Nature Geoscience 6, 303–307 (2013); published online 24 March 2013; corrected after print 24 March 2013.

In the print version of this Article, in the 7th author affiliation the US state is incorrect; it should read 'Arizona'. The 'accepted' date of the Article is also incorrect; it should read 8 February 2013. These errors are correct in the HTML and PDF versions.