Feature

In Days of Cold

What happened 13,000 years ago?

Dr Jamie Shulmeister

Twenty thousand years ago, about one-third of the South Island was covered in ice. Tall forests were confined to Northland, with patchy scrub and grassland covering most of the rest of the country. By 13,500 years ago, ice was retreating rapidly in the Southern Alps and tall forest had spread south to cover most of the North Island. This matched conditions around the world where the great ice sheets that covered much of northern Europe and North America were melting away.

However, at that time the retreat of the ice sheets stopped in Europe, and some glaciers started to readvance. In addition, the climate reverted to near full glacial conditions. This climatic deterioration was first identified from pollen records in Europe as a change from tall trees (representing warm conditions) to ground-covering plants with preferences for Arctic or mountain environments. Of these, the most important was the mountain avens (Dryas octopetala), which lent its name to the time period called the Younger Dryas (YD), at the end of the last Ice Age and spanning between about 13,500 and 11,000 years ago.

There is a long history of research into this event in New Zealand, with a number of early workers suggesting a link between the New Zealand glacial record and the Younger Dryas in Europe. However, lack of a satisfactory correlation and the absence of any evidence of the event in pollen records led to the abandonment of a YD correlation in New Zealand. In particular, the careful dating of the Canavan's Knob moraine at Franz Josef by John Mercer in 1988, which was the most likely candidate for a big YD glacial re-advance, indicated that while a glacial advance occurred, it happened before the Younger Dryas.

While research in New Zealand languished, major advances were taking place overseas in understanding the YD event. These findings turned the YD from an interesting footnote on climate change at the end of the last glaciation into one of the major issues in the subject.

Rapid Changes, Hemispheric Changes

There were two reasons for this. Firstly, it became apparent that the event was caused in northern Europe by very rapidly occurring changes in the North Atlantic Ocean. We now know that the YD cooling in the North Atlantic, which caused summer temperatures to decline by up to 10^oC, took only a few decades, at most, for the cooling to occur. In fact, I and others believe that the 10^oC cooling occurred in less than one year, probably triggered by massive flooding of the North Atlantic by glacial meltwater from the North American ice sheet.

Secondly, there is a global ocean circulation system that cycles water from the Pacific to the Atlantic, via the Indian Ocean in surface currents, and then returns it in the opposite direction by deep currents. This is known as the "thermohaline circulation". The key to this ocean circulation is the massive evaporation of warm water travelling north in the North Atlantic, which concentrates the salt in the water and causes the water to sink as it becomes very dense. This provides the source of the return current to the Pacific.

If the North Atlantic was flooded by freshwater, it would sit as a cap on top of the ocean water and stop the evaporation. This should "turn off" the circulation and thus, changes in the North Atlantic should have effects in the Pacific Ocean and Southern Hemisphere. This threw the spotlight back on the Southern Hemisphere and New Zealand. In 1994, George Denton from the US and Chris Hendy of the University of Waikato re-examined the Canavan's Knob Moraine at Franz Josef. Using new lab procedures they recalculated the age of the moraine and determined it to be of Younger Dryas age.

Since the moraine represents a large readvance of the Franz Josef Glacier, they argued that this showed a major YD event in New Zealand. Since 1994, this work has been widely reported to support claims of YD influence in the Southern Hemisphere.

Did it Happen Here ?

The difficulty with this is twofold. Firstly, there is no clearcut supporting evidence for this event from either glaciers or pollen records in New Zealand. There are some glacial sites which may show the same readvance but in the absence of good age control, the correlation is at best uncertain. The pollen records suggest no changes in vegetation through the YD, unlike the strong changes in Europe.

Secondly, even if the Franz Josef advance is a result of the YD in New Zealand, it does not automatically mean that it represents a cold snap like the one Europe experienced.

In New Zealand, the mountain glaciers are at least as susceptible to changes driven by the amount of snowfall as they are to temperature change. This can be seen today in the readvances of the West Coast glaciers in the 1990s. Few would suggest that this is a particularly cold period.

There was a need to get more evidence to check the Franz Josef result, and that is what we undertook. We looked for a site that should be comparable to Westland.

We ended up in the Cobb Valley in northwest Nelson. Here there was a glacier during the last glaciation and almost certainly during the YD, though it has not survived to the present day.

The kettle holes (small swamps) at the southern end of Lake Sylvester provided a source for pollen cores which indicate that the Cobb Valley area became ice-free about 13,000 years before present (BP)
Photo courtesy of Jamie Shulmeister

We were lucky enough to recover cores from melt-out hollows called kettle holes on the moraines in the valley. These cores contained pollen records that cover the YD period in some detail. Our records clearly show that in northwest Nelson there is no evidence of a change in vegetation that would suggest a cold period.

Analysis of the pollen shows sediment changes during the period of the Younger Dryas (11,000-9,991 years BP). Vegetation changes occurred, most notably between beech and bog pine (Halocarpus). These changes, however, suggest they resulted from flood events or rainfall changes, rather than temperature changes. If it had gotten colder, daisies and lilies would have expanded.
Pollen diagram courtesy of Christine Singer

This does not mean that the Franz Josef work is wrong. It just makes it difficult for the glacial readvance to be the result of a drop in temperature. It could still easily relate to a change in snow fall.

Our work raises more questions than it answers and further research is needed to sort out what really happened in our back yard 13,000 years ago.

Why Look?

One final word. Why should we care about an apparently obscure climate change that happened thousands of years ago?

The reason is simple. The evidence suggests that some climate change does not occur as a gradual shift from one climate to another but rather that big changes can occur in as little as a year.

In north-west Europe, the YD caused a 10^oC change in summer temperatures. If that happened today it would cause the complete failure of north-west European agriculture. The YD lasted for several hundred years, so it would not be a question of just riding it out.

The Denton and Hendy view suggests that a similar change could happen in New Zealand.

We had better understand our climate history to see if these types of changes could happen and if they could, to try and understand what triggers them so that we can either prevent the change or, more likely, at least plan for them.

Dr Jamie Shulmeister is in the Research School of Earth Sciences at Victoria University, Wellington.