Fluorescence spectroscopy studies of humic substance fractions isolated from permanently frozen sediments of Yakutian coastal lowlands

Abstract

Predictions of global warming render studies of organic matter stored in permanently frozen sediments of northernmost areas particularly topical. This paper offers results of fluorescence spectroscopy applied to the study of humic and/or humic-like fractions gained from polygenetic syncryogenic sediments in northern Yakutia in comparison with humic substances (HS) from recent soil and the Nordic (NOM) reference of aquatic natural organic matter. To isolate HS fractions from sediment and soil samples the method of sequential extraction with sulfonate resin (HS-S fraction), carboxylate resin (HS-C fraction) and 0.1 M NaOH (HS-A fraction) was used. The HS extracts were filtered through a 0.45 μm membrane filter, following which part of each HS fraction was broken down into humic acids (HA) and fulvic acids (FA). Synchronous fluorescence spectra were obtained on aqueous solutions of all specific HS fractions as well as HA and FA fractions isolated thereof and NOM. Variations in the main peak position and intensity seen on the synchronous fluorescence spectra of HS-S, HS-C and HS-A extracted from the same soil/sediment sample are apparently indicative of the distinct degree of humification of the isolated fractions. It was found that HA fractions, compared to FA fractions, show a more varied array of fluorescence observations which change depending on the source of HS. Consistent with fluorescence-produced characteristics for HS and HA fractions, sediment samples can be classified into two groups, one comprising Pleistocene sediments of the ice complex, the other sediments of the Holocene thickness and Pleistocene buried soil. The HS and HA of the first group manifest dominating peaks in the low- and intermediate-wavelength spectral regions, which is indicative of a relatively low degree of humification and certain similarity to NOM. The HS and HA fractions of the second group show a fluorescence signals set typical of recent soil, where the dominating peak of a particular fraction occurs in the long-wavelength region peculiar to a high degree of humification. An examination of experimental data suggests essential bioclimatic fluctuations in the region over the period of the studied sediments formation.