The coupled oxygen and carbon dynamics in the subsurface waters of the Gulf and Lower St. Lawrence Estuary and implications for artificial oxygenation

Abstract. The Gulf and Lower St. Lawrence Estuary have experienced major environmental change over the past century, including the development of hypoxic bottom waters and their simultaneous warming and acidification. Here, we use biogeochemical observations collected during the 2021–2023 TReX project as well as historical data, integrated with a tracer-calibrated 1D Advection-Diffusion model with variable boundary conditions to represent dissolved oxygen (DO) and dissolved inorganic carbon (DIC) dynamics within the core of the oxygen minimum zone (27.15–27.3 kg m−3 isopycnals) of the Laurentian Channel. The rate of in-channel oxygen utilization in the deep layer was nearly invariant from 2003 to 2023 at 21.1 ± 2.5 µmol kg−1 yr−1 and the DIC accumulation rate was estimated to be 18.3 ± 2.5 µmol kg−1 yr−1. Using δ13CDIC data, we assess the effect of microbial organic matter remineralization processes and dilution of the 13CDIC pool (-6.6×10-3 ‰ µmol−1 of added metabolic DIC). This combination of long-term observations and tracer-informed transport modeling helps reconcile differences in prior estimates of biogeochemical transformation rates and improves our ability to predict deepwater DO and DIC cycling. Finally, we apply the model to the mitigation scenario proposed by Wallace et al. (2023) for artificial re-oxygenation of the Laurentian Channel bottom waters using pure oxygen. We estimate that the injection of ∼ 8.3 × 105 t yr−1 of oxygen, equivalent to an additional 55 µmol kg−1 relative to the 2023 boundary concentration proximal to the Cabot Strait, would be required to achieve and maintain above hypoxic levels (>62.5 µmol kg−1) at the head of the Laurentian Channel. Using the model, we estimate the time required to re-establish steady-state along-channel distributions of DO and DIC following a change in offshore boundary conditions to be about 10 years, or twice the along-channel transit time. This study provides new long-term characterizations of deepwater DO and DIC cycling and offers a first-order assessment of the feasibility of large-scale re-oxygenation in the Gulf and Lower St. Lawrence Estuary.