Drilling reveals climatic secrets

By SIMON COLLINS

Scientists have found a unique record of past climates underneath a Northcote park, showing that our current pattern of alternating "El Nino" and "La Nina" weather may have switched on and off at roughly 10,000-year intervals.

A series of holes up to 61m deep drilled in the Onepoto Domain, near the Onewa Rd motorway turnoff, has found that the pattern of warm, wet "La Nina" years and cold, dry "El Nino" years switched on suddenly about 14,800 years ago.

Some time after that it switched off again, then switched on again between 4000 and 5000 years ago, and is continuing.

Canterbury University geologist James Shulmeister said the evidence pointed towards a very long-term cycle. "We don't know how long it ran the first time, but given that the basic conditions - the shape of the Pacific and how the currents operate - have been in place for millions of years, the suspicion must be that the El Nino/La Nina southern oscillation switches on and off regularly," he said.

The Onepoto Domain has turned out to be one of the best records of past climate change in the Southern Hemisphere because it marks the spot of an ancient volcano, also called Onepoto ("short beach").

Auckland University geologist Paul Augustinus said the volcano erupted between 250,000 and 350,000 years ago.

Onepoto was a "maar" volcano, where hot molten rock rising from deep inside the Earth hit a layer of water just below the surface, causing a massive explosion with the force of an atomic bomb and creating a huge crater up to 80m deep.

"If it hits an aquifer under the ground, it turns the water in the aquifer into supercharged steam, so the pressure builds up and it blows its top," Dr Shulmeister said.

For geologists an ancient maar volcano is a godsend, because it has preserved layers of sediment laid down over succeeding years in a deep lake which was isolated from rivers or the sea until a rising sea level breached the crater about 9000 years ago.

At Onepoto, Dr Augustinus and Dr Shulmeister have been able to make out 2943 alternating light and dark layers of material laid down in individual summer and winter seasons in three samples of the earth "core" which they extracted from the domain with an oil-rig-style drill.

In the summer, seasonal blooms of algae grew on the prehistoric lake, alternating with cold winters when the material washed into the lake contained hardly any life forms.

In the most recent sample, laid down about 9500 years ago just before the sea breached the crater, the annual seasonal layers were all about the same thickness, indicating that the El Nino/La Nina cycle was not operating.

In the next sampling of material laid down about 14,800 years ago, blocks of roughly seven years of thick annual layers alternated with about seven years of relatively thin layers when El Nino weather reduced the amount of algae forming in each summer. This pattern lasted for only about 150 years, in two separate periods, before reverting to the more even pattern of the most recent sample.

Finally, the oldest sampling taken from material laid down about 25,000 years ago showed consistent El Nino/La Nina fluctuations at intervals of around two and a half years - more like today's pattern.

Dr Augustinus said there was not enough evidence to say how frequently the El Nino/La Nina pattern switched and off. But Dr Shulmeister's hunch is that the pattern might be switched on and off by the long-term Milankovitch Cycle - a gradual shift in the wobble of the Earth's rotation which tilts the Southern Hemisphere closer to the sun in summer and away from it in winter for about 10,000 years, and then swaps over for the next 10,000 years.

When the tilt reinforces the seasonal extremes by tilting the south towards the sun in summer and away in winter, as at present, there is a bigger gap in temperatures between the Equator and the South Pole. This stirs up stronger winds, and Dr Shulmeister believes that it may trigger the El Nino/La Nina pattern.

In about 5000 years, it will be the Northern Hemisphere's turn to tilt towards the sun in summer and away in winter, and then the winds in the Southern Hemisphere will be calmer.

If Dr Shulmeister is right, the El Nino/La Nina pattern will then switch off again for another 10,000 years.

El Nino and La Nina

* In normal conditions, such as this year, trade winds blow westward across the Pacific, piling up warm water north of New Zealand and bringing us relatively wet weather.

* In El Nino years the trade winds weaken, giving northern New Zealand relatively cold, dry weather. The reverse occurs in La Nina years, bringing warmer, wetter weather.

* These fluctuations are overlain by 20- to 30-year cycles called the Inter-decadal Pacific Oscillation. We have just ended a 30-year period dominated by El Ninos and started a period likely to be dominated by La Ninas.

* Scientists have found even longer periods of about 10,000 years when the whole system of fluctuating El Nino and La Nina years switches on and off.

American Geophysical Union abstracts
Pepper, Shulmeister, Nobes & Augustinus:
Possible ENSO signals prior to the Last Glacial Maximum, during the last deglaciation and early Holocene, from NZ