1,721,068 research outputs found
Role of microbial populations in the release of reduced iron to the water column from marine aggregates
No full textThe release of dissolved iron from artificial aggregates formed from oxic natural coastal water and senescent phytoplankton material was demonstrated under dark conditions. The rate of release was controlled by the amount of reducible Fe(III) available, and appears to be limited by the competing oxidation of Fe(II). Molecular (16S rRNA gene) analyses showed the bacterial community associated with the aggregates, originating from estuarine water, to be dominated by oxic heterorophs. However, it was possible to culture NO 3- and Fe(III)-reducing bacteria from the artificial aggregates, and marine particles incubated with Fe(III) under anaerobic conditions contained microorganisms belonging to the genera Desulfovibrio and Marinobacter, bacteria known to reduce Fe(III). Whilst the precise mechanism of reduction is not clear, it is evident that marine aggregates can be a source of Fe(II), and thus ultimately iron in other forms, in coastal waters and most probably other natural water systems. © Inter-Research 2009
Spatial variability of diploptene δ<sup>13</sup>C values in thermokarst lakes: the potential to analyse the complexity of lacustrine methane cycling
No full textCryospheric changes in northern high latitudes are linked to significant greenhouse gas flux to the atmosphere, including methane release that originates from organic matter decomposition in thermokarst lakes. The connections between methane production in sediments, transport pathways and oxidation are not well understood and this has implications for any attempts to reconstruct methane production from sedimentary archives. We assessed methane oxidation as represented by methane oxidising bacteria across the surface sediments of two interior Alaska thermokarst lakes in relation to methane emissions via ebullition (bubbling). The bacterial biomarker diploptene was present and had low ?13C values (lower than ?38 ‰) in all sediments analysed, suggesting methane oxidation was widespread. The most ?13C-depleted diploptene was found in the area of highest methane ebullition emissions in Ace Lake (?13C diplotene values between ?68.2 and ?50.1 ‰), suggesting a positive link between methane production, oxidation, and emission in this area. In contrast, significantly less depleted diploptene ?13C values (between ?42.9 and ?38.8 ‰) were found in the area of highest methane ebullition emissions in Smith Lake. Lower ?13C values of diploptene were found in the central area of Smith Lake (between ?56.8 and ?46.9 ‰), where methane ebullition rates are low but methane diffusion appears high. Using ?13C-diplotene as a proxy for methane oxidation activity, we suggest the observed differences in methane oxidation levels among sites within the two lakes could be linked to differences in source area of methane production (e.g. age and type of organic carbon) and bathymetry as it relates to varying oxycline depths and changing pressure gradients. As a result, methane oxidation is highly lake-dependent. The diploptene ?13C values also highlight strong within-lake variability, implying that single-value, down-core records of hopanoid isotopic signatures are not secure indicators of changing methane flux at the whole-lake scale
Investigating past methane cycle perturbations through the lens of novel polyfunctionalised hopanoids
No full textEnhanced aerobic methanotrophy has been inferred in wetland environments during past warm climates, notably the Paleocene-Eocene Thermal Maximum (PETM; ∼56 Ma). This is based on a negative carbon isotope excursion in bacterial-derived hopanes (d13Chop). However, these compounds have multiple source organisms and their d13Chop can also reflect changes in the metabolic pathway used to assimilate carbon. In contrast, bacteriohopanepolyols (BHPs) are polyfunctionalised hopanoids that may be a more diagnostic tracer of methane oxidation, but their preservation potential is unclear. In this study, we aim to: (i) determine the preservation of novel BHPs during the PETM; and (ii) explore the potential of BHPs as a proxy to reconstruct past methane cycle dynamics. Overall, our study presents the most diverse bacteriohopanepolyol profile found thus far in sedimentary deposits beyond 50 million years ago, suggesting excellent preservation. Interestingly, a peak in 35-aminobacteriohopane-30,31,32,33,34-pentol (aminopentol) coincides with the most negative d13Chop value during the PETM. An increase in aminopentol during the PETM was also previously observed, further supporting the utility of aminopentol as a proxy for methane cycling during transient warming events
Refining the global branched glycerol dialkyl glycerol tetraether (brGDGT) soil temperature calibration
Get PDFThis is the author accepted manuscript. The final version is available from Elsevier via the DOI in this record.Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are increasingly used to
reconstruct past terrestrial temperature and soil pH. Here we compare all available
modern soil brGDGT data (n=350) to a wide range of environmental parameters to
obtain new global temperature calibrations.
