Relationships among Ni, Cu, Zn, and major nutrients in the Southern Ocean

Abstract

Abstract During consecutive transects at the 6°W meridian including the Polar Frontal region (PFr), the southern Antarctic Circumpolar Current area (sACC area) and the Weddell Gyre Boundary Front, the spatial and temporal distribution of copper (Cu), nickel (Ni), zinc (Zn) and dissolved silica (Si) was related to phytoplankton activity and hydrography. In the PFr, a diatom spring bloom coincided with reduced trace metal and Si concentrations. The trace metal/Si ratios increased during bloom development due to preferential Si net uptake. Within the surface water of the sACC area, a continuous increase in trace metal and Si concentration towards the south was observed. The increase in concentration towards the south is attributed either to a constant flux of trace metals and Si mediated by sinking biogenic particles out of the AASW, or by a combination of the southward increasing upwelling of Upper Circumpolar Deep Water (UCDW) and the continuous downward particle flux. The observed subsurface maxima in the sACC area are probably caused by leaching of sea ice diatoms sedimenting after sea ice melting. Minima in transmission above the pycnocline point to the formation, sinking and dissolution of marine snow responsible for the concentration maxima of trace metals. At the deep sampling stations, Cu correlated strongest with Si among the trace metal/major nutrient correlations, whereby the Cu/Si slopes were significantly lower at the stations in the relatively high productive PFr (0.013–0.018 nM/μM) than at the stations in the relatively low productive sACC area (0.020–0.022 nM/μM). The relatively low Cu/Si slopes at the stations in the PFr are probably due to preferential Si uptake by diatoms in the upper water column and Cu scavenging in the deeper water column. A longer retention of Cu compared to Si during the dissolution of the diatom frustules in the sediment may have contributed to the relatively low Cu/Si slopes as well. Within the UCDW, the trace metals showed maxima in concentration similar to those of phosphate and nitrate, indicating the core of the UCDW. At one station near the Cape Basin, the concentrations of trace metals, phosphate and nitrate reflect the input of North Atlantic Deep Water into the Antarctic Circumpolar Current. In the Low Circumpolar Deep Water (LCDW), trace metal maxima were ascribed to hydrothermal input.