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Storing carbon in the deep ocean is a key-feedback mechanism that allows astronomical forcing to drive the late Pleistocene glacial/interglacial variations. As carbon storage is intrinsically linked to oxygenation, proxies for sediment oxygenation have been used to quantify changes in carbon storage during the late Pleistocene. However, evidence for astronomically-paced changes in carbon storage beyond the late Pleistocene is limited, hindering our understanding of the stability of this feedback mechanisms. Here we used molecular fossils (biomarkers) in marine sediment cores that span the last ~3.5 million years to assess the long-term impact of astronomical forcing on deep ocean oxygenation, and hence carbon storage, and explore the stability of this deep ocean feedback mechanism. Using high-resolution records from three independent cores from the North Atlantic, we find that the concentration of biomarkers from anaerobic bacteria is eccentricity paced during the middle and late Pleistocene with high abundances during glacials and absence during interglacials. We interpret this data to reflect a decrease in oxygenation and hence increase in carbon storage during the most recent glacials. Across the MPT this pacing changes to obliquity forcing and we show that this forcing is persistent into the late Pliocene, highlighting the stability of this feedback mechanism. However, prior to 2.7 Myr we find no biomarkers of anaerobic bacteria across the North Atlantic, suggesting reduced carbon storage prior to the intensification of the glaciation of the Northern Hemisphere. Our findings indicate that the lowering of atmospheric CO2 by the sequestration of carbon in the deep ocean in response to astronomical forcing persisted throughout the Quaternary and was essential for the development of Plio/Pleistocene ice ages, but this feedback mechanisms did not persist into the warm Pliocene.

A multiproxy Holocene record from a bog in the Hudson Bay Lowlands, northern Ontario, Canada, was used to evaluate how ecohydrology relates to carbon accumulation. The study site is located at a somewhat higher elevation and on coarser grained deposits than the surrounding peatlands. This promotes better drainage and thus a slower rate of carbon accumulation relative to sites with similar initiation age. The rate of peat vertical accretion was initially low as the site transitioned from a marsh to a rich fen. These lower rates took place during the warmer temperatures of the Holocene thermal maximum, confirming the importance of hydrological controls limiting peat accretion at the local scale. Testate amoebae, pollen, and plant macrofossils indicate a transition to a poor fen and then a bog during the late Holocene, as the carbon accumulation rate and reconstructed water table depth increased. The bacterial membrane lipid biomarker indices used to infer paleotemperature show a summer temperature bias and appear sensitive to changes in peat type. The bacterial membrane lipid biomarker pH proxy indicates a rich to a poor fen and a subsequent fen to bog transition, which are supported by pollen, macrofossil, and testate amoeba records.

Bacteria play key roles in the carbon cycle. In many sediments and peatlands, methanotrophic bacteria consume a portion of released methane, reducing the emissions of this potent greenhouse gas. In marine oxygen minimum zones (OMZ) and other anoxic settings, anaerobic ammonium oxidizing (anammox) bacteria remove bioavailable nitrogen while performing chemoautotrophic carbon fixation. Methanotrophic and anammox bacteria synthesize a wide number of complex bacteriohopanepolyols (BHPs), comprising notably several stereoisomers of bacteriohopanetetrols (BHT), which are used as biomarker lipids. While BHT-17β(H), 21β(H), 22R, 32R, 33R, 34S (BHT-34S) is ubiquitous in the environment, its 34R stereoisomer (BHT-17β(H), 21β(H), 22R, 32R, 33R, 34R; BHT-34R) has only five known producers: the freshwater anammox genera ‘Candidatus Brocadia’, the aerobic acidic peatland methanotroph Methylocella palustris, the nitrogen-fixing aerobic bacteria Frankia spp., and the aerobic acetic acid-producing bacteria Acetobacter pasteurianus and Komagataeibacter xylinus. BHT-x—another BHT isomer of unknown stereochemistry—has only one known producer, the marine anammox bacteria ‘Candidatus Scalindua’ (Schwartz-Narbonne et al., 2020). The occurrence and extent of these different carbon cycle processes can be assessed by measuring the concentrations of these BHT stereoisomers and changes in their δ13C values (Hemingway et al., 2018; Lengger et al., 2019).However, the 13C fractionation associated with the different carbon assimilation pathways of these bacteria has been minimally assessed, resulting in poorly constrained ranges in δ13C values and difficulty in interpreting isotope results.We used a gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) method to measure the δ13C of BHT-34S, BHT34R, and BHT-x of cultured bacteria (‘Ca. Scalindua’, ‘Ca. Brocadia’, Methylocella tundrae, Frankia spp., and Komagataeibacter xylinus). These δ13C values were combined with bulk isotopic measurements of the bacterial biomass and δ13C analyses of the bacterial growth substrates to establish carbon isotopic fractionation from substrate to biomass to BHT lipid. We demonstrated that bacteria using different metabolic pathways produced distinct fractionation factors between substrate and BHTs, which potentially allows for distinguishing BHT-34R produced by ‘Ca. Brocadia’ and methanotrophs from other freshwater producers (e.g. in peatlands). Measurement of BHT-specific fractionation factors allowed us to better constrain the contribution of anammox bacteria to fixed carbon in OMZ. This work expands the application of BHT isomers to isotopically identify carbon cycle processes. ReferencesHemingway, Jordon D., et al. "A novel method to measure the 13C composition of intact bacteriohopanepolyols." Organic Geochemistry 123 (2018): 144-147.Lengger, Sabine K., et al. "Dark carbon fixation in the Arabian Sea oxygen minimum zone contributes to sedimentary organic carbon (SOM)." Global Biogeochemical Cycles 33.12 (2019): 1715-1732.Schwartz-Narbonne, Rachel, et al. "A unique bacteriohopanetetrol stereoisomer of marine anammox." Organic Geochemistry (2020): 103994.

