The sun is shining, and air feels surprisingly warm when we walk on a 35cm ice that covers a frozen lake in central Finland. The heavy sledges move nicely, because there is not much snow on the ice today. The journey isn’t far either, as we’re by the city of Kuopio, which is surrounded by Finland’s 10th largest lake. Despite the temperature of -10˚, I need to take off my hat – the sun in early March is already warm, or it could be the fact that the heavy sledge is following me obediently.
We’re crossing the icy bay not for sport or holiday activities, nor is it part of a plan to hike to north Pole. Instead, our focus is in research. We are determinedly walking to the middle of the bay to collect a sediment core from its bottom. Mud – or sediments, as geologists call them – are deposited slowly at the bottom of lakes. How quickly mud accumulates varies greatly depending on the body of water, but at this bay in Lake Kallavesi, about 1 centimetre of sediment is deposited annually. Logically, the new sediment is deposited on top of earlier layers, and so sediments are like time machines – the deeper you dig into the older sediments, the further you reach into the past. You can think of sediments like libraries of a lake’s untold stories, and if you can read the words of the sediment core, they can tell amazing stories.
