Kevin W. Mandernack

Obliquity-paced Pliocene West Antarctic ice sheet oscillations

Thirty years after oxygen isotope records from microfossils deposited in ocean sediments confirmed the hypothesis that variations in the Earth’s orbital geometry control the ice ages, fundamental questions remain over the response of the Antarctic ice sheets to orbital cycles. Furthermore, an understanding of the behaviour of the marine-based West Antarctic ice sheet (WAIS) during the ‘warmer-than-present’ early-Pliocene epoch (∼5–3 Myr ago) is needed to better constrain the possible range of ice-sheet behaviour in the context of future global warming. Here we present a marine glacial record from the upper 600 m of the AND-1B sediment core recovered from beneath the northwest part of the Ross ice shelf by the ANDRILL programme and demonstrate well-dated, ∼40-kyr cyclic variations in ice-sheet extent linked to cycles in insolation influenced by changes in the Earth’s axial tilt (obliquity) during the Pliocene. Our data provide direct evidence for orbitally induced oscillations in the WAIS, which periodically collapsed, resulting in a switch from grounded ice, or ice shelves, to open waters in the Ross embayment when planetary temperatures were up to ∼3 °C warmer than today and atmospheric CO2 concentration was as high as ∼400 p.p.m.v. (refs 5, 6). The evidence is consistent with a new ice-sheet/ice-shelf model that simulates fluctuations in Antarctic ice volume of up to +7 m in equivalent sea level associated with the loss of the WAIS and up to +3 m in equivalent sea level from the East Antarctic ice sheet, in response to ocean-induced melting paced by obliquity. During interglacial times, diatomaceous sediments indicate high surface-water productivity, minimal summer sea ice and air temperatures above freezing, suggesting an additional influence of surface melt under conditions of elevated CO2.

Subsurface Microbial Diversity in Deep-Granitic-Fracture Water in Colorado

ABSTRACT A microbial community analysis using 16S rRNA gene sequencing was performed on borehole water and a granite rock core from Henderson Mine, a >1,000-meter-deep molybdenum mine near Empire, CO. Chemical analysis of borehole water at two separate depths (1,044 m and 1,004 m below the mine entrance) suggests that a sharp chemical gradient exists, likely from the mixing of two distinct subsurface fluids, one metal rich and one relatively dilute; this has created unique niches for microorganisms. The microbial community analyzed from filtered, oxic borehole water indicated an abundance of sequences from iron-oxidizing bacteria (Gallionella spp.) and was compared to the community from the same borehole after 2 weeks of being plugged with an expandable packer. Statistical analyses with UniFrac revealed a significant shift in community structure following the addition of the packer. Phospholipid fatty acid (PLFA) analysis suggested that Nitrosomonadales dominated the oxic borehole, while PLFAs indicative of anaerobic bacteria were most abundant in the samples from the plugged borehole. Microbial sequences were represented primarily by Firmicutes, Proteobacteria, and a lineage of sequences which did not group with any identified bacterial division; phylogenetic analyses confirmed the presence of a novel candidate division. This “Henderson candidate division” dominated the clone libraries from the dilute anoxic fluids. Sequences obtained from the granitic rock core (1,740 m below the surface) were represented by the divisions Proteobacteria (primarily the family Ralstoniaceae) and Firmicutes. Sequences grouping within Ralstoniaceae were also found in the clone libraries from metal-rich fluids yet were absent in more dilute fluids. Lineage-specific comparisons, combined with phylogenetic statistical analyses, show that geochemical variance has an important effect on microbial community structure in deep, subsurface systems.

A stable sulfur and oxygen isotopic investigation of sulfur cycling in an anoxic marine basin, Framvaren Fjord, Norway

