Impact of reworked foraminifera from an eroding salt marsh on sea-level studies, New Zealand

Abstract

Quantitative sea-level reconstructions using Holocene foraminifera are usually based on transfer function estimates of elevation derived from the tidal-elevation-related zonation of modern salt marsh foraminifera at the same site. Many New Zealand salt marshes are actively eroding as a result of recent sea-level rise exceeding sediment supply, making it difficult to find transects for modern analogue studies that are unaffected by erosion at their seaward ends. At Waikawa Harbour, South Island, New Zealand we show that reworked salt marsh foraminifera from the eroding bank have been mixed in significant quantities with in situ faunas: (1) in the sand flat seaward of the marsh; (2) in the spring tidal marsh on top of the 80-cm-high bank (together with mid tide foraminifera from the sand below the bank); and (3) transported laterally into a nearby non-eroding rush marsh at lower elevation. The fibrous organic-rich salt marsh mud of the bank has been undermined by erosion and large blocks have collapsed to a lower level on the sand flat where the in situ relict faunas are being mixed with newly colonised lower elevation salt marsh foraminifera. These studies show that eroding salt marshes may result in substantial reworking and mixing of foraminiferal tests into the modern faunas thereby compromising their use in sea-level reconstructions. This study prompted us to revisit part of a cored sequence at nearby Catlins Lake and reinterpret a previously inferred earthquake displacement (5-4.5 ka) to be the result of an eroding and collapsing salt marsh bank instead.