Natural and Artificial Radionuclides as Tracers of Ocean Processes

Changes in the provenance and composition of waters exported from the Arctic Ocean have the potential to impact large-scale ocean circulation processes in the sub-polar North Atlantic. The main conveyor of waters from the Arctic Ocean to lower latitudes is the East Greenland Current (EGC), flowing southward through Fram Strait. It is therefore crucial to determine and monitor the composition of the EGC, a mixture of polar waters of different origins. Here we present a pilot study on the potential of the long-lived anthropogenic radionuclides ¹²⁹I and ²³⁶U as tracers of the EGC water mass composition, based on a time series of ²³⁶U and ¹²⁹I concentrations measured across Fram Strait in the years 2016, 2018, and 2019. The overall spatial distribution of ²³⁶U and ¹²⁹I was similar among the three sampling years, but a decrease in concentration was observed in the upper water column of the EGC. The observed changes could only partly be attributed to the transient nature of the radionuclide signals, but instead pointed to changes in the EGC water mass composition. To investigate these changes, ²³⁶U and ¹²⁹I were first combined in a mixing model featuring the endmembers expected in the upper EGC. We distinguished between Pacific Water (PAC), Atlantic Water advected from the Arctic Ocean (ATL), and Atlantic Water recirculating in Fram Strait (RAC). In ²³⁶U-¹²⁹I tracer space, PAC and RAC showed similar tracer signatures, but were well distinguished from ATL. From 2016 to 2018/19, a decrease in the ATL fraction was evident for the upper EGC. Secondly, the respective combination of ²³⁶U and ¹²⁹I with salinity showed differences in absolute water mass fractions, but similar temporal trends. Both suggested an increase in PAC of about 20% for the uppermost layer of the EGC (samples with potential densities below 26.5) and an increase in RAC of about 10−20 % for denser samples. ¹²⁹I and ²³⁶U, in combination with salinity, were shown to be suitable tracers to investigate water mass composition in Fram Strait, with the advantage that they can distinguish Atlantic Water advected from the Arctic Ocean from that recirculating in Fram Strait.

The Canadian Arctic is warming three times faster than the rest of the planet. The impact of climate change on the Arctic carbon cycle, and in particular for Baffin Bay, remains poorly constrained. Sinking particulate organic matter (POM_sink) is a key component of the biological carbon pump and provides a direct linkage between surface productivity and the preservation of carbon in marine sediments. While POM_sink provides a rapid POM shunt to the deep ocean (days) the majority of marine POM is suspended (POM_susp) persists for years in the water column. Stable carbon (δ¹³C), nitrogen (δ¹⁵N) and radiocarbon (Δ¹⁴C) measurements are powerful tools for evaluating sources and cycling of POM. In this study, we measure depth-integrated (0-400m) POM_susp stable carbon (δ¹³C), nitrogen (δ¹⁵N) and radiocarbon (Δ¹⁴C) values at 11 stations in Baffin Bay. We use POM C:N_a ratios to evaluate its diagenetic state. POM_susp δ¹³C and δ¹⁵N values ranged from −21.6‰ to −29.5‰, and +3.84‰ to +7.21‰, respectively. POM_susp Δ¹⁴C values ranged from −41.8‰ to +76.8‰. Together, our results suggest Baffin Bay POM_susp has multiple carbon and nitrogen sources. POM_susp δ¹³C and Δ¹⁴C are strongly correlated to surface salinity, indicating surface water has a strong influence on POM_susp isotopic composition and ¹⁴C-age. POM_susp from coastal Greenland stations have similar Δ¹⁴C values to surface dissolved inorganic carbon (DIC) and POM concentrations, consistent with primary production as the predominant POM_susp source in Eastern Baffin Bay. Positive POM_susp Δ¹⁴C values in central Baffin Bay suggest an accumulation of atmospheric “bomb” ¹⁴C in the sub-polar gyre. POM_susp exiting Davis Strait via the Baffin Island Current was determined to have a significant degraded, resuspended sediment component. Finally, POM_susp δ¹⁵N values highlight two distinct N sources in Baffin Bay: Pacific and Arctic nutrients at Northern gateway stations vs. the influx of Atlantic nutrients via Davis Strait along coastal Greenland. Overall, these first Baffin Bay POM_susp Δ¹⁴C values provide useful baseline data for rapidly changing Arctic ecosystem.

The North Atlantic and Arctic oceans, along with the North Pacific, are the main reservoirs of anthropogenic radionuclides introduced in the past 75 years. The POSEIDON-R compartment model was applied to the North Atlantic and Arctic oceans to reconstruct ¹³⁷Cs contamination in 1945–2020 due to multiple sources: global fallout, exchange flows with other oceans, point-source inputs in the ocean from reprocessing plants and other nuclear facilities, the impact of the Chernobyl accident and secondary contamination resulting from river runoff and redissolution from bottom sediments. The model simulated the marine environment as a system of 3D compartments comprising the water column, bottom sediment, and biota. The dynamic model described the transfer of ¹³⁷Cs through the pelagic and benthic food chains. The simulation results were validated using the marine database MARIS. The calculated concentrations of ¹³⁷Cs in the seaweed and non-piscivorous and piscivorous pelagic fish mostly followed the concentration of ¹³⁷Cs in water. The concentration in coastal predator fish lagged behind the concentration in water as a result of a diet that includes both pelagic and benthic organisms. The impact of each considered source on the total concentration of ¹³⁷Cs in non-piscivorous fish in the regions of interest was analyzed. Whereas the contribution from global fallout dominated in 1960–1970, in 1970–1990, the contribution of ¹³⁷Cs released from reprocessing plants exceeded the contributions from other sources in almost all considered regions. Secondary contamination due to river runoff was less than 4% of ocean influx. The maximum total inventory of ¹³⁷Cs in the Arctic Ocean (31,122 TBq) was reached in 1988, whereas the corresponding inventory in the bottom sediment was approximately 6% of the total. The general agreement between simulated and observed ¹³⁷Cs concentrations in water and bottom sediment was confirmed by the estimates of geometric mean and geometric standard deviation, which varied from 0.89 to 1.29 and from 1.22 to 1.87, respectively. The approach used is useful to synthesize measurement and simulation data in areas with observational gaps. For this purpose, 13 representative regions in the North Atlantic and Arctic oceans were selected for monitoring by using the “etalon” method for classification.