(CO2)-C-14 analysis of soil gas: Evaluation of sample size limits and sampling devices

Radiocarbon (C-14) analysis of CO2 respired from soils or sediments is a valuable tool to identify different carbon sources. The collection and processing of the CO2, however, is challenging and prone to contamination. We thus continuously improve our handling procedures and present a refined method for the collection of even small amounts of CO2 in molecular sieve cartridges (MSCs) for accelerator mass spectrometry C-14 analysis. Using a modified vacuum rig and an improved desorption procedure, we were able to increase the CO2 recovery from the MSC (95%) as well as the sample throughput compared to our previous study. By processing series of different sample size, we show that our MSCs can be used for CO2 samples of as small as 50 mu g C. The contamination by exogenous carbon determined in these laboratory tests, was less than 2.0 mu g C from fossil and less than 3.0 mu g C from modern sources. Additionally, we tested two sampling devices for the collection of CO2 samples released from soils or sediments, including a respiration chamber and a depth sampler, which are connected to the MSC. We obtained a very promising, low process blank for the entire CO2 sampling and purification procedure of similar to 0.004 (FC)-C-14 (equal to 44,000 yrs BP) and similar to 0.003 (FC)-C-14 (equal to 47,000 yrs BP). In contrast to previous studies, we observed no isotopic fractionation towards lighter delta C-13 values during the passive sampling with the depth samplers

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