South­ern Alps dis­solv­ing al­most be­fore our eyes

The Press - - News - Paul Gor­man

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.

JOHN KIRK-AN­DER­SON/STUFF

The up­per reaches of the Rakaia River, which con­tains dis­solved chem­i­cals from rocks along the Main Di­vide.

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