Alaska: Ancient cave sediments provide new climate clues

(Press-News.org) Paul Wilcox, a geologist at the University of Innsbruck, has discovered the first land-based evidence of meltwater pulses from the Cordilleran Ice Sheet during the last ice age about 20,000 years ago. The age of the cave sediments was constrained via optical dating techniques, which is crucial to help piece together the sequence of climate events leading to a warming planet. The results were published in the journal Nature Geoscience.

Around 20,000 years ago, the Cordilleran Ice Sheet in western North America reached its maximum extent. This was followed by warming climate conditions, causing the ice-sheet to melt. While it is generally accepted that increasing energy from a change in Earth’s position relative to the sun played an important role in shaping the climate at this time, the energy from this change was simply not strong enough to be singularly responsible for the ice-sheet to melt and retreat backwards. Additional climate forces were needed to kick-start ice-sheet retreat. However, these climate forces remain enigmatic. 

A team led by Paul Wilcox from the Quaternary Research Group at the Institute of Geology recently came one step closer to solving the mystery with the discovery of 20,000-year-old cave sediment on Prince of Wales Island in Alaska. Michael Meyer, head of the laboratory for optically stimulated luminescence dating and also a member of the Quaternary Research Group, and Daniela Festi, a pollen researcher at GeoSphere Austria, were closely involved in analysing the cave sediment samples.

Examining past climate change is one of the most effective ways to understand the mechanisms and impacts of current human-induced climate change. Insights from geological and historical climate records help refine projections of future climate scenarios – critical for informing effective strategies for both mitigation and adaptation.

Surprising Discovery

Wilcox found the sediment evidence described in the study during field research in remote caves on Prince of Wales Island. ‘The cave sediments struck me as odd, as they contained no organic debris – which is highly unusual in a temperate rainforest setting,’ says Wilcox. In addition, the deposits showed anomalies in weathering: While cave sediments are usually weathered chemically, the rocks discovered by Wilcox appeared like they had been exposed to mechanical weathering processes.

‘I systematically took some samples to find out how old these sediments are,’ says Wilcox, describing the research process. Dating cave sediments is a difficult task, especially when there is no organic material available for the more commonly used radiocarbon dating (14C dating) method.

Cave as Time Capsules

An alternative method of numerically dating sediments is to directly analyse their intrinsic light signals that accumulate over time, using optically stimulated luminescence (OSL) dating. Such an OSL dating approach relies on the presence of sand-sized quartz grains in the sediment and determines the time of the last exposure of the sediment to light. ‘A limiting factor in OSL analysis is the availability of quartz. Although the site is situated in a limestone region, we found just enough quartz residues to make this dating approach viable,’ says Michael Meyer, who has been working intensively on this dating method in various geological contexts for more than ten years.

 ‘OSL dating is a powerful technique for constraining the recent geological past but has been rarely applied to cave sediments for various methodological reasons. As part of this research work, we succeeded for the first time in dating sediment samples from a cave using OSL,’ explains Meyer.

The sediments analysed are between 20,000 and 17,000 years old and were deposited in the cave towards the end of the last ice age. They have remained there untouched to this day: ‘In a way, the cave acted as a kind of time capsule that preserved the sediments for thousands of years,’ says Wilcox.

Preserved pollen was also found within the sediments: ‘This indirectly proves that there may have been ice-free biological refugia during maximum ice-sheet extent in which plants could survive even under extreme climatic conditions,’ explains Daniela Festi, who analysed the pollen data.

Melting Ice Masses

The quartz deposit in the middle of a limestone region proves that the sediments must have been transported over long distances to reach the cave. This is where it links with climate change during the last ice age: The cave sediments are evidence of meltwater events from the Cordilleran Ice Sheet. This is extraordinarily rare, as the vast majority of sediment transported by ice-sheet meltwater ends up in the ocean. ‘This is the first evidence of land-based meltwater events from this ice-sheet,’ says Wilcox.

Cascade Effect in Climate Change

Current research into the mechanisms and drivers of anthropogenic climate change has identified the North Atlantic overturning circulation (AMOC) as a potential tipping point for the global climate system. Paleoclimatic data show that strong meltwater pulses into the North Atlantic – known as Heinrich events – occurred repeatedly during the last glacial cycle, significantly disrupting the region’s heat budget and triggering global climate consequences.

Recent evidence from cave sediments in Alaska reveals that meltwater events in the Northeast Pacific – referred to as Siku events – preceded those in the North Atlantic. ‘This supports the hypothesis that climate changes in the Northeast Pacific were a prerequisite for the shifts observed in the North Atlantic,’ explains Wilcox.

Understanding the spatial and temporal dynamics of these meltwater events, and their impact on both the Atlantic and Pacific Oceans, is critical. Future climate warming may lead to increased freshwater input into these basins – especially the North Atlantic – which could push oceanic overturning circulation systems beyond critical thresholds.

The next step is to investigate additional caves in the region with comparable sediment archives, in order to reconstruct ice-sheet dynamics during the last ice age more comprehensively. Such research will improve our understanding of freshwater input processes in the past and help anticipate their role in shaping ocean and climate systems under future warming scenarios.

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