Warm intervals in geological time are increasingly used to understand feedbacks, processes and impacts of climates warmer than society has ever experienced. The Late Pliocene (3.6-2.6 million years ago) is one such warm interval that has been the focus of considerable research in the past 5 years. Pollen preserved in the same sedimentary sequences as the gold of the Klondike mining region of the Yukon, Canada provides evidence of how warm the Pliocene was. Pollen fossilises really easily and as it is small (on average around 0.005cm) you don't need to process a lot of sediment to extract it. Pollen is also incredibly diverse and different structures and morphologies allow it to be connected back to it's parent plant. Giving palynologists (palaeontologists who study pollen) the opportunity to reconstruct the vegetation that once grew in an area.
Today the Klondike Mining region has a mean annual temperature of -5C, long cold winters of -23 to -32C and short mild summers of 10-15C. The vegetation that produced the fossil pollen could only have grown in a climate with a mean annual temperature of 1-12C, with winters close to 0C and longer summers. So how much CO2 do you need to generate this level of warming? Well Pliocene CO2 reconstructions are typically between 350-400 ppmv. Our current atmospheric levels? 399 ppmv (www.co2now.org).
The Late Pliocene (3.6–2.6 Ma) was a period of significant global warmth, considered a potential analogue for future anthropogenic climate change. Newly discovered fine-grained sediments from between the gold-bearing lower and upper White Channel Gravels show the presence of a wetland or lake within Bonanza Creek, Dawson Mining District, Yukon. This environment was surrounded by a diverse Pinaceae-dominated boreal forest with significant stands of angiosperms in favourable sites. Quantitative climate reconstructions derived from pollen and spores reveal a mean annual temperature at least 6 °C warmer than today with warm summers and relatively mild winters. Finally, the new pollen assemblage is used to discuss the age of the White Channel Gravels.