Summary Here, we explore the use of bacterial and archaeal lipid carbon and hydrogen isotopic compositions to explore carbon cycling processes in modern settings and in response to ancient warming events. Investigations of Holocene peat and Eocene lignites reveal both stability over long time-scales but rapid, transient responses to climate perturbations.

Peatlands are impacted by climate and land-use changes, with feedback to warming by acting as either sources or sinks of carbon. Expert elicitation combined with literature review reveals key drivers of change that alter peatland carbon dynamics, with implications for improving models. The carbon balance of peatlands is predicted to shift from a sink to a source this century. However, peatland ecosystems are still omitted from the main Earth system models that are used for future climate change projections, and they are not considered in integrated assessment models that are used in impact and mitigation studies. By using evidence synthesized from the literature and an expert elicitation, we define and quantify the leading drivers of change that have impacted peatland carbon stocks during the Holocene and predict their effect during this century and in the far future. We also identify uncertainties and knowledge gaps in the scientific community and provide insight towards better integration of peatlands into modelling frameworks. Given the importance of the contribution by peatlands to the global carbon cycle, this study shows that peatland science is a critical research area and that we still have a long way to go to fully understand the peatland-carbon-climate nexus.

Summary Elementar UK Ltd present present two pioneering new techniques for the analysis of larger and/or more polar organic molecules of biogeochemical interest which are traditionally considered unamenable for gas chromatography stable isotope ratio mass spectrometry (GC-IRMS) analysis, which have been developed by and in collaboratoration with Elementar GC-IRMS users from a variety of laboratories. This includes a rapid one-step derivatization procedure for the isotope analysis of the non-exchangeable hydrogen in mono and disaccharides and subsequent conversion to H2 by chromium reduction ( Abrahim et al, 2020 ), and the development of a high-temperature GC-IRMS (HTGC-IRMS) methodology employed for the analysis of a suite of compounds of interest ( Lengger et al., 2021 ).

Summary Bacteria play key roles in the carbon cycle. In many sediments and peatlands, methanotrophic bacteria consume a portion of released methane, reducing the emissions of this potent greenhouse gas. In marine oxygen minimum zones (OMZ) and other anoxic settings, anaerobic ammonium oxidizing (anammox) bacteria remove bioavailable nitrogen while performing chemoautotrophic carbon fixation. Methanotrophic and anammox bacteria synthesize a wide number of complex bacteriohopanepolyols (BHPs), comprising notably several stereoisomers of bacteriohopanetetrols (BHTs), which are used as biomarker lipids. We used a gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) method to measure the δ13C of BHTs of cultured bacteria. These δ13C values were combined with bulk isotopic measurements of the bacterial biomass and δ13C analyses of the bacterial growth substrates to establish carbon isotopic fractionation from substrate to biomass to BHT lipid. We demonstrated that bacteria using different metabolic pathways produced distinct fractionation factors between substrate and BHTs, which potentially allows for distinguishing BHTs produced by ‘Ca. Brocadia’ and methanotrophs from other freshwater producers (e.g. in peatlands). Measurement of BHT-specific fractionation factors allowed us to better constrain the contribution of anammox bacteria to fixed carbon in OMZ. This work expands the application of BHT isomers to isotopically identify carbon cycle processes.

