbiota and microbial processes, can provide a window into isolated and geologically ancient systems. These habitats are earth analogs for extraterrestrial systems that have yet to be sampled, though potentially harbor, or have harbored life at some time during their past. They are also small-scale models of the numerous sub-glacial lake systems, which have been identified across Antarctica and in Iceland. Methods are needed to sample these ecosystems with environmental stewardship in mind, in which human impact on the ecosystem is mitigated before and during sampling. This report describes an entry and sampling approach that was executed at Lake Vida, East Antarctica, a permanently ice-sealed lake that has never been sampled. Best practice sampling procedures were developed with emphasis on mitigating introduction of trace organics or microbiota to the ecosystem. The conceptual approach is transferable to other isolated pristine aquatic ecosystems on Earth and elsewhere.

Hydrocarbon mixtures too complex to resolve by traditional capillary gas chromatography display gas chromatograms with dramatically rising baselines or “humps” of coeluting compounds that are termed unresolved complex mixtures (UCMs). Because the constituents of UCMs are not ordinarily identified, a large amount of geochemical information is never explored. Gas chromatograms of saturated/unsaturated hydrocarbons extracted from Late Archean argillites and greywackes of the southern Abitibi Province of Ontario, Canada contain UCMs with different appearances or “topologies” relating to the intensity and retention time of the compounds comprising the UCMs. These topologies appear to have some level of stratigraphic organization, such that samples collected at any stratigraphic formation collectively are dominated by UCMs that either elute early- (within a window of C15–C20 n-alkanes), early- to mid- (C15–C30 n-alkanes), or have a broad UCM that extends through the entire retention time of the sample (from C15–C42 n-alkanes). Comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry (GC×GC–MS) was used to resolve the constituents forming these various UCMs. Early- to mid-eluting UCMs are dominated by configurational isomers of alkyl-substituted and non-substituted polycyclic compounds that contain up to six rings. Late eluting UCMs are composed of C36–C40 mono-, bi-, and tricyclic archaeal isoprenoid diastereomers. Broad UCMs spanning the retention time of compound elution contain nearly the same compounds observed in the early-, mid-, and late-retention time UCMs. Although the origin of the polycyclic compounds is unclear, the variations in the UCM topology appear to depend on the concentration of initial compound classes that have the potential to become isomerized. Isomerization of these constituents may have resulted from hydrothermal alteration of organic matter.

ARTICLE I NFO The Lomagundi (2.22-2.1 Ga) positive carbon isotope excursion in shallow-marine sedimentary carbonates has been associated with the rise in atmospheric oxygen, but subsequent studies have demonstrated that the carbon isotope excursion was preceded by the rise in atmospheric oxygen. The amount of oxygen released to the exosphere during the Lomagundi excursion is constrained by the average global fractionation between inorganic and organic carbon, which is poorly characterized. Because dissolved inorganic and organic carbon reservoirs were arguably larger in the Paleoproterozoic ocean, at a time of lower solar luminosity and lower ocean redox state, decoupling between these two variables might be expected. We determined carbon isotope values of carbonate and organic matter in carbonates and shales of the Silverton Formation, South Africa and in the correlative Sengoma Argillite Formation, near the border in Botswana. These units were deposited between 2.22 and 2.06 Ga along the margin of the Kaapvaal Craton in an open-marine deltaic setting and experienced lower greenschist facies metamorphism. The prodelta to offshore marine shales are overlain by a subtidal carbonate sequence. Carbonates exhibit elevated 13 C values ranging from 8.3 to 11.2‰ vs. VPDB consistent with deposition during the Lomagundi positive excursion. The total organic carbon (TOC) contents range from 0.01 to 0.6% and δ 13 C values range from −24.8 to −13.9‰. Thus, the isotopic fractionation between organic and carbonate carbon was on average 30.3±2.8‰ (n=32) in the shallow-marine environment. The underlying Sengoma shales have highly variable TOC contents (0.14 to 21.94%) and δ 13 C values (−33.7 to −20.8‰) with an average of −27.0±3.0‰ (n=50). Considering that the shales were also deposited during the Lomagundi excursion, and taking δ 13 C values of the overlying carbonates as representative of the δ 13 C value of dissolved inorganic carbon during shale deposition, a carbon isotope fractionation as large as ~37‰ appears to characterize the production of bulk organic matter in the deeper part of the Pretoria Basin at that time. This enhanced fractionation relative to that observed in shallow-water environments likely reflects heterotrophic (secondary) and chemotrophic productivity at and below a pronounced redoxcline, consistent with the euxinic conditions inferred from independent evidence for the deeper part of the Pretoria Basin. Greater variability in organic carbon vs. carbonate carbon isotopic values on the shallow-marine carbonate platform suggests that the carbon cycling was dominated by a large dissolved inorganic carbon reservoir during the Lomagundi excursion. Our study suggests that in contrast to the Late Neoproterozoic and Phanerozoic, when carbon isotope fractionation between carbonate and organic carbon in the open ocean was mostly controlled by primary producers, in the Paleoproterozoic redox-stratified ocean heterotrophic and chemotrophic productivity overprinted a signal of primary productivity below the redoxcline. This strong imprint of heterotrophic and chemotrophic productivity on organic carbon isotope records complicates the reconstruction of spatial patterns and secular trends in the δ 13 C values of dissolved inorganic carbon in the Paleoproterozoic seawater.

