Rich D Pancost

Archaeal lipids in Mediterranean cold seeps: molecular proxies for anaerobic methane oxidation

Abstract We investigated the distributions and δ13C values of biomarkers for Archaea associated with anaerobic methane oxidation in disparate settings throughout two Eastern Mediterranean mud dome fields. All major classes of archaeal lipids are present in the studied sediments, including isoprenoid glycerol diethers, isoprenoid glycerol dialkyl glycerol tetraethers, and irregular isoprenoid hydrocarbons. Of the compounds present, many, including a novel glycerol tetraether and sn-3-hydroxyarchaeol, have not been previously reported for settings in which methane oxidation is presumed to occur. Archaeal lipids are depleted in 13C, indicating that the Archaea from which they derive are either directly or indirectly involved with methane consumption. The most widespread archaeal lipids are archaeol, PMI, and glycerol tetraethers, and these compounds are present at all active sites. However, archaeal lipid abundances and distributions are highly variable; ratios of crocetane, PMI, and hydroxyarchaeol relative to archaeol vary from 0 to 6.5, from 0 to 2, and from 0 to 1, respectively. These results suggest that archaeal communities differ amongst the sites examined. In addition, carbon isotopic variability amongst archaeal biomarkers in a given mud breccia can be as large as 24 ‰, suggesting that even at single sites multiple archaeal species perform or are supported by anaerobic methane oxidation.

A comparative study of lipids in Sphagnum species

Abstract The free lipid compositions of twelve species of Sphagnum were determined by capillary gas chromatography/mass spectrometry as part of a study to identify characteristic lipids for Sphagnum in peat bogs. Complex mixtures of lipids, comprised of C28–C29 sterols, C30 triterpenoids, C16–C30 fatty acids, C22–C30 fatty alcohols, C21–C33n-alkanes and isoprenoid and straight-chain wax esters, were identified and quantified. Sterols are dominated by the C29 sterols, 24-ethylcholesta-5,22-dien-3β-ol and 24-ethylcholest-5-en-3β-ol, whilst in some species C28 sterols are also abundant. Summed concentrations of triterpenoids varied widely (20–3500 μg/g dry weight), with only small concentrations present in the mesotrophic species, S. fimbriatum and S. palustre. Ursolic acid is always the major triterpenoid detected. Although absolute concentrations vary significantly, the carbon number distributions of fatty acids, fatty alcohols and n-alkanes are similar in all examined species. Thus, the distributions of these compounds, and especially the dominance of C23 and C25n-alkanes, are a useful chemotaxonomic fingerprint for Sphagnum species and can thus be used in compound-specific 13C and 14C studies of peat bogs.

Carbon isotope geochemistry of the Frasnian^Famennian transition

Abstract Carbon isotope records for inorganic and total organic carbon (TOC) as well as for individual organic biomarkers show two positive excursions in the Late Frasnian with comparable shape and amplitude in δ13C. The positive shifts in δ13C can be correlated with the deposition of the Kellwasser horizons. The inorganic carbon isotope excursions are explained by an enhanced organic carbon burial that is expected to lower the concentration of oceanic dissolved CO2. TOC δ13C values do not exhibit a larger shift than that recorded by inorganic carbon δ13C values; this is unexpected since a decrease in the dissolved CO2 concentration should result in lower carbon isotope fractionation during photosynthesis (ep). δ13C values of presumed algal biomarkers (low-molecular-weight n-alkanes, pristane, phytane), although offset by ∼2.5‰, exhibit essentially identical records as TOC, and confirm this unexpected result. It is proposed that high atmospheric and oceanic CO2 concentrations during the Devonian resulted in maximum photosynthetic fractionation. Any change in the CO2 concentration would thus not affect ep. As such, δ13C of primary organic carbon and δ13Ccarb will exhibit parallel excursions. The data imply that carbon isotopes and the relationship between ep and [CO2] will not be effective to study changes in pCO2 levels during the Devonian since CO2 concentrations were too high.

