Abstract:
A method comprising an improved seawater collection protocol and subsequent Inductively Coupled
Plasma Mass Spectrometry (ICP-MS) based analytical technique was validated through an intercalibration
exercise performed with the University of Plymouth (UK), multiple cross-over stations and analyses of
certified reference materials (SAFe, GEOTRACES and NASS-5). The commercially available seaFAST-pico
preconcentration module was employed for the simultaneous extraction of a suite of trace metals (Mn,
Fe, Ni, Cu, Zn, Co, Cd and Pb) from their seawater matrix prior to ICP-MS analysis. Extremely low detection
limits (< 0.228 nmol/kg) combined with low blank values ensured quantitive recovery on ICP-MS and
minimal interferences arising from alkali and alkaline earth metals (Na, K, Mg and Ca) present in the saline
matrix. The results of the certified reference materials were in excellent agreement with their
corresponding consensus values and validated the methods precision and accuracy. During ICP-MS
analysis, repeatability and reproducibility were monitored through analysis of an internal Stellenbosch
University (SU) TM4 control and various commercially available quality controls, the results of which
further confirmed a high level of precision. The distribution of Dissolved Copper (DCu) and Dissolved Zinc
(DZn) was investigated in the Atlantic sector of the Southern Ocean. DCu displayed typical nutrient type
behaviour reflected by sub-nanomolar surface concentrations increasing steadily until maximum
observed concentrations of 2 – 3 nmol/kg in the Antarctic Bottom Waters (AABW). DZn concentrations
ranged between approximately 1 and 12 nmol/kg and exhibited characteristic nutrient-type behaviour
although intermediate and deepwater distributions were more conservative compared to DCu. Local
subsurface minima coincided with elevated levels of chlorophyll-a (chl-a) indicating biological utilisation
by phytoplankton in the euphotic zone. Remineralisation of sinking organic matter, predominantly diatom
frustules, from Antarctic Surface Water (AASW) resulted in deeper sub-surface maxima for DZn. The
dominant supply of trace metals to surface waters south of the Antarctic Polar Front (APF) was advective
upwelling of nutrient rich Upper Circumpolar Deep Water (UCDW) and AABW. Atmospheric inputs and
melting ice accounted for minor surface influxes where there was a poor DCu/salinity correlation. Both
trace elements displayed significant correlations with the macronutrient silica, evidence of their role in
the biological cycle. An overall Cu:Si relationship of Cu (nM) = 0.011 Si (μM) + 0.851 (R2 = 0.85, n=98) was
obtained for this study while the corresponding Zn:Si relationship was Zn (nM) = 0.043 Si (μM) + 1.021 (R2
= 0.80, n=98). The APF exerted a strong control over nutrient distributions separating low nutrient low chlorophyll (LNLC) subtropical waters to the north from high nutrient low chlororphyll (HNLC) waters to
the south.
