Figure 2: a) Initiating an in situ iron experiment by releasing dissolved iron along an expanding track (blue line) around
a central drifting buoy (red line). b) Adding iron sulphate to a tank of acidified seawater. c) Surface mapping using
concentration of the dissolved tracer sulphur hexafluoride in surface waters. The ship’s track is indicated by the black dots
overlying the contour plot of sulphur hexafluoride concentration.