Molecular characterization of organic matter in Canadian Arctic paleosols for paleoecological applications

Abstract

The preservation of soil organic matter (SOM) in loess–paleosol sequences from the Klondike goldfields in the Canadian Arctic has recorded several Quaternary glacial–interglacial cycles. Both non-targeted and targeted molecular organic geochemical techniques are employed to provide an additional approach to confirm and expand on existing reconstructions of late Quaternary environments in this region. Solid-state nuclear magnetic resonance (NMR) techniques performed on 31 paleosol samples formed during marine isotope stage (MIS) 2 and MIS 4 showed similar SOM characteristics. However solution-state NMR of the base-soluble extracts detected a large amount of vascular plant inputs and preservation. Biomarker analysis revealed a large predominance of constituents from higher plants such as long chain n-alkyl lipids, hydroxyl n-alkyl lipids, sterols and terpenoids. Vegetation source proxies based on the distribution of n-alkanes were inconclusive, however when combined with additional targeted extractions of hydrolysable lipids and lignin-derived phenols, it was revealed that the major vegetative sources originated from woody and non-woody angiosperms. In addition, a novel thiazole–sulfur biomolecular structure in a paleosol from the Goldbottom Creek, 14C dated to 25.3kyr before present (BP), was observed by solution-state NMR. This type of molecule has not been previously detected in base-soluble extracts of SOM. Although the origin and preservation of this structure is unknown, a mechanism based on microbial biomolecular intermediates is proposed. The spectra also contained large resonances attributed to plant cuticle functional groups providing further evidence of plant-derived material preservation. Collectively, this comprehensive dataset agrees with paleoecological hypotheses derived from physical morphology and plant macrofossils suggesting that these molecular methods may be applied as an alternative technique for Arctic paleoclimate studies as well as identification of molecular constituents not previously identified in modern soils.