Counting Swedish varves using µXRF analyses

This month, PhD Student Rachel Devine details her work using the ITRAX to count Swedish Varves, and constraining the drainiage of the Baltic Ice Lake.


Picture: Varves from the Baltic Ice Lake in Östergötland – Image Credit Rachel Devine

During the last deglaciation, outbursts of cold water into the North Atlantic have been proposed to alter climate by modifying the salinity of the Atlantic Meridional Overturning Circulation, causing significant cooling events 1. One such cooling event is the Younger Dryas (12,900-11,700 yr BP), which was probably triggered by the injection of freshwater to the North Atlantic from a catastrophic meltwater outburst from the Laurentide Ice Sheet in North America 2,3. However, an alternative source of freshwater release into the North Atlantic has been proposed recently, where meltwater from the Fennoscandian Ice Sheet, which was stored in the Baltic Ice Lake, drained catastrophically and was of sufficient magnitude to trigger the Younger Dryas event 4. But, there are several conflicting ages for the timing of Baltic Ice Lake drainage spanning approximately 2,500 years 5, 6, 7. This research seeks to resolve outstanding chronological uncertainties of Baltic Ice Lake drainage through analysis of annually laminated (varved) glacial lake sediments deposited in the Östergötland region of south-eastern Sweden. Several key varve sites have been resampled from Östergötland, which span the onset, duration and termination of the Younger Dryas. The glaciolacustrine varves are composed of two layers – a silt/sand summer layer and a clay winter layer. Whilst the conventional method for counting Swedish varves from the Baltic Ice Lake is through macro-scale observations from open-core surfaces, the boundary between layers is not always distinct, although continuous geochemical data from ITRAX has been shown to have great potential in delimiting the boundaries between these layers and thus delimit the seasonal layers. The ability to distinguish these layers using μ-XRF data will: a) enable more precise definition of the thickness of the annual layers to be identified; b) improve counts of the annual layers and produce a revised annually resolved chronology. These data will then be integrated with other high-resolution analyses (thin section sedimentology, tephrochronology and radiocarbon dating) to produce a more precise estimate of the timing of Baltic Ice Lake drainage.

You can follow Rachels updates on Twitter @RachelDevine_

[1] Clark, P. U., Marshall, S. J., Clarke, G. K., Hostetler, S. W., Licciardi, J. M., and Teller, J. T. (2001). Freshwater forcing of abrupt climate change during the last glaciation, Science, 293(5528), 283-287.

[2] Fairbanks, R. G. (1989). A 17,000-year glacio-eustatic sea level record: influence of glacial melting rates on the Younger Dryas event and deep-ocean circulation. Nature, 342(6250), 637-642.

[3] Murton, J. B., Bateman, M. D., Dallimore, S. R., Teller, J. T. & Yang, Z. (2010). Identification of Younger Dryas outