Tidal zonation of surface biota in a salt marsh and an assessment of their utility for developing a sea-level curve for the late Holocene awaroa inlet, New Zealand

Abstract

The salt marsh in Awaroa Inlet, a technically stable site with a 1700-year sedimentation record, was studied to assess its appropriateness to reconstruct a finescale, late Holocene sea-level curve for New Zealand. This study focused on understanding the tidally controlled surface zonation of the Inlet. An assessment was made of the applicability of relating modern tidal zonation of vegetation and foraminiferal assemblages on the marsh surface to assemblages possibly preserved in subsurface sediments. Tide levels in the Inlet were estimated to a high level of accuracy using data from a temporary tide gauge installed in Awaroa Inlet, and a tide survey in the Inlet. Comparison of tides in Awaroa Bay to those in the Inlet highlighted a time lag and tide elevation difference at high tide due to friction of the marsh surface on the flood tide, but showed no significant difference at low tide. Tidal cycles in Awaroa Bay corresponded very closely to those at the permanent tide gauge at the Port of Nelson, meaning that tide levels from Nelson could be extrapolated to the study site. A detailed survey of Awaroa Inlet was undertaken, and the surface biota and sediment were analysed to assess the modern tidal zonation on the marsh surface. The three modern agglutinating foraminiferal associations described in Awaroa Inlet were found to have moderately- to well-constrained vertical ranges, between mean high water neaps and highest astronomical tide. Two calcareous foraminiferal associations were well constrained to below mean high water neaps. Six zones of vegetation were described. The vegetation zones corresponded well to modern tide levels of mean high water neaps, mean high water, mean high water springs, highest astronomical tide, and above tidal inundation respectively. The distribution of sediment showed a general pattern of fining of grain size with increased elevation through the Inlet, but no clear zonation. Although defined surface zonation was found, Awaroa Inlet would not be an ideal site to reconstruct a fine-scale late Holocene sea-level curve because of potential poor preservation of the modern zones in subsurface sediments. The methodology used in this study would be useful for assessing surface biota zonation at other sites around New Zealand.