Using foraminiferal faunas as proxies for low tide level in the estimation of Holocene tectonic subsidence, New Zealand

Abstract

Abstract Fossil foraminifera provide some of the most precise reconstructions of former Holocene sea-levels, but these are based on their narrow high tidal zonation. In this study we identify two foraminifera-based proxies for low tide levels in sheltered harbour or estuarine mudflats. One proxy is the increasing relative abundance of two calcareous foraminifera, Elphidium advenum and Haynesina depressulus , which comprise > 5% of the fauna below mean low water neap level in salinities > 30 psu. The second proxy is based on the periodic occurrence of a wedge of low salinity, low pH surface water trapped in the heads of sheltered elongate bays. This water promotes the abundance of intertidal, brackish, agglutinated foraminifera, mostly Miliammina fusca and Haplophragmoides wilberti , and the dissolution of calcareous shelled foraminifera down to ∼ 0.5 m below extreme low water spring level. We use these two foraminiferal proxies, in association with the modern analog technique, to estimate former sea-level heights in four cores of Holocene sediment from two sites in the northern South Island of New Zealand. On the basis of these, we calculate a gradual subsidence rate of 0.55–1.05 m kyr − 1 over the last 4 kyr at Anakoha Bay in the outer Marlborough Sounds, and no significant vertical displacement over the last 6 kyr in Nelson Haven, 70 km to the southwest. The Anakoha Bay results provide independent evidence in support of long-held hypotheses favouring a recent subsidence origin for the deeply-drowned valley systems of the Marlborough Sounds. The Nelson Haven results provide constraints on recent vertical movement across the Waimea-Flaxmore Fault system, which forms the western boundary of the actively uplifting east Nelson mountains.