Testing the use of bomb radiocarbon to date the surface layers of blanket peat

The recently formed surface layers of peatlands are archives of past environmental conditions and can have a temporal resolution considerably greater than deeper layers. The low density and conditions of fluctuating water table have hindered attempts to construct chronologies for these peats. We tested the use of the radiocarbon bomb pulse to date recently accumulated peat in a blanket mire. The site was chosen because the peat profiles contained independent chronological markers in the form of charcoal-rich layers produced from known burning events. We compared chronologies derived from accelerator mass spectrometry C-14 analysis of plant macrofossils against these chronological markers. The bomb C-14-derived chronologies were in broad agreement with the charcoal dating evidence. However, there were uncertainties in the final interpretation of the C- 14 results because the pattern of C-14 concentration in the peat profiles did not follow closely the known atmospheric C-14 record. Furthermore, samples of different macrofossil materials from the same depth contained considerable differences in C-14. Suggested explanations for the observed results include the following: i) minor disturbance at the site, ii) in-situ contamination of the C-14 samples by carbonaceous soot, and iii) differential incorporation of plant material during blanket peat growth

Isotope (¹⁴C and ¹³C) analysis of deep peat CO₂ using a passive sampling technique

We developed and tested a new method to collect CO<sub>2</sub> from the surface to deep layers of a peatland for radiocarbon analysis. The method comprises two components: i) a probe equipped with a hydrophobic filter that allows entry of peat gases by diffusion, whilst simultaneously excluding water, and, ii) a cartridge containing zeolite molecular sieve that traps CO<sub>2</sub> passively. We field tested the method by sampling at depths of between 0.25 and 4 m at duplicate sites within a temperate raised peat bog. CO<sub>2</sub> was trapped at a depth-dependent rate of between ∼0.2 and 0.8 ml d<sup>−1</sup>, enabling sufficient CO<sub>2</sub> for routine <sup>14</sup>C analysis to be collected when left in place for several weeks. The age of peatland CO<sub>2</sub> increased with depth from modern to not, vert, similar170 BP for samples collected from 0.25 m, to ∼4000 BP at 4 m. The CO<sub>2</sub> was younger, but followed a similar trend to the age profile of bulk peat previously reported for the site (Langdon and Barber, 2005). δ<sup>13</sup>C values of recovered CO<sub>2</sub> increased with depth. CO<sub>2</sub> collected from the deepest sampling probes was considerably <sup>13</sup>C-enriched (up to not, vert, similar+9‰) and agreed well with results reported for other peatlands where this phenomenon has been attributed to fermentation processes. CO<sub>2</sub> collected from plant-free static chambers at the surface of the mire was slightly <sup>14</sup>C-enriched compared to the contemporary atmosphere, suggesting that surface CO<sub>2</sub> emissions were predominantly derived from carbon fixed during the post-bomb era. However, consistent trends of enriched 13C and depleted <sup>14</sup>C in chamber CO<sub>2</sub> between autumn and winter samples were most likely explained by an increased contribution of deep peat CO<sub>2</sub> to the surface efflux in winter. The passive sampling technique is readily portable, easy to install and operate, causes minimal site disturbance, and can be reliably used to collect peatland CO<sub>2</sub> from a wide range of depths

A passive sampling method for radiocarbon analysis of atmospheric CO₂ using molecular sieve

Radiocarbon (14C) analysis of atmospheric CO2 can provide information on CO2 sources and is potentially valuable for validating inventories of fossil fuel-derived CO2 emissions to the atmosphere. We tested zeolite molecular sieve cartridges, in both field and laboratory experiments, for passively collecting atmospheric CO2. Cartridges were exposed to the free atmosphere in two configurations which controlled CO2 trapping rate, allowing collection of sufficient CO2 in between 1.5 and 10 months at current levels. 14C results for passive samples were within measurement uncertainty of samples collected using a pump-based system, showing that the method collected samples with 14C contents representative of the atmosphere. δ13C analysis confirmed that the cartridges collected representative CO2 samples, however, fractionation during passive trapping means that δ13C values need to be adjusted by an amount which we have quantified. Trapping rate was proportional to atmospheric CO2 concentration, and was not affected by exposure time unless this exceeded a threshold. Passive sampling using molecular sieve cartridges provides an easy and reliable method to collect atmospheric CO2 for 14C analysis

Chronologies for Recent Peat Deposits Using Wiggle-matched Radiocarbon Ages: Problems with Old Carbon Contamination

