Characterization of submarine glacial landforms and lowstand fluvial systems from western Campbell Island, New Zealand

Campbell Island is the southernmost of New Zealand’s subantarctic islands, located about 600 km south of the South Island at 52.33°S, 169.09°E. The volcanic strata of this remote, unpopulated ~113 km2 island are eroded by a series of steep-sided valleys that are assumed to be glacial in origin. This is evidenced by their U-shapes, ground moraine, and rocky hills along the sides of the valleys with roches-moutonées geometries. At least two of these valleys, Perseverance Harbour and Northeast Harbour, have basal levels that are beneath current sea level. This enables the investigation of the floors of these fiords with high-frequency marine seismic imaging techniques. Perseverance Harbour is ~9 km long with water depths of 35 to 45 m in the center. Northeast Harbour is ~3.5 km long with water depths of 15 to 25 m in the center. Sea level during the last glacial maximum is expected to have been ~120 m below the current level. The shoreline east of Campbell Island therefore would have been 6 - 10 km east of the present day coast. Water depths on this coast rapidly fall to 60 to 70 m and then follow a gentler gradient outward and beyond the inferred lowstand shoreline. Detailed investigations of seafloor features around Campbell Island are lacking. The relatively-shallow water depths on the leeward (east) side of Campbell Island provide an opportunity to examine the floors of the fiords and the adjacent shelf for evidence of glacial processes and associated sedimentation. Of particular interest are (1) determining the extent of past glacial cover on and around the island, and (2) observing glacial and periglacial erosional processes on the seafloor. In March 2009, a detailed high-frequency seismic survey was undertaken in Perseverance and Northeast Harbours and on the eastern shelf of the island. Data recorded included single-channel Chirp and electro-acoustic (boomer) sub-bottom imaging, and interferometric side scanning sonar (C3D). A network of ~42 lines was collected that provided full C3D bathymetric coverage of the seafloor within the harbours and good coverage on the shelf. Data were collected in water depths from <10 m in some parts of the harbours to >150 m on the eastern part of the shelf. Unusually calm and stable weather conditions resulted in ideal conditions for data collection; data quality is high. Boomer data successfully imaged the upper 20 to 60 m of sub-seafloor sediments and sedimentary rocks. Chirp data imaged a maximum of 20 m. Preliminary results suggest that terminal moraines co-incide with the mouths of Perseverance and Northeast Harbours. Present erosion and sediment transport off the island are expected to be minimal. This would suggest that glaciers did not extend out onto the present-day shelf of the island during the last glacial lowstand. Observations also support the existence of sub-aerially eroded channels on the eastern shelf of the island that appear to be sourced from glacial valleys onshore. Further investigations are required to link glacial and periglacial processes with the development of these channels.