Iron‐stimulated changes in 13C fractionation and export by equatorial Pacific phytoplankton: Toward a paleogrowth rate proxy

We present δ13C values for phytol, an algal biomarker, which document up to 7 per mil isotopic enrichment during the IronEx II iron fertilization experiment. We evaluate these data using a laboratory-derived 13C fractionation model and show this variability is largely the result of elevated growth rates. Isotopic enrichment and stimulation of growth rate were accompanied by a sevenfold increase in the export of particulate organic carbon as estimated from 234Th activities. This is the first direct evidence that enhanced productivity following iron enrichment can lead to both increased export of organic matter and an associated isotopic signal in an algal biomarker. On the basis of these results, we propose biomarker isotopic data be used in conjunction with paleo-CO2 records to reconstruct paleogrowth rates. This approach provides a means to test for iron-stimulated changes in algal growth in sedimentary records.

Arsenic release and attenuation in low organic carbon aquifer sediments from West Bengal

High arsenic concentrations in groundwater are causing a humanitarian disaster in Southeast Asia. It is generally accepted that microbial activities play a critical role in the mobilization of arsenic from the sediments, with metal-reducing bacteria stimulated by organic carbon implicated. However, the detailed mechanisms underpinning these processes remain poorly understood. Of particular importance is the nature of the organic carbon driving the reduction of sorbed As(V) to the more mobile As(III), and the interplay between iron and sulphide minerals that can potentially immobilize both oxidation states of arsenic. Using a multidisciplinary approach, we identified the critical factors leading to arsenic release from West Bengal sediments. The results show that a cascade of redox processes was supported in the absence of high loadings of labile organic matter. Arsenic release was associated with As(V) and Fe(III) reduction, while the removal of arsenic was concomitant with sulphate reduction. The microbial populations potentially catalysing arsenic and sulphate reduction were identified by targeting the genes arrA and dsrB, and the total bacterial and archaeal communities by 16S rRNA gene analysis. Results suggest that very low concentrations of organic matter are able to support microbial arsenic mobilization via metal reduction, and subsequent arsenic mitigation through sulphate reduction. It may therefore be possible to enhance sulphate reduction through subtle manipulations to the carbon loading in such aquifers, to minimize the concentrations of arsenic in groundwaters.

The role of indigenous microorganisms in the biodegradation of naturally occurring petroleum, the reduction of iron, and the mobilization of arsenite from west bengal aquifer sediments.

High levels of naturally occurring arsenic are found in the shallow reducing aquifers of West Bengal, Bangladesh, and other areas of Southeast Asia. These aquifers are used extensively for drinking water and irrigation by the local population. Mechanisms for its release are unclear, although increasing evidence points to a microbial control. The type of organic matter present is of vital importance because it has a direct impact on the rate of microbial activity and on the amount of arsenic released into the ground water. The discovery of naturally occurring hydrocarbons in an arsenic-rich aquifer from West Bengal provides a source of potential electron donors for this process. Using microcosm-based techniques, seven sediments from a site containing naturally occurring hydrocarbons in West Bengal were incubated with synthetic ground water for 28 d under anaerobic conditions without the addition of an external electron donor. Arsenic release and Fe(III) reduction appeared to be microbially mediated, with variable rates of arsenic mobilization in comparison to Fe(III) reduction, suggesting that multiple processes are involved. All sediments showed a preferential loss of petroleum-sourced n-alkanes over terrestrially sourced sedimentary hydrocarbons n-alkanes during the incubation, implying that the use of petroleum-sourced n-alkanes could support, directly or indirectly, microbial Fe(III) reduction. Samples undergoing maximal release of As(III) contained a significant population of Sulfurospirillum sp., a known As(V)-reducing bacterium, providing the first evidence that such organisms may mediate arsenic release from West Bengali aquifers.

A 400-year record of environmental change in an euxinic fjord as revealed by the sedimentary biomarker record

A 400-year sedimentary record of the euxinic Kyllaren Fjord was investigated using organic biogeochemical, geochemical and sedimentological approaches. Accumulation rates of a suite of biomarkers, and in some cases their stable carbon isotopic composition, were determined. Biomarkers were separated into three different groups: terrestrial derived, general aquatic/marine derived, and aquatic/marine derived but related to high primary productivity. Variations in bulk sediment parameters and biomarker accumulation rates with depth were used to interpret the sedimentary record. We infer that in the first half of the 19th century an environmental change occurred, caused by a change in either the precipitation regime, vegetation and/or land use. The data also show that in the last century, and especially in the last decades, a distinct increase in primary productivity occurred. Most likely this is caused by a ‘natural’ eutrophication due to the transformation of the fjord from an open to a confined system by the building of a partially open dam in 1954, possibly combined with anthropogenic eutrophication. A concurrent shoaling of the fjord chemocline may have had a mediating effect in the change towards a higher productivity by enhancing nutrient and carbon recycling. Enhanced carbon recycling is reflected by decreased δ13C values of several biomarkers.

