Claude Largeau

Comparison of quantification methods to measure fire‐derived (black/elemental) carbon in soils and sediments using reference materials from soil, water, sediment and the atmosphere

Black carbon (BC), the product of incomplete combustion of fossil fuels and biomass (called elemental carbon (EC) in atmospheric sciences), was quantified in 12 different materials by 17 laboratories from different disciplines, using seven different methods. The materials were divided into three classes: (1) potentially interfering materials, (2) laboratory-produced BC-rich materials, and (3) BC-containing environmental matrices (from soil, water, sediment, and atmosphere). This is the first comprehensive intercomparison of this type (multimethod, multilab, and multisample), focusing mainly on methods used for soil and sediment BC studies. Results for the potentially interfering materials (which by definition contained no fire-derived organic carbon) highlighted situations where individual methods may overestimate BC concentrations. Results for the BC-rich materials (one soot and two chars) showed that some of the methods identified most of the carbon in all three materials as BC, whereas other methods identified only soot carbon as BC. The different methods also gave widely different BC contents for the environmental matrices. However, these variations could be understood in the light of the findings for the other two groups of materials, i.e., that some methods incorrectly identify non-BC carbon as BC, and that the detection efficiency of each technique varies across the BC continuum. We found that atmospheric BC quantification methods are not ideal for soil and sediment studies as in their methodology these incorporate the definition of BC as light-absorbing material irrespective of its origin, leading to biases when applied to terrestrial and sedimentary materials. This study shows that any attempt to merge data generated via different methods must consider the different, operationally defined analytical windows of the BC continuum detected by each technique, as well as the limitations and potential biases of each technique. A major goal of this ring trial was to provide a basis on which to choose between the different BC quantification methods in soil and sediment studies. In this paper we summarize the advantages and disadvantages of each method. In future studies, we strongly recommend the evaluation of all methods analyzing for BC in soils and sediments against the set of BC reference materials analyzed here.

Botryococcus braunii: a rich source for hydrocarbons and related ether lipids

This paper presents a review on Botryococcus braunii, a cosmopolitan green colonial microalga characterised by a considerable production of lipids, notably hydrocarbons. Strains like wild populations of this alga differ in the type of hydrocarbons they synthesise and accumulate: (1) n-alkadienes and trienes, (2) triterpenoid botryococcenes and methylated squalenes, or (3) a tetraterpenoid, lycopadiene. In addition to hydrocarbons and some classic lipids, these algae produce numerous series of characteristic ether lipids closely related to hydrocarbons. This review covers the algal biodiversity, the chemical structures and biosynthesis of hydrocarbons and ether lipids and the biotechnological studies related to hydrocarbon production.

A reappraisal of kerogen formation

Abstract Comparative microscopical and chemical studies of recognizable entities in kerogen and their extant counterparts suggest a new and simple mechanism of kerogen formation which clarifies the interrelationships between extant biomass, kerogen, and fossil fuels.

A Review of Macromolecular Organic Compounds That Comprise Living Organisms and Their Role in Kerogen, Coal, and Petroleum Formation

It is generally accepted that the formation of kerogens is the result of the so-called depolymerization-recondensation pathway (Tissot and Welte, 1984; Durand, 1980), Thus, naturally occurring macromolecular substances such as polysaccharides and proteins are enzymatically depolymerized to oligo- and monomers, which for the most part are mineralized. However, a small part of them are thought to condense with other substances such as low-molecular-weight lipids in a random way (Fig. 1). During diagenesis, the “geopolymers” thus formed continuously undergo chemical transformations by which they become more and more insoluble and resistant. Kerogens are being formed, which—depending on the nature of the original organic matter contributions—can generate various amounts and sorts of oil under thermal stress.

