Two diatom oxygen isotope records from Naimakka in northern Fennoscandia: implications for Holocene palaeohydrology and atmospheric circulation dynamics
(English)Manuscript (preprint) (Other academic)
This study present past changes in lake water oxygen isotope composition (δ18Olakew) calculated from diatom oxygen isotope composition (δ18Odiatom) from two lake sediment records. The two sediment sequences (9500 and 2800 years) are from open-basin lakes, although with different hydrologies, and are located close to a meteorological station in Naimakka, Northern Fennoscandia. The isotope composition of lake waters was analysed to determine the modern regional hydrological setting. Our results show that changes in the isotope composition of precipitation (δ18Op) is the primary forcing mechanism decreasing or increasing the δ18Olakew values in both lakes. Changes in the isotope signals are interpreted as reflecting regional variations in atmospheric circulation and climatic seasonality. δ18Odiatom from Lake Keitjoru, covering the last 9500 years, shows a depletion trend of 1.3‰ from c. 8000 until around 1500 cal yr BP. This decrease is thought to be due to a long term decrease in the influence of a zonal airflow (strong westerlies, relatively high δ18Op) over Fennoscandia in favor of an increasing proportion of colder meridional airflow (weak westerlies, lower δ18Op) from the north. A synchronous shift to lower δ18O values was recorded in Lake Keitjoru and lake Oikojärvi around 700-600 cal yr BP (~AD 1250-1350), a time when European climate deteriorated into the “Little Ice Age”. We argue that a circulation pattern dominated by meridional airflow, with reduced westerlies over Northern Fennoscandia and high amounts of winter precipitation from southeast or north, replaced a zonal atmospheric circulation pattern at this time. The δ18Odiatom minima c. 400 and 50 cal yr BP (~AD 1550 and AD 1900) is likely also a response to shifts in circulation, which is also seen in lower summer temperatures at that time recorded in tree-rings and from pollen data. The fact that the same isotope shifts have been detected in δ18O records from hydrological different lakes lends support to our conclusion that these records reflect variations in atmospheric circulation pattern.
oxygen isotopes, diatom biogenic silica, atmospheric circulation, Holocene, northern Fennoscandia, North Atlantic Oscillation
IdentifiersURN: urn:nbn:se:su:diva-29440OAI: oai:DiVA.org:su-29440DiVA: diva2:233057