Universität zu Köln

Melchert, Jan Olaf ORCID: 0000-0002-1548-340X (2022). AMS Radiocarbon analysis of Greenhouse gases: Method development and application. PhD thesis, Universität zu Köln.

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Abstract

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.

Item Type:	Thesis (PhD thesis)
Translated title:	Title Language UNSPECIFIED German
Creators:	Creators Email ORCID ORCID Put Code Melchert, Jan Olaf jan.melchert@uni-koeln.de orcid.org/0000-0002-1548-340X UNSPECIFIED
URN:	urn:nbn:de:hbz:38-550583
Date:	January 2022
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Geosciences > Institute of Geology and Mineralog
Subjects:	Natural sciences and mathematics Earth sciences
Uncontrolled Keywords:	Keywords Language greenhouse gases English radiocarbon English yedoma English carbon cycle English accelerator mass spectrometry English carbon isotopy English permafrost English
Date of oral exam:	20 January 2022
Referee:	Name Academic Title Rethemeyer, Janet Professor Melles, Martin Professor
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/55058