AMS Radiocarbon analysis of Greenhouse gases: Method development and application

The northern circumpolar permafrost regions are warming faster compared to the global mean. As a result, the duration of annual thaw seasons is increasing, so that the permafrost is subjected to increasingly deep thaw. Within the permafrost zones large amounts of carbon were buried over thousands of years and previously freeze locked due to the cold climate. These carbon sinks are affected by the consequences of global warming and become available for microbial degradation, and thereby potentially turning from carbon sinks into carbon pools. As a result of the microbial degradation of organic matter, the greenhouse gases carbon dioxide and methane are released into the atmosphere, amplifying the global warming, and forcing a positive climate feedback. Among the permafrost deposits, Pleistocene sediments, termed Yedoma, are especially vulnerable to climate change induced rapid thaw and subject to thermoerosion because of their high ice-content. While chemical characterizations of permafrost organic matter and laboratory incubation experiments give information about the degradability of Yedoma organic matter, actual field-studies documenting the extent and exact sources of greenhouse gas release are limited. The analysis of carbon isotopes (13C, 14C) can be used to estimate the age of the permafrost organic matter that is being degraded into greenhouse gases and trace their sources. This is done by collecting carbon dioxide during either field expeditions or during analogue laboratory incubation experiments. Samples are handled in laboratory vacuum rigs, during which the amount of carbon dioxide is quantified and purified from other gases that have been collected along the carbon dioxide. While these methods are overall established and reliable, the analyses are, however, sensitive towards contamination from exogenous carbon sources. The sensitivity towards contamination is amplified towards increasingly smaller sample sizes, down to a few micrograms of carbon. Thus, assessments are necessary to quantify how much contamination is introduced during the preparation of carbon dioxide samples for isotopic analysis and to determine what is the smallest sample size that still delivers meaningful results. The aims of this thesis were to evaluate the laboratory methods applied for the isotopic analysis of carbon dioxide and apply these methods and sampling devices to a field study investigating the degradability of freshly thawed Yedoma organic matter. The evaluation of our existing laboratory methods to handle carbon dioxide demonstrated that routine measurements can safely be performed on samples as small as 20 μg C. Samples as small as 2.5 μg C can also be analyzed, however the associated uncertainty is very large and prevents the analysis of 14C-depleted samples. Further, new methods for the isotopic analysis of methane via accelerator mass spectrometry were established. This required the development of a workflow and pre-treatment routine to convert methane samples to carbon dioxide. After establishing a satisfying conversion rate, first test runs of different standards and gas mixtures further allowed the identification and quantification of sources of exogenous carbon, potentially contaminating samples. During the application study, carbon dioxide emissions were collected along with Yedoma sediment samples from an active retrogressive thaw slump in the Lena River Delta in Siberia. The sediment samples were incubated in the laboratory for a 1.5-year aerobic incubation experiment, during which the amount of produced carbon dioxide was closely monitored and sampled at fixed intervals. The carbon isotopy of the carbon dioxide sampled during the field campaign was compared with the carbon dioxide produced during the incubation experiment and a mass balance approach was applied to identify carbon sources based on isotopic values. The results show that labile organic matter pools are being degraded first and that their contribution to carbon dioxide emissions is decreasing after the initial thaw. Mostly ancient organic matter is being degraded at sites where Pleistocene-aged Yedoma is exposed, the admixture of Pleistocene Yedoma sediments with younger Holocene sediments through erosion does not favor the degradation of ancient organic matter through positive priming. Instead, younger substrates are degraded preferentially, if available. Most surprisingly, the results of the mass balance approach suggest, that a significant proportion of the carbon dioxide, that was released during the field measurement as well as during the laboratory incubation, was derived from inorganic carbon. The release of carbon dioxide from inorganic carbon is likely favored by low pH values and organic acids contained in the thaw slump sediments. The results of the carbon isotope analysis further suggest that a fraction of the carbon dioxide derived from secondary carbonates. Therefore, it is hypothesized that these secondary carbonates precipitated from microbially respired carbon dioxide and thus would not contribute to a net positive carbon emission budget

