International audienceGlycerol dialkyl glycerol tetraethers (GDGTs) are membrane-spanning lipids from Bacteria and Archaea that are ubiquitous in a range of natural archives and especially abundant in peat. Previous work demonstrated that the distribution of bacterial branched GDGTs (brGDGTs) in mineral soils is correlated to environmental factors such as mean annual air temperature (MAAT) and soil pH. However, the influence of these parameters on brGDGT distributions in peat is largely unknown. Here we investigate the distribution of brGDGTs in 470 samples from 96 peatlands around the world with a broad mean annual air temperature (−8 to 27 °C) and pH (3–8) range and present the first peat-specific brGDGT-based temperature and pH calibrations. Our results demonstrate that the degree of cyclisation of brGDGTs in peat is positively correlated with pH, pH = 2.49 x CBTpeat + 8.07 (n = 51, R2 = 0.58, RMSE = 0.8) and the degree of methylation of brGDGTs is positively correlated with MAAT, MAATpeat (°C) = 52.18 x MBT5me’ – 23.05 (n = 96, R2 = 0.76, RMSE = 4.7 °C). These peat-specific calibrations are distinct from the available mineral soil calibrations. In light of the error in the temperature calibration (∼ 4.7 °C), we urge caution in any application to reconstruct late Holocene climate variability, where the climatic signals are relatively small, and the duration of excursions could be brief. Instead, these proxies are well-suited to reconstruct large amplitude, longer-term shifts in climate such as deglacial transitions. Indeed, when applied to a peat deposit spanning the late glacial period (∼15.2 kyr), we demonstrate that MAATpeat yields absolute temperatures and relative temperature changes that are consistent with those from other proxies. In addition, the application of MAATpeat to fossil peat (i.e. lignites) has the potential to reconstruct terrestrial climate during the Cenozoic. We conclude that there is clear potential to use brGDGTs in peats and lignites to reconstruct past terrestrial climate

A combination of XPS and solid-state 13C NMR techniques have been used to characterize organic oxygen species and carbon chemical/structural features in peats, pyrolyzed peats, lignites, and other coals. Both the 13C NMR and XPS results show the same ordering for the amount of aromatic carbon, higher ranking coals > lignites > peats. In general the value for H/C decreases as the percent of aromatic carbon increases. For pyrolyzed peats, the H/C level is higher than lignites and other coals of comparable levels of aromatic carbon. This is likely due to significant differences in the carbon structural framework of these materials. A van Krevelen plot, based on elemental H/C data and XPS derived O/C data, shows the well-established pattern for peats, lignites, and other coals. In general, O/C decreases as the percent of aromatic carbon increases, with the expected magnitude ordering, peats > lignites > higher ranking coals. Most of the H/C values of pyrolyzed peats are higher than coals at comparable O/C. A range of O/C levels (0.23−0.13) were produced from pyrolysis of peats; however, these data, when plotted versus the percent aromatic carbon, fall below the values for lignites and other coals. These results indicate that simple pyrolysis does not appear to fully capture the chemical transformations encountered during the natural formation of coals. Both XPS and 13C NMR results are sensitive to the basic difference in the kinds of organic oxygen species found in peats and coals. The advantages of using a combination of XPS and 13C NMR along with the pitfalls of using a single technique for organic oxygen speciation are discussed. For peats, pyrolyzed peats, lignites, and other coals, XPS results for the total amount of organic oxygen fall between upper and lower limit estimates based on 13C NMR derived parameters associated with different oxygen species. For lignites and other coals, there is a sharp drop in the number of carbonyl and carboxyl groups near 60% aromatic carbon. The amount of carbon− oxygen single-bonded species reflected in the NMR parameters falO and faOCH3 and the XPS parameter C−O oxygen, decrease as the percent aromatic carbon increases. The highest levels of phenolic and phenoxy oxygen are found near 60% aromatic carbon. NMR results show that the amount of phenolic and phenoxy carbon (faP) and aliphatic carbon−oxygen single-bonded species (falO) are very similar for pyrolyzed peats, lignites, and other coals at comparable levels of aromatic carbon. These results indicate that thermal decarboxylation/decarbonylation and demethoxylation pathways exist for peat and suggest that similar pathways occur during natural coalification processes.

