Environmental and sea-level changes on Banks Peninsula (Canterbury, New Zealand) through three glaciation–interglaciation cycles
Abstract A greater than 200 ka record of marine transgressions and regressions is recorded from a 75 m core from Banks Peninsula, Canterbury, New Zealand. This record comprises thick suites of muddy sediments attributed to back barrier, lake and lagoonal environments alternating with thin soil and loess complexes. These deposits have been dated using radiocarbon and thermoluminescence (TL) techniques supported by proxy data (diatoms, phytoliths, pollen and sedimentology). The aqueous deposits are attributed to three interglacials and an interstadial (Marine Isotope Stages 1, 5a, 5c, 6, and 7). The loesses and paleosols date to the intervening stadials (Isotope Stages 2, 5d (or 6?) and probably 8). On the basis of transgressive beach facies, back barrier swamps and barrier-blocked lake deposits, a partial sea-level curve including data from Isotope Stage 5 is presented. Our data indicate that Banks Peninsula has been tectonically stable over that period and we provide sea-level points that support the existing isotope curve during Stages 5 and 6. Detailed diatom records are limited to Isotope Stage 1 and the latter part of Stage 5. Diatom histories recorded from these stages are remarkably consistent. Both indicate a progressive floral change from marine types through freshwater colonising species to freshwater planktonic assemblages. These reflect parallel histories of coastal evolution during the two interglacials. In both cases, marine transgression in the early part of the isotope phase was followed by lagoon development implying that a gravel spit extended across the embayment from the west. This was succeeded by lake development when the lagoon was cut off by the juncture of the spit with Banks Peninsula. This lake deepened as the coast rotated into swash alignment and the spit was converted into a gravel barrier. The vegetation history of the site indicates that mixed podocarp broadleaf forests, similar to the pre-European flora of Banks Peninsula, occupied the region during Isotope Stages 1 and 7. This contrasts with the palynological interpretation of a marine record (DSDP Site 594) from off the Canterbury coast which suggested that Isotope Stage 7 was markedly cooler than the Holocene. During glacial periods, forest was eliminated and replaced by a tall shrubland of mixed montane and coastal affinities.