ECOLOGICAL DISTRIBUTION OF THE FORAMINIFERA IN A TIDAL LAGOON-BRACKISH LAKE, NEW ZEALAND, AND ITS HOLOCENE ORIGINS

Abstract

The shallow subtidal to intertidal sandy mudflats of an unusual, 6 km 2 , alternating tidal lagoon–brackish lake (Lake Onoke, New Zealand) has a remarkably consistent foraminiferal fauna dominated (>80%) by Miliammina fusca . During intervals when the gravel barrier across its mouth is closed, the salinity in the lake declines and the level may rise to 1 m above the extreme astronomical tide level (EHWS) of the sea outside. A transect of samples through and above the salt marsh on the edge of Lake Onoke identifies three elevation-related foraminiferal zones: M. fusca (up to mid tide level, MSL); M. fusca-Trochamminita salsa (MSL to mean high water level, MHW); and T. salsa-Haplophragmoides wilberti (MHW to 1 m above EHWS). This is the first record of foraminiferal faunas living above EHWS in New Zealand. Foraminiferal faunas in Holocene sediment cores indicate that a much larger, ancestral Lake Onoke (~20 km 2 ) was created 4000–3000 cal yr BP, presumably by the growth of a gravel barrier across the previously open mouth to the sea. During the late Holocene, river sand prograded southwards into ancestral Lake Onoke creating swampy alluvial flats that separate modern Lake Onoke from its freshwater counterpart, Lake Wairarapa. Foraminiferal faunas indicate the early Holocene (6500-4000 cal yr BP) presence of an elongate (30 km-long), mostly intertidal embayment extending up the lower Wairarapa Valley beneath present-day Lakes Onoke and Wairarapa. Foraminifera-based elevation estimates suggest that during the early Holocene, this embayment subsided at a rate of about 1–2 mm per yr, sufficient to accommodate the accumulating mud and retain an extensive area (>20 km 2 ) of tidal flats. A slowing rate of subsidence accompanied by eustatic sea-level fall probably account for late Holocene aggradation.