In this Letter, the position of the y axes in Fig. 3b was altered during the production process, and the carbon isotope label ‘δ13C’ was inadvertently changed to ‘δ18C’. In addition, the x axis in Fig. 3b is on a different scale; whereas this extends to 40 kyr in Fig. 3a and c, it extends to 50 kyr in Fig. 3b. This figure has been corrected online.