Preparation and Handling of Methane for Radiocarbon Analysis at Cologneams

CH₄ is the second most important anthropogenic greenhouse gas and originates from different sources. The use of radiocarbon (¹⁴C) analysis of CH₄ opens up the possibility to differentiate geological and agricultural origin. At the CologneAMS facility, the demand for ¹⁴C analysis of CH₄ required the development of a sample handling routine and a vacuum system that converts CH₄ to CO₂ for direct injection of CO₂ into the AMS. We evaluated the processing of CH₄ using several series of gas mixtures of ¹⁴C-free and modern standards as well as biogas with sample sizes ranging from 10 to 50 µg C. The results revealed a CH₄ to CO₂ conversion efficiency of 94–97% and blank values comparable to blank values achieved with our routinely used vacuum system for processing CO₂ samples. The tests with a near modern CH₄:CO₂ biogas mixture gave reproducible results with a near modern ¹⁴C content of 0.967–1.000 F¹⁴C, after applying the background correction.ISSN:0033-822

EXPLORING SAMPLE SIZE LIMITS OF AMS GAS ION SOURCE C-14 ANALYSIS AT COLOGNEAMS

Increasing demands for small-scale radiocarbon (C-14) analyses required the installation of a SO-110 B type ion source (HVE Europa B.V.) at our 6 MV Tandetron AMS (HVE) dedicated for the direct injection of CO2 using either the gas injection system (GIS) from Ionplus AG or a EuroVector EA 3000 elemental analyzer (EA). We tested both systems with multiple series of C-14-free and modern standards (2.5-50 mu g C) combusted in quartz ampoules or EA containers and were able to quantify exogenous C introduced. In EA-GIS-AMS analysis exogenous C is mainly derived from the EA sample containers. Blank values for 50 mu g C combusted in solvent-cleaned tin (Sn) vessels were 0.0127 +/- 0.0012 (FC)-C-14 (boats) and 0.0090 +/- 0.0010 (FC)-C-14 (capsules), while they were much higher for thermally cleaned silver (Ag) capsules. The processing of gas samples for GIS-AMS yields similar blank values corresponding to 0.30 +/- 0.08 mu g exogenous C with 0.93 +/- 0.23 (FC)-C-14 consisting of 0.28 mu g C modern and 0.02 mu g C fossil C. The combustion of larger amounts of blank material (1 mg C) in a single quartz tube split into aliquots gives lower blanks (0.0064 +/- 0.0008 (FC)-C-14; 50 mu g C). Thus, C-14 analysis of small, gaseous samples is now possible at CologneAMS

Sources of CO2 Produced in Freshly Thawed Pleistocene-Age Yedoma Permafrost

The release of greenhouse gases from the large organic carbon stock in permafrost deposits in the circumarctic regions may accelerate global warming upon thaw. The extent of this positive climate feedback is thought to be largely controlled by the microbial degradability of the organic matter preserved in these sediments. In addition, weathering and oxidation processes may release inorganic carbon preserved in permafrost sediments as CO2, which is generally not accounted for. We used C-13 and C-14 analysis and isotopic mass balances to differentiate and quantify organic and inorganic carbon released as CO2 in the field from an active retrogressive thaw slump of Pleistocene-age Yedoma and during a 1.5-years incubation experiment. The results reveal that the dominant source of the CO2 released from freshly thawed Yedoma exposed as thaw mound is Pleistocene-age organic matter (48-80%) and to a lesser extent modern organic substrate (3-34%). A significant portion of the CO2 originated from inorganic carbon in the Yedoma (17-26%). The mixing of young, active layer material with Yedoma at a site on the slump floor led to the preferential mineralization of this young organic carbon source. Admixtures of younger organic substrates in the Yedoma thaw mound were small and thus rapidly consumed as shown by lower contributions to the CO2 produced during few weeks of aerobic incubation at 4 degrees C corresponding to approximately one thaw season. Future CO2 fluxes from the freshly thawed Yedoma will contain higher proportions of ancient inorganic (22%) and organic carbon (61-78%) as suggested by the results at the end, after 1.5 years of incubation. The increasing contribution of inorganic carbon during the incubation is favored by the accumulation of organic acids from microbial organic matter degradation resulting in lower pH values and, in consequence, in inorganic carbon dissolution. Because part of the inorganic carbon pool is assumed to be of pedogenic origin, these emissions would ultimately not alter carbon budgets. The results of this study highlight the preferential degradation of younger organic substrates in freshly thawed Yedoma, if available, and a substantial release of CO2 from inorganic sources