The Molecular Composition of Humic Substances Isolated From Yedoma Permafrost and Alas Cores in the Eastern Siberian Arctic as Measured by Ultrahigh Resolution Mass Spectrometry

• Published in: Journal of geophysical research. Biogeosciences 2019-08, Vol.124 (8), p.2432-2445
• Main Authors: Zherebker, A., Podgorski, D. C., Kholodov, V. A., Orlov, A. A., Yaroslavtseva, N. V., Kharybin, O., Kholodov, A., Spector, V., Spencer, R. G. M., Nikolaev, E., Perminova, I. V.
• Format: Article
• Language: English
• Subjects: Aliphatic compounds; Aromatic compounds; Biodegradation; Chemical composition; Climate change; Cores; Degradation; FT‐ICR MS; Fulvic acids; Humic acids; Humic substances; Lakes; Mass spectrometry; Mass spectroscopy; Microbial degradation; Microorganisms; molecular composition; Organic carbon; Organic matter; Permafrost; Pleistocene; Resolution; Scientific imaging; Soil; Spectroscopy; Tannins; thermokarst; Total organic carbon; Transformations; Water analysis; yedoma
• ISSN: 2169-8953; 2169-8961
• DOI: 10.1029/2018JG004743

Ongoing climate change is making the large pool of organic matter (OM) stored in Arctic permafrost vulnerable to mobilization; thus, garnering a deeper understanding of molecular transformations within the abundant pool of soil OM, specifically humic substances, is crucial. Here we present the first high‐resolution mass‐spectrometry examination of molecular compositions of humic acid (HA) and fulvic acid (FA) isolated from organic‐rich deep yedoma (Pleistocene age ice‐rich permafrost) and alas (thermokarst deposit formed during permafrost thaw) cores. The FA fractions were dominated by oxygen‐rich unsaturated compounds, whereas the HA fractions were mostly composed of relatively reduced saturated and aromatic moieties. A substantial increase in contribution of both CHO‐only and N‐containing aliphatic compounds was observed in the HA fractions of the yedoma OM with depth, whereas the alas HA fractions were depleted in aliphatics but enriched with condensed and hydrolyzable tannins. The observed differences in compositional space of the immobile OM stored in the deep yedoma versus alas deposits were consistent with evolution of OM during thermokarst lake genesis, implying intense microbial degradation of N‐rich OM released from the yedoma deposits and accumulation of highly degraded, plant‐derived OM. The patterns of molecular transformations of OM were apparent in compositional space of the least degraded HA fractions as compared to much more oxidized FA fractions. This shows great promise of molecular exploration of the alkali‐extracted OM, comprising up to 50% of the total organic carbon in deep permafrost both for paleoreconstructions and predictions of climate feedback to released OM due to permafrost thaw. Key Points Application of high‐resolution mass spectrometry revealed significant contribution of reduced saturated compounds in permafrost humic acids Comparison of yedoma to alas humic fractions indicated substantial alteration of organic matter during thermokarst depression Extraction of humic acids enables much better imprint of evolutionary transformation of immobile organic matter during thaw events