Late Quaternary vegetation and climate history reconstructed from palynology of marine cores off southwestern New Zealand

Abstract

<p>Little is known about how mid-latitude Southern Hemisphere terrestrial vegetation responded during glacial terminations and the warmer phases of the Late Quaternary, especially beyond the last glacial cycle where records are commonly fragmentary and poorly-dated. The timing, magnitude and sequence of environmental changes are investigated here for terminations (T) I, II and V and their subsequent warm interglacials of MIS 1, 5e and 11 by direct correlation of terrestrial palynomorphs (pollen and spores) and marine climate indicators in marine piston cores MD06-2990/2991 recovered from the East Tasman Sea, west of South Island, New Zealand. The climate there is strongly influenced by the prevailing mid-latitude westerly wind belt that generates significant amounts of orographic rainfall and the proximity of the ocean which moderates temperature variability. Chronological constraint for the cores is provided by δ¹⁸O stratigraphy, radiocarbon chronology and the identification of two widespread silicic tephra horizons (25.6 ka Kawakawa/Oruanui Tephra (KOT); ~345 ka Rangitawa Tephra (RtT)) sourced from the central North Island. Similar vegetation changes over the last two glacial cycles at MD06-2991 and in the adjacent nearby on land record of vegetation-climate change from Okarito Bog permit transfer of the well resolved Marine Isotope Stage (MIS) chronology to Okarito for the pre radiocarbon dated interval (~139-28 ka). Placing both sequences on a common age scale nonetheless assumes there is minimal lag between pollen production and final deposition on the seafloor. However, the timing of Late Pleistocene palynomorph events and KOT between independently dated marine and terrestrial sedimentary sequences are found in this study to be indistinguishable, which supports the direct transfer of terrestrially derived ages to the marine realm and vice versa. Vegetation change in southwestern New Zealand is of similar structure during T-I and T-II, despite different amplitudes of forcing (i.e., insolation rise, CO₂ concentrations). In a climate amelioration scenario, shrubland-grassland gave rise to dominantly podocarp-broadleaf forest taxa, with accompanying rises in mean annual air temperature (MAAT) estimated from Okarito pollen typically synchronous with nearby ocean temperatures. The T-II amelioration commenced after ~139 ka in response to increasing boreal summer insolation intensity, with prominent ocean-atmosphere warming over the period from ~133-130 ka. In contrast, northern mid-high latitude paleoclimate records display cooling over Heinrich Stadial 11 (~135-130 ka), and are prominently warm from ~130-128 ka, while southwestern New Zealand and the adjacent ocean displays cooling. Such millennial-scale climate asynchrony between the hemispheres is most likely a result of a systematic, but non-linear re-organisation of the ocean-atmosphere circulation system in response to orbital forcing. The subsequent MIS 5e climatic optimum in Westland was between ~128-123 ka, with maximum temperatures reconstructed in the ocean and atmosphere of 2.5°C and 1.5°C higher than present. Similarities revealed between land and sea pollen records in southwestern New Zealand over the last ~160 ka offer confidence for assessing vegetation and climate for older intervals, including T-V/MIS 11, for which no adjacent terrestrial equivalents currently exist. Vegetation change over T-V is similar to T-II and T-I, with southern warming antiphased with northern mid-high latitude cooling. Tall trees and the thermophilous shrub Ascarina lucida define interglacial conditions in the study region between ~428-396 ka. East Tasman Sea surface temperatures rose in two phases; 435-426 ka (MIS 12a-MIS 11e) and 417-407 ka (MIS 11c climatic optimum), reaching at least ~1.5-2°C warmer than present over the latter. Similarly, Ascarina lucida dominance over MIS 11c is akin to that displayed during the early Holocene climatic optimum (11.5-9 ka) in west-central North Island, where MAAT average ~3°C higher today. This contrasts markedly with the dominance of the tall tree conifer Dacrydium cupressinum for the Holocene (MIS 1) and last interglacial (MIS 5e) in southwestern New Zealand. Biogeographic barriers are proposed to have inhibited the migration of species from more northerly latitudes better adapted to warmer climatic conditions over MIS 5e and MIS 11.</p>