What Does the Stable Carbon Isotope Ratio (δ13C-DIC) Tell Us About the Coastal Ocean Carbon Cycle Along the North American East Coast?

The ratio of ¹³C/¹²C in oceanic dissolved inorganic carbon (δ¹³C-DIC) is an effective tracer for exploring aquatic carbon cycles influenced by net biological production and respiration, lateral transport from land to ocean, and alongshore ocean currents. Additionally, δ¹³C-DIC is a valuable tool for estimating anthropogenic CO₂ accumulation rates in the ocean. We developed and validated an automated, high-precision (±0.03 ‰) method for ship-based simultaneous analysis of DIC and δ¹³C-DIC using Cavity Ring-Down Spectroscopy (CRDS). This approach enabled the analysis of over 1,600 discrete seawater samples, along with numerous duplicates and standards, during a 40-day cruise along the North American eastern ocean margin in summer 2022.

Our findings revealed distinct air-sea δ¹³C disequilibrium due to short water residence times. A clear latitudinal gradient in surface δ¹³C-DIC distribution was also observed, with highly positive δ¹³C-DIC values in the northern sub-regions attributed to hotspots of net biological production and very low values in estuaries due to terrestrial inputs. The biological mechanism driven these variations is supported by a strong linear correlation between δ¹³C-DIC, the biological component of DIC deviation, and O₂ supersaturation data. Using a box model, we further examined the interplay of various timescales for biological production, gas exchange (months for CO₂ and DIC, years for δ¹³C-DIC), and water residence time in shaping the distributions of DIC and δ¹³C-DIC along the ocean margin.

Moreover, we observed a notable decrease in δ¹³C-DIC over the past 25 years, particularly within the upper ocean mixed layer, with the decline progressively diminishing with depth to approximately 1,500 m. These decadal changes in δ¹³C-DIC are substantially larger than those of DIC in the context of natural spatial and temporal variability. This pattern underscores the growing influence of anthropogenic carbon in surface and subsurface waters, suggesting δ¹³C-DIC may serve as a more sensitive indicator than DIC concentration for detecting anthropogenic CO₂ accumulation given its more pronounced decadal variability. The characteristic δ¹³C-DIC signals also help identify carbon sources affecting ocean pH and acidification.