Ice ages unearthed

Clues to New Zealand’s past have been unearthed in a peat bog on the west coast of the South Island. Dr Marcus Vandergoes of GNS Science has taken 10 m deep cores (5 cm diameter columns of material) from Ōkārito Pākihi bog near Franz Joseph Glacier in Westland. These contain a record of vegetation and climate change during the two most recent glacialinterglacial climate cycles. Marcus has used different dating methods to find out when, and how fast, environmental changes happened.

Reconstructing environments

Ōkārito Pākihi is now a peat bog, but until 10,000 years ago, it was a swampy lake. Sediments washed into this lake from the surrounding land, along with plant material from nearby vegetation. Over the years, layer after layer of sediment built up, trapping and preserving pollen, leaves and wood. When the lake became a peat bog, the layers continued to build, but mainly formed from organic remains rather than sediments.

Marcus collected 165 pollen samples, at approximately 10cm intervals down the core. The species found at different depths told him how the vegetation had changed over time. Because different plants live in different conditions, he was also able to reconstruct how the environment had changed over time.

Marcus could recognise four main climate periods in this pollen evidence, from the top of the core (youngest) to the bottom of the core (oldest) – 2nd interglacial (warm), 2nd glacial (cold), 1st interglacial (warm) and 1st glacial (cold).

Dating environmental events

Marcus wanted to know if climate changes at Ōkārito Pākihi happened at the same time as other places in New Zealand and round the world. To do this, he needed to know some actual dates.

There was a layer of tephra (volcanic ash) nearly 4 m down the core from a volcanic eruption during the 2nd glacial period. The chemical make-up matched ash from the Kawakawa tephra found and dated at other places. This was from an eruption of the Taupō volcano about 27,000 years ago. Layers above the tephra were therefore younger than 27,000 years old, and layers below it were older.

The change from 2nd glacial to 2nd interglacial, with a retreat of ice, was about 3m down the core. Plant material from several layers about this point was dated using conventional and accelerator mass spectrometry radiocarbon dating. To get enough material for dating, Marcus developed a technique for concentrating the pollen from sediments. Results showed that the climate change was quite fast – between about 14,500 and 11,000 years ago.

The 2nd glacial period started over 1m below the 27,000 year old tephra, so would be quite a bit older. The sediments contained less organic material and were too old for precise radiocarbon dating. Marcus therefore sent samples to the University of Cologne, Germany, for dating by optically stimulated luminescence (OSL). This method is used on quartz grains trapped in buried sediments. A number of dates were obtained, from 75,000 to 48,000 years ago.

Vegetation and sediment evidence for the end of the 1st glacial was found nearly 9 m down the core. This was dated to 127,000 years ago using OSL.

Marcus has put together a well dated picture of climate change in southern New Zealand during the last two glacial/interglacial cycles. This helps scientists see how quickly climate has changed in the past and adds information to models of future changes. The dating is also important because it can be compared with similar research in other countries. Scientists can then find out if glacial/interglacial climate cycles happened at the same time all over the Earth.