Abstract

A review of hydrographic section, single-channel seismic profiles, bottom photographs, and nephelometer profiles collected by the USNS Eltanin, plus new seismic and sedimentary data from the RV Rapuhia, provide an insight into the sedimentary regime beneath the Pacific deep western boundary current (DWBC) where it enters the SW Pacific off southernmost New Zealand at Macquarie Ridge and Campbell Plateau. There the DWBC is reinforced by the Antarctic Circumpolar Current (ACC) largely though the generation of deep-reaching eddies formed by interaction of the ACC with the pronounced bathymetry. It is a region of high eddy kinetic energy, which induces widespread abyssal erosion. It is also where the DWBC receives its first major injection of terrigenous sediment, which is transferred from southern New Zealand along the 450+ km-long Solander Channel. From Macquarie Ridge the ACC-DWBC travels northeast onto the Subantarctic Slope of Campbell Plateau. This high, steep, western boundary intensifies the flow, causing erosion along the slope base. East of the scour the 800 km-long Campbell Drift has been deposited. This drift extends to 51 degrees S, where the ACC swings east to continue its circum-Antarctic journey, while the DWBC continues northeast to Bounty Trough, where the combination of abundant sediment supplied by Bounty Channel, a subdued western boundary, and a less energetic DWBC, have allowed the Bounty Fan to form directly across the current path. Apart from localized winnowing near zones of high relief, evidence for widespread DWBC influence on the fan is inconclusive. However, current effects become more apparent north of Bounty Fan, where the steep western boundary resumes in the form of Chatham Rise. Eroded fan sediments have contributed to a series of small drifts at 4400-5000 m depth, whereas at 2400-3400 m depth on the flank of Chatham Rise, the DWBC has molded a sinuous terrace-like drift. Viewed regionally, sedimentation beneath the boundary current south of Chatham Rise contrasts strongly with the regime to the north. The southern current is energetic and terrigenous supply is intermittent, being restricted to glacial lowstands. Consequently, erosion is common, and drifts are few and are themselves subject to erosion. By comparison, the current north of the rise is generally slower and has a more consistent terrigenous supply, particularly near Hikurangi Channel, which has been active irrespective of sea level. Thus widespread drift deposition is favoured.

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