Inorganic carbon system dynamics of Southern Hudson Bay and James Bay

Abstract

Driven by the difference in the partial pressure of CO₂ (pCO₂) between the atmosphere and the ocean, 26% of anthropogenic carbon dioxide (CO₂) emissions have been absorbed by the world’s oceans. The Arctic Ocean is responsible for 5-14% of the ocean CO₂ sink despite accounting for 4.3% of the global surface ocean area due to its cool temperatures and low salinity relative to other oceans. As CO₂ is absorbed by seawater, it forms carbonic acid resulting in reduced carbonate ion availability and lower pH. Increasing anthropogenic CO₂ emissions have impacted seawater carbonate chemistry, resulting in acidic conditions in which calcifying organisms have difficulties forming hard structures. The peripheral Arctic seas Southern Hudson Bay and James Bay (SHB-JB) are experiencing rapid climate change, including decreased sea ice cover, altered river runoff, and increased temperatures. However, our ability to assess ecosystem response to changing Arctic conditions is limited as SHB-JB remain understudied with respect to the inorganic carbon system. The objective of this thesis was to evaluate the marine inorganic carbon system of SHB-JB using three years (2021–2023) of ship-based measurements. Continuous surface mixed layer measurements indicated that SHB-JB were weak sources of CO₂ to the atmosphere during August. Temperature was the main driver of surface water pCO₂, while dissolved inorganic carbon (DIC) and total alkalinity (TA) decreased with proximity to river inflows. Saturation state (Ω) indicated undersaturation (Ω<1) of aragonite across James Bay, while calcite undersaturation was restricted to the Moose and Eastmain River estuaries. Freshwater impacts on seawater inorganic carbon differed based on watershed properties; rivers entering eastern James Bay were more acidic than rivers entering western James Bay, which caused a greater reduction in coastal Ω. This difference reflects watershed geology; western James Bay is underlain by carbonate till, whereas eastern James Bay is underlain by silicate bedrock. The results of this thesis provide a baseline assessment of summertime marine inorganic carbon and acidification state within James Bay while supporting previous carbon system studies in Hudson Bay. Continued monitoring of the SHB-JB carbonate system is required to provide insight into how Arctic ecosystems respond to advancing climate change.