Bioavailability of Ancient Terrestrial Organic Carbon in the Mackenzie River System

Amanda Spivak, Marine Chemistry & Geochemistry
Marco Coolen, Marine Chemistry & Geochemistry


Arctic Research Initiative
2011 Funded Project


Arctic permafrost soils are estimated to contain vast pools of organic carbon, much of which may become biologically available or mobilized as the soils thaw with continued global warming. Recent studies indicate that a portion of thawed permafrost carbon is exported from the landscape to rivers and coastal waters. For instance, the age of suspended carbon in North America’s largest arctic river (Mackenzie River) is 4430 - 7970 radiocarbon (14C) years old, and is predominantly derived from permafrost thawing and river-bank erosion. If the river borne permafrost-derived organic carbon is biologically available, then it may be microbially decomposed, assimilated, and respired as CO2, a major greenhouse gas. This ancient carbon may also be channeled into the food web via invertebrate grazing of microbial biomass. Although sediment invertebrates likely constitute a small fraction of the fate of carbon exported from the landscape, they represent important trophic links to larger animals, such as fish. Despite the potential importance of these two pathways to atmospheric CO2 pools and microbial and animal biomass production, there is little information about the biological availability of permafrost derived organic carbon. Through this proposal, we will address this knowledge gap with a series of laboratory experiments and a field investigation focused on the Mackenzie River system. The experimental component will focus on processes controlling ancient organic carbon decomposition and incorporation into microbial biomass in both terrestrial soils and river sediments. More specifically, Co-PI Coolen will (1) examine microbial decomposition of riverborne permafrost soil organic carbon to smaller compounds that can be easily assimilated by the microbial population, (2) identify the major players involved in these processes, and (3) determine the extent to which microbes can assimilate and respire ancient carbon as CO2. The field investigation will examine potential entry points of ancient carbon into benthic food webs in the river system. PI Spivak will determine (4) the sources and approximate age of particulate organic carbon in the water column and sediments and whether ancient permafrost carbon subsidizes sediment invertebrate animal communities. Results from this combination of experimental and field-based work will provide unique insight to the bioavailability of ancient permafrost carbon, the processes controlling its decomposition and subsequent release of CO2 into the atmosphere, and the potential importance of this globally significant pool of organic matter as a carbon source to higher trophic levels. This research builds on previous and on-going studies in arctic habitats and the Mackenzie River system and will contribute to our collective understanding of carbon dynamics in high-latitude ecosystems.