The chemical pathways for nitrogen and sulfur transformations during coalification are elucidated by comparing the chemical forms of unaltered peats, lignites, and coals and pyrolyzed peats. Nitrogen forms are characterized by a combination of X-ray photoelectron spectroscopy (XPS) and 15 N nuclear magnetic resonance (NMR). In unaltered peats, the 15 N NMR and XPS nitrogen (1s) spectra are consistent with the presence of amide nitrogen. When peat is pyrolyzed, the main peak in the 15 N NMR spectrum broadens and shifts from -260 ppm to -245 ppm, which is consistent with the loss of some amide nitrogen and the appearance of pyrrolic nitrogen forms. The pyrolyzed peat shows a new XPS peak that appears at 398.6 eV, which is characteristic of pyridinic nitrogen. These results indicate that a thermal transformation of amide nitrogen into pyrrolic and pyridinic forms occurs after thermal stress that is roughly equivalent to lignitification. High total nitrogen levels are found in pyrolyzed peats relative to lignites and higher-rank coals, suggesting that some amides initially found in peat are lost via nonthermal pathways during coalification. Lignites contain the highest levels of quaternary nitrogen, and they are associated with protonated pyridinic structures. The relatively low levels of quaternary nitrogen in pyrolyzed peats in the presence of pyridinic nitrogen indicates that there are fewer acidic sites in pyrolyzed peats relative to lignites. Hence, most quaternary nitrogen is formed during lignitification as a result of the creation and interaction of basic nitrogen species with acidic functionalities and is lost completely during bitumenization. Sulfur X-ray absorption near-edge structure spectroscopy (S-XANES) of unaltered peats detect the presence of disulfide, mercapto, aliphatic sulfide, and aromatic forms of organically bound sulfur. The level of organic sulfur in pyrolyzed peats is comparable to that in lignites and higher-rank coals, suggesting that much of the organic sulfur in coals is derived from sulfur species incorporated during peatification. XPS and S-XANES results show that the relative level of aromatic sulfur increases as the severity of peat pyrolysis increases. The relative level of aromatic sulfur increases through the selective loss of disulfide, aliphatic sulfide, and SO 3 groups and through the transformation of aliphatic sulfur forms. Aliphatic sulfur is present mostly as mercapto and disulfide species in peats pyrolyzed to an equivalence vitrinite reflectance of R o ) 0.5 and in lignites but not in higher-rank coals. These results indicate that mercapto and disulfide species are lost after lignitification. Organic sulfur in peats pyrolyzed to R o ) 1.0 exist mainly as aromatic forms, consistent with the level of aromatic sulfur increasing with the increasing degree of coalification.

The purpose of this paper is to illustrate some of the kinds of information about coals that are at present being generated from studies of peat deposits and also to point out some possible new areas of research that might be undertaken in the future. Some notable examples of new ideas about coal seam composition or formation that have been generated from studies of modern peat deposits include: (1) discovery of the probable progenitors of certain coal macerals and elucidation of the processes by which they may have formed; (2) evidence that some types of mineral matter may be dissolved out of peat deposits; (3) verification of the role of marine waters in emplacement of sulphur in peat; (4) discovery of the importance of ‘doming’ in peat deposit development; (5) discovery of a new way to form a split in a coal seam, i.e. development of a ‘fire splay’; (6) elucidation of the mechanisms responsible for producing stratification in coal seams; (7) recognition of the world-wide importance of ‘back-barrier’ coal-forming environments. These kinds of discoveries are important in themselves; however, they can also be shown to have many additional practical applications, especially if woven into models to predict the economic characteristics of coal seams.

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