Nature Geoscience 9, 361–365 (2016); published online 18 April 2016; addendum published after print 8 August 2016.

In the version of this Letter originally published the data availability statement was not included and it should have read: All seawater data are publicly and freely available at Bolin Centre Database; δ18O-data plotted in Fig. 4 are from ref. 60 and are available at the British Oceanographic Data Centre web site.