Abstract In 1993 we measured the δ34S values of total dissolved sulfide (δ34S∑H2S) and sulfate (δ34SSO4−) and the δ18O of sulfate (δ18OSO4−) from water samples collected across the oxic–anoxic interface and in the deep permanently anoxic waters of the stratified Framvaren fjord in southern Norway. Near the chemocline, variations in the δ34SSO4− and δ18OSO4− values were generally less than 1‰ from ambient seawater values. However, a minimum δ34SSO4− value of +19.7‰ was detected at 20 m depth, which coincided with the depth that sulfide first appeared and may reflect sulfide oxidation. Small increases in δ34SSO4− and δ18OSO4− values 3 m below this depth are consistent with a zone of sulfur disproportionation. The δ34S∑H2S value near the interface at 22 m was −19.8‰, which is 41.2‰ depleted in 34S relative to the sulfate collected at that depth. In close agreement with earlier measurements made at Framvaren in 1982, the δ34SSO4− values collected from the deeper anoxic waters showed a marked 34S enrichment with depth, which corresponded with a decrease in the sulfate concentration. These results are interpreted to be the result of active dissimilatory sulfate reduction. A Rayleigh plot for the sulfate data measured in 1993 provides estimates for the sulfur and oxygen isotope enrichment factors (es and eo, respectively) for sulfate reduction of −41.5‰ and −9.8‰, respectively, with the former value matching closely the observed difference in δ34S between the dissolved sulfide and sulfate near the interface. Our results from 1993, however, contrast with δ34SSO4− and δ34S∑H2S data in the water column made in 1983 by Anderson et al. [Mar. Chem. 23 (1988) 283). We conclude that the results of 1983 may be anomalous, and as a result this may offer additional interpretations than what was previously provided for the origin of reduced inorganic sulfur in the sediments of Framvaren based on their measured δ34S values. We hypothesize that the lower δ34Strs values in the sediments relative to δ34S∑H2S values in the water column could also result from different rates of sulfate reduction, or in shallower sediments just beneath the chemocline, also from disproportionation of S∘, S2O3−, or SO3−. We hypothesize that the observed ratio of 4.4:1 for the measured changes in δ34SSO4− versus δ18OSO4− within the anoxic waters approximates the 4:1 atom ratio of oxygen to sulfur in the residual sulfate as a result of dissimilatory sulfate reduction and reflects little oxygen isotope exchange between intermediates of sulfur metabolism and water either during bacterial sulfate reduction or from sulfide reoxidation processes. Based on comparisons with other studies, we further propose that this lack of isotopic exchange with water, and the subsequent ∼4:1 ratio of δ34SSO4− versus δ18OSO4−, occurs under conditions that promote a unidirectional biochemical reaction for sulfate reduction during which kinetic isotope effects are fully expressed and are consequently reflected in the δ34SSO4− and δ18OSO4− values.

The biogeochemical controls of the δ15N and δ18O of N2O produced in landfill cover soils

We document an enrichment of both the δ18OAtm.O2 and δ15NAtm.N2 values of soil-derived N2O collected from landfill cover soils relative to tropospheric N2O. The isotopic values of N2O vary from −5.1‰ to +l9.4‰ and from +19.0‰ to +33.5‰ for δ 15NAtm.N2 and δ18OAtm.O2,, respectively. A tight linear correlation for δ18OAtm.O2 versus δ15NAtm.N2 is apparent, reflecting coupled microbial processes that produce N2O that may be isotopically enriched or depleted in relation to tropospheric N2O. Several explanations are provided to explain this correlation, including evidence for NH3 limitation during nitrification, which would be expected to diminish isotopic fractionation and consequently result in more enriched isotopic values of N2O. Desiccation effects on nitrification were also observed, which contribute to NH3 limitation and thus could influence the isotopic signature of N2O. Our results indicate that the N2O isotopic composition from soils may vary greatly depending on the season and soil moisture conditions and may at times be enriched in 15N and 18O relative to tropospheric N2O.

Bacterial abundance and composition in marine sediments beneath the Ross Ice Shelf, Antarctica

Marine sediments of the Ross Sea, Antarctica, harbor microbial communities that play a significant role in the decomposition, mineralization, and recycling of organic carbon (OC). In this study, the cell densities within a 153-cm sediment core from the Ross Sea were estimated based on microbial phospholipid fatty acid (PLFA) concentrations and acridine orange direct cell counts. The resulting densities were as high as 1.7 × 10⁷ cells mL⁻¹ in the top ten centimeters of sediments. These densities are lower than those calculated for most near-shore sites but consistent with deep-sea locations with comparable sedimentation rates. The δ¹³C measurements of PLFAs and sedimentary and dissolved carbon sources, in combination with ribosomal RNA (SSU rRNA) gene pyrosequencing, were used to infer microbial metabolic pathways. The δ¹³C values of dissolved inorganic carbon (DIC) in porewaters ranged downcore from -2.5‰ to -3.7‰, while δ¹³C values for the corresponding sedimentary particulate OC (POC) varied from -26.2‰ to -23.1‰. The δ¹³C values of PLFAs ranged between -29‰ and -35‰ throughout the sediment core, consistent with a microbial community dominated by heterotrophs. The SSU rRNA gene pyrosequencing revealed that members of this microbial community were dominated by β-, δ-, and γ-Proteobacteria, Actinobacteria, Chloroflexi and Bacteroidetes. Among the sequenced organisms, many appear to be related to known heterotrophs that utilize OC sources such as amino acids, oligosaccharides, and lactose, consistent with our interpretation from δ¹³CPLFA analysis. Integrating phospholipids analyses with porewater chemistry, δ¹³CDIC and δ¹³CPOC values and SSU rRNA gene sequences provides a more comprehensive understanding of microbial communities and carbon cycling in marine sediments, including those of this unique ice shelf environment.

