Archaeal and bacterial tetraether lipids in tropical ponds with contrasted salinity (Guadeloupe, French West Indies): Implications for tetraether-based environmental proxies

International audienceThe occurrence and distribution of archaeal and bacterial glycerol dialkyl glycerol tetraether lipids (GDGTs) in continental saline environments have been rarely investigated. Here, the abundance and distribution of archaeal isoprenoid GDGTs (iGDGTs) and archaeol, and of bacterial branched GDGTs (brGDGTs) in four tropical water ponds of contrasting salinity in two islands from the French Western Indies, Grande-Terre and La Désirade, have been determined. The sedimentary distribution of the GDGTs strongly differed between the two islands. Caldarchaeol was largely predominant among iGDGTs in the (hyper)saline ponds from Grande-Terre, suggesting a substantial contribution of iGDGTs derived from methanogenic Archaea. In contrast, both caldarchaeol and crenarchaeol were present in high relative abundance in the low salinity ponds from La Désirade, suggesting that iGDGTs were derived from mixed archaeal communities. In addition, the relative proportion of the most methylated brGDGTs was much higher in Grande-Terre ponds than in La Désirade ponds. The applicability of different proxies based on GDGTs and archaeol was tested for these specific environments. The relative abundance of archaeol vs. caldarchaeol (ACE index) was comparable for the four ponds, independent of salinity, showing that the ACE might not necessarily track salinity change. Moreover, the relative proportion of caldarchaeol vs. total iGDGTs was unexpectedly observed to increase with salinity, suggesting production of this compound by halophilic Archaea. The supposed high abundance of methanogenic Archaea in Grande-Terre ponds prevented the application of TEX86 as a temperature proxy, whereas the TEX86 could be successfully used for local temperature reconstruction in La Désirade ponds. BrGDGTs seem to be produced predominantly in situ (water column and/or sediment) in hypersaline ponds from Grande-Terre, but in La Désirade ponds likely result from a mixture of soil and aquatic sources. In Grande-Terre ponds, brGDGT-derived temperature estimates generated using either soil or lacustrine calibrations were much lower than expected. The mismatch between expected and estimated temperature might be explained by the presence of halophilic microbial communities producing specific brGDGT distributions in the saline ponds from Grande-Terre. The study shows that the sources of brGDGTs, iGDGTs and archaeol (i) may strongly differ in aquatic environments of varying salinity, even at a regional scale, and (ii) have to be constrained before tetraether-derived proxies in such settings can be confidently applied

Protection of organic matter by mineral matrix in a Cenomanian black shale

L'UMR CNRS 6531, Laboratoire de Géologie de la Matière Organique a été intégré dans l'ISTO - UMR6113 - CNRS-Université d'OrléansThree types of pathways (degradation-recondensation, natural sulphurization and selective preservation) are commonly considered for the formation of kerogen dispersed in sedimentary rocks. A fourth pathway has been recently put forward, however, from studies on Recent marine sediments, the so-called sorptive protection mechanism. This pathway is based on the adsorption of otherwise labile organic compounds onto minerals, thus preventing their diagenetic degradation and promoting their subsequent condensation into kerogen. The main results of the present study are derived from a combination of microscopic and pyrolytic methods applied on a Cenomanian kerogen. They provide (i) evidence, on an ancient material, for a crucial role of the mineral matrix both in organic matter (OM) preservation during kerogen formation and in kerogen stability once formed, (ii) indications that the dominant protective process likely involves physical protection by minerals, resulting from alternation of organic and clay nanolayers of approximately 100 nm in thickness, rather than OM adsorption as molecular monolayers and (iii) observations of the relatively poor stability of an isolated kerogen, contrary to the inertness commonly assumed for fossil macromolecular organic matter

Effects of a short-term experimental microclimate warming on the abundance and distribution of branched GDGTs in a French peatland