Rationale: The hydrogen isotopic composition of lipids (δ 2Hlipid) is widely used in foodscience and as a proxy for past hydrological conditions. Determining the δ 2H values of large,well-preserved triacylglycerides and other uniquely microbial lipids, such as glycerol dialkylglycerol tetraether (GDGT) lipids, is thus of widespread interest but has so far not been possible due to their size which prohibits analysis by traditional gas chromatographypyrolysis isotope ratio mass spectrometry (GC-P-IRMS).Methods: We determined the δ 2H values of large, polar molecules and applied hightemperature gas chromatography (GC) methods on a modified GC-P-IRMS system. Themethods were validated using authentic standards of large, functionalised molecules(triacylglycerides, TAG), purified reference standards of GDGTs, and compared to δ 2Hvalues determined by elemental analyser pyrolysis isotope ratio mass spectrometry (EA-PIRMS); and subsequently applied to the analysis of GDGTs in a sample from a methaneseep and a Welsh peat.Results: δ 2H values of TAGs agreed within error between different between GC-P-IRMS andEA-P-IRMS, with GC-P-IRMS showing 3-5 ‰ precision for 10 ng H injected. Archaeal lipidGDGTs with up to three cyclisations could be analysed: δ 2H values were not significantlydifferent between methods with standard deviations of 5 to 6 ‰. When environmentalsamples were analysed, δ 38 2H values of isoGDGTs were 50 ‰ more negative than those ofterrestrial brGDGTs.Conclusions: Our results indicate that the high temperature GC-P-IRMS (HTGC-P-IRMS)method developed here is appropriate to determine the δ 2H values of TAGs, GDGT lipidswith up to two cyclisations, and potentially other high molecular weight compounds. Themethodology will widen the current analytical window for biomarker and alimentary lightstable isotope analyses. Moreover, our initial measurements suggest that bacterial andarchaeal GDGT δ 2H values can record environmental and ecological conditions

The hydrogen isotopic composition of lipids (δWe determined the δThe δOur results indicate that the HTGC/P/IRMS method developed here is appropriate to determine the δ

Molecular fossils (biomarkers) are abundant in organic rich natural archives such as peats and lignites (fossilized peat), where their distribution is governed by their biological source, environmental factors, such as temperature and pH, and diagenetic reactions. As a result, biomarkers in peat have become an important toolto study past variations in vegetation, environment and climate in terrestrial settings, as well as biogeochemistry on time-scales of hundreds to millions of years ago. In recent years, significant progress has been made in understanding the controls on biomarker distributions, especially those derived from microorganisms and peat- forming plants, allowing for example, the quantification of past temperature and vegetation history during peat formation. Herein, we provide a review of a range of commonly applied biomarker proxies in peats, discuss the latest proxy developments, and explore the potential of using biomarkers in peat and lignite as paleoenvironmental proxies. We provide a framework for biomarker analyses in peat and identify possible future research directions.

In response to rising CO2 concentrations and increasing global sea surface temperatures, oxygen minimum zones (OMZ), or “dead zones”, are expected to expand. OMZs are fueled by high primary productivity, resulting in enhanced biological oxygen demand at depth, subsequent oxygen depletion, and attenuation of remineralization. This results in the deposition of organic carbon‐rich sediments. Carbon drawdown is estimated by biogeochemical models; however, a major process is ignored: carbon fixation in the mid‐ and lower water column. Here, we show that chemoautotrophic carbon fixation is important in the Arabian Sea OMZ; and manifests in a 13C‐depleted signature of sedimentary organic carbon. We determined the δ13C values of Corg deposited in close spatial proximity but over a steep bottom‐water oxygen gradient, and the δ13C composition of biomarkers of chemoautotrophic bacteria capable of anaerobic ammonia oxidation (anammox). Isotope mixing models show that detritus from anammox bacteria or other chemoautotrophs likely forms a substantial part of the organic matter deposited within the Arabian Sea OMZ (~17%), implying that the contribution of chemoautotrophs to settling organic matter is exported to the sediment. This has implications for the evaluation of past, and future, OMZs: biogeochemical models that operate on the assumption that all sinking organic matter is photosynthetically derived, without new addition of carbon, could significantly underestimate the extent of remineralization. Oxygen demand in oxygen minimum zones could thus be higher than projections suggest, leading to a more intense expansion of OMZs than expected.

In the version of this Analysis originally published, the following affiliation for A. Lohila was missing: ‘Finnish Meteorological Institute, Climate System Research, Helsinki, Finland’. This affiliation has now been added, and subsequent affiliations renumbered accordingly, in the online versions of the Analysis.

Glycerol 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 degrees 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, R-2 = 0.58, RMSE = 0.8) and the degree of methylation of brGDGTs is positively correlated with MAAT, MAAT(peat) (degrees C) = 52.18 x MBT'(5me) - 23.05 (n = 96, R-2 = 0.76, RMSE = 4.7 degrees C). These peat-specific calibrations are distinct from the available mineral soil calibrations. In light of the error in the temperature calibration (similar to 4.7 degrees 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 (similar to 15.2 kyr), we demonstrate that MAAT(peat) yields absolute temperatures and relative temperature changes that are consistent with those from other proxies. In addition, the application of MAAT(peat) 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. (C) 2017 The Authors. Published by Elsevier Ltd.