Bituminous cobbles collected from the Fossil Forest site (Geodetic Hills, Axel Heiberg Island, Canadian Arctic) have been subjected to petrographic and geochemical characterization. Transmitted light microscopy and pyrolytic analyses indicate that these samples are derived from algal biomass and are in most ways, generally comparable with other boghead coals and torbanites. However, fluorescence microscopy results in the surprising observation that these samples do not fluoresce under normal blue-light excitation conditions. Strong fluorescence is generally considered to be characteristic of alginites and Type I (and II) kerogens, and the absence of this property in these samples suggests that even petrographic analyses with blue-light excitation (fluorescence) may underestimate the occurrence of algal-derived materials in some samples.

The period 800-717 million years (Ma) ago, in the lead-up to the Sturtian Snowball glaciation, saw an increase in the diversity of eukaryotic microfossils. To afford an independent and complementary view of this evolutionary period, this study presents the distribution of eukaryotic biomarkers from three pre-Sturtian successions across the supercontinent Rodinia: the ca. 780 Ma Kanpa Formation of the Western Australian Officer Basin, the ca. 800-740 Ma Visingso Group of Sweden, and the 740 Ma Chuar Group in Arizona, USA. The distribution of eukaryotic steranes is remarkably similar in the three successions but distinct from all other known younger and older sterane assemblages. Cholestane was the only conventional structure, while indigenous steranes alkylated in position C-24, such as ergostane, stigmastane, dinosterane and isopropylcholestane, and n-propylcholestane, were not observed. This sterane distribution appears to be age diagnostic for the pre-Sturtian Neoproterozoic. It attests to the distinct evolutionary state of pre-Snowball eukaryotes, pointing to a taxonomic disparity that was still lower than in the Ediacaran (635-541 Ma). All three basins also show the presence of a new C28 sterane that was tentatively identified as 26-methylcholestane, here named cryostane. The only known extant organisms that can methylate sterols in the 26-position are demosponges. This assignment is plausible as molecular clocks place the appearance of the earliest animals into the pre-Sturtian Neoproterozoic. The unusual 26-methylsterol may have protected sponges, but also other eukaryotes, against their own membranolytic toxins. Some protists release lytic toxins to deter predators and kill eukaryotic prey. As conventional membrane sterols can be the site of attack for these toxins, sterols with unusual side-chain modification protect the cell. This interpretation of cryostane supports fossil evidence of predation in the Chuar Group and promotes hypotheses about the proliferation of eukaryophagy in the lead-up to the Cryogenian.