The chemical structure of Gloeocapsomorpha prisca microfossils: implications for their origin

Abstract Two Estonian Kukersites (Ordovician) and two samples from the Guttenberg Member (Ordovician) of the Decorah formation (North America) containing botryoidal aggregates of Gloeocapsomorpha prisca were investigated by RuO4 chemical degradation, FTIR, and flash pyrolysis-GC/MS to obtain information about the polymeric structure of these microfossils. The products formed upon oxidation by RuO4 were analysed by GC/MS and revealed the presence of a wide range of carboxyl and/or carbonyl moiety containing compounds with carbon skeletons ranging from C5 to C20. The Estonian Kukersites reveal the presence of a characteristic set of mono-, di-, and tricarboxylic acids. These compounds suggest that the Estonian Kukersites are composed of a polymer consisting of mainly C21 and C23 n-alkenyl resorcinol building blocks. Similarly, although the tricarboxylic acids are not present, the RuO4 degradation product mixtures of the Guttenberg Member samples, suggest a poly(n-alkyl resorcinol) structure. The higher thermal maturity is most likely responsible for the different chemistry and morphology of the G. prisca microfossils in these samples. Because compounds like n-alkenyl resorcinols are known to polymerise under oxygenated conditions even in an aqueous environment, it is not per se necessary that these microfossils are composed of a selectively preserved biopolymeric cell wall. It is also possible that G. prisca microfossils are composed of a cell wall or sheath component that polymerised during senescence or diagenesis of the organism.

Controls on the carbon-isotope compositions of compounds in Peru surface waters

We sampled suspended organic matter from two east–west transects across the Peru upwelling region and determined the taxonomic and regional variability of the relationships between photosynthetic fractionation (ep) and environmental conditions for diatoms, dinoflagellates, and total photosynthate. We also examined controls on the δ13C values of zooplankton-derived sterols and suspended particulate organic carbon (SPM). Ratios of chlorophyll-a (chl-a) to SPM suggest that light limited diatom growth rates along the northern transect, while nutrients apparently limited growth along the southern transect. In the southern transect, three different diatom-derived sterols exhibit similar ep–growth rate relationships, consistent with previous results indicating that diatoms in Peru waters employ active transport of bicarbonate and were growth-limited by silicate. Phytol, largely synthesized by primary producers, and zooplanktonic sterols exhibited relationships similar to the diatom sterols, but correlation coefficients between their respective ep values and environmental conditions were significantly lower. Values of ep calculated from particulate organic carbon (POC) did not correlate with specific environmental variables, possibly due to the overprint of spatial variations in phytoplankton assemblages on δ13CPOC. These results suggest that the quality of environmental interpretation based upon carbon-isotopic composition decreases as particulate organic matter and target biomarkers become less taxon-specific due to contributions from multiple sources.

Temperature sensitivity of methane production in the permafrost active layer at Stordalen, Sweden: A comparison with non-permafrost northern wetlands

Abstract Relationships were determined between methane (CH4) production and in situ conditions within the permafrost active layer during a single melt season at Stordalen, Sweden, with a specific emphasis on temperature sensitivity of methanogenesis. In situ temperature, moisture, pH, dissolved organic carbon, and CH4 concentration data were measured at three contrasting active layer sites (sedge mire, Sphagnum mire, and ombrotrophic bog), and laboratory incubations of active layer material were subsequently employed to determine the sensitivity of CH4 production to temperature. Q10 values, describing the CH4 production response of peat to a temperature change of 10 °C, ranged from 1.9 to 3.5 and 2.4 to 5.8 for the sedge and Sphagnum mire sites, respectively. A wider review of the literature on Q10 responses of methanogenesis in northern peatlands shows similar features to the temperature response of CH4 production in the active layer at Stordalen. In general, Q10 values are not significantly different in Arctic permafrost wetlands than non-Arctic northern wetlands; however, Sphagnum sites display Q10 responses (mean Q10 = 8) that are notably greater than that of wetter minerotrophic-sedge environments (mean Q10 = 4.3). This finding has implications for the parameterization of Q10 factors in numerical carbon cycling models, and suggests that the use of spatially variable Q10 values could be a useful approach for more accurate modeling of CH4 fluxes from northern wetlands under different climatic change scenarios.