Shira lake sediments help unveil the secrets of erstwhile climate
Scientists from the Institute of Geology and Petroleum Technologies and their overseas colleagues have studied the bottom sediments of Lake Shira in Khakassia, Russia, and found out how climatic and environmental conditions in the region have changed over 150 years.
For this purpose, they conducted a number of geochemical, isotopic, magnetic, seismo-acoustic and radiocarbon studies of the lake, which is 9.4 km long, with a maximum width of 5 km and an area of about 39 km. The results of the research are presented in a paper published in Radiocarbon.
"We studied the bottom sediments of Lake Shira, which is located in the Khakassky state natural biosphere reserve, to find out how the climate has changed in the area of this body of water," says senior researcher Dilyara Kuzina. "The samples were collected during the expedition in 2020. Layer by layer, centimeter by centimeter, we studied one of the core columns with a capacity of 30 centimeters, extracted from the bottom of the lake. Two methods were used to determine the age of the sediments: radiocarbon dating by measuring the radiocarbon (14C) content and determining the 210Pb and 137Cs isotope ratios, which provided a very good age model. In addition, we used elemental and isotopic geochemistry methods to refine the core chronology, as well as measuring magnetic susceptibility and consulting historical records. The sediment layers we studied span the period from approximately 1870 to 2020."
Lake sediments store information from the time of their formation. Like the annual rings of trees, a lake's layers contain data on what the surrounding conditions were like, past climate, and whether it has experienced human impact at certain points in time.
"Lake sediments are excellent archives of past climate change because environmental conditions directly affect the sediment that accumulates on the bottom of bodies of water. For example, in dry and hot weather, lakes receive less water, evaporation is abundant, the concentration of elements that dissolve in water increases, and eventually they can precipitate in the form of salts or carbonates," Kuzina adds.
By studying such sediments, it is possible to decipher what were the conditions of sedimentation.
The senior researcher explains what exactly the international group managed to find out, "We found that the region of this lake had a warm and humid climate from 1870 to 1900, then it changed to a completely opposite type of climate. Between 1900 and 1940, the climate was cold and dry. During this period, Lake Shira became saline and holomictic (vertical mixing of waters to the bottom occurred). In 1940-1963, the lake was stratified (temperature and density of water at different depths differed), and in some places meromictic (this is the name given to a body of water in which there is practically no circulation of water between layers of different mineralization). From 1963 to 2000 there was an increase in the salinity of the lake. At the same time, between 1994 and 2003, the lake became saline-meromictic with oxygen-free conditions in the deep water layers. In the bottom sediments of the lake, anthropogenic impact and industrial pollution have been recorded in recent years."
According to one of the authors of the article, Pavel Krylov, deputy director for ressearch of the Institute of Geology and Petroleum Technologies, the team was the first to obtain data on the thickness of the bottom sediments of Lake Shira and their structure.
"Before that, there was only a depth map obtained with a household echo sounder," he says. "We conducted seismo-acoustic studies and obtained unique transects. The seismic sections were used to determine the bedding structure of the sediments and the most favorable location for coring."
Krylov emphasizes that the places of core sampling of lake sediments should be chosen with special care so that the results of their laboratory studies are the most reliable.
"Preliminary seismo-acoustic studies are carried out very rarely due to the lack of equipment that allows to work on small bodies of water," Krylov shares. "Our geophysicists have developed an electrodynamic acoustic wave emitter, called Boomer, which we use at each site before selecting core columns."
According to paleoclimatologists, lake sediments have been able to store data on environmental and climate changes for thousands of years. Now scientists are studying another core, selected in 2020 during the expedition; its length is 5 meters. It contains a record of how sedimentation conditions in Lake Shira have changed over the past 9,000 years. To date, the magnetic properties of the core have been studied, and it has been found out what changes in the magnetic field occurred during this period.
The work is funded by a Russian Science Foundation grant "General picture of climate change and its impact on the environment and society in the northern and central parts of the Eurasian continent since the last glaciation: reconstruction from geological and paleobiological data", headed by Vice-Rector for Earth Sciences Danis Nurgaliev.
More information:
Accessing old carbon influence on TOC 14C age and environmental change from the recent sediments in Lake Shira, Russia
www.cambridge.org/core/journal … 779165CDE6BE522D3C03
Provided by Kazan Federal University