Nature Geoscience 3, 728–734 (2010); published online: 12 September 2010; corrected after print: 14 October 2010.

In the version of this Letter originally published, “−0.8 ± 0.5 Sv” in the fifth sentence of the abstract should have read “+0.8 ± 0.5 Sv”. This error has now been corrected in the HTML and PDF versions of the text.