NWT RESEARCH DATABASE

Tags: physical sciences, carbon budget, organic carbon, hydrography, erosion

Objective(s): To better quantify the carbon source, age and fate in one of the largest rivers draining the high latitudes.

Project Description: The research team will collect a new sample set to better quantify the carbon source, age and fate in one of the largest rivers draining the high latitudes. Based on previous work, the team have identified three priority objectives for sample collection in the Mackenzie River: 1) Collect river depth profile sediment samples across the major changes in the annual hydrograph. 2) Sample paired river water and sediment to allow the linkages between dissolved organic carbon (DOC) and particulate organic carbon (POC) age to be made for the first time. 3) Collect samples to establish the link between organic carbon (OC) age and the DIC (and CO2) pools to better understand cycling of old carbon during river transport. The research team will sample river sediments and waters to establish temporal variability in OC source and age. The team will target high and falling flow (in June) when most sediment and water are exported. The team will directly measure velocity profiles, channel cross-sections and water discharge using an Acoustic Doppler Current Profiler (ADCP), providing detailed constraints on the hydrodynamics of sediment transport to compliment the Environment Canada water discharge measurements. At all sites the team aim to collect at least 6 river depth profiles (i.e. 1 per week). The team will use custom built Van-Dorn depth samplers to collect samples near the surface which reflect fine grained, clay mineral dominated sediments, and the progressively coarser material carried closer to the river each profile consists of ~5-6 discrete samples, whose depths are selected based on the water velocity profile data provided by the ADCP at time of sampling. These have been previously used on the Mackenzie River and other large rivers to provide unique insight on sediment and POC source. They also allow the team to collect large samples (~8 L) with abundant material (~4 g per sample) for a range of analyses. River water samples will be filtered within 24 hours at using custom built 142 mm diameter filter units developed by the research team. These provide samples suitable for inorganic and organic isotopic analyses on sediments and water which can be stored for decades for future research projects. Filtered water will be stored in acid pre-cleaned bottles and shipped to the UK for storage. Suspended sediments will be decanted to pre-combusted glass vials, shipped to the UK and freeze dried. These methods are suitable for a large range of organic and inorganic elemental and isotopic analyses, including analysis of trace metals which may be of environmental interest to those in the community. When the research team visit local communities they will take time to explain the approach and the methods and answer any questions people may have. The research team will revisit communities to explain findings as the project moves forward. The team will provide summaries in non-technical language to explain the main findings we have made. The fieldwork for this study will be conducted from June 7, 2019 to September 15, 2019.