Abstract:
Foraminiferal microfossils from salt-marsh sediments provide a record of former tidal elevations that can be used to produce sea-level reconstructions and extend the sea-level record back in time through preceding centuries, serving as valuable ‘substitutes’ where tide-gauge records are not available (e.g. pre-1900 for New Zealand). Rates of sea-level rise or fall vary around the world and thus understanding how sea level behaved around New Zealand for the past 500 years is of importance in predicting and preparing for future changes and their impacts on coastal areas around the country. This study was also designed to test the Northern Hemisphere-based hypothesis that a global acceleration of sea-level rise coincided with late 19th century increases in greenhouse gas emissions, which might be driving climate warming. This is the second New Zealand-based and third Southern Hemisphere-based study to use saltmarsh foraminifera to provide an extended record of sea-level for the late Holocene. Three South Island localities with a total of six core sites were studied: Waikawa Harbour (Southland, 3 core sites), Whanganui Inlet (NW Nelson, 2 core sites) and Akaroa Harbour (Banks Peninsula, 1 core site). For each locality, age models were constructed using different dating methodologies, which include AMS 14C, 137Cs, 210Pb, local stratigraphic markers, and palynology. Using modern analogue datasets (local modern foraminiferal transects) containing the assemblage distribution according to elevation from each area, transfer functions were derived to reconstruct past salt marsh heights and derive sea level curves for each site. Using these data, sea level curves were derived, showing that at Waikawa Harbour (WH) and Whanganui Inlet (WI), a steep fall occurred before 1350AD, followed by a slow rise between the 1550’s and around 1900, and that for the last 120 years an average sea level rise of 3.4 ± 0.5 mm/yr is found. These values are within error from the value found for Pounawea, Catlins (2.8 ± 0.5 mm/y – Gehrels et al., 2008). Due to local disturbances, such as storm surges, Akaroa Harbour could not be considered a reliable study site. The values found for the past 120 years in WH and WI do not match the values from New Zealand tide gauges, which could be due to one or a combination of many factors, and include: tide gauge inaccuracy; tectonic uplift at the tide gauge sites; tectonic subsidence at my study sites; foraminiferal proxy inaccuracy; compaction-induced subsidence at the study sites or increased storminess resulting in extreme high water level rising faster than mean sea level.