Dissolved Fe in the upper waters of the Pacific sector of the Southern Ocean

Abstract

The concentration of dissolved Fe was determined in upper ocean waters along 170 degreesW between 53 and 72 degreesS during four cruises between October 1997 and March 1998 as part of the US-JGOFS Southern Ocean Antarctic Polar Frontal Zone cruises. Fe concentrations were extremely variable in both space and time as a result of advection of meso-scale eddys through the region. Observed values ranged from a maximum of 0.34 nM at 64 degreesS in November next to the retreating ice-edge to 0.075 nM in March at 71 degreesS in the northern Ross Sea gyre. In general, the highest average mixed-layer Fe concentrations and the largest seasonal changes were observed in the two frontal zones at similar to 60 degreesS and similar to 64 degreesS. Formation of deep mixed layers during winter and the entrainment of sub-surface waters enriched in Fe is the primary source of this element to surface waters in this region of the Southern Ocean. Additionally, upwelling of circumpolar deep water in the circumpolar current along its northern boundary at the Polar Front and its southern boundary at the Southern Antarctic Circumpolar Current Front is an important mechanism supplying Fe to surface waters in these areas throughout the summer. Comparison of integrated Fe concentrations and Th-derived POC export and N drawdown all suggest that the maximum changes in these properties are comparable to those that would be predicted by Fe availability. Fe concentrations determined by shipboard Flow Injection Analysis (Measures et al., Mar. Chem. 50 (1995) 3) were significantly higher than those found in replicate samples determined by shored based flameless atomic absorption spectrophotometry after preconcentration by chelation-solvent extraction (Johnson et al., Mar. Chem. 57 (1997) 137). The concentration differences, which appear to be restricted to the upper 200-300 m, do not appear to arise from different sampling or handling processes but instead are believed to result from the different sensitivity of the methods to organic fractions of Fe in seawater. (C) 2001 Elsevier Science Ltd. All rights reserved.