Detecting the Signature of Permafrost Thaw in Arctic Rivers

Abstract:

Arctic permafrost soils contain vast quantities of ancient organic matter. Numerous studies have shown extensive permafrost thaw and degradation in the Arctic, but dissolved organic carbon (DOC) exported from the mouths of large Arctic rivers – which are expected to integrate processes and changes occurring through their watersheds – has been shown to be predominantly modern. This raises the question, where is the ancient DOC that is mobilized from permafrost thaw and the deepening of the active layer? This study examines DOC radiocarbon age, biolability and dissolved organic matter (DOM) composition via FT-ICR-MS in permafrost thaw streams and the Kolyma River mainstem (Northeast Siberia). Ancient permafrost thaw stream DOC is observed to be highly biolabile particularly in comparison to modern Kolyma River mainstem DOC. In conjunction with this high biolability the permafrost thaw stream DOM exhibits large changes in molecular structure, loss of hydrogen rich (energy rich) aliphatic molecules, and production of molecules in the classical area in van Krevelen space associated with riverine DOM. Modern Kolyma River mainstem DOM conversely appears very stable in bioincubations in comparison to ancient permafrost thaw DOM. Thus the apparent offset between mobilization of ancient permafrost derived organic matter and the current predominantly modern age of DOC at the mouth of major Arctic rivers may be explained due to microbial degradation of permafrost derived DOC within the river’s hydrologic residence time.