Marion Island’s last ice age happened earlier than we thought. Why it matters

The Prince Edward Island group lies about 2300km south-east of South Africa in the Southern Indian Ocean. There are two islands in this group: Prince Edward and Marion Island, both of which are peaks of oceanic shield volcanoes.

Marion Island, the larger and older of the two, is about 293 km² in surface area and is estimated to be younger than 1 million years old. The island is still considered active, with volcanic eruptions recorded in 1980 and 2004. Marion’s average annual air temperature is 6℃ and it receives about 2200 mm rainfall a year.

These islands are significant. They’ve been declared a South African special nature reserve; and, given that they are the only specks of land across thousands of kilometres of open ocean, they are also extremely important sentinels for palaeo-climate research.

This branch of study focuses on investigating how earth surface processes and ecological systems responded to changes in ancient climatic patterns. For example, changes in air circulation (winds), the position of ocean currents and changes in rainfall patterns have a direct effect on the island’s landscape development. Understanding the links between landscape responses and climate change of the past can help us to better predict some of the climate change processes that currently threaten the planet.

Marion Island’s last ice age has been a topic of scientific enquiry since a research expedition discovered glacial striations in 1965. Striations are grooves eroded, or scratched, in the rock surface as a glacier – a large, heavy ‘stream’ of ice – moves over the rock. Before this remarkable discovery, little consideration was given to the fact that the island may have been glaciated, or covered in ice, in the past.

Since then, scientists have worked to learn when that ice age happened and how extensive the glaciation was. More glacial striations have been found on the island and, with the tools and knowledge available to them, research teams have, until now, estimated that Marion Island’s last ice age occurred between 20 000 and 15 000 years ago.

Now, thanks to improved dating techniques and some cutting edge technology, we believe we’ve found the real – and surprising – answer. Our findings suggest that Marion Island’s last major ice age ended prior to 35 000 years ago, and that most of the glaciers had retreated by 17 000 years ago.

These findings are in stark contrast with the original worldview that suggested the entire globe was at a maximum glaciated state around 20 000 years ago. Instead, we see increasingly that the Northern and Southern Hemisphere did not respond equally to climate changes in the past. This mean they most probably do not respond to current climate change in a similar manner.

This can help researchers to better understand the current physical landscape’s age, the history of island’s ecology and the response of Marion Island to climate change since the last glaciation, as well as understanding the island’s response to current and future climate change. These findings also contribute to the greater picture of palaeo-climate change and response across the Southern Ocean, for other sub-Antarctic islands like the Kerguelen Archipelago, Auckland and Campbell Islands or South Georgia.

Calculating the ice age

Our research was made possible by recent improvements in cosmogenic nuclide dating techniques, in particular that of cosmogenic chlorine-36 (³⁶Cl).

Cosmogenic ³⁶Cl is an isotope – an atom with the same number of protons, but a different number of neutrons – of otherwise naturally occurring chlorine ³⁵Cl and can only be produced within a rock surface when cosmic rays (extremely high-energy particles) from outer space pass through our atmosphere and collide with the atoms in the rock. These ³⁶Cl isotopes are produced as long as the rock lies exposed to the open atmosphere. Measuring the quantity of isotopes in the rock surface allows us to calculate how long the rock has been left exposed.

A roche mountonnée is a French name for glacially eroded bedrock that looks like a sheep’s back, such as in this photo. These features are also found on Marion Island.
Elizabeth Rudolph

This principle can be applied to, for example, bedrock that was once glaciated but has since become exposed after the glaciers have retreated (melted out). That means the timing of the last glacial retreat can be established by determining the exposure age of a once glaciated bedrock.

We are a team of geomorphologists – earth scientists – from the universities of Fort Hare, the Free State and South Africa. We sampled rock surfaces of numerous glaciated features on Marion Island to determine their ³⁶Cl exposure ages.

The quantity of ³⁶Cl isotopes was determined using an accelerator mass spectrometer at the Scottish Universities Environmental Research Centre. This showed that Marion Island’s most recent ice age ended prior to 35 000 years ago, and that most of the glaciers had retreated off the island by 17 000 years ago. These dates show that the climate at Marion Island changed from glacial conditions (cold and wet) to interglacial conditions (warmer and drier, or precipitation as rain instead of snow) much earlier than thought.

Our findings agree with more recent hypotheses that suggest the Northern and the Southern hemispheres do not respond synchronously to climatic changes. Researchers have argued in recent years that the Southern Hemisphere may have been at a glacial maximum long before the Northern Hemisphere reached its maximum glaciated state.

This emphasises the importance of studying and understanding the Southern Hemisphere’s response to climate change, and not making assumptions based on Northern Hemispheric studies.

Review

These new insights call for a review of existing theories regarding Marion Island’s landscape history and ecology, mainly because the island surface after glaciation has existed far longer than previously thought.

We also hope our study will serve as a reminder that collaboration across research disciplines, technical expertise and geographical boundaries are imperative to gain a full picture on our earth’s past – and future – climate.

by : Elizabeth M Rudolph, Lecturer in Geography, University of the Free State

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