Permafrost is undergoing rapid thaw, especially across the sub-Arctic discontinuous landscape. Carbon in the form of natural organic matter (NOM) has the potential to be released from permafrost upon thaw, whereby it may transform and/or incorporate with modern aquatic sediment and surface water carbon. Deriving from different source material, permafrost NOM is expected to have differing composition and therefore reactivity compared to overlying modern NOM. However, the extent and consistency of variations in chemical composition and reactivity are not well understood. In this study, soil cores were collected from two discontinuous permafrost sites in boreal interior Alaska and separated into sections to analyze NOM from both active layer and permafrost sections. Sampled permafrost was dated back to a maximum of 7,200 years by radiocarbon dating. Results show that aromatic content increased with depth past the active layer into the permafrost, whereas carbohydrate composition overall decreased with depth. In the permafrost, aliphatic groups were observed to be retained in the soil during leaching, indicating preferential leaching behaviors of specific functional groups upon thaw. Chemical reactivity was probed through photobleaching kinetics experiments and analyzed by UV/Vis and fluorescence upon the addition of various reactive species scavengers to probe for triplet and radical reactivity. Results indicate variation in both composition and photobleaching reactivity among permafrost NOM as the chemical composition varies remarkably with depth and location in the watershed. Overall, these results help to highlight the complex heterogeneity of NOM composition and reactivity in discontinuous permafrost soils, and to better understand the link between permafrost composition and potential biogeochemical contribution to aquatic systems upon thaw.

Natural organic matter characterization of composition and reactivity from permafrost soils in the sub-Arctic

2019 Fall National Meeting

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