Cycling of zinc and its isotopes across multiple zones of the Southern Ocean: Insights from the Antarctic Circumnavigation Expedition

Zinc (Zn) is an essential micronutrient, playing an important role in several key processes in marine phytoplankton. Here, we present the first high-resolution depth profiles for dissolved Zn and δ⁶⁶Zn from all major zones of the Southern Ocean, collected during the Antarctic Circumnavigation Expedition in Austral Summer 2016/2017. The dataset reveals that Zn cycling changes between different regions of the Southern Ocean. Within the Antarctic Circumpolar Current (ACC), Zn cycling is closely linked to phosphate (PO₄), governed by uptake and regeneration, seasonal mixing, and upwelling. Here, uptake by phytoplankton is associated with a very small fractionation (α = 0.99995), resulting in slightly elevated surface δ⁶⁶Zn (up to +0.67‰), overlying a shallow subsurface δ⁶⁶Zn minimum (+0.36‰ at ∼200 m). South of the ACC, a partial coupling of Zn and silicate (Si) results in a shift of the Zn isotope systematics and a deep enrichment of Zn and Si. Two possible mechanisms could potentially cause this change: 1) reversible scavenging onto sinking particulates, or, 2) association of isotopically heavy Zn with diatom frustules. We also observe the effects of regional processes on Zn in the surface Southern Ocean: firstly, natural Fe fertilization near the Balleny Islands appears to lead to reduced Zn uptake (relative to phosphate) by phytoplankton, that is associated with a greater apparent Zn isotope fractionation than elsewhere in the Southern Ocean (α = 0.99987); secondly, meltwater inputs from the Mertz Glacier add small amounts of isotopically light Zn to surface waters near the Antarctic shelf. Overall, we propose that the lack of distinct δ⁶⁶Zn signatures transported in intermediate waters of the lower latitude global oceans is due to near-complete uptake of Zn by phytoplankton in the surface Southern Ocean with only a small isotope fractionation, in contrast to elements like cadmium and silicon.