Towards a Better Understanding of Coastal Carbon Cycling and Sequestration: Quantifying Sinking Carbon in the Bedford Basin and Beyond

The transport of particulate carbon from surface waters to the seafloor is a key mechanism of carbon burial in open ocean and coastal regions, yet coastal regions play a proportionally larger role in carbon export given their limited extent. It has been suggested that one fifth of the total carbon entering coastal areas off eastern North America (from the atmosphere and through rivers) is subsequently buried in coastal areas. However, despite their importance to the cycling of carbon, and associated elements such as nitrogen, direct measurements of carbon fluxes and carbon accumulation in coastal systems remain a challenge.

Here, we quantify seasonal changes in carbon export in Bedford Basin, an estuary on the Scotian Shelf off Eastern Canada during the 2019 spring bloom. We use the deficit of naturally occurring, particle reactive thorium-234 with respect to its conservative parent uranium-238 to quantify sinking carbon fluxes. While this method is well established for quantifying seasonal changes in the open ocean’s biological carbon pump, it’s use in complex coastal regimes is still rare.

In order to test the feasibility of quantifying carbon fluxes via the thorium-234/uranium-238 disequilibrium method, we measured thorium-234 and carbon in the water column, sinking particulates and the top most sediment in central Bedford Basin. Under steady state conditions, carbon fluxes determined from the water column deficit should match those derived from thorium-234 excess in the topmost sediments. If confirmed, this will open up the possibility of an alternative method for measuring carbon fluxes and deposition in coastal settings on time-scales of weeks to months, which could significantly aid our understanding of common environmental concerns, such as hypoxia, in these vulnerable regions. Preliminary results indicate a good agreement between water column and sediment fluxes of thorium-234 throughout the spring and summer 2019.