International audienceBranched glycerol dialkyl glycerol tetraethers (GDGTs) are complex lipids of high molecular weight, recently discovered in soils and increasingly used as palaeoclimate proxies. Their degree of methylation, expressed in the MBT, was shown to depend on mean annual air temperature (MAAT) and to a lesser extent on soil pH, whereas the relative abundance of cyclopentyl rings of branched GDGTs, expressed in the CBT, was related to soil pH. To date, only a few studies were interested in the application of the MBT and CBT proxies in peatlands. In order to validate the applicability of branched GDGTs as temperature proxies in these environments, it is essential to investigate the effect of temperature on branched GDGT-producing bacteria and especially on the speed of adaptation of these microorganisms to temperature changes. The aim of this work was to study the effects of in situ experimental climate warming on the abundance and distribution of branched GDGTs in a Sphagnum-dominated peatland (Jura Mountains, France). Branched GDGTs either present as core lipids (CLs; presumed of fossil origin) or derived from intact polar lipids (IPLs, markers for living cells) were analysed. Air temperature was experimentally increased using a passive warming system consisting of open mini-greenhouses (Open-Top Chamber - OTC). The effect of the OTCs was especially apparent in spring and summer, with (i) an increase in maximal air temperature of ca. 3°C during these two seasons and (ii) an increase in average air temperature of ca. 1°C in summer. Despite the short duration of the climate experiment (26 months), branched GDGT distribution was significantly affected by this temperature rise, with higher MBT values in the OTCs than in the control plots, supporting the empirical relationship between MBT and MAAT established from a large range of soils. The difference in branched GDGT-derived temperatures between control and OTC plots (2-3 °C) was in the same range as the increase in maximal (daytime) temperature induced by the OTCs in spring and summer, suggesting that branched GDGT-producing bacteria might be more active during the warmest months of the year. The OTC treatment had no significant effect on the abundance of branched GDGTs, which were mainly present as "fossil" CLs (70 to 85% of the total branched GDGT pool). Furthermore, no significant differences in branched GDGT distribution were observed between the CLs and IPLs, which both provided higher MBT and MAAT values for the OTCs. This suggests that the fossil pool of branched GDGTs has a very fast turnover (less than the 2 year duration of the experiment) at the peat surface and that branched GDGT distribution may rapidly reflect changes in environmental conditions (at least air temperature) occurring in peat environments

Sulfur sedimentary processes and fossilization

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Occurrence and distribution of glycerol dialkyl glycerol tetraethers in a French peat bog

International audienceThe present study was aimed at examining the distribution and abundance of glycerol dialkyl glycerol tetraethers (GDGTs) of archaeal and bacterial origin in peat samples from surface and deep (ca. 50 cm) horizons of a peat bog in the Jura Mountains (northeastern France). Two principal types of GDGTs are present: extractable GDGTs, recoverable by solvent extraction, and non-extractable GDGTs, linked to the soil matrix. Within the extractable pool, “free” (i.e. core lipids) and “bound” (i.e. intact polar and/or ester-bound lipids) GDGTs can be distinguished. Extractable “free” and “bound” GDGTs were extracted using both accelerated solvent extraction (ASE) and a modified Bligh and Dyer technique. Both methods were shown to allow adequate extraction of “free” archaeal and bacterial GDGTs from soil samples. Both extraction protocols afforded similar relative distributions of archaeal and bacterial GDGTs, although poorer extraction of “bound” GDGTs was observed for ASE relative to Bligh and Dyer. Even though only low amounts of bacterial GDGTs were released after acid hydrolysis of solvent-extracted samples, non-extractable and total extractable GDGTs showed different distribution patterns in some samples. Consequently, these two lipid pools potentially reflect different proxy records of mean annual air temperature (MAAT) and pH. Last, the distribution of bacterial GDGTs differed between the different samples. Samples from deep horizons gave lower GDGT-derived MAAT values than those from surficial horizons, in agreement with measured soil temperatures at 7 cm and 50 cm depths from April to September. MAAT estimates more closely resemble spring and summer temperatures rather than annual soil temperature. The variability in bacterial GDGT distribution and resulting MAAT estimates probably also reflects the heterogeneity of peat samples and the variation in several environmental factors such as peat moisture level and oxygen availability

Electron Paramagnetic Resonance Study of a Photosynthetic Microbial Mat and Comparison with Archean Cherts

