Whanganui rocks and climate cycles

The study of fossils, combined with accurate dating, gives us a detailed picture of climate and environment changes in New Zealand over the last few million years.

Just west of Whanganui are cliffs made of layer upon layer of sedimentary rocks, formed during the Pleistocene period. This is one of only a few places in the world where a great thickness of rocks of this age can be studied on land. What’s more, many of the layers are full of fossils. These help geologists date the rocks and provide clues to what the environment was like at the time.

Relative dating

The rock layers at Whanganui are stacked up with the oldest at the bottom and the youngest at the top, but tectonic activity has tilted the layers slightly. This means that, if you head west along the beach in your 4WD, which you can at low tide, the rocks you drive past get older.

Absolute dating

Dr Alan Beu from GNS Science is one of several scientists who study the rocks and fossils near Whanganui. To sort out changes over time, Alan needs accurate dates.

One way to get them is to use the fission track method on zircon crystals in thin layers of tephra that have been trapped in the rocks. The tephra is made of ash from ancient eruptions of volcanoes in the central North Island. Two examples are the Onepuhi tephra (0.57 million years ago) and the Kupe tephra (0.64 million years ago). Dates of tephra layers allow Alan to pin down the ages of some of the other layers above and below them.

Climate cycles

Many of the fossil shells in the Whanganui rocks are the same as species alive today, so we know what environment they lived in. Alan uses the fossils in each layer to help work out if the water the sediments were laid down in was deep or shallow and if it was warm or cool.

It turns out that there wasn’t a simple build-up of one layer of sediment on top of another here. The rocks actually record cycles of changes between glacial periods (ice ages with cold temperatures and low sea level) and interglacial periods (warm temperatures and high sea levels).

Each of the main layers near Whanganui represents rocks laid down during interglacials, whereas the boundaries between the layers mark where rock was eroded away during glacial periods. These boundaries are called unconformities.

Oxygen isotopes in plankton

How long did each glacial/interglacial cycle last? To answer this, Alan made use of research carried out on deep-sea cores around the world. These cores provide uninterrupted sequences of rocks going back millions of years. Scientists have measured oxygen isotopes present in the shells of microscopic fossil plankton, called foraminifera, in these rocks.

The isotopes in the fossils show patterns of increase and decrease through time. These changes reflect ocean temperature during glacial and interglacial climate cycles. Seawater during glacial periods contains more oxygen-18, because the lighter oxygen-16 is taken up more by ice. Since living foraminifera take up oxygen from seawater, during glacial periods, they too will contain more oxygen-18.

The rocks containing the foraminifera fossils were dated using radiometric methods and paleomagnetism. They showed that something had happened to the length of the climate cycles. Until a million years ago, each climate cycle lasted about 41,000 years. Since then, the cycles have lasted about 100,000 years. The cycles are related to the Earth’s rotation and its orbit around the Sun, but reasons for the change a million years ago are not known.

Alan has matched the layers of rock at Whanganui to dated oxygen isotope cycles from deep-sea cores.