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Two diatom oxygen isotope records from Naimakka in northern Fennoscandia: implications for Holocene palaeohydrology and atmospheric circulation dynamics
Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
NERC Isotope Geosciences Laboratory, BGS, UK.
NERC Isotope Geosciences Laboratory, BGS, UK.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

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.

Keyword [en]
oxygen isotopes, diatom biogenic silica, atmospheric circulation, Holocene, northern Fennoscandia, North Atlantic Oscillation
National Category
Physical Geography
Identifiers
URN: urn:nbn:se:su:diva-29440OAI: oai:DiVA.org:su-29440DiVA: diva2:233057
Available from: 2009-08-27 Created: 2009-08-27 Last updated: 2010-04-22Bibliographically approved
In thesis
1. Holocene climate and atmospheric circulation changes in northern Fennoscandia: Interpretations from lacustrine oxygen isotope records
Open this publication in new window or tab >>Holocene climate and atmospheric circulation changes in northern Fennoscandia: Interpretations from lacustrine oxygen isotope records
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis investigates how variations in the oxygen isotopic composition of lake waters in northern Fennoscandia are recorded in lake sediment archives, especially diatoms, and how these variations can be used to infer past changes in climate and atmospheric circulation. Results from analyses of the oxygen isotopic composition of lake water samples (δ18Olakew) collected between 2001 and 2006 show that δ18O of northern Fennoscandian lakes is mainly controlled by the isotopic composition of the precipitation (δ18Op). Changes in local δ18Op depend on variations in ambient air temperature and changes in atmospheric circulation that lead to changes in moisture source, vapour transport efficiency, or winter to summer precipitation distribution. This study demonstrates that the amount of isotopic variation in lake water δ18O is determined by a combination of the original δ18Olakew, the amount and timing of the snowmelt, the amount of seasonally specific precipitation and groundwater, any evaporation effects, and lake water residence time. The fact that the same isotope shifts have been detected in various δ18Olakew proxies, derived from hydrologically different lakes, suggests that these records reflect regional atmospheric circulation changes. The results indicate that diatom biogenic silica isotope (δ18Odiatom) records can provide important information about changes in atmospheric circulation that can help explain temperature and precipitation changes during the Holocene. The reconstructed long-term Holocene decreasing δ18Op trend was likely forced by a shift from strong zonal westerly airflow (relatively high δ18Op) in the early Holocene to a more meridional flow pattern (relatively low δ18Op). The large δ18Olakew depletion recorded in the δ18O records around ca. 500 cal yr BP (AD 1450) may be due to a shift to more intense meridional airflow over northern Fennoscandia resulting in an increasing proportion of winter precipitation from the north or southeast. This climate shift probably marks the onset of the so-called Little Ice Age in this region.

Place, publisher, year, edition, pages
Stockholm: Department of Physical Geography and Quaternary Geology, Stockholm University, 2009. 30 p.
Series
Dissertations from the Department of Physical Geography and Quaternary Geology, ISSN 1653-7211 ; 18
Keyword
oxygen isotope, diatom silica, lake sediment, atmospheric circulation, North Atlantic Oscillation, northern Fennoscandia, The Holocene, Little Ice Age
National Category
Physical Geography
Research subject
Physical Geography
Identifiers
urn:nbn:se:su:diva-29343 (URN)978-91-7155-904-3 (ISBN)
Public defence
2009-10-02, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 13:00 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 1: In press. Paper 2: Submitted. Paper 5: In progress.Available from: 2009-09-10 Created: 2009-08-24 Last updated: 2010-04-22Bibliographically approved

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