Particulate iron seasonality between winter and spring in the open Southern Atlantic Ocean is primarily driven by non-biological processes

Abstract

In the open Southern Ocean, primary productivity is mainly driven by limiting concentrations of iron. However, limited available iron data during winter leave a gap in our understanding of how seasonality affects the iron cycle, and primary productivity. In this context, two cruises followed the same transect in the Atlantic sector of the Southern Ocean, from the SubTropical zone to the Antarctic zone, during spring and winter. This study aims to determine whether particulate iron concentrations showed seasonal changes, and to identify the processes driving these variations. Particulate iron ranged between 0.04–0.99 nM and 0.04–1.17 nM during spring and winter, respectively, with the lowest concentrations measured in surface waters. Overall, seasonality did not consistently affect total pFe distributions, and when differences existed, total pFe concentrations were higher in winter than in spring. Our findings indicate that biogenic particulate iron is higher in spring than in winter, and represents only a minor component of total pFe in surface waters during both seasons. Therefore, biological processes do not account for the observed seasonal changes in total pFe concentrations. At certain stations, higher pFe concentrations in winter relative to spring appear to be related to episodic external inputs of lithogenic particles, such as iron-rich dust deposition and inputs from the ACC. The authigenic fraction is the primary contributor to total pFe. These particles seem to be formed through the precipitation of dissolved iron derived from the dissolution of lithogenic particles or hydrothermal activity.