Effects of Polynesian and European land use on Sedimentation in Coromandel Estuaries

The Coromandel Peninsula has historically been subjected to large scale catchment clearance and waste disposal to water courses associated with early European kauri logging, gum digging and mining. As most sediment runoff eventually ends up in coastal waters, cores of sea floor sediments can provide one of the best records of the impact of such past land use activities. This study was commissioned by the Department of Conservation to provide a preliminary appraisal of the impacts of historical mining and logging activities on sediments in Coromandel estuaries and tests methods for assessing the impact of mining and logging activities on the marine environment. Purpose: To determine sediment impacts on Coromandel estuaries from past land uses for comparison with future forestry clearance and potential mining activities.

Cores were taken from the Firth of Thames (3 sites), Coromandel Harbour (3 sites) and Whangapoua Harbour (2 sites). Core stratigraphy was determined using pollen and radiocarbon dating with particular attention being given to identifying time horizons in the cores corresponding to pre-Polynesian settlement, Polynesian occupation and European times. The sediment texture, sedimentation rates and heavy metal levels from these various stratigraphic levels were then measured and compared to identify anthropogenic influences.

Firth of Thames, Coromandel and Whangapoua Harbours.

NZMG, 1:100,000, POINT Firth of Thames – E2730821 N6448060 Coromandel – E2732138 N6489127 Whangapoua Harbour – E2744479 N6492139

1988 and 1991?

Cores of 0.6-2.0 m length were taken from the Firth of Thames and Coromandel Harbour sites using a Livingston-type piston corer (50 mm ID) from the DSIR research vessel 'Kutere'. Cores were taken from sites in the Whangapoua Harbour by driving pre-split PVC water pipe (50 mm ID) into the tidal flats at low tide. The pipe was then dug out. Split cores were sliced into 1-2 cm thick sections while wet. Any shell layers were bagged as a unit unless they were greater 2 cm thickness. Pollen analyses were undertaken on 1 cm thick sections sliced from the cores at various intervals. Detailed pollen counting was undertaken in some cores, while in others only the types of pollen present and their importance in qualitative terms was noted. Radiocarbon dating was undertaken on shell (mostly cockle Chione). Textural descriptions of the cores are given, and bulk density and water content determined. Sedimentation rates were calculated from the core thickness and time horizons (mm/y) and from the core bulk density and time horizons (gm/cm/y). Selected trace metal concentrations were determined by X-ray fluorescence spectropscopy (XRF). Frequency of collection: One off study

Radiocarbon dating – sample size, redeposition, limitations on age range, age corrections Gaps in collection: Not reported Data quality: Not reported Attribute accuracy: standard techniques were followed. Compression of sediment during coring is difficult to determine, hence sediment units may be thicker and the actual sedimentation rates may be larger than reported here. A new and relatively inexpensive atomic absorption technique was used to determine levels of Ag, Au, Hg and Cd. However, there are reservations about the accuracy of the results and the data is not presented in the report. Biological and physical mixing of sediment layers can affect results. Pollen identification was made difficult by the large amount of undigested material obscuring the grains and by grains that are commonly corroded, contorted or broken. Hence the accuarcy of the pollen percentages is not as high as can be expected in highly organic sediments. Completeness: Limited samples due to expense of dating techniques and chemical analysis

report, tables, figures Digital Format: MS Word, MS Excel

Freely available Sensitivity/Confidentiality: None?

Finished

The abstract reports that four samples are taken from Coromandel Harbour when in fact only three are. Additional comments: Given the potential problem with sediment mixing, the report suggests that more useful and less ambiguous information can be obtained from detailed pollen analysis on a few cores rather then from a less detailed analysis on a larger number of cores. Other dating methods are a valuable cross-check. Radiocarbon dating is useful for sediments older than about 250 years but future investigations should consider the use of stratigraphic techniques for cross-checks on pollen dating over the last 150 years.