A 9.14 m long sediment sequence was recovered from Lake Fryxell, Taylor Valley, southern Victoria Land, Antarctica, and investigated for its chronology and sedimentological, mineralogical, and biogeochemical changes. The basal part of the sequence is dominated by coarse clastic matter, i.e., mainly sand. The sediment composition suggests that a lake existed in Fryxell basin during the Middle Weichselian by ca. 48,000 cal. year BP. After a short period of lake-level lowstand ca. 43,000 cal. year BP, lower Taylor Valley became occupied by the proglacial Lake Washburn, which was at least partly supplied by meltwater and sediments from the Ross Ice Sheet that was advanced to the mouth of Taylor Valley. Evaporation of Lake Washburn to lower levels started during the Last Glacial Maximum at ca. 22,000 cal. year BP, long before the Ross Ice Sheet retreated significantly. Lake-level lowering was discontinuous with a series of high and low stands. From ca. 4000 cal. year BP environmental conditions were similar to those of today and lower Fryxell basin was occupied by a small lake. This lake evaporated to a saline or hypersaline pond between ca. 2500 and 1000 cal. year BP and refilled subsequently.

Lipid extracts from 28 modern soils collected across different climate zones in China and Mongolia have been analyzed systematically using gas chromatography–mass spectrometry (GC–MS). These extracts contain C16–C22 n-alkan-2-ones with a predominance of even carbon number homologues and n-alkan-3-ones with exclusively odd carbon numbers. The predominance of n-alkan-2-ones is attributed to microbial reworking of soil organic matter in warm and wet southeast and southwest China. Lower molecular weight (LMW) n-alkan-3-ones (C15–C21), maximizing at C17 or C19, dominate in southeast and southwest China, while the higher molecular weight (HMW) n-alkan-3-ones (C23–C33) with a mode at C27 or C29 dominate in cold, arid northwest China and Mongolia. The n-alkan-2-ones and n-alkan-3-ones exhibit strong regional variations, in agreement with the spatial distribution of climate and vegetation. Their C15–21/C22–33 ratios and the carbon preference indices of the HMW (C23–C35) n-alkan-2-ones (CPI(H) KET) sensitively reflect the variation in soil ecosystem response.

With a view to the authentication of an unusual series of branched alkanes with quaternary centers (BAQCs) isolated from geological samples, and whose structures rest on the interpretation of EI-mass spectral fragmentation patterns, the total synthesis of 3-ethyl-3-methylheptadecane, 3,3-dimethylheptadecane, 3,3,11,11-tetraethyltridecane, and 5,5,7,7-tetraethylundecane is described. The GC-MS data of the first two samples are identical with those of the isolates and confirm their structures. However, the GC-MS data of the two more highly branched structures do not match those of the geological isolates leading to the conclusion that these structures were erroneously assigned.

Lipids extracted from 26 modern soils collected from different climate zones in China and Mongolia are analyzed using gas chromatographymass spectrometry. These extracts contain pseudohomologous families of 5,5-diethylalkanes (5,5-DEAs), 6,6-diethylalkanes, 5-butyl-5-ethylalkanes and 6-butyl-6-ethylalkanes, which are first detected in modern soils. All these pseudohomologous series display exclusively odd or exclusively even carbon numbers, clearly indicating a biosynthetic origin, and their carbon number distributions generally covary. The distributions of 5,5-DEAs exhibit strong regional variations, correlated with the spatial distributions of climate and vegetation. The lower molecular weight 5,5-DEAs, maximizing at C21 or C23, dominate in southeast and southwest China samples, while the higher molecular weight 5,5-DEAs with a mode at C29 dominate in cold, arid northwest China and Mongolia. The results demonstrate that the distribution of 5,5-DEAs can serve as a new independent proxy, indicating variations in soil ecosystems and climates, just like the well-known proxy of n-C15–21/n-C22–33 ratios.

