FIGURE 1. Measured concentrations from the EPICA Dome C ice core.
From the following article:
Southern Ocean sea-ice extent, productivity and iron flux over the past eight glacial cycles
E. W. Wolff, H. Fischer, F. Fundel, U. Ruth, B. Twarloh, G. C. Littot, R. Mulvaney, R. Röthlisberger, M. de Angelis, C. F. Boutron, M. Hansson, U. Jonsell, M. A. Hutterli, F. Lambert, P. Kaufmann, B. Stauffer, T. F. Stocker, J. P. Steffensen, M. Bigler, M. L. Siggaard-Andersen, R. Udisti, S. Becagli, E. Castellano, M. Severi, D. Wagenbach, C. Barbante, P. Gabrielli and V. Gaspari
Nature 440, 491-496 (23 March 2006)
doi:10.1038/nature04614
Data are on an ice depth scale. 
D are averaged over 3.85-m sections6; chemical concentrations are averaged over 2.2-m sections.