Figure 2. Composite record of semi-quantitative ²⁰⁶Pb/²⁰⁷Pb isotopic ratios from this study (geometric average) compared with previously measured values from Law Dome⁶ (blue, error bar indicates standard error).

We consider the measurements semi-quantitative because no calibration specific for lead isotopic ratios was made during our continuous measurements. Grey shading indicates the standard error in the composite mean values. The number of ice cores included in the composite incorporates measurements from two coastal and seven inland sites spanning both East and West Antarctica. Temporal evolution of the Antarctic-wide isotopic composite record also is consistent with discrete measurements from the previously studied coastal sites (see also Supplementary Fig. 6 online), with only small differences in magnitude during the late 19^th and early 20^th centuries.