Southern Alps dissolving almost before our eyes
We’ve seen the massive boulders and rockslides tumbling down the slopes of the Southern Alps after torrential rain or earthquakes.
But part of the Alps is also disappearing into thin air.
Or more accurately, simply dissolving and going down the river.
For her master of science thesis, University of Otago geography postgraduate student Sophie Horton did the calculations.
She found on average about 5 per cent of the total material eroded from the Alps each year becomes unseen chemicals coursing down the major alpine rivers of the South Island.
Horton told the joint Meteorological Society-Hydrological Society conference in Christchurch that in some sampling she and her colleagues carried out, up to 44 per cent of those weathered bits of New Zealand dissolved into the waters and appeared downstream, never to be seen again.
The largest proportion of the chemicals in this dissolved sediment load was calcium bicarbonate, but calcium and sulphate were also significant constituents.
Alpine rivers had a bedload of rocks tumbling along and becoming physically weathered, she said.
‘‘As that breaks down, that creates what we know as suspended sediment, which has for a long time been the cool kid on the block.
‘‘Everybody has kind of wanted to get their hands on suspended sediment and quantify it – where it’s come from, how much we’ve got.
‘‘The Southern Alps has a lot of suspended sediment, relative to global standards. That’s because of our tectonic uplift – exhuming all that rock and chucking it down the rivers every year.
‘‘I’m not interested in any of that, that much. What I care about is, as we get the rain coming through over the West Coast, intense rain, we get ion exchange, we get carbonation, we get hydrolysis. And that suspended sediment then ends up being dissolved quite quickly.’’
In the central western part of the Alps alone – close to the Franz Josef and Fox glaciers, where the most intense rain fell and the uplift along the Alpine Fault was at its greatest – the total dissolved sediments from greywacke and schist rocks came to about 1700 tonnes per square kilometre annually, Horton said.
However, in the Otago schist region it was much lower, most likely due to different geology, lower rainfall and other processes involved in the weathering.
Eighty-four river catchment sites were sampled along and on either side of the Alps, from the Paparoa River in the north to the Greenstone River in the south.
‘‘I wanted to get regional. There have been studies done on dissolved load, and its importance in the Southern Alps, either single catchments or specific areas. I wanted to be a bit ambitious and go across the whole Southern Alps.’’
Colleagues had been collecting dissolved load data in rivers since 2013, she said.
The upper reaches of the Rakaia River, which contains dissolved chemicals from rocks along the Main Divide.