C21 and C22 highly branched isoprenoid (HBI) alkanes occurring in high relative abundance in lagoonal sediments of Abu Dhabi have been unambiguously identified as 2,6,10-trimethyl-7-(3-methylpentyl)dodecane and 3,7,11-trimethyl-6- (3-methylpentyl)tridecane, respectively, using NMR spectroscopy. A second C21 HBI isomer is tentatively identified as 3,7,11-trimethyl-6-(3-methylbutyl)tridecane, on the basis of comparison of its mass spectral fragmentation with those of fully identified HBIs. The structures of these three components are formed by extension of the "parent" C20 HBI alkane, first identified in Rozel Point Oil by Yon et al. [Yon, D.A., Maxwell, J.R., Ryback G., 1982. 2,6,10-trimethyl- 7-(3-methylbutyl)dodecane, a novel sedimentary biological marker. Tetrahedron Letters 23, 2143–2146], by one or two carbon atom(s), respectively, at the terminal carbon atom of one or two of the T-branches. In contrast to the C20 HBI alkane present in the oil, 13C NMR spectroscopy did not indicate the presence of diastereoisomers for the C21 and C22 HBIs in sediments, indicating stereospecific enzyme-controlled biosynthesis of these components by an unidentified biological source. The stable carbon isotopic compositions of the major C20–C22 HBIs are identical, suggesting a common biological source.

Twelve pseudohomologous series of branched alkanes with one or two quaternary carbon atoms (BAQCs; C14–C33 range) are present in the extractable organic matter of Cenomanian and Turonian black shales of Pasquia Hills (Saskatchewan, Canada). Five of these pseudohomologous series are also observed in a Holocene sample from the Russian Arctic. Three of the pseudohomologous series were identified by Kenig et al. [PNAS 100 (2003) 12554–12558] as 3,3- and 5,5-diethylalkanes (DEAs) and 2,2-dimethylalkanes (DMAs). The nine other pseudohomologous series of BAQCs are tentatively identified on the basis of their characteristic mass spectral fragmentation and retention indices as 3,3-dimethylalkanes (DMAs), 3-ethyl-3-methyl- and 5-ethyl-5-methylalkanes, 9,9-, 7,7-, and 6,6-diethylalkanes (DEAs), 3,3,ω3,ω3- and 5,5,ω5,ω5-tetraethylalkanes, and 5-butyl-5-ethylalkanes. Pseudohomologous series of branched alkanes with tertiary carbon atoms, such as monomethyl- and monoethylalkanes are also present in the same carbon-number range. All these pseudohomologous families share the particularity of including isomers with exclusively odd or exclusively even carbon numbers, clearly indicating a biosynthetic origin. BAQCs such as 5,5-DEAs, 3,3-DEAs and 3-ethyl-3-methylalkanes seem to be widespread in sediments and sedimentary rocks (ranging from modern to ∼2.1 billion years). A xenobiotic source for BAQCs can be excluded at this point. BAQCs seem to have a very low biodegradability, in part explaining their widespread occurrence in ancient sediments.

The extractable organic matter of organic-rich black shales and associated organic-poor bioturbated green/gray shales of the Illinois and Michigan Basins of North America is analyzed to constrain the water column structure during their deposition. All black shale samples contain derivatives of the biomarker isorenieratene, a diaromatic carotenoid pigment produced exclusively by the brown strain of the green sulfur bacteria Chlorobiaceae. Chlorobiaceae are phototrophic organisms requiring euxinic (anoxic and sulfidic) conditions to survive, and the presence of isorenieratene derivatives indicates that the Middle Devonian–Early Mississippian epeiric seas of North America experienced bottom water euxinic conditions during black shale deposition. Analysis of green/gray shales also reveals the presence of isorenieratene derivatives, and in some cases isorenieratane. The highly bioturbated fabric of these sediments indicates the presence of oxygen in bottom waters during their deposition. Thus, we suggest that short-term shoaling of the oxycline across the basin slope may have occurred during their deposition, though they were deposited predominantly under an oxygenated water column (i.e. above the chemocline). Most green/gray shales contain three pseudohomologous series of branched alkanes with quaternary substituted carbon atoms (BAQCs), which are absent in black shale samples. Recent studies suggest that BAQCs are produced by benthic chemoautotrophs dwelling at the redox boundary separating anoxic sediments from moderately oxygenated waters. Comparison of the biomarker content (i.e. regular isoprenoid, BAQCs) between black and green/gray shales indicates different sources of organic matter, and suggests that phytoplanktonic productivity in the upper water column was enhanced during black shale deposition. The results of this study indicate that the alternation of green/gray and black shales in these basins results from the vertical fluctuations of the chemocline. These fluctuations seem to be driven by changes in phytoplanktonic productivity. Our data, when associated to previous studies of other black shales from Canada and Europe confirm that photic zone euxinia was a widespread feature of low-latitude epeiric seas during the Middle Devonian–Lower Mississippian.

