Dissolved zinc and cadmium isotope systematics in the Amundsen and Weddell coastal Antarctic marginal seas

Coastal Antarctica is experiencing rapid environmental change with potential effects on regional marine trace element biogeochemistry. Here, we investigate the biogeochemistry of two dissolved bioactive trace elements, zinc (Zn) and cadmium (Cd), and their isotope ratios (δ⁶⁶Zn and δ¹¹⁴Cd) in two coastal marginal seas with distinct oceanographic features - the Amundsen Sea with the intrusion of Circumpolar Deep Water (CDW) onto the Antarctic continental shelf, and the Weddell Sea where formation of Antarctic Bottom Water occurs. In the Amundsen Sea, our isotope data show CDW predominantly controls δ⁶⁶Zn and δ¹¹⁴Cd on the continental shelf. This result is consistent with previous concentration-focused studies that suggested only a negligible addition of Zn and Cd from continental sediments and ice shelf meltwater, and other processes (e.g., scavenging) play a limited role in their cycling on the shelf region. In the Weddell Sea, homogeneous δ⁶⁶Zn and δ¹¹⁴Cd within different water masses across the Antarctic Peninsula shelf, while Zn and Cd concentrations increase via physical mixing with deep water masses, suggest a preformed isotope signature on the continental shelf. In surface waters of both regions, δ¹¹⁴Cd exhibited isotope fractionation linked to biological uptake, with different Rayleigh closed system fractionation factors (α = R_biomass/R_seawater) for regions dominated by haptophytes (0.99930-0.99960) and diatoms (0.99970-0.99995) and we speculate that such differences may be associated with variability between species. In contrast, estimated fractionation factors for Zn in haptophytes (0.99995) and diatoms (0.99980-0.99995) dominated blooms are similar and comparable to reported values in the Southern Ocean (0.99995 ± 0.00001). At the intermediate depth (250-1500 m) in the Weddell Sea, significantly lower δ¹¹⁴Cd in the inner gyre compared to the outer gyre implies Cd regeneration and reduced ventilation. This pattern was not observed for δ⁶⁶Zn, likely due to its smaller biological fractionation in the surface. These findings confirm the role of CDW as the main source of Zn and Cd to the Amundsen Sea and the importance of physical mixing in setting global dissolved Zn and Cd distributions during the formation of deep waters in the Weddell Sea, providing insights into the impacts of regional coastal systems on the biogeochemistry of Zn and Cd.