Radiocarbon (Δ14C) and Stable Carbon (δ13C) Isotopic Composition of Dissolved Inorganic Carbon (DIC) in Baffin Bay

Abstract

It has been estimated that approximately half of all anthropogenic fossil fuel carbon dioxide (CO2) emissions have been absorbed by the oceans. Air-sea gas exchange of CO2 and the buffering capacity of seawater allows the oceans to store significant amounts of dissolved inorganic carbon (DIC; ~38,000 GtC). The Arctic Ocean is currently warming at double the rate of the rest of the planet, yet the effect of climate change on the Arctic marine carbon cycle remains unconstrained. Recent work suggests that Arctic marine environments are a carbon sink for the majority of the year, and plays a key role in storing anthropogenic carbon below the mixed layer. Baffin Bay is a semi-enclosed, Arctic basin that supplies cold surface water to the Labrador Sea; a critical region for North Atlantic deep-water formation. While the physical oceanography of surface Baffin Bay is well characterized, less is known about deep water formation mechanisms and its ventilation age. The few residence times for Baffin Bay Deep Water (BBDW) range widely from 20-1450 years. Improved residence time estimates are essential for understanding the role Baffin Bay plays in the Arctic carbon cycle and how this region will respond to climate change. Radiocarbon (D14C) and stable carbon (δ13C) measurements of DIC are powerful tools for parameterizing water mass sources, aging and residence times. However, very few DIC Δ14C and d13C data have been reported in the Arctic Ocean, comprising only a handful of stations in the Eurasian Basin, Canadian Basin, and Beaufort Sea. With this goal in mind, we conducted a study in which DIC samples were collected aboard the CCGS Amundsen in 2019 for δ13C and Δ14C analysis. DIC δ13C and D14C values ranged from 0.68‰ to +1.90‰ and -90.0‰ to +29.8‰, respectively. Surface DIC δ13C values were +0.69‰ to +1.90‰, while deep (>100m) d13C values ranged -0.01 to -0.68‰. Significant linear correlations were found for δ13C and potential density, suggesting DIC δ13C is an effective water mass and carbon source tracer in Baffin Bay. Surface DIC Δ14C values ranged -5.4‰ to +22.9‰, while deep DIC (>1400m) DIC Δ14C averaged -82.2 8.5‰ (n = 9). Much larger surface to deep gradients in DIC Δ14C are observed in Baffin Bay vs. that of the North Atlantic Ocean, suggesting significant aging of BBDW. Next, we used the potential alkalinity method (Palk) and the ΔC* method to quantify the amount of “bomb” 14C and anthropogenic C (DICanth) to model “natural” DIC Δ14C profiles. Both Palk and ΔC* proxies had high errors in cold, low salinity surface water. In particular, surface (<400m) Δ14Cbomb was overestimated at all stations. However, both proxies did not indicate Δ14Cbomb or DICanth contributions below 1000m. Two 14C residence times were estimated based on two proposed mechanisms of BBDW formation. A residence time of 690 +/- 35 years was estimated assuming surface brine rejection in Nares Strait is the main source of BBDW. Another plausible source of BBDW is the entrainment of dense north Atlantic Water over Davis Strait mixed with brine enriched surface water. A comparison of DICanth and Δ14Cbomb corrected deep DIC Δ14C values from the North Atlantic (GO SHIP A16N) to BBDW, resulted in a residence time of 360 +/- 35 years. These residence times (360-690 years) provide new constraints on the ventilation age of deep Baffin Bay and suggest this basin has the potential to store carbon for centuries.