The perennially ice-covered lakes in the McMurdo Dry Valleys, Antarctica, are part of the coldest and driest ecosystem on earth. To understand lacustrine carbon and nitrogen cycling in this end-member ecosystem, and to define paleolimnological proxies for ice-covered lakes, we measured the stable carbon and nitrogen isotopic composition of particulate organic matter (POM) and benthic organic matter (BOM) within the lakes of Taylor Valley. The δ13C compositions of seasonally ice-free edges of the lakes (moats) are enriched relative to under-ice organic matter. Thus, the organic carbon isotopic composition of buried sediments may be a proxy for sample position within the lake. In the moats, δ13C values are governed by limited CO2diffusion across benthic cyanobacterial cell membranes. During a high glacial melt (2001–2002) season, both δ13CPOM and δ13CBOM in the moats were more depleted than during previous low melt years. We propose that this occurred in response to higher [CO2](aq) and/or reduced growth rates resulting from turbidity-induced light limitation. Though moats and under-ice environments are usually poorly connected, during the 2001–2002 season, the enrichment of the δ13CPOM values at 6 m depth in the stream-proximal sites relative to deep-profile sites implies enhanced connectivity between these environments. The δ13C compositions of BOM and POM profiles in Lake Hoare and Lake Fryxell indicate that these lakes are dominated by benthic productivity. In contrast, in Lake Bonney, the similarity of the δ13C values of BOM and POM indicates the pelagic component dominance in the carbon cycle.

A pseudohomologous series of branched aliphatic alkanes with a quaternary substituted carbon atom (BAQCs, specifically 2,2-dimethylalkanes and 3,3- and 5,5-diethylalkanes) were identified in warm (65°C) deep-sea hydrothermal waters and Late Cretaceous black shales. 5,5-Diethylalkanes were also observed in modern and Holocene marine shelf sediments and in shales spanning the last 800 million years of the geological record. The carbon number distribution of BAQCs indicates a biological origin. These compounds were observed but not identified in previous studies of 2.0 billion- to 2.2 billion-year-old metasediments and were commonly misidentified in other sediment samples, indicating that BAQCs are widespread in the geological record. The source organisms of BAQCs are unknown, but their paleobiogeographic distribution suggests that they have an affinity for sulfides and might be nonphotosynthetic sulfide oxidizers.

Bulk-geochemical and biomarker data were collected for three Cenomanian-Turonian (Late Cretaceous) sections in the Western Canada Sedimentary Basin (Western Interior Seaway, Canada). Rock-Eval and biomarker analyses indicate an east-west thermal maturity trend due to progressive deeper burial associated with the tectonic flexure of the foreland basin. The source of extractable organic matter is dominantly marine with a minor terrestrial contribution. A pseudohomologous family of 2,2-dimethylalkanes (2,2-DMAs), in the C 16 –C 28 range with an even number of carbon atoms exclusively, was tentatively identified from mass spectral data in samples from the Pasquia Hills region (easternmost section). The specific origin of 2,2-dimethylalkanes remains unclear. The presence of isorenieratene derivatives indicates that the northern part of the Western Interior Seaway experienced photic zone euxinic events throughout the transgressive, highstand, and regressive phases of the Greenhorn Cycle. The differential occurrence of isorenieratane between the Canadian sections and the United States transect at maximum sea-level high stand indicates that the water column of the northern part of the WIS was less oxygenated, whereas the water column in the southern WIS was more dynamic and well-mixed.

