A foraminiferal sea-level reconstruction from Pauatahanui Inlet, southern North Island, New Zealand

Vertical land movement associated with interseismic subsidence and slow-slip events pose a major complicating factor when understanding how sea-level rise is likely to affect New Zealand's capital city of Wellington. To understand how these factors have affected sea level in the long-term, and thereby gain geological context for these movements, the nearest undisturbed salt marsh (Pauatahanui Inlet) was surface sampled and cored. These samples were used to assess the relationship between species of foraminifera (a group of testate protists) and elevation, then apply these relationships down-core to calculate past sea level. The nearest continuous GPS station (Paekakariki Hill) displays 1.7±0.35 mm/yr subsidence since its installation in the year 2000 (Tenzer and Fadil, 2016). However, according to a variety of transfer functions (statistical techniques which relate foraminiferal assemblage to elevation), our sediment core data indicate that, since the 1855 earthquake (which leaves a very distinct signature in the record), relative sea level has risen at Pauatahanui by ~1.4 to 1.6 mm/yr. This is less than what would be expected from subsidence alone, yet also must incorporate a significant signal from anthropogenic sea-level rise. We therefore interpret that the slow subsidence in the cGPS record likely indicates a short-term signal which, over the course of the past century, has been counteracted by the combined influence of events such as slow-slip associated with the Kapiti Coast source region, post-and co-seismic uplift, and possibly variations in subsidence rate. This, combined with the lower than expected rates of sea-level rise at the Wellington tide gauge from nearby cGPS stations, suggests that it is unwise to base local sea level projections on the observed recent net subsidence alone without factoring in the long-term effect of slow-slip and co-seismic uplift