We show that soil moisture index (MI), a modeled parameter that also takes
potential evapotranspiration into account, is correlated to the 6-methyl brGDGT
distribution but does not significantly control the distribution of 5-methyl brGDGTs.
Instead, temperature remains the primary control on 5-methyl brGDGTs. We propose the following global calibrations: MAAT soil = 40.01 x MBT’5me − 15.25 (n=350, R2 22 = 23 0.60, RMSE = 5.3 °C) and growing degree days above freezing (GDD0 soil) = 14344.3 x MBT’5me - 4997.5 (n=350, R2 24 = 0.63, RMSE = 1779 °C).
Recent studies have suggested that factors other than temperature can impact
arid and/or alkaline soils dominated by 6-methyl brGDGTs. As such, we develop new
global temperature calibrations using samples dominated by 5-methyl brGDGTs only
(IR6me<0.5). These new calibrations have significantly improved correlation
coefficients and lower root mean square errors (RMSE) compared to the global
calibrations: MAATsoil’ = 39.09 x !"#!!"
! − 14.50 (n=177, R2 30 = 0.76, RMSE =
4.1 °C) and GDD0 soil’ = 13498.8 x !"#!!"
! − 4444.5 (n=177, R2 31 = 0.78, RMSE =
1326). We suggest that these new calibrations should be used to reconstruct terrestrial
climate in the geological past; however, care should be taken when employing these
calibrations outside the modern calibration rangThis research was funded through the advanced ERC grant `The greenhouse earth
412 system' (T-GRES, project reference 340923). R.D.P. acknowledges the Royal Society
413 Wolfson Research Merit Award
Biogeochemical controls on microbial diversity in seafloor sulphidic sediments
No full textThe ultimate fate of hydrothermal sulphides on the seafloor depends on the nature and rate of abiotic and microbially catalysed reactions where sulphide minerals are exposed to oxic seawater. This study combines organic and inorganic geochemical with microbiological measurements across a suboxic transition zone of highly altered sulphidic sediments from the Trans-Atlantic Geotransverse hydrothermal field to characterize the reaction products and microbial communities present. There is distinct biogeochemical zonation apparent within the sediment sequence from oxic surface layers through a suboxic transition zone into the sulphide material. The microbial communities in the sediment differ significantly between the biogeochemical horizons sampled, with the identified microbes inferred to be associated with Fe and S redox cycling. In particular, Marinobacter species, organisms associated with circumneutral Fe oxidation, are dominant in a sulphide lens present in the lower core. The dominance of Marinobacter-related sequences within the relict sulphide lens implies that these organisms play an important role in the alteration of sulphides at the seafloor once active venting has ceased. © 2010 Blackwell Publishing Ltd
The role of prokaryotes in supergene alteration of submarine hydrothermal sulfides
No full textWe combine mineralogical, stable isotope and organic biomarker data to understand the role of prokaryote activity in supergene reactions within submarine hydrothermal sulfidic sediments. Data are presented for two adjacent cores from the periphery of the inactive Alvin hydrothermal mound. The limit of oxygenated seawater penetration into the sulfidic sediments is expressed as a sharp peak in solid phase Cu (atacamite and secondary Cu sulfides) associated with supergene alteration of the sulfide pile. Total prokaryote numbers are low throughout the upper few metres of sediment relative to published data for deep-sea sites. However, there is a statistically significant enrichment of prokaryote numbers at the redox front that coincides with abundant Fe oxide filaments and a unique distribution of microbial biomarkers. The dominance of quaternary-branched alkanes in the oxidized transition zone immediately above the redox front (and their absence below) suggests a significant role of the source organisms in iron or sulfide oxidation under the more circumneutral conditions associated with the redox transition zone. The morphology of the Fe oxide filaments preserved within late stage silica and gypsum mineralization is consistent with a biogenic origin of the filaments. Gypsum sulfur isotopes are in equilibrium with fluids that are derived from quantitative sulfide oxidation and gypsum nucleation is inferred to be biologically induced. These new data suggest that supergene alteration of sulfidic sediments generates sharp redox and pH gradients that stimulate prokaryotic activity, in particular iron and sulfide oxidisers, which in turn govern the distribution of secondary mineral phases and the abundance of redox sensitive trace metals
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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