Transfer of terrestrial organic carbon in the Mackenzie River system


Arctic Research Initiative
2010 Funded Project


Arctic soils are thought to contain more than 50% of the carbon stored in soils globally.  This very large stock of carbon is likely very sensitive to changes of environmental conditions such as permafrost thawing. It has therefore been proposed that future warming of the Arctic regions might destabilize the organic carbon that has been stored in soils for several thousands of years, creating a source of CO2 and CH4 for the atmosphere, exacerbating current anthropogenic emissions of greenhouse gases. However, there are major unresolved questions about this possible positive feedback mechanism. Specifically, the extent of the greenhouse gases source still needs to be determined at large spatial scales. Moreover, the fate of soil carbon when affected for instance by permafrost thawing is uncertain as a large fraction might simply be exported by rivers and transferred to other carbon reservoirs such as marine sediments.

Thanks to spatial integration provided by erosion processes, the study of river water and sediments can yield valuable insights into the exchange of carbon at the scale of river basins.  The Mackenzie River (4241 km long, drainage area of 1.8x106 km2) is one of the largest rivers draining into the Arctic Ocean and is its largest single supplier of sediments. It is therefore of paramount importance in the context of organic carbon cycling in the Arctic.

The proposed research is based on detail sampling of water and sediments at several locations in the Mackenzie River basin. Using geochemical analysis of organic and inorganic species we aim at characterizing sources and composition of dissolved and particulate organic carbon and the mineral matrix that controls the fate of organic carbon during passage through the Mackenzie River system. Specifically, we seek to determine 1) the proportion of the organic carbon exported by the rivers derived from ancient soils, 2) how this proportion varies along the course of the river, i.e. across different ecosystems and, 3) how this ancient carbon is affected by transport processes on its way to the Arctic Ocean.

We expect that the outcome of our study will be a basin scale estimation of current greenhouse gasses emissions generated by destabilization of organic carbon in response to Arctic climate warming. In addition, the results of our proposal will provide a detailed characterization of the current flux and age of riverine organic carbon in the Mackenzie River system.  This is a necessary first step, which will permit the study of historical sedimentary records to assess whether the flux and age of organic carbon discharged by Arctic rivers has increased from the pre-industrial era to the present day. Finally, we expect this project will provide data and infrastructure necessary to the integration of the Mackenzie River system in an ongoing larger initiative to study dynamics of terrestrial organic carbon cycling during continental erosion.