Using modern intertidal zonation of benthic foraminifera as a proxy for estimating former sea-level: an example from Whanganui Inlet, Northwest Nelson

Abstract

Whanganui Inlet, Northwest Nelson is New Zealand's second largest estuary (2744 hectares). It is structurally bound within the Pakawau sub-basin, which has remained tectonically stable for the last 690ka. Eight cores up to five metres in length were recovered and radiocarbon dated, providing a useful sediment record for foraminiferal assemblage analysis that allow inference of sea levels in Whanganui Inlet for the past ~7000 years. Statistical analysis of modern zonation of benthic foraminifera showed consistently well-constrained vertical ranges within the intertidal zone in southern Whanganui Inlet. Subsequent similar analysis on samples collected from subsurface samples recognised two fossil sample associations, one being dominated by Ammonia parkinsoniana and the other by species Miliammina fusca. These associations were compared to modern indicators and provided the best constraint of MSL (± 0.30m) Furthermore, a marked increase in concentrations (up to 15%) of agglutinated foraminifera M. fusca were used to constrain past MSL to within ±0.50m. Past sea level indicators recognised in this study have been plotted against Gibbs' (1986) sea level curve for comparison. Three sample associations, established from cluster analysis, indicates past MSL was at -4m (± 0.30m) approximately 7300 years BP; a second indicator plots MSL at -1.10m (± 0.30m) around ~5500 years BP and a third, also with an error of ± 0.30m, puts MSL at -0.70m with an inferred age of ~4200 years BP. In addition, the two abundance spikes of M. fusca infer past MSL to have been at -0.85m at ~4750 years BP and -0.50m at an age of ~3900 years BP respectively.