Algae offer insights to tsunami risks

The tsunami cottage at Little Pigeon Bay on Canterbury’s Banks Peninsula continues to give up its secrets.

It was badly damaged by two inundations some hours after the November 2016 magnitude 7.8 Kaiko¯ ura earthquakes.

For this research, Niwa scientists went looking for diatoms – tiny single-celled algae that live in water– in the sedimentary deposits left inside the cottage after the water drained away.

They found diatoms that prefer salt water, brackish water, and fresh water.

Seawater ran up the beach and into the cottage. Seawater also surged up a fresh water stream that ran beside the cottage and went into the cottage from behind. This explained the presence of brackish and fresh water diatoms.

‘‘It provided us a unique opportunity to look at sediments that we know were solely the result ... of a tsunami,’’ Niwa’s Darren Ngaru King said.

That was an ‘‘opportunity to understand what a diatom assemblage or a flora looks like from a tsunami’’.

While scientists and civil defence professionals have been alert to tsunami dangers for decades, there is new interest in diatoms as they can help to identify tsunami behaviour.

‘‘If you want to understand your tsunami risk, then you need to understand your tsunami history and that’s ultimately what drives this work,’’ King said.

When researchers examine other sedimentary deposits that might have been caused by a tsunami, they can expect to find similar assemblages of diatoms, he said.

In short, if researchers find seawater and brackish diatoms where they shouldn’t be, then they have to ask how they got there.

‘‘There’s an enormous amount of work taking place in the four corners of the globe looking to better understand tsunami hazard through geologic records,’’ King said.

‘‘When looking to reconstruct events … we use multiple forms of evidence to build up a comprehensive and believable story’’ and diatoms are emerging as a useful form of evidence.

‘‘Descriptions of diatom flora in modern tsunami deposits have not previously been reported in Aotearoa-New Zealand and have only been reported in a handful of international studies,’’ King and colleagues wrote in the December 2020 edition of the journal Marine Micropaleontology.

Previous Niwa research found that one wave arrived about 1am and a second wave arrived about 2.30am on November 14, 2016. The quakes started just after midnight.

Importantly, for researchers, the cottage trapped the sediments, and they did not behave as they usually do in a natural environment.

Also, the deposits were not contaminated after the event. ‘‘Once the sediments were trapped and the water had receded, those sediments lay there preserved for us to use and that hasn’t been experienced in any other part of the world,’’ King said.

King and colleagues found more fresh water diatoms in the back of the cottage and more salt water diatoms in the front of the cottage, which made sense if the tsunamis ran up the stream and mixed waters entered the cottage from the rear.

‘‘Given the growth in importance of diatoms as proxies for reconstructing past earthquakes and tsunamis, this work provides an analogue from Aotearoa-New Zealand to assist the identification and interpretation of tsunami transported diatoms in future palaeoenvironmental studies,’’ the authors concluded.

The cottage was demolished after the tsunami and King understood it had not been rebuilt. It was not occupied when the tsunami arrived.

Little Pigeon Bay is on the north side of Banks Peninsula and opens to the northeast.

Tsunami damage to the historic Little Pigeon Bay cottage.