Evaluation of a protocol for the quantification of black carbon in sediments

Formation of highly condensed black carbon (BC) from vegetation fires and wood fuel combustion presumably transfers otherwise rapidly cycling carbon from the atmosphere-biosphere cycle into a much slower cycling geological form. Recently reported BC fractions of total organic carbon (TOC) in surficial marine sediments span a wide range (2-90%), leaving it presently unclear whether this variation reflects natural processes or is largely due to method differences. In order to elucidate the importance of BC to carbon burial the specificity of applied methods needs to be constrained. Here the operating range and applicability of a commonly used chemothermal oxidation (CTO) method is evaluated using putative BC standards, potentially interfering substances, and natural matrix standards. Test results confirm the applicability of the method to marine sediments. Integrity tests with model substrates suggest applicability to low-carbon soils but only with a lower specificity to seawater particulate matter. The BC content of marine sediment samples in a set of studies employing the CTO method proved to be consistent with associated geochemical information. The radiocarbon content of the BC isolate in an environmental matrix standard was shown to be similar to the radiocarbon signature of pyrogenic polycyclic aromatic hydrocarbons (PAHs), here serving as molecular markers of combustion (fraction modern fM of BC was 0.065 ± 0.014 and of PAHs 0.056 ± 0.020), while being clearly distinct from the radiocarbon content of the bulk TOC (fM = 0.61 ± 0.08). Urgent questions such as the global accumulation rate of black carbon in soils and sediments may prove approachable with the chemothermal oxidation technique of BC quantification.

Sites of accumulation and composition of hydrocarbons in Botryococcus braunii

Abstract Raman spectrometry and electron microscopy show that, in the hydrocarbon-rich alga Botryococcus braunii , hydrocarbons accumulate in two distinct sites; internally in cytoplasmic inclusions and externally in successive outer walls and derived globules. No other classes of lipid are present in noticeable amounts in the cytoplasmic inclusions and in the external globules. The same hydrocarbons are observed in the internal and external pools but with different relative abundances, the shorter hydrocarbons being more abundant in the internal pool. The bulk of B. braunii hydrocarbons ( ca 95%) is located in the external pool. Such an extracellular location allows this species to exhibit both an unusually high hydrocarbon content (15% of dry wt) and a normal level (0.75%) within the cells. The hydrocarbon pattern and location of B. braunii were compared with that of other organisms; a close relation appears between higher plant epidermal cells and this green alga. The trilaminar outer walls of B. braunii , at whose contact external hydrocarbon globules accumulate, contain a sporopollenin-like compound.

Pyrolysis of immature Torbanite and of the resistant biopolymer (PRB A) isolated from extant alga Botryococcus braunii. Mechanism of formation and structure of torbanite

Abstract Immature Torbanite and the resistant biopolymer (PRB A ) isolated from extant B. braunii were previously compared using bulk spectroscopic methods. In the present work, analysis of 400°C pyrolysis products and pyrolysis residues provided further information on their structure and possible relationships. It appears that such polymers are based upon unbranched, saturated, cross-linked hydrocarbon chains up to C 31 . In addition to these bridging structures, a substantial part of the alkyl chains is singly bound, as esters of unbranched, saturated or cis unsaturated, even fatty acids. These esters are sterically protected, against chemical degradations, by the network of the bioand geopolymer. Quantitative and qualitative observations derived from 400°C pyrolysis confirm that the chemical structure of PRB A and immature Torbanite are closely related. The pyrolysis residues show a similar evolution and numerous common features are noted, with respect to the nature and the distribution of the major constituents of the pyrolysates (hydrocarbons and fatty acids). Accordingly, Botryococcus provides what seems to be the first example of a close structural relationship between a biopolymer produced in large amounts by an extant alga and the geopolymer of an immature kerogen. The essential role of PRB A in Torbanite formation is ascertained. Moreover, it is found that the resistant biopolymer does not undergo important structural changes during the first stages of diagenesis. Thus, owing to steric protection, the esters of immature Torbanite show a distribution quite close to the one of PRB A esters, with exclusively even constituents and a large contribution of unsaturated acids. Recent observations pointed to the possible genesis of some algal kerogens principally by selective preservation of resistant macromolecules. Such a type of formation is clearly predominant in Torbanite, where the bulk of the fossil organic matter corresponds to a selectively preserved and weakly altered, resistant biopolymer, while incorporation of lipids into the kerogen structure during diagenesis seems to play a minor role.

