Seismic and cultural influences on the sedimentary record, Wellington Harbour, New Zealand

Abstract

Wellington Harbour is a sub-circular basin with an average water depth of 14 m and a surface area of 85 km2. The harbour basin was drowned by the post-glacial rise in sea level some 10,000 years ago. Since that time it has been trapping sediment, largely transported via the Hutt River, which has a catchment 642 km2 in area. Mud sized sediment is dispersed throughout the harbour, whilst sand is confined to the harbour margins to depths of around 5 m. This study of seismic and cultural influences on sedimentation in the harbour is based on eight cores from five distinct sedimentary environments. The cores vary in length from 2.5 to 6 m representing a sedimentary record from <1000 years at Petone to ~10,000 years (early Holocene, at Seaview) and beyond (Evans Bay). Chronological control has been established for the cores using a variety of techniques including 14C dating, palynology (especially the first appearance of the exotic family Pinaceae), trace element chemistry (principally Pb, Zn and Cu) and the recognition of the 1855 earthquake event in nearshore cores from Petone Wharf. The base of a mussel layer surrounding the wharf immediately post dates its construction in 1907 and provides another chronological marker. Two cores from Petone Wharf each show an abrupt decrease in mud content of 15 - 20% at 3.75 and 4.25 m below sea floor. This drop in mud content is interpreted in terms of seafloor shallowing caused by the 1855 earthquake. The amount of uplift is estimated by using mud content as a proxy for water depth (that is, increasing mud content equates to increasing water depth) through a relationship established directly by taking samples from the beach to 12 m water depth. The data were sufficient to estimate the amount of uplift at 1.5 - 2.7 m. It is also evident that the mud/depth relationship has probably been affected in European times (post 1840) due to much greater sediment input into the harbour. Chronological markers in the two cores suggest that sedimentation has increased by a factor of at least 10, from ~2 mmyr-1 prior to 1855 to 20 - 30 mmyr-1 since then. Two cores from the centre of the harbour record sedimentation entirely from settling with a uniform mud content of 98%. Dating is heavily dependent on a single 14C age of 3941 - 3709 cal BP at the base of WH93-3 and on palynological and pollution chronological markers around the turn of the century. Sedimentation rate prior to this was less than 1 mmyr-1, but afterwards lay in the 8 - 15 mmyr-1 range. Two cores towards the harbour mouth, on the edge of the Eastbourne platform, show more variation in sediment texture than those from the centre of the harbour, although over the long term an equilibrium level is apparent. Sedimentation at these sites is subject to both the influence of the Hutt River and longshore transport of sediment through the harbour entrance from the south coast. The sedimentary record at Seaview, on the rocky eastern shore of the harbour, consists of early Holocene gravelly sands beneath a 3 m thick shell bed. A radiocarbon age of 8833 - 8484 cal BP was obtained from a lower shell bed comprised primarily of the intertidal species Mytilus edulis aoteanus. This layer is presently located 17 m below current sea-level and is considered to have been deposited close to the sea-level that existed 8500 years ago. A eustatic sea-level curve for New Zealand shows that at this time sea-level was some 17 m below that of the present day. This indicates that little net vertical tectonic movement has occurred at this site during the Holocene. Seabore-1 was cored in Evans Bay, a sheltered embayment at the southern end of Wellington Harbour. A lithologically distinct organic layer 4 m bsf in Seabore-1 is considered to have formed during estuarine conditions quite different from the marine environment found in Evans Bay today. A break in deposition occurred after the deposition of this layer, possibly associated with a channel between Evans Bay and the open sea. The channel then closed allowing deposition of the overlying sandy silt. A study of pollen around the harbour margins and in harbour sediment shows that most pollen enters the harbour via the Hutt River and is well mixed and widely dispersed. The complete clearing of native timber trees surrounding the harbour and subsequent replacement with grassland has not yielded a corresponding change in the pollen spectra, although the appearance of exotic pollen and an increase in grass pollen was noted. This is thought to be due to the supply of native tree pollen from the entire Hutt River catchment to the harbour which is a much greater contributor of pollen than the immediate surrounds of the harbour. The ten fold increase in sedimentation rate in the mid - late nineteenth century is considered to be principally due to accelerated erosion in the Hutt River catchment caused by extensive forest clearance by early settlers. As the distribution of pollen shows that Hutt River sediment influences the entire harbour, such clearing would increase sedimentation rates everywhere within the harbour basin. No indication was found of any effect of earthquakes on sedimentation in the harbour apart from uplift of the sea floor in 1855.