South­ern Alps dis­solv­ing al­most be­fore our eyes

Marlborough Express - - FRONT PAGE -

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.

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