Southern Alps dissolving almost before our eyes
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.