The land is fickle. Uplift and denudation can make paleoclimate science on land difficult in some cases and impossible in others. Oh, to think of all the beautiful terrestrial climate records that have been lost to the sands (or silts or clays) of time! Much more reliable is the ocean, where change is measured not in years or decades, but millenia.
In central Westland, the pollen and spores of terrestrial plants find their way out to sea in the suspended sediment of rivers and their marine counterparts, submarine canyons (think giant underwater rivers snaking their way down the steep continental slope). At one point on the true right levee of the Hokitika Canyon, 3.2 metres of sediment (including entombed pollen and marine microfossils) has slowly and steadily accumulated over the past 210,000 years. Collected by NIWA’s RV Tangaroa in 2005, this record became the basis the longest paleovegetation record for Westland to date.
The MSc study, recently completed by Matt Ryan at the ARC, documented central Westland vegetation succession throughout the last two glacial cycles. By counting the proportions of various pollen taxa in samples taken down the full length of the core, a picture emerged of changing land vegetation over time. However, relating these changes to known climatic events required definitive age control.
δ18O or ‘delta-O-18′ is a measure of the ratio between the stable isotopes of oxygen (18O:16O). This ratio in the shells of benthic foraminifera is an indirect proxy for global ice volume: evaporation preferentially removes the lighter isotope, 16O from the ocean, so when the resulting precipitation falls in the form of snow on an ice sheet, the ocean is left enriched in oxygen’s heavier isotope, 18O. Since this proxy measures a global signal, new records can be confidently tied to existing, independently dated oxygen isotope curves. A δ18O curve was generated for this core from the benthic foram species, Globigerina bulliodes, and was tied to the global benthic isotope stack of Lisiecki and Raymo (2005).
By comparing the vegetation story to sea surface temperature records, Matt was able to see which tree species colonised the post-glacial valleys of Westland first, replacing the herb and shrub taxa characteristic of ice ages. While many podocarp/hardwood species respond immediately to interglacial warming on the West Coast, a re-dated pollen record from off the coast of South Canterbury shows that there is a significant delay in vegetation response to the east of the South Island’s main divide. Also, the ‘beech-gap’, a mysterious present day absence of southern beech forest across central Westland, is explained in terms of out-competing by tall tree taxa during the last deglaciation.
The large scope and workload of this Master’s project has been good practice for Matt, who is about to embark on a PhD in which he will once again greatly extend the paleovegetation record of Westland, this time as far back as the intriguing warm interglacial and Holocene analogue, Marine Isotope Stage 11. Good on ya Matt and all the best!
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Figures by Matt Ryan. Turbidity flow schematic modified from Peakall et al. (2000). Hokitika Canyon multibeam bathymetry data courtesy of NIWA. This research was made possible by the supervision of Dr. Gavin Dunbar and Dr. Michael Hannah.
