Holocene sediments of Tamaki Estuary: characterisation and impact of recent human activity on an urban estuary in Auckland, New Zealand

Abstract

The present day Tamaki Estuary is a 17 km long, funnel shaped mesotidal, estuary imposed on the old Tamaki River. The estuary was initially formed by fluvial denudation, and subsequently drowned by the later rise in sea level and then modified by fluvial and marine depositional processes. The estuary is underlain by Miocene Waitemata Group sedimentary rocks and Quaternary rhyolitic and basaltic tephra. Multiproxy analysis, including grain size, magnetic susceptibility, heavy metal, pollen, glass shard, Cl4 and Csl37 analyses were carried out on six long cores and three short cores collected along the estuary. Thirty-six surface samples were also collected for spatial analysis of heavy metals and grain size. Glass shard analysis of cored sediments was used to establish key tephra time markers. Radiocarbon dating and pollen analyses were also used to date cored sediments and calculate sedimentation rates. During the early to late Holocene the rate of sedimentation was around 0.ll-l.6mm/year. During this period, regional vegetation was podocarp/hardwood forest. Maori settlement in the estuary catchment was associated with forest clearance as indicated by increased sedimentation rates (2.4mm/year) and a sharp decline in forest taxa pollen coinciding with an increase in bracken and grass pollens. Land clearance following European settlement (~1840 AD onwards) was accompanied by a sharp increase in sedimentation rates (6.25mm/ year) and the appearance of exotic pollen taxa such as Pinus. Major fossil organisms are foraminifera, ostracods and mollusks. Foraminifera assemblages are mainly calcareous benthonic. Ostracod assemblages are more diverse and indicate brackish water conditions. The most abundant bivalve shell is the cockle Austrovenus stutchburyi. The most identified gastropod was the horn shell Zeacumantus lutulentus. These life forms indicate marine conditions with brackish and fluvial freshwater influxes. Based on radiocarbon dating, marine transgression in the Tamaki estuary started at least 7500 years B.P. The Transgressive System Tract (TST) sediments transgressed the pre-Early Holocene sediments formed of plastic, massive compact grey mud. Sediments of this early transgressive stage consist of sandy mud with shell materials in the lower estuary and mud to muddy sand with thin peat layers in the upper estuary. Deposition of Highstand System Tract (HST) sediments coincided with sea-level stabilisation at or near its present day level c 6500 years BP, and HST sedimentation continues to the present day. Quartz and feldspar form the main sand and silt sized minerals. Clay minerals include smectite, and mixed layer smectite, illite, chlorite and chlorite/kaolinite. The quartz/feldspar (Q/F) ratio can be related to the depositional system tracts with the highest feldspar content associated with the Lowstand System Tract, and reduced feldspar and increasing quartz contents associated with Transgressive and High System Tracts. This relationship needs additional testing but may prove to be a useful tool in identifying depositional system tracts in clastic successions. The concentration of heavy metals in bulk surface sediments can be related to catchment pollutant inputs and grain size variations. A systematic increase in the bulk mud content towards the upper reaches of the estuary is associated with the accumulation of heavy metals in the tidal flats of this upper area. High levels of heavy metals also occur in the fine sediment fraction (< 63µm) in the upper estuary. Significant fine fraction enrichments for Cd, Cu, Pb, and Zn were found in the uppermost layers of all of the cores as compared to the much lower concentration "pristine background" values found at depth. Spatial association of contaminants with industrial areas and yacht anchorages, and the temporal enrichments associated with the significant urbanisation and development of the catchment since 1945, indicate that these pollutants are strongly related to anthropogenic activities. Three indices of contamination (Igeo, EF, and Cd) were calculated to assess the pollutant impact on the cored fine fraction sediments. The average Igeo lies within class 1 suggesting uncontaminated to moderately contaminated sediment quality. EF values indicate a three fold metal enrichment. A modified and generalised equation for the calculation of the degree of contamination (Cd) is proposed in this thesis, and the resulting Cd values indicate a moderate degree of contamination. The bulk sediment concentrations are all below the ANZECC ISQG-H guidelines. However, Pb and Zn concentrations do exceed the ISQG-L values in some of the surface bulk samples in the upper estuary proximal to long established sources of catchment pollutants. The thesis is a multi-disciplinary contribution to the study of Tamaki Estuary in terms of historic development, catchment processes and biological activity, as well as the post-Holocene sedimentary infilling and the associated widespread enrichment of heavy metals in the estuary over the last 50 years.