Karelia, ¹ 46, June 12, 2014
Global warming seems to be getting more and more obvious each year. Yet, what are the causes of climate change? Is it the result of human activities or of natural factors? To make this out one should look into the past – hasn’t anything like this happened on the Earth before, in eras bygone? A helpful tool here is palaeolimnology, the scientific discipline investigating bottom sediments of lakes and other freshwater bodies. Such studies are underway at the Northern Water Problems Institute of the Karelian Research Centre RAS. According to the Institute’s Director Dmitry Subetto, by studying bottom sediments one can try to remodel the climate changes of the remote past. Dmitry Alexandrovich told his colleague scientists about some of such studies in his recent presentation made at the Karelian Research Centre.
bottom sediment drilling rig (D. Subetto in the middle)
Climate fluctuations are something quite ordinary. For instance, the relatively warm era of 100-150 thousands years BP was superseded by a prolonged ice age. Some 10-11 thousands years BP the glacier finally retreated, and the climatic temperature curve started rising. Note that natural temperature fluctuations during this time slice were range extensive. The world climate was at times getting cooler, like in the Middle Ages, when Europe suffered of Siberian frosts, and at times warmer, almost like today. Knowing this, should the humankind be exaggerating the role its plays in climate change?
Bottom sediments can be regarded as a kind of archives from the past, as Dmitry Subetto put it. The material brought over by wind, meltwater, discharges is deposited on the bottom of reservoirs. This material bears information about changes that have occurred in vegetation, landscapes, climate parameters. For instance, gray, silty sediment layers with little organic matter form when the climate is cold, when glaciers and tundra landscapes prevail. When the climate gets warmer, organic-rich silts begin to accumulate on the bottom of reservoirs. Changes in the temperature, precipitation volumes, other climatic parameters in certain eras can be judged by the characteristics and composition of bottom sediments.
Lake El’gygytgyn in Chukotka is 3.6 million years old. It appeared as a great fireball hit the ground, and although located within the permafrost zone, its 12-kilometre basin remained outside the ice shield during the cold period. The glacier did not affect this Chukotka lake, leaving the contents of its basin intact. The lake therefore contains an impressive body of bottom sediments. This body is a replete depository of information, including facts of environmental change. Hydrologists from a number of countries carried out palaeolimnological surveys on the lake from 2008 to 2010. The deposits were drilled to a depth of hundreds of metres. The materials derived will suffice for months or years of treatment and interpreting.
Unlike the Chukotka long-liver, lakes Onega and Ladoga are comparatively young. They’ve existed the way we know them for some 12-15 thousands years. At one point in time, the waterbodies that had been there before were covered by large ice sheets. The glaciers persisted, moving and shaving the landscape, wiping off everything on their way, including the bottom sediments of prehistoric lakes. Some remnants of ancient sediments scientists would be so happy to find could only survive in some depressions or be trapped in some bedrock fissures.
The palaeolimnology of nearly all large and relatively large lakes of Western Europe has by now been quite well investigated. Ladoga and Onega are the only large European lakes which bottom sediments have not yet been thoroughly studied. In fact, it turns out the Russian database is rather poor and just being assembled.
First interest in investigating bottom sediments in lakes Ladoga and Onega appeared in the 1930s. In the following years the studies were performed by researchers from the Karelian Research Centre’s institutes of Geology, Biology, Northern Water Problems. In 2013, Karelian scientists together with colleagues from the Arctic and Antarctic Research Institute (St. Petersburg) carried out palaeolimnological surveys of bottom sediments in Lake Ladoga.
What have we learned as a result? An important finding was the traces of modern tectonic activity, confirming that Ladoga and Onega lie in an active zone (meaning we can still experience some shocks!). Having pierced twenty metres of sediments (in some sites the ‘Old Testament’ silts were even thicker) the drill reached the bedrock. Quite expectedly, the lower part of the extracted core corresponded to sediments typical of the ice age. Pollen analysis of the deepest part of the core yielded also information about the vegetation now missing from Ladoga shores. For example, oak and elm used to grow there. Hence, there was a period warm enough for their development. Researchers believe some cores may contain the remains of ancient sea silts which could have been retained in fissures, although the glacier has ‘licked off’ most of the sediments.
One can only regret that the bottom sediment ‘chronicle book’ of young Karelian lakes has not so many pages, since the glacier had emptied lake basins, mixed all the material together and carried it away. We can however count on the possibility to decode the information encrypted in deposits from the past ten to twelve thousands years. Further studies may help discover more ‘gold keys’ unveiling the mysteries of the remote past.