|dc.description.abstract||The Coromandel Harbour is located on the eastern side of the Coromandel Peninsula, North Island, New Zealand. To date, sedimentological research conducted in the harbour has been confined to nearshore areas, with limited data existing for the subtidal regions of the harbour. The primary aim of this thesis is to identify whether and how various human activities in the catchment have altered harbour-wide, intertidal and subtidal, sedimentation rates and sediment geochemistry. A secondary aim is to identify the sedimentary evolution of the whole Coromandel Harbour over broad time scales (i.e. thousands of years).
Sedimentological data has been collected from 17 intertidal and subtidal sediment cores. Cores have been analysed for down-core changes in sediment texture, mineralogy, observational characteristics and geochemistry measured through portable X-ray fluorescence (pXRF). A facies model constructed from this data has been used to interpret the sedimentary development of the harbour. Pre-human and anthropogenic sediment accumulation rates (SARs) have been estimated using radiocarbon dating, qualitative pollen analysis and facies analysis. Anthropogenic heavy metals have been interpreted against pre-human baselines to identify influences on natural contaminant levels, with specific values compared with regional contaminant guidelines to assess ecological threats.
Deeply weathered soils developed in a subaerial environment somewhere between the last interglacial at c.120 ka and the extended last glacial maximum (eLGM) at 29 ka. These soils were overtopped by streambank and floodplain deposits at the eLGM to the onset of the mid-Holocene sea level rise at c.7500 cal yr B.P. As sea level rose, inundated eLGM and early estuarine sediments were initially pyritised in a stratified, restricted marine setting. Over time, sea level rose and the stratification of the harbour was destroyed, ceasing pyritisation. Streams began to rapidly aggrade at the harbour with the positive change in base level, giving early estuarine (c.7500-5000 cal yr B.P) subtidal SARs of ~0.31-0.45 mm/yr. As streams reached stable profiles, SARs decreased to generally conformable rates of 0.25-0.47 mm/yr in the intertidal regions and ~0.1-0.25 mm/yr in the subtidal regions during the pre-Polynesian phase (c.7500-700 cal yr B.P). Polynesian SARs (700-130 cal yr B.P) decreased to ~0.05-0.13 mm/yr. Whole European (1820 A.D-present) SARs in the northern parts of the harbour are ~0.52-0.77 mm/yr and appear to be chiefly related to mining and deforestation. Recent European (1975 A.D-present) SARs are ~3.52-10.37 mm/yr in the southern parts of the harbour and are chiefly related to pine plantation erosion. A secondary depocentre for pine plantation sediments appears to be at the inlet where rates of ~4.98 mm/yr occur.
Only arsenic and mercury exist over Australian and New Zealand Environment and Conservation Council (ANZECC) Interim Sediment Quality Guidelines (ISQG) Low concentrations in anthropogenic sediments analysed. Maximum harbour-wide arsenic concentrations of up to 33.5 mg/kg that exceed the ISQG-Low value of 20 mg/kg are associated with mining related sediments near the Whangarahi Stream mouth. Maximum arsenic concentrations in pine plantation sediments is 22.3 mg/kg. Mercury may also exceed ISQG-Low/High values throughout all harbour sediments, though it is unclear whether mercury has been incorrectly measured by pXRF.||