Stable carbon isotope fractionation of trans-1,2-dichloroethylene during co-metabolic degradation by methanotrophic bacteria

Abstract Changes in the carbon isotope ratio ( δ 13 C) of trans -1,2-dichloroethylene ( t -DCE) were measured during its co-metabolic degradation by Methylomonas methanica , a type I methanotroph, and Methylosinus trichosporium OB3b, a type II methanotroph. In closed-vessel incubation experiments with each bacterium, the residual t -DCE became progressively enriched in 13 C, indicating isotopic fractionation. From these experiments, the biological fractionation during t -DCE co-metabolism, expressed as e , was measured to be −3.5‰ for the type I culture and −6.7‰ for the type II culture. This fractionation effect and subsequent enrichment in the δ 13 C of the residual t -DCE can thus be applied to determine the extent of biodegradation of DCE by these organisms. Based on these results, isotopic fractionation clearly warrants further study, as measured changes in the δ 13 C values of chlorinated solvents could ultimately be used to monitor the extent of biodegradation in laboratory or field settings where co-metabolism by methanotrophs occurs.

Isolation and Characterization of Pseudomonas Strains Capable of Fe(III) Reduction with Reference to Redox Response Regulator Genes

Two bacterial strains, KNA-6-3 and KNA-6-5, were isolated from groundwater of the Tono uranium mine, Gifu Prefecture, central Japan, and showed slight but persistent ferric iron (Fe(III)) reduction. The 16S rDNA sequences of the strains were > 96% similar to that of the facultative denitrifier Pseudomonas stutzeri, a species that has not been regarded as a reducer of Fe(III). The genes of redox stress response regulators involved in switching between aerobic and anaerobic metabolisms, arcA, arcB and anr, were amplified by PCR and sequenced. Phylogenetic analyses of the amplified sequences showed high similarities to known arcA, arcB and anr genes from other bacterial species. Expression of the putative arcA, arcB and anr genes was monitored by real-time reverse transcription PCR during aerobic and anaerobic growth. Transcripts of the putative arcA and anr increased with slight but persistent Fe(III) reduction, whereas the putative arcB transcript showed no significant correlation with Fe(III) reduction. The putative anr sequences from the two strains were less similar, and therefore the putative arcA sequences were targeted to estimate the abundance of related bacteria in the Tono uranium mine groundwater. Real-time genomic PCR using universal arcA primers and an arcA probe showed that arcA-bearers would occur at as low as 0.06% (50 cells ml−1) of the total microbial population of 8.05 × 104 cells ml−1.

The relative contribution of methanotrophs to microbial communities and carbon cycling in soil overlying a coal‐bed methane seep

Seepage of coal-bed methane (CBM) through soils is a potential source of atmospheric CH4 and also a likely source of ancient (i.e. (14) C-dead) carbon to soil microbial communities. Natural abundance (13) C and (14) C compositions of bacterial membrane phospholipid fatty acids (PLFAs) and soil gas CO2 and CH4 were used to assess the incorporation of CBM-derived carbon into methanotrophs and other members of the soil microbial community. Concentrations of type I and type II methanotroph PLFA biomarkers (16:1ω8c and 18:1ω8c, respectively) were elevated in CBM-impacted soils compared with a control site. Comparison of PLFA and 16s rDNA data suggested type I and II methanotroph populations were well estimated and overestimated by their PLFA biomarkers, respectively. The δ(13) C values of PLFAs common in type I and II methanotrophs were as negative as -67‰ and consistent with the assimilation of CBM. PLFAs more indicative of nonmethanotrophic bacteria had δ(13) C values that were intermediate indicating assimilation of both plant- and CBM-derived carbon. Δ(14) C values of select PLFAs (-351 to -936‰) indicated similar patterns of CBM assimilation by methanotrophs and nonmethanotrophs and were used to estimate that 35-91% of carbon assimilated by nonmethanotrophs was derived from CBM depending on time of sampling and soil depth.

