The depth to the upper surface of an ideal South Pacific oceanic lithosphere is calculated by applying a well-known thermal subsidence equation to a grid of ocean floor ages that is generated from published isochrons interpreted from magnetic and gravity anomaly data. When these depths are removed from observed bathymetry, a grid of residual bathymetry results, which at long wavelengths can be related to the presence of seafloor sediments. By employing the concept of an isostatic correction for sedimentary loads, which describes the depth difference between observed bathymetry of a sedimented ocean and the thermal subsidence surface, it is possible to model sediment thickness using the residual bathymetric anomaly. From forward considerations, the calculated sediment thicknesses are assessed to be significant at amplitudes of 5001000 metres and more. Local flexural effects, abyssal hill topography, and seamount volcanism are likely to give rise to much larger uncertainty at short and medium wavelengths. At the West Antarctic margin, sediment volumes estimated in this way are larger than those based on isopachs constructed from seismic data, which might be underestimated by extrapolation into poorly sampled ice-covered waters.