Glacial geology of Cape Bird, Ross Island, Antarctica

Located on the northwest coast of Ross Island in McMurdo Sound, Cape Bird features virtually unweathered surface drift with erratics from the Transantarctic Mountains (TAM) and with enclosed marine shell fragments. This Cape Bird drift is cut by Holocene beaches. The chronology of drift deposition and beach formation comes from AMS radiocarbon dates of associated marine shells, and of penguin bones and skin. The results indicate that a grounded ice sheet with a surface elevation in excess of 590 m deposited Cape Bird drift after 26,860 14 C yr BP. The enclosed erratics, shells, and foraminifers indicate that a component of the ice within this sheet flowed through the TAM, grounded on the Ross Sea floor, and ultimately advanced landward onto the lower slopes of Mount Bird. On the basis of similar physical weathering characteristics, far-traveled erratics, radiocarbon chronology, and geomorphic setting, Cape Bird drift is correlated with Ross Sea drift elsewhere in the McMurdo Sound region. Reconstructed surface contours for the grounded ice sheet are based on the areal distribution and upper limit of Cape Bird/Ross Sea drift on ice-free areas alongside McMurdo Sound. The ice surface sloped down to the west from the Ross Embayment toward the TAM. A major flowline within this sheet passed westward around northern Ross Island southward over Capes Bird and Royds, and then again westward across the sound. This ice-flow direction is required because of the paucity of kenyte erratics in Cape Bird drift. Kenyte, a distinctive bedrock lithology, is common on the west coast of Ross Island, south of Cape Bird. Therefore, the lack of widespread kenyte erratics in Cape Bird drift precludes northward flow of grounded glacier ice during the last glacial maximum (LGM). Wave-washed sediment and beach ridges at Cape Bird extend to 7.6 m above mean high tide (MHT). A set of five low-elevation beaches, all less than 4.0 m above MHT, is parallel with the modern coastline. A higher set features six beach ridges transverse to the modern coastline between 4.9m and 7.6 m above MHT. Storm waves probably produced both sets of beaches. AMS radiocarbon dates of collagen from buried penguin bones indicate that the lower ridges are modern storm beaches. The upper beaches formed around 3585 14 C yr BP, when relative sea level stood about 3.6 m above MHT. Given that the mapped upper limit of Cape Bird drift exceeds 590 m elevation and that the marine limit at Cape Bird is only 7.6 m above MHT, the raised beaches at Cape Bird probably reflect only the isostatic uplift since seasonally open water appeared off Cape Bird following recession of an ice-shelf front about 3585 14 C yr BP, rather than the total isostatic uplift after retreat of the ice-sheet grounding line. It is less likely that the lowness of the beaches at Cape Bird reflects tectonic subsidence on this volcanically active island. In either case, the beaches at Cape Bird do not represent the total isostatic uplift since deglaciation. Therefore, the best measure of the thickness of glacier ice at Cape Bird during the LGM comes from the areal distribution, upper limit, and AMS radiocarbon chronology of Cape Bird drift.