Extended Data Fig. 4: Map of approximate ɛ_Nd values in rocks and offshore sediments from around the Ross Sea embayment.

Epsilon Nd values are overlain on MODIS imagery²⁰⁰ and the BedMachine Antarctica V1 modern bed topography^43,44, with the MEaSUREs grounding line and ice sheet margin shown^45,46. The approximate boundary between West and East Antarctic lithosphere is shown using a white dashed line⁴⁷. Modern/late Holocene and terrestrial till samples are represented by circles with the same colour bar^28,30,55. Although ice flow patterns have changed since their deposition, Last Glacial Maximum tills in offshore sediments are also plotted as squares to improve spatial coverage²⁸. Individual samples and references are reported in Supplementary Table 1. The bedrock map was produced by Kriging between sample locations within a group, then masking to the outcrop area. Beacon and Ferrar Group (Fig. 1) rocks are often not differentiated in geological mapping, but are roughly equal volumetrically¹³⁶, with the uppermost Beacon Supergroup formations having a Ferrar-like isotopic signature¹³⁹. We hence assume a 60% Ferrar, 40% Beacon mixture is representative.