Nature Geoscience 2 842–850 (2009); published online: 22 November 2009; corrected after print: 13 December 2009.

In the version of this Progress Article originally published, the colour scale for Fig. 3e and f was incorrect and should have been as shown on the left below. Fig. 3h and i were incorrect and should have been as shown here. These errors have been corrected in the HTML and PDF versions of the Progress Article.