Lake Vida, located in Victoria Valley, is one of the largest lakes in the McMurdo Dry Valleys. Unlike other lakes in the region, the surface ice extends at least 27 m, which has created an extreme and unique habitat by isolating a liquid-brine with salinity of 195 g L−1. Below 21 m, the ice is marked by well-sorted sand layers up to 20 cm thick, within a matrix of salty ice. From ice chemistry, isotopic abundances of 18O and 2H, ground penetrating radar profiles, and mineralogy, we conclude that the entire 27 m of ice formed from surface runoff, and the sediment layers represent the accumulation of fluvial and aeolian deposits. Radiocarbon and optically stimulated luminescence dating limit the maximum age of the lower ice to 6300 14C yr BP. As the ice cover ablated downwards during periods of low surface inflow, progressive accumulation of sediment layers insulated and preserved the ice and brine beneath; analogous to the processes that preserve shallow ground ice. The repetition of these sediment layers reveals climatic variability in Victoria Valley during the mid- to late Holocene. Lake Vida is an excellent Mars analog for understanding the preservation of subsurface brine, ice and sediment in a cold desert environment.

Monoethylalkanes (MEA, C16–C22) with an even number of carbon atoms are relatively abundant in the saturated hydrocarbon fraction of the basal Graneros Shale (Cenomanian, Western Interior Seaway, USA). On the basis of isomer distribution and isotopic composition, it is proposed that MEA derive from eubacterial organisms, possibly cyanobacteria, living in fresh to brackish water environments.

Upper Cretaceous strata in the Pasquia Hills of the northern Manitoba Escarpment, eastern Saskatchewan, Canada provide a detailed paleoenvironmental and sea-level record of the eastern margin of the Western Interior Seaway. Sediments deposited during the Cenomanian/Turonian Greenhorn marine cycle are dominantly black mudstones deposited in a stratified water column, with bottom-water anoxia recurrently reaching into the photic zone. A middle Cenomanian sea-level lowstand event followed by transgression left a series of bonebeds within the Belle Fourche Member of the Ashville Formation, indicating a sedimentary environment starved of coarse siliciclastics. Maximum sea level resulted in the formation of limestone beds within the Favel Formation, further favoured by reduced terrigenous sediment input compared to the western margin. Limestone sedimentation was followed by a phase of increased freshwater input under lower sea level conditions, and reducing zoo- and phytoplankton diversities. During final Greenhorn regression, eastern Saskatchewan probably turned into a restricted basin severely limiting marine circulation. Poor or absent benthic foraminiferal assemblages and biomarker analysis suggest prevailing watermass stratification throughout the Cenomanian/Turonian transgressive/regressive cycle. This was caused either by a freshwater lid, stratification of Boreal and Tethyan-derived watermasses, or both, to various intensities affected by changing sea level. Basin oxygenation during Niobrara time varies between localities along the eastern margin as documented by presence/absence of benthic and planktic foraminifera.

Bulk-geochemical and biomarker (molecular fossil) data were collected for four Cenomanian–Turonian sections in the Western Interior Basin, USA. The four sections represent a 900 km wide transect across the basin from Kansas to New Mexico. Rock–Eval and biomarker analyses indicate an east–west thermal maturity trend due to progressive deeper burial associated with the tectonic flexure of the foreland basin since the Cenonamian–Turonian. At each site the source of extractable organic matter is dominantly marine with minor terrestrial contribution. The presence of isorenieratene derivatives in samples from Bunker Hill (KS), Pueblo (CO), and Red Wash (NM) indicates the presence of the green sulfur bacterium Chlorobium in the water column, and shows that the southern part of the Western Interior Seaway (WIS) experienced events of photic zone anoxia. At maximum transgression, sedimentary features indicative of water column oxygenation occur within single beds with geochemical evidence of photic zone anoxia. This is consistent with a scenario in which bottom waters experienced alternating oxic/dysoxic and anoxic conditions. These so-called ‘intermittent anoxic events’ underline the high variability in redox conditions in the WIS. The distribution of isorenieratene derivatives shows a dynamic water column structure of the WIS, and indicates that anoxia was a permanent feature of the transgressive and regressive intervals of the Greenhorn Cyclothem, rather than at maximum transgression. Hence, the oceanographic circulation is dominated by near field effects inherent to the seaway, rather than by direct proto-Atlantic elements such as the oceanic anoxic event (OAE).