Dating sediments which have accumulated over the last few hundred years is critical to the calibration of longer-term paleoclimate records with instrumental climate data. We attempted to use wiggle-matched radiocarbon ages to date 2 peat profiles from northern England which have high-resolution records of paleomoisture variability over the last ~300 yr. A total of 65<sup>14</sup>C accelerator mass spectrometry (AMS) measurements were made on 33 macrofossil samples. A number of the age estimates were older than expected and some of the oldest ages occurred in the upper parts of the sequence, which had been dated to the late 19th and early 20th century using other techniques. We suggest that the older <sup>14</sup>C ages are the result of contamination by industrial pollution. Based on counts of spheroidal carbonaceous particles (SCPs), the potential aging effect for SCP carbon was calculated and shown to be appreciable for samples from the early 20th century. Ages corrected for this effect were still too old in some cases, which could be a result of fossil CO<sub>2</sub> fixation, non-SCP particulate carbon, contamination due to imperfect cleaning of samples, or the "reservoir effect" from fixation of fossil carbon emanating from deeper peat layers. Wiggle matches based on the overall shape of the depth-<sup>14</sup>C relationship and the <sup>14</sup>C minima in the calibration curve could still be identified. These were tested against other age estimates (<sup>210</sup>Pb, pollen, and SCPs) to provide new age-depth models for the profiles. New approaches are needed to measure the impact of industrially derived carbon on recent sediment ages to provide more secure chronologies over the last few hundred years

Old carbon contributes to aquatic emissions of carbon dioxide in the Amazon

Knowing the rate at which carbon is cycled is crucial to understanding the dynamics of carbon transfer pathways. Recent technical developments now support measurement of the <sup>14</sup>C age of evaded CO<sub>2</sub> from fluvial systems, which provides an important "fingerprint" of the source of C. Here we report the first direct measurements of the <sup>14</sup>C age of effluxed CO<sub>2</sub> from two small streams and two rivers within the western Amazonian Basin. The rate of degassing and hydrochemical controls on degassing are also considered. We observe that CO<sub>2</sub> efflux from all systems except for the seasonal small stream was <sup>14</sup>C -depleted relative to the contemporary atmosphere, indicating a contribution from "old" carbon fixed before ~ 1955 AD. Further, "old" CO<sub>2</sub> was effluxed from the perennial stream in the rainforest; this was unexpected as here connectivity with the contemporary C cycle is likely greatest. The effluxed gas represents all sources of CO<sub>2</sub> in the aquatic system and thus we used end-member analysis to identify the relative inputs of fossil, modern and intermediately aged C. The most likely solutions indicated a contribution from fossil carbon sources of between 3 and 9% which we interpret as being derived from carbonate weathering. This is significant as the currently observed intensification of weather has the potential to increase the future release of old carbon, which can be subsequently degassed to the atmosphere, and so renders older, slower C cycles faster. Thus <sup>14</sup>C fingerprinting of evaded CO<sub>2</sub> provides understanding which is essential to more accurately model the carbon cycle in the Amazon Basin

Living roots magnify the response of soil organic carbon decomposition to temperature in temperate grassland.

Increasing atmospheric carbon dioxide (CO2) concentration is both a strong driver of primary productivity and widely believed to be the principal cause of recent increases in global temperature. Soils are the largest store of the world's terrestrial C. Consequently, many investigations have attempted to mechanistically understand how microbial mineralisation of soil organic carbon (SOC) to CO2 will be affected by projected increases in temperature. Most have attempted this in the absence of plants as the flux of CO2 from root and rhizomicrobial respiration in intact plant-soil systems confounds interpretation of measurements. We compared the effect of a small increase in temperature on respiration from soils without recent plant C with the effect on intact grass swards. We found that for 48 weeks, before acclimation occurred, an experimental 3 °C increase in sward temperature gave rise to a 50% increase in below ground respiration (ca.0.4 kg C m−2; Q10=3.5), whereas mineralisation of older SOC without plants increased with a Q10 of only 1.7 when subject to increases in ambient soil temperature. Subsequent 14C dating of respired CO2 indicated that the presence of plants in swards more than doubled the effect of warming on the rate of mineralisation of SOC with an estimated mean C age of ca.8 y or older relative to incubated soils without recent plant inputs. These results not only illustrate the formidable complexity of mechanisms controlling C fluxes in soils, but also suggest that the dual biological and physical effects of CO2 on primary productivity and global temperature have the potential to synergistically increase the mineralisation of existing soil C

Should aquatic CO2 evasion be included in contemporary carbon budgets for peatland ecosystems?

