Pliocene pollen grains reveal a warm, wet Klondike
The Yukon’s gold fields are famous for yielding up the remains of large creatures that roamed the mammoth steppes of Beringia during the Pleistocene - the chilly epoch that lasted from about two and a half million years ago to 11,700 years ago. But scientists are hard-pressed to describe Yukon mammals of the epoch that immediately preceded the Pleistocene.
“We know very little about the physical environment of the Pliocene” - about 5.3 to 2.6 million years ago - says Grant Zazula, Yukon paleontologist. “We have tons and tons of bones from the Pleistocene - mammoth, bison, horses and all these guys - but we know nothing about what mammal life was like during the Pliocene.”
We are, however, beginning to learn about Pliocene plants and the climatic conditions that would have supported them, according to a paper published last year in the academic journal Palynology: “Palynological evidence for a warmer boreal climate in the Late Pliocene of the Yukon Territory, Canada.”
The most intriguing surprise from the Klondike research, for the paper’s lead author, Northumbria University palynologist (pollen scientist) Matthew Pound, is how warm the boreal Yukon was during the epoch just before the Pleistocene. Trees and other plants of the time and place were those of today’s temperate regions, not of our familiar, if threatened, frigid North.
These plant species, as indicated from fossilized pollen, show that annual temperatures in the Pliocene were warmer by about six degrees than today, and that global mean temperatures were two to three degrees warmer, says Pound.
That may not sound like much, but such a rise is sufficient to alter our present world in ways we might not like, he adds.
“Pliocene climates may provide plausible comparative scenarios for interpreting the path of future climate warming during the 21st century,” Pound writes in the introduction to the paper.
The U.K. palynologist has not yet visited the Yukon, but a small but significant bit of the Yukon came to him in 2013, when his friend Robert Lowther, of the University of Leeds, returned from Canada with samples of dark fine-grain earth found in the Klondike gold fields.
Lowther had been surveying the distribution of gold and White Channel Gravel in the central Yukon. Much of what he found was coarse gravel, which doesn’t hold pollen grains well, but a dark band of earth running through that gravel stirred his curiosity. He turned to Pound for help.
Palynology is a branch of paleontology that focuses, literally, on microscopic fossils with organic walls, especially pollen and spores, says Pound. “Most plants produce them in large amounts, so they easily get into sedimentary systems, are easily fossilized and are wonderfully resistant to decay,” he says.
By taking a sample from that dark band in Klondike and dissolving the enveloping minerals and clay in hydrochloric and hydrofluoric acid, these pollens can be concentrated and viewed individually under a microscope.
Pollen grains display characteristic features, depending on the species. “For instance, birch pollen has a sort of rounded triangular shape, with three openings in the corners of these triangles, with little chambers ... Whereas pine pollen has a central body that carries genetic material and has tufts, like Mickey Mouse ears, that allow it to drift in the wind quite easily.”
Beneath his microscope Pound found pollen from vegetation that lived in a much warmer Klondike than we have known since the mammoth heyday.
That warmth plays havoc with Pliocene animal research. If a mammal drops dead in a warm, damp landscape, explains Zazula, its bones are going to be more readily weathered, crushed and redistributed by water than the bones of a creature that has been encased in ice wedges and permafrost, such as those from the chilly, drier Pleistocene.
During the Pliocene, the bones of anything that died on the landscape were probably destroyed, Zazula says. “The kind of soil-formation processes that can take quartz bedrock and crumble it to bits probably did a number on the bones as well.”
“But there are a number of things we can learn from the Pliocene,” adds Pound.
From observing densities of pollen and spores in samples, looking at the climate tolerances of each plant, and then reconstructing an “envelope” in which these plants co-existed, scientists can tell if the region was predominantly forested or open tundra.
Corylus, or hazel, a temperate-region tree, has been especially significant in helping scientists date the mud and soil samples from Cheechako Hill.
Pine was predominant in the Pliocene in central Yukon but disappeared with the onset of the ice age. With the current warming trend, we will likely see pine forests moving north again, says Pound.
Spores from some plants, like Sphagnum moss, reveal the presence of bogs, lakes or rivers or an inland sea, as well as a higher and more geologically active water table.
Vegetation is what we’ve got the biggest grasp on for the Pliocene; it would be fantastic to be able to make predictions about what animals lived then, such as Pliocene equivalents of mammoth, or beaver, says Pound.
Soil samples from the Klondike (Cheechako Hill, Adams Hill and French Hill) have so far revealed no animal matter at all, to Pound’s surprise - no chitinous bits of insects, no tiny rodent teeth, just grains, spores and a few slivers of wood. But the palynologist hopes to be included in any upcoming research into animal life in the Pliocene.
The more we can learn about Yukon’s Pliocene plants, climate and ecosystems, the more we can dare to speculate about the fate of mammalian life in a warm North ... and about the climate challenges we may face in a not-so-distant future, says Pound.
This column is co-ordinated by the Yukon Research Centre at Yukon College with major financial support from Environment Yukon and Yukon College. The articles are archived at http://www.yukoncollege.yk.ca/research/publications/your-yukon