Drivers of Nickel Distribution and Seasonality in the Southern Ocean: New Perspectives From the GEOTRACES GIpr07 Transect

Abstract

Winter dissolved nickel (dNi) and particulate nickel (pNi) concentrations were measured in the Southern Ocean (GEOTRACES GIpr07 transect) to investigate biogeochemical cycling within the water column and over seasonal timescales. Concentrations of dNi ranged from 1.98 to 8.21 nmol kg⁻¹ with low surface concentrations and maxima in deepest sampled water masses. Combining our winter data with the GEOTRACES Intermediate Data Product (2021) shows insignificant seasonal dNi variation in surface waters north of the Antarctic Polar Front, indicating the dominance of year-round mixing processes. However, lower summer concentrations than winter in the Antarctic Zone (∆0.23 nmol kg⁻¹) suggest a role for biological processes at high latitudes. For pNi, concentrations ranged from 5 to 49 pmol kg⁻¹ with higher values in surface/near-surface water masses. Vertical attenuation factors (b values) for pNi (0.19 ± 0.06) and particulate phosphorus (pP; 0.43 ± 0.10) suggest a greater retention of Ni in particles than P, invoking scavenging processes or refractory Ni phases. Water mass analysis shows that remineralization of pNi contributes a maximum of 6% of the highest measured dNi. Instead, dNi distributions and macronutrient relationships were largely explained by phytoplankton uptake in surface waters, and mixing and advection of Atlantic and Antarctic origin water masses, each with different preformed nutrient compositions. Winter trace metal measurements provide new perspectives regarding the balance between biological and physical drivers in the Southern Ocean. For Ni, the biological component is small with respect to physical mixing processes and over the timescales in which water masses accumulate Ni during their transport.