Migration of the Siple Dome ice divide, West Antarctica

The non-linearity of the ice-flow law or a local accumulation low over an i ce divide can cause isochrones (internal layers) to be shallower under the divide relative to the flanks, forming a "divide bump" in the internal laye r pattern. This divide signature is analyzed using ice-flow models and inve rse techniques to detect and quantify motion of the Siple Dome ice divide,W est Antarctica. The principal feature indicating that migration has occurre d is a distinct tilt of the axis of the peaks of the warped internal layers beneath the divide. The calculated migration rate is 0.05-0.50 m a(-1) tow ard Ice Stream D and depends slightly on whether the divide bump is caused by the non-linearity of ice flow or by a local accumulation low. Our calcul ations also suggest a strong south-north accumulation gradient of 5-10 x 10 (-6) a(-1) in a narrow zone north of the divide. A consequence of divide mi gration is that pre-Holocene ice is thickest about 0.5 km south of the pres ent divide position. Divide motion indicates that non-steady processes, pos sibly associated with activity of the bounding ice streams, are affecting t he geometry of Siple Dome. The migration rate is sufficiently slow that the divide bump is maintained in the internal layer pattern at all observable depths. This suggests that major asynchronous changes in the elevation or p osition of the bounding ice streams are unlikely over at least the past 10( 3)-10(4) years.