Effects of the fungicides mancozeb and chlorothalonil on fluxes of CO2, N2O, and CH4 in a fertilized Colorado grassland soil

[1] Management of agricultural soil plays an important role in present and future atmospheric concentrations of the greenhouse gases carbon dioxide (CO 2 ), nitrous oxide (N 2 O), and methane (CH 4 ). Pesticides are used as management tools in crop production, but little is known about their effects on soil-atmosphere exchange of CO 2 , N 2 O, and CH 4 . Field studies described in this paper determined the effect of two commonly used fungicides, mancozeb and chlorothalonil, on trace gas exchange. Separate experimental plots, 1 m 2 , were established in nitrogen fertilized no-tilled native grassland and tilled soils with and without fungicide application. Two studies were conducted. The first study was initiated in June 1999 and lasted for 1 year with monthly flux measurements from tilled and no-till soils. The second study commenced in August 2001 with twelve weekly measurements from tilled soils only. From both studies mancozeb suppressed emissions of CO 2 and N 2 O in the tilled soil by an average of 28% and 47%, respectively. This suppression corresponded with efficacy periods of 14-29 and 56-77 days, respectively. From the no-till soils mancozeb decreased CO 2 and N 2 O emissions by 33% and 80% for periods of 29 and 94 days, respectively. Mancozeb inhibited CH 4 consumption in the first study by 46% and 71% in the tilled and no-till soil for periods of 8 and 29 days, respectively, but had no effect in the second study. From both studies chlorothalonil initially suppressed CO 2 and N 2 O emissions and enhanced CH 4 uptake in the tilled soil by an average of 37%, 40%, and 115%, respectively. These effects corresponded with efficacy periods of 14-29, 21-56, and 1-14 days, respectively. In the no-till soil chlorothalonil inhibited CO 2 and N 2 O emissions and enhanced CH 4 uptake by 29%, 48%, and 86% for periods of 29, 56, and 56 days, respectively. Following the initial period of suppression, chlorothalonil subsequently enhanced N 2 O emissions in the tilled soil by an average of 51% and in the no-till soil by 81% before returning to near background levels. The beginning of increased N 2 O emissions from the chlorothalonil-amended plots corresponded with a maximum soil concentration of the chlorothalonil degradate, 4-hydroxy-2, 5, 6-trichloroisophthalonitrile. The site specific global warming potential (GWP) resulting from the fluxes of CO 2 , N 2 O, and CH 4 from all soils was determined to decrease by an average 26% and 21% as a result of a single application of mancozeb or chlorothalonil, respectively. The decrease in CO 2 emissions in the fungicide-amended plots potentially could result in the conservation of as much as 1200 and 2400 kg C ha -1 yr -1 organic carbon in the tilled and no-till plots, respectively. Therefore it is feasible that application of certain fungicides to agricultural soil might lead to enhanced soil carbon sequestration and thus have additional positive effects on atmospheric CO 2 concentrations.

Acetoclastic Methanosaeta are dominant methanogens in organic-rich Antarctic marine sediments

Despite accounting for the majority of sedimentary methane, the physiology and relative abundance of subsurface methanogens remain poorly understood. We combined intact polar lipid and metagenome techniques to better constrain the presence and functions of methanogens within the highly reducing, organic-rich sediments of Antarctica’s Adélie Basin. The assembly of metagenomic sequence data identified phylogenic and functional marker genes of methanogens and generated the first Methanosaeta sp. genome from a deep subsurface sedimentary environment. Based on structural and isotopic measurements, glycerol dialkyl glycerol tetraethers with diglycosyl phosphatidylglycerol head groups were classified as biomarkers for active methanogens. The stable carbon isotope (δ13C) values of these biomarkers and the Methanosaeta partial genome suggest that these organisms are acetoclastic methanogens and represent a relatively small (0.2%) but active population. Metagenomic and lipid analyses suggest that Thaumarchaeota and heterotrophic bacteria co-exist with Methanosaeta and together contribute to increasing concentrations and δ13C values of dissolved inorganic carbon with depth. This study presents the first functional insights of deep subsurface Methanosaeta organisms and highlights their role in methane production and overall carbon cycling within sedimentary environments.