Microfossil record of the Holocene evolution of coastal wetlands in a tectonically active region of New Zealand

Abstract

The shallow tidal Wairau coastal lagoons, New Zealand, are in a prime location for investigating the relative roles of tectonic and eustatic sea level on their palaeogeographic evolution. The Wairau lagoons are unique in New Zealand for their wide seasonal and tidal salinity range, from hyposaline (10—20 psu) to hypersaline (35—54 psu). Foraminiferal and ostracod associations are recognised, using Q-mode cluster analysis, living in and around these lagoons and detrended canonical correspondence analysis (DCCA) shows that their distributions are strongly correlated with tidal elevation and salinity. Analyses of the modern analogue faunal data combined with Holocene microfaunal data from five 2.5—9 m deep cores enables direct palaeoenvironmental interpretation of the fossil faunas and elucidation of the lagoons’ palaeogeographic evolution. The area was inundated by rising eustatic sea level from 8.5 ka onwards, forming a fully marine, sheltered, subtidal bay. Sediment supply outpaced local tectonic subsidence and the bay filled with mud, shallowing to intertidal by 4.5—3.5 ka, still with an open mouth to the sea. Since then sediment supply has kept pace with 3—4 m of inferred tectonic subsidence. At ~1.5 ka the calcareous-dominated foraminiferal faunas suddenly changed to agglutinate-dominated faunas, indicating a switch to a semi-closed lagoon linked to the Wairau River estuary, with highly varied salinity like today. We infer this was caused by northwards extension of the Wairau Boulder Bank across the bay’s mouth in response to a sharp eustatic sea-level fall after 2 ka. Sediment supply switched to fluvially derived sand which built a flood-delta into the lagoon dividing it into three water bodies. Relative sea-level rise in the last 600 years from earthquake-related compaction (AD 1855) and accelerating eustatic rise (0.6 m) has resulted in increased marginal erosion of the lagoons and their re-amalgamation into one linked water body.