Bed Ribbing Instability Explanation: Testing a numerical model of ribbed moraine formation arising from coupled flow of ice and subglacial sediment

摘要

Ribbed moraines are large (up to 16 km long) ridges of sediment produced transverse to ice flow direction that formed widely beneath palaeo-ice sheets. Since ice sheet stability is sensitive to conditions operating at the bed, an understanding of ribbed moraine genesis will provide critical information on ice sheet dynamics. Currently, there is no consensus on ribbed moraine formation and various competing hypotheses have been presented to account for their genesis. Only one of these theories has been developed into a physically based numerical model that quantitatively describes ribbed moraine formation. This theory, known as the Bed Ribbing Instability Explanation (BRIE), argues that ribbed moraines are produced by a naturally arising instability in the coupled flow of ice and till. BRIE demonstrates that transverse subglacial ridges (i.e., ribbed moraine) spontaneously grow under certain parameter combinations, and it predicts their wavelength (spacing between ridges). The model represents a significant advance because it is the first time a theory of subglacial bedform generation has been developed to make quantitative predictions which can be formally tested. This paper discusses the types of tests that are currently possible and reports the results from the first testing of BRIE. This analysis centers on the ability of BRIE to predict the primary characteristics of ribbed moraine, which are patterning and wavelength. Results show that BRIE successfully predicts the correct ribbed moraine pattern and appropriate wavelengths. The tests fail to falsify the model, and it is concluded that BRIE remains a viable explanation of ribbed moraine formation.