Recent sedimentation beneath the Deep Western Boundary Current off northern New Zealand

The triangular shaped region defined by Chatham Rise in the south, the Louisville Seamount Chain in the east and the Pacific/Australian plate boundary in the west is one of the most active sedimentary regions of the world ocean. The SW Pacific Deep Western Boundary Current (DWBC), with a transport of 20 Sv, flows through the area producing regions of scour, nepheloid layers and typical depositional bedforms. Sediment is supplied to the region by turbidity currents directly via Hikurangi Channel and by the DWBC, which removes material from Bounty Fan south of Chatham Rise, as well as by fallout of volcanic ash and pelagic biogenic material. The CaCO3 content of the sediments is very much controlled by terrigenous dilution, with Chatham Rise having ∼ 60%, but Hikurangi Plateau (near the Channel), sited well above the 4750 m-deep CCD, having only ∼ 20%. Although there is much evidence of scour around pinnacles and in scoured, possibly furrowed, mud deposits seen on 3.5 kHz profiles, dilute nepheloid layers, slow geostrophic and measured velocities, photographic and sediment grain-size evidence do not indicate fast flows. Mudwaves are of the irregular, vertically migrating type (rather than progressive antidunes), suggesting flows < ∼ 0.10 m s−1. In common with other Southern Ocean areas, flows may have been strongest during glacial times. Hikurangi Channel delivers its turbidity currents into the path of the DWBC, which sweeps them along to create a deep-sea fan with the characteristics of a contourite drift, here termed a fan-drift. This is mantled with migrating, climbing mudwaves, which are ascribed to deposition from episodic turbidity currents travelling at more than 0.1 m s−1. The fan-drift forms on the right-hand side of a boundary channel along the foot of the 1000 m-high Rapuhia Scarp. It is interpreted as a large right-bank levee formed by turbidity currents overspilling the Channel, which also owes its origin to scour by the DWBC. The turbidite deposits are dominantly mud and the evidence of scour/deposition patterns around a swath-mapped seamount indicate the depositing flows to be consistent with the DWBC flow direction, demonstrating entrainment of the turbidity currents by the deep geostrophic flow. Patterns of sedimentation around the Louisville Seamount Chain indicate action of a strong filament of the DWBC on the east side of the seamounts at least as far north as 37°S, and a relative intensification of the current on the west side of the seamounts north of 38°S.