Preparative high pressure liquid chromatography on US-Y zeolite shape-selective molecular sieve was studied for carbon isotopic fractionation effects. We tested a standard mixture [17β, 21β(H)-hopane, 5α-cholestane] and complex natural hydrocarbon mixtures dominated by tetra- and pentacyclic triterpenoids extracted from Oxford Clay shales. We confirmed that steroids and hopanoids were separated on the basis of stereochemical configuration while isotopic analysis of eluents indicated that shape-selective chromatography did not result in isotopic fractionation. US-Y zeolite chromatography can be used to simplify hydrocarbon mixtures and prepare well resolved mixtures of molecular fossils for compound-specific isotopic analyses (CSIA).

Preparative open pyrolysis products of kerogen isolated from a Holocene microbial mat collected in Abu Dhabi (United Arab Emirates) contain homologous series of monomethylalkanes (MMAs, C16‐C29). The isomer and carbon number distributions of these branched alkanes closely match those of MMAs in ancient sediments and oils. It is proposed that monomethylalkanes in oils and in thermally mature ancient sediments derive from non-extractable organic matter produced by one or more members of the cyanobacterial assemblage, eubacterial primary producers and/or heterotrophic bacteria. # 2000 Elsevier Science Ltd. All rights reserved.

Carbon isotopic compositions of suspended organic matter and biomarker compounds were determined for 59 samples filtered from Southern Ocean surface waters in January 1994 along two north-south transects (WOCE SR3 from Tasmania to Antarctica, and across the Princess Elizabeth Trough (PET) east of Prydz Bay, Antarctica). Along the SR3 line, bulk organic matter show generally decreasing 13C contents southward, which are well correlated with increasing dissolved molecular carbon dioxide concentrations, CO2(aq). This relationship does not hold along the PET transect. Using concentrations and isotopic compositions of molecular compounds, we evaluate the relative roles of several factors affecting the δ13C of Southern Ocean suspended particulate organic matter. Along the WOCE SR3 transect, the concentration of CO2(aq) plays an important role. It is well described by a supply versus demand model for the extent of cellular CO2 utilization and its associated linear dependence of isotopic fractionation (EP) on the reciprocal of CO2(aq). An equally important factor appears to be changes in algal assemblages along the SR3 transect, with their contribution to isotopic fractionation also well described by the supply and demand model, when formulated to include the cell surface/volume control of supply. Changes in microalgal growth rates appear to have a minor effect on EP. Along the PET transect, algal assemblage changes and possibly changes in microalgal growth rates appear to strongly affect the carbon isotopic variations of suspended organic matter. These results can be used to improve the formulation of modern carbon cycle models that include phytoplankton carbon isotopic fractionation.

We present the first results of U37K′ and ealkenone from Emiliania huxleyi grown in chemostat cultures at constant temperature but at variable growth rates. A small change in U37K′ with growth rate was found for a noncalcifying strain of E. huxleyi, whereas results from a calcifying strain showed no systematic variation between U37K′ and growth rate. We conclude that nutrient-limited growth rate effects will not produce serious errors in paleotemperature determinations using U37K′ or U37K′. However, U37K′ values for the two strains differed significantly and apparent temperatures calculated using unsaturation ratios obtained from chemostat experiments are very different from those derived from batch culture experiments. Results of isotopic analyses of alkenones and bulk cells indicate no effect of growth rate on ealkenone (ealkenone=4.2±0.2‰). Constancy of ealkenone values is a requirement for environmental and paleoenvironmental interpretations. These results indicate that isotopic analyses of alkenones are ideal markers for paleo-[CO2(aq)] determinations.