Foraminiferal Sea-Level Reconstructions From Major New Zealand Cities: Implications for Long-Term Vertical Land Movement Trends from Centennial Baselines

Vertical land movement variability around the coasts of New Zealand has introduced a great deal of uncertainty to projections of future sea-level rise around major coastal cities. To gain an understanding of how this movement has occurred and changed over time, as well as the other factors driving sea-level change around major cities, four new sea-level reconstructions are presented. These records are from salt marshes in Greater Wellington (Pāuatahanui), Dunedin (Aramoana) and Auckland (Catalina Bay and Rangiototo Island. At Pāuatahanui, relative sea-level rise has risen by 1.50 ± 0.59 mm/yr since 1855, with either a gradual or abrupt deceleration during the twentieth century, followed by a rapid acceleration from ~2000-present to >3 mm/yr, which is not observed at the Wellington tide gauge. This change indicated that modern ~1.7 mm/yr land subsidence rates recorded by global positioning systems from adjacent to Pāuatahanui salt marsh over the last 20 years is a recent feature, the onset of which is recorded in the salt marsh record. Had this not been the case, the rate of relative sea-level rise reconstructed at Pāuatahanui would have been much greater. At Aramoana, which lies at the mouth of Otago Harbour, relative sea level has risen by 2.18 ± 0.83 mm/yr since 1881, showing excellent agreement with the Dunedin tide gauge, with a specific ~60-year recurring trend of accelerations and decelerations replicated at both the salt marsh and the tide gauge. This trend is not observed at other southern South Island foraminiferal sea-level reconstructions, and appears to be due to the influence of Southern Annular Mode (SAM) variability, on the basis of comparisons between the rate of relative sea-level rise and SAM trends. The close agreement between the tide gauge (subsiding by only ~0.69 mm/yr) and the salt marsh (whose sea-level record indicates only very marginally more subsidence) suggests that the high degree of subsidence measured in the city centre is localised to the city, and the result of the compression of soft underlying sediments by large man-made structures. The Pāuatahanui and Aramoana sea-level reconstructions yield valuable insights into the processes that have driven relative sea-level change around Greater Wellington and Dunedin, which will assist in understanding how sea-level rise will affect these sites in the future. One key finding has been the revelation of spatial variability in land movement even on what was considered to be the local scale, to the extent that neither Aramoana truly reflects the scale of sea-level rise at the coast of Dunedin city, nor does Pāuatahanui at the coast of Wellington city. The two sea-level reconstructions from Auckland appear to be less reliable than those from elsewhere, but yield valuable insights into the influence of local processes including freshwater runoff triggered by catchment disturbance, species infaunality, and dissolution on foraminiferal sea-level reconstructions.