Sedimentation Response to Holocene Landscape Disturbance on the Poverty Bay Continental Margin, East Coast New Zealand

Since human settlement, dramatic landscape changes have occurred on the Raukumara Peninsula, East Coast North Island of New Zealand. In particular, European destruction of native forests for pasture caused accelerated erosion of the mudstone and sandstone dominated hinterland. Sediment eroded from the Raukumara Ranges is primarily carried by three small-catchment river systems, which collectively deliver approximately 70 Mt/y of suspended sediment, representing about 0.3% of total global input to the ocean. Today, the Waipaoa River delivers 15 Mt/y of mud to coastal Poverty Bay, accumulating in an actively subsiding mid-shelf basin and outer shelf lobe. The shelf is bordered along its seaward edge by two emergent ridges, but a significant component of hemipelagic sediment leaks through the 13 km-wide Poverty Gap between the ridges, and is deposited on the slope in a large structural indentation that is heavily incised by the Poverty submarine canyon system. Using Holocene tephrochronology, and accepting near-full capture of Holocene riverine sediment on the shelf and slope, accumulation rates indicate that the modern (post-colonisation) sediment input from the Waipaoa River is probably an order of magnitude higher than the average for the Holocene. Previous studies suggest that a five-times increase in accumulation rates by the early 1900's on the shelf is contemporaneous with deforestation. Modern sediment mass accumulation rates determined from excess ²¹⁰Pb profiles suggest that shelf sedimentation increases seaward, reaching a maximum of 0.9 cm/y on the outer shelf, with no net accumulation apparent on the inner-middle shelf. In general, accumulation rates are an order of magnitude lower on the slope, around 0.1 cm/y, decreasing slightly down-slope. Palynological data show a succession of destruction of native forests by burning, extensive land clearance for pasture, and the establishment of exotic forests. These markers date the arrival of Polynesian settlers and suggest that the sediment accumulation rate on the Poverty slope doubled upon European deforestation.