What is a glacier? Understanding the glacial history of the western Arctic
To interpret what we see today both on land and at the seabed, we need to understand how the landscape was different in the past. When we say “past”, we mean on a geologic timeframe!
A key period we are interested in better understanding is a period about 10,000-20,000 years ago, when the climate was much colder and glaciers covered much of Canada. There have been several other geological periods in the last million years, when glaciers covered the landscape; indeed Antarctic and Greenland and parts of Baffin Island are still covered.
But we are most interested in this “recent” one, and in particular, we are hoping to collect some of the first data on the western side of the Mackenzie Trough to understand the imprint that that glaciation has had on the Yukon Shelf. Scientists have had to speculate on the extent and timing of glaciations because they’ve never been out there with such equipment.
Glaciers are huge ice sheets which can be up to 1-2 km thick. Moving “at a glacial pace” the glaciers flow shaping the landscape beneath them – carving out materials (sediment and rocks) in some areas, grinding and depositing mixes of these materials in others. The patterns of removal and deposition of these materials are the key pieces of evidence that we are looking for to reconstruct what happened thousands of years ago… a scientific time-machine!
Why is it important to understand the glacial history?
The glacial history has direct influence on the strength and properties of the seabed sediments. This has implication for foundation stability (for any seabed engineering), the occurrence of landslide events, the temperature of the seabed which influences permafrost and gas hydrate occurrence, seabed habitat, and a host of other spin-offs from our improved understanding of the natural system in the Beaufort Sea can contribute in some way to many local and even global issues.
Investigating the glacial history from the Araon
Once again, we are using all the tools on board the Araon to build up the picture of the seabed and below it and hopefully develop a better understanding of the geological history of the Beaufort Sea. Using the sonar instruments onboard, we can recognize the removal of the sediments as the glacier advanced by the broad and deep cuts in the soft rocks and sediments across the continental shelf. This process created the Mackenzie Trough, a long scar over 100 m deep and 10s of km wide gouged over 100 km out across the shelf. Then, as the glacier retreats, it leaves behind massive deposits of mud, sand and gravel mixes, sometimes entirely mixed-up in what is called a till deposit, and sometimes in well layered sediment beds, like a layer-cake.
Figure 1 - Some areas have a "layer-cake" cover of muds deposited in front of the glacier as it retreated to the coast. The black arrow points to the seabed. The white arrow shows over 40 metres thickness. Below this are mixed sediments, probably from beneath the glacier when it filled the Mackenzie Trough.
Each successive bed of sediment records the conditions of the environment of the time, and we attempt to tease out a history of environmental changes, where the beds represent the “pages” of a history book. We have observed more than 80 m of this mud (with some stones) in some places but 20 to 40 m is commonplace.
Figure 2 - A sediment core has been captured in the steel barrel below the 1500 kg lead weights and retrieved at the Araon’s stern.
While our sediment cores can only reach a few metres into the muds, not the 10s or 100s of metres that the glaciers carved and deposited, they still provide very important information. The cores allow us to physically examine the sediment and better identify the types of deposits. And with a bit of luck, we will find some “datable” material, such as small fossil shells or tiny “foraminifera”. The material can be sent to a lab for radiocarbon dating which tells us how old the shell is.
Figure 3 - Finding a fossil shell (red arrow) in the glacial muds is a bonus for age dating; otherwise we have to “dig” to collect 100s of tiny microfossils in this mud, a tedious task!
Our first impression is that we have collected some high quality data that will build upon existing knowledge. It will take plenty of work to put together a coherent picture of glaciations largely through interpreting the extent and behaviour of the glaciation from our new seismic records.
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