A Palynological Record of the Vegetation and Climate of Westland Since 210 ka

Abstract

A continuous 210 kyr-long, pollen and spore (palynomorph) profile extracted from a 3.2 m long piston core (TAN0513-14) from the eastern Tasman Sea, ~100 km west of the South Island of New Zealand, is examined. Chronological control is provided by a detailed δ^18O curve measured on Globigerina bulloides from the same core. Although the absolute abundance of palynomorphs is relatively low (~2000 grains/g) in TAN0513-14, the assemblage is unusually diverse throughout the core, and large scale changes in palynomorph content are very similar to that from a terrestrial record from nearby Okarito Pakihi (swamp).

This close correlation between the sites is attributed to (a) sediment (and palynomorphs) having been supplied to the core site via a long narrow feeder canyon with its head adjacent to a single river; (b) the relatively small catchment area discharging sediment (and palynomorphs) from West Coast rivers into the Tasman Sea (c) a narrow continental shelf which limits potential change in catchment size in response to sea level change; and (d) the relatively large pollen source-area and low altitude of Okarito Pakihi. The combination of (c) and (d) suggests that the two sites are primarily recorders of the same “regional” pollen rain. This allows both Okarito and TAN0513-14 to be placed on the same LR04 timescale by correlation of variations in Dacrydium cupressinum and Fuscospora between sites.

The net result is the longest detailed picture of vegetation change in nearby Westland presently available, extending previous paleovegetation reconstructions by 60 kyr. Glacial–Interglacial changes in vegetation are well-defined and generally synchronous with periods of Northern Hemisphere warmth as recognised from foraminiferal δ^18O. Glacial periods are characterised by higher relative abundances of cool climate taxa such as Poaceae and Asteraceae. By contrast interglacials 7a, 5a, c and 3 are characterised by higher levels of beech pollen and lower levels of D. cupressinum and Ascarina lucida suggesting that conditions were cooler relative to those of the present day.

The vegetation during MIS 5e is characterised by the highest proportions of tall tree and shrub pollen and the highest proportions of D. cupressinum in the core (~99%), suggesting conditions similar to or slightly warmer (~2°C) than present. Coastal climate is strongly influenced by ocean temperature due to the great specific heat capacity of the ocean, therefore it is not surprising that atmospheric temperature, as interpreted from palynomorphs, and SST as measured from alkenones and faunal assemblages are strongly correlated. Increases in warmth associated with both records precede negative shifts in δ^18O, which is taken to be indicative of Northern Hemisphere ice sheet melting, by ~4- 6 kyr. Although periods of enhanced Southern Hemisphere insolation intensity (a quantity 180° out of phase with Northern Hemisphere insolation intensity) might be expected to have their strongest effect during periods of low CO2 (e.g. MIS 2, and MIS 6), there is no evidence for this in the TAN0513-14 record. Instead the strongest correlation of insolation intensity is with CO2. This suggests that the primary control on Southern Hemisphere climate resides in the mechanism that regulates the uptake and release of CO2 from the ocean.