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 400023000 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.

Late Holocene geomorphic history of Lake Wairarapa, North Island, New Zealand

“…Sometime between 4 and 3 cal kyr BP, foraminifera-based reconstructions indicate that the rate of subsidence in the southern parts of the Wairarapa Valley drastically decreased and that the Ruamahanga River may have shifted course to discharge further south along the eastern side of the elongated embayment. Together, reduced subsidence and the river's altered course resulted in a rapid infilling of the middle of the embayment with sandy sediment, creating ancestral lakes Wairarapa and Onoke (Hayward et al 2011). This rapid infilling is consistent with the sedimentation rate derived from NZA34897 and NZA34661.…”

“…The lake was once a shallow marine extension of Palliser Bay (Leach & Anderson 1974;Pickrell & Irwin 1978;Kamp 1992;Hayward et al 2011), resulting from the postglacial marine transgression that reached maximum incursion sometime during 7-3 kyr BP (Leach & Anderson 1974;Heath 1979). As the shoreline subsequently receded, the Lower Wairarapa Valley developed into an extensive estuary with the Ruamahanga River prograding south along the eastern side until 3.5-3.1 kyr BP.…”

“…Between 7 and 4 cal kyr BP, foraminiferal faunas indicate an elongate, intertidal bay extended at least 30 km into the southern Wairarapa Valley and sediment accumulation kept pace with subsidence (Hayward et al 2011). Sometime between 4 and 3 cal kyr BP, foraminifera-based reconstructions indicate that the rate of subsidence in the southern parts of the Wairarapa Valley drastically decreased and that the Ruamahanga River may have shifted course to discharge further south along the eastern side of the elongated embayment.…”

Evidence for Past Subduction Earthquakes at a Plate Boundary with Widespread Upper Plate Faulting: Southern Hikurangi Margin, New Zealand

“…Synchroneity. There are no known similar paleosol-silt contacts during this time period at other sites in the region Hayward et al, 2011;Clark, Hayward, et al, 2011).…”

“…5. Synchroneity: In terms of regional synchroneity of submergence, similar paleosol-silt contacts have not been described at other sites in the region Hayward et al, 2011;Clark, Hayward, et al, 2011), but as we shall discuss in the Seismic Sources section, there are correlations to regional tectonic events.…”