The ocean’s biological carbon pump plays a large role in the global carbon cycle. A fraction of the photosynthetically fixed organic carbon produced in the sunlit zone of the upper ocean is exported to depth as particulate organic carbon (POC). Here, we investigate seasonal changes in POC export in Bedford Basin, a semi-restricted coastal basin located on the east coast of Canada using the ²³⁸U-²³⁴Th radioisotope system. This isotope system is widely used in the open ocean, however, it’s application in complex coastal regimes is still relatively new. Results from sampling in 2019 and 2021 indicate general agreement between water column fluxes of ²³⁴Th and excess ²³⁴Th in the uppermost sediment layers of the basin. In addition, observations of ²³⁴Th fluxes in spring of 2021 show an increase in ²³⁴Th activity in the water column, and thus a smaller deficit with respect to ²³⁸U, compared to spring of 2019. The smaller deficit suggests a reduction in POC export to depth in spring 2021 compared to spring 2019. If confirmed, this reduction in POC flux during spring is likely driven by the ongoing warming of the basin and the related reduction in water column mixing during the winter. Data from this study will be incorporated into a model to validate steady-state assumptions in the basin. Continued advances in the application of the ²³⁴Th-²³⁸U method in coastal zones will improve our understanding and quantification of marine carbon fluxes and thus, open the door for addressing other complex questions and environmental concerns that face coastal areas in the Anthropocene.

Primary Presenter: Madeline Healey, Dalhousie University (mhealey@dal.ca)

Authors:

Stephanie Kienast, Dalhousie University  (stephanie.kienast@dal.ca)

Erin Black, University of Rochester  (e.black@rochester.edu)

Christopher Algar, Dalhousie University  (chris.algar@dal.ca)

Maria Armstrong, Dalhousie University  (maria.armstrong@dal.ca)