Quantifying the sink strength of northern hemisphere peatlands requires measurements or realistic estimates of all major C flux terms. Whilst assessments of the net ecosystem carbon balance (NECB) routinely include annual measurements of net ecosystem exchange and lateral fluxes of dissolved organic carbon (DOC), they rarely include estimates of evasion (degassing) of CO2 and CH4 from the water surface to the atmosphere, despite supersaturation being a consistent feature of peatland streams. Instantaneous gas exchange measurements from temperate UK peatland streams suggest that the CO2 evasion fluxes scaled to the whole catchment are a significant component of the aquatic C flux (23.3±6.9 g C m-2 catchment yr-1) and comparable in magnitude to the downstream DOC flux (29.1±12.9 g C m-2 catchment yr-1). Inclusion of the evasion flux term in the NECB would be justified if evaded CO2 and CH4 were isotopically “young” and derived from a “within-ecosystem” source, such as peat or in-stream processing of DOC. Derivation from “old” biogenic or geogenic sources would indicate a separate origin and age of C fixation, disconnected from the ecosystem accumulation rate that the NECB definition implies. Dual isotope analysis (δ13C and 14C) of evasion CO2 and DOC strongly suggest that the source and age of both are different and that evasion CO2 is largely derived from allochthonous (non-stream) sources. Whilst evasion is an important flux term relative to the other components of the NECB, isotopic data suggest that its source and age are peatland-specific. Evidence suggests that a component of the CO2-C evading from stream surfaces was originally fixed from the atmosphere at a significantly earlier time (pre-AD1955) than modern (post-AD1955) C fixation by photosynthesis

Radiocarbon analysis of methane at the NERC Radiocarbon Facility (East Kilbride)

Methane is the second most important anthropogenically produced greenhouse gas, and radiocarbon (14C) analysis is extremely valuable in identifying its age and source in the environment. At the NERC Radiocarbon Facility (East Kilbride, UK) we have developed expertise in analysis of methane 14C concentration and methodological approaches to field sampling over the past 20 years. This has opened a wide range of applications, which have mainly focused on (1) the age and source of methane emitted by peatlands and organic soils (e.g. to quantify the release of ancient carbon), (2) the source of aquatic emissions of methane, and (3) the age of methane generated by amenity and illegal landfill. Many of these scientifically important applications involve challenging sampling and measurement considerations, which our development program has continually aimed to overcome. Here, we describe our current methods, and recent improvements to aid field collection of samples in remote locations. We present the results of tests which (1) show the effectiveness of our methods to remove contaminants, especially CO2, (2) quantify the 14C background contribution, and (3) demonstrate the reliability of metal gas storage canisters for sample storage

Radiocarbon analysis of methane emitted from the surface of a raised peat bog

We developed a method to determine the radiocarbon (14C) concentration of methane (CH4) emitted from the surface of peatlands. The method involves the collection of ~ 9 L of air from a static gas sampling chamber which is returned to the laboratory in a foil gas bag. Carbon dioxide is completely removed by passing the sample gas firstly through soda lime and then molecular sieve. Sample methane is then combusted to CO2, cryogenically purified and subsequently processed using routine radiocarbon methods. We verified the reliability of the method using laboratory isotope standards, and successfully trialled it at a temperate raised peat bog, where we found that CH4 emitted from the surface dated to 195-1399 years BP. The new method provides both a reliable and portable way to 14C date methane even at the low concentrations typically associated with peatland surface emissions

Predicting climate change impacts on maritime Antarctic soils: A space-for-time substitution study

We report a space-for-time substitution study predicting the impacts of climate change on vegetated maritime Antarctic soils. Analyses of soils from under Deschampsia antarctica sampled from three islands along a 2,200 km climatic gradient indicated that those from sub-Antarctica had higher moisture, organic matter and carbon (C) concentrations, more depleted δ13C values, lower concentrations of the fungal biomarker ergosterol and higher concentrations of bacterial PLFA biomarkers and plant wax n-alkane biomarkers than those from maritime Antarctica. Shallow soils (2 cm depth) were wetter, and had higher concentrations of organic matter, ergosterol and bacterial PLFAs, than deeper soils (4 cm and 8 cm depths). Correlative analyses indicated that factors associated with climate change (increased soil moisture, C and organic matter concentrations, and depleted δ13C contents) are likely to give rise to increases in Gram negative bacteria, and decreases in Gram positive bacteria and fungi, in maritime Antarctic soils. Bomb-14C analyses indicated that sub-Antarctic soils at all depths contained significant amounts of modern 14C (C fixed from the atmosphere post c. 1955), whereas modern 14C was restricted to depths of 2 cm and 4 cm in maritime Antarctica. The oldest C (c. 1,745 years BP) was present in the southernmost soil. The higher nitrogen (N) concentrations and δ15N values recorded in the southernmost soil were attributed to N inputs from bird guano. Based on these analyses, we conclude that 5–8 °C rises in air temperature, together with associated increases in precipitation, are likely to have substantial impacts on maritime Antarctic soils, but that, at the rates of climate warming predicted under moderate greenhouse gas emission scenarios, these impacts are likely to take at least a century to manifest themselves