The modern organic carbon cycle in Hudson Bay, an Arctic coastal sea undergoing change

Arctic coastal seas are important sites of organic carbon cycling. Projected changes in Arctic temperatures, river inflows and sea ice conditions will affect this cycling, with consequences for local ecosystems and global carbon cycles. To predict and measure the effects of change requires in-depth understanding of organic matter (OM) sources and processes controlling production, transport and burial. In this thesis, various geochemical tools were applied to study the modern organic carbon cycle in Hudson Bay, a large but poorly-known Arctic inland sea, which is undergoing change more rapidly than other Arctic areas. Organic compositional data for sediments and suspended particulate samples from marine and river waters, together with a biogeochemical box model for nitrate, revealed that new marine primary production is concentrated in inshore surface waters, where there is increased upwelling of deep, nutrient-rich waters. This is probably supported in part by nutrient entrainment related to the large volume of river inflow to the Bay, which circulates through the inshore region. River inflow also provides the largest source of allochthonous (terrigenous) OM. Bulk (C/N, delta13C, delta15N) and specific organic biomarkers (lignin) showed that heterogeneous terrestrial materials undergo hydrodynamic sorting in the coastal zone, resulting in the coarse fraction being retained near river mouths while a fine fraction undergoes transport by marine currents. Seasonal sea ice cover interacts with winter/spring river inflow to influence OM production and transport indirectly. Sediment and particulate organic carbon budgets showed that resuspension and lateral transport of fine-grained coastal sediments is also an important process, supplying most of the sediment for contemporary burial of OM and as much terrigenous OM as river inflow and subaerial coastal erosion combined. Resuspension also supplies offshore Hudson Bay with old (glacigenic) marine carbon, supporting slightly enhanced burial of marine OM in the Bay's sediments, compared to other Arctic shelves. The importance of resuspension likely reflects the exceptional postglacial isostatic rebound (relative sea-level fall) ongoing in Hudson Bay. Hypothesized transitional sedimentary and OC regimes in Hudson Bay pose challenges for interpreting responses to climate change and using Hudson Bay as a sentinel for change in Arctic coastal seas.