Dr Alan Beu, a paleontologist at GNS Science, explains why rocks near Whanganui are of world significance. There are few other places on land with such a depth of Pleistocene rocks in a single sequence. The rocks, and the fossils in them, provide evidence for sea-level changes during ice ages and the warm periods between.
DR ALAN BEU
The rocks and cliffs at Whanganui provide only one of about three places in the world or four where there’s a big thickness of rocks of that age, deposited in shallow water, where you can recognise sea-level change in great detail in particular, so there’s a single sequence of rocks extends all the way from Waiōuru in the central North Island to the coast – 4,000 metres of thickness of Pliocene and Pleistocene rock, which is incredibly thick on the world scale.
Well, the important point on the world scale is that the rock layers in the cliffs represent sea-level change through the Pleistocene glacial periods commonly known as the ice ages. For most of geological time, it’s been changing of the order of 50 metres between glacial - interglacial periods, but for the last 800,000 or 900,000 years, it’s been changing on a much bigger scale with a slower time scale and sea-level change of at least 100 metres between glacial - interglacial periods.
So there are lines visible through the rock, descending the cliffs, which are the time gaps, where during the glacial period, the sea went out of the basin altogether, so all you have represented there is the interglacial periods, but you can see these what geologists call unconformities – gaps in the strata representing the glacial periods – and you can map those throughout the basin. So they’re important for confirming world climate change, and the fossils themselves of course identify the environment and we use those to study climate change in more detail.
Glacial - interglacial cycles, I should say, are the main things that control the Earth’s climate. About 2.6 million years ago, for reasons nobody’s entirely clear about but there are probably all sorts of contributing factors, glaciations started to form on the land masses in the Northern Hemisphere. These take up huge volumes of water out of the ocean, so you get large areas of the Earth’s surface covered in ice. Initially, this took roughly 50 metres of water out of the worlds’ ocean, but later the glaciations have been so large and taking so much more water out that sea level drops 130 metres between glacial and interglacial cycles, so these take a long while to build up to a peak and then they rather suddenly melt again, and quite quickly the world goes back into a warm climate.
Department of Earth and Ocean Sciences, University of Waikato
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