A paleoenvironmental history of Lake Kohangapiripiri, Fitzroy Bay, Wellington

Abstract

Lake Kohangapiripiri is a small coastal lake in Fitzroy Bay, Wellington. It is situated at the seaward end of Cameron Creek and is separated from the sea by a gravel barrier. The lake is partially surrounded by wave-cut cliffs indicating it was once an inlet of the sea.

Five cores were taken from a wetland on the edge of the lake to a maximum depth of 9.25m. Six radiocarbon dates give chronologic control to the record. Sediment beneath the lake is brownish black and >88% mud. Grain size and organic carbon content are uniform throughout the core sequence indicating a stable depositional environment for the last 7000 (calibrated) years BP.

Ninetyfive species of diatoms were identified from 65 counted samples. The diatom flora of the sediment shows that a relatively fresh water lake has been in place since at least 7000 years BP. Salt spray has been blown into the lake throughout the 7000 years permitting brackish diatoms to survive. The lake was closer to sea level and the barrier lower and narrower between 7000 and 5850 years BP, allowing storm waves to wash marine diatoms into the lake. The lake was relatively deep between 5100 and 3500 years BP and has been shallowing since 2500 years BP. Wetlands have been forming around the lake edge since about 800 years BP.

Lake Kohangapiripiri lies between Turakirae Head and Wellington Harbour both of which have well documented Holocene earthquake histories. Comparisons of the lake record with these locations shows that the lake environment is relatively insensitive to earthquake events. Uplift in the 1855 earthquake resulted in a raised beach on the barrier in front of the lake. Recent uplift is likely to have aided in the removal of the lake from direct marine influence and in the shallowing of the lake. However, the lake sediment records no obvious indication of earthquake events. Changes in the diatom assemblage, especially at 5850 years BP could be the result of an earthquake but do not demonstrate the event absolutely.