International audienceOrganic radicals in artificially carbonized biomass dominated by oxygenic and non-oxygenic photosynthetic bacteria, Microcoleus chthonoplastes-like and Chloroflexus-like bacteria respectively, were studied by Electron Paramagnetic Resonance (EPR) spectroscopy. The two bacteria species were sampled in mats from a hypersaline lake. They underwent accelerated ageing by cumulative thermal treatments to induce progressive carbonization of the biological material, mimicking the natural maturation of carbonaceous material of Archean age. For thermal treatments at temperatures higher than 620 °C, a drastic increase in the EPR linewidth is observed in the carbonaceous matter from oxygenic photosynthetic bacteria and not anoxygenic photosynthetic bacteria. This selective EPR linewidth broadening reflects the presence of a catalytic element inducing formation of radical aggregates, without affecting the molecular structure or the microstructure of the organic matter, as shown by Raman spectroscopy and Transmission Electron Microscopy. For comparison, we carried out an EPR study of organic radicals in silicified carbonaceous rocks (cherts) from various localities, of different ages (0.42 to 3.5 Gyr) and having undergone various degrees of metamorphism, i.e. various degrees of natural carbonization. EPR linewidth dispersion for the most primitive samples was quite significant, pointing to a selective dipolar broadening similar to that observed for carbonized bacteria. This surprising result merits further evaluation in the light of its potential use as a marker of past bacterial metabolisms, in particular oxygenic photosynthesis, in Archean cherts

Origin of compositional differences in organic matter abundance and oil potential of cherty and clayey Cenomanian black levels in the Umbria-Marche basin (Italy).

International audienceRock-Eval pyrolysis of a large set of Cenomanian samples, collected from the black levels (clayey, cherty and mixed) in three sections of the Umbria-Marche basin, showed large differences in organic matter (OM) quantity and quality. The chert samples systematically exhibit much lower TOC contents, markedly lower HI and higher OI. This reflects the extensive oxidative destruction of the initial kerogen that occurred upon the chertification of some clayey sediments. A comparative study, by a combination of microscopic, spectroscopic and pyrolytic methods, was performed on kerogens of the chert and clay layers of a representative mixed level. The various fractions of the initial kerogen underwent differential destruction or alteration during chertification, resulting in (i) relative enrichments of microfossils and woody debris although lignin was altered by demethoxylation and (ii) extensive destruction of the amorphous fraction while it remained predominant. The amorphous fraction retained in the chert kerogen showed large changes in composition related to oxygen incorporation and probably escaped complete destruction owing to oxidative reticulation. The above features account for the pronounced systematic differences in OM abundance and oil potential between the chert and clay layers in the black levels

Organic matter sources and early diagenetic degradation in a tropical peaty marsh (Tritrivakely, Madagascar). Implications for environmental reconstruction during the Sub-Atlantic

Peat samples from a one metre core and living Cyperaceae, collected in Tritrivakely marsh in Madagascar, were studied to determine the organic matter (OM) composition and extent of OM degradation in this core. The study was carried out combining light microscopy observations, bulk analyses, infra-red spectroscopy, hydrolyses of sugars, oxidation of lignin and pyrolyses. In the surface peat, organic matter derived from Cyperaceae undergoes extensive degradation of its basic cell wall components, morphologically revealed by destructuration of plant tissues and their transformation into reddish amorphous organic matter occurring in large amounts all along the core. Two ratios (cinnamic units/lignin and xylose+arabinose/total sugars) were determined as markers of Cyperaceae. It appeared that the vegetation of the marsh remained probably unchanged during the considered accumulation period, i.e. the last 2300 years B.P. Rhamnose, mannose and non-cellulosic glucose probably have a common origin and are mostly derived from bacteria. In contrast, galactose is likely to be a marker of algal source, especially of the diatoms that occur only in the upper part of the core (0-ca. 50 cm). Acid/aldehyde ratios of syringic and vanillic monomers (index of lignin oxidative depolymerisation) and mannose+rhamnose+non-cellulosic glucose/total sugars ratios (reflecting bacterial degradation of hemicelluloses) are positively correlated, and can thus be considered as markers of microbial degradation of the Cyperaceae tissues. The n-alkane/n-alk-1-ene doublets that dominate the pyrolysates of hydrolysed peat samples reflect the contribution of B. braunii algaenan and higher plant suberans, and of condensed lipids mostly derived from higher plants and microalgae. The upper part of the core is characterised by a greater dilution of Cyperaceae-derived compounds by organic matter from microalgae when compared with deeper samples, as recorded by peat bulk features, hydrolysable sugars, lignin oxidation products and pyrolysis products. Two accumulation periods can thus be distinguished in the core: a peaty phase between 2300 years B.P. and ca. 1500 years B.P. (low watertable and strongly limited microalgal growth); a waterlogged marsh, from ca. 1500 years B.P. to the present time, in which a higher water table was longer lasting with a substantial algal production. The environmental variation thus recorded could correspond to a regional climatic change occurring around 1500 years B.P