Chemical evidence of kerogen formation in source rocks and oil shales via selective preservation of thin resistant outer walls of microalgae: Origin of ultralaminae

Abstract New structures, termed ultralaminae, were recently observed by Transmission Electron Microscopy, usually in high amounts, in a number of kerogens from oil shales and source rocks. Morphological similarities were noted between ultralaminae and the thin (ca. 15 nm) resistant outer walls, composed of non-hydrolyzable macromolecules (algaenans), commonly occurring in extant Chlorophyceae, especially in the cosmopolitan genus Scenedesmus . Identification of the pyrolysis products of S. quadricauda algaenan showed (i) a highly aliphatic structure based on a macromolecular network of long (up to C 32 ) polymethylenic chains probably cross-linked by ether bridges, and (ii) a close correlation based on the formation of n- alkylnitriles , between this algaenan and two ultralaminar kerogens, the Rundle Oil Shale (mainly composed of ultralamina accumulations) and the Green River Shale (ultralaminae dispersed within an amorphous matrix). These fossil ultralaminae, therefore, likely originated from the selective preservation of the thin, algaenan-containing, outer walls of Scenedesmus and/or of other Chlorophyceae containing outer walls of a similar morphology and composition. Relative distributions of n- alkylnitriles and of n- alkanoic acids, in the pyrolysates of S. quadricauda algaenan and of the Rundle Oil Shale, indicated that nitriles are not derived from secondary reactions of carboxylic acids but originate from preexisting nitrogen functions, likely amides. Previous evidence of kerogen formation via selective preservation of algaenans was restricted to rather uncommon kerogens; the present results, added to ultralamina common occurrence and abundance, point to a wide involvement and to a large contribution of the selective preservation of algaenan-containing thin outer walls of Chlorophyceae in the formation of kerogens in a number of lacustrine source rocks and oil shales. All the available information suggest that the three-way correlation (selective preservation of algaenans from thin resistant chlorophycean outer walls, formation of fossil ultralaminae, presence of n- alkylnitriles with a typical distribution in the pyrolysates of ultralaminar kerogens) observed in this study might be of a general character.

A REVIEW OF SOME IMPORTANT FAMILIES OF REFRACTORY MACROMOLECULES: COMPOSITION, ORIGIN, AND FATE IN SOILS AND SEDIMENTS

Extensive knowledge about refractory biomacromolecules has been accumulating for 20 years. The components, characterized by a conspicuous resistance to drastic base and acid laboratory hydrolyses, also exhibit a relatively high resistance to degradation under natural conditions. These refractory biomacromolecules, identified in vascular plants and microalgae, probably play a major role in living organism protection, and they are thought to be important for organic matter sources, composition, and turnover in soils and sediments as well. In addition, some refractory macromolecules are formed in natural environments as the result of condensation/aromatization processes. This review is concerned with six families of refractory macromolecules: lignins, sporopollenins, aliphatic macromolecules (algaenans, cutans, suberans), tannins, black carbon, and proteins. The origin and composition of each family and its contribution to organic matter in soils and sediments are discussed, focusing on recent advances and on questions that are still pending.

Distribution of aliphatic, nonhydrolyzable biopolymers in marine microalgae

The recent discovery of several marine microalgal species which biosynthesize nonhydrolyzable, aliphatic biomacromolecules, termed algaenans, has provided new insights about the nature and origin of the organic matter preserved in marine sediments. This paper reports the examination of 12 new species of marine microalgae from five different classes; the results are discussed together with previous observations on marine algaenans. None of the diatoms and prymnesiophytes and only one of the dinoflagellates examined in the present study produced algaenan, whereas all the eustigmatophytes biosynthesize a very specific algaenan composed of polyether-linked long-chain alkyl units. Many of the chlorophytes investigated, as with their freshwater counterparts, also produce algaenan. The location of the algaenan in the algal cells and the resistance of these materials to chemical and bacterial attack are discussed with respect to their role as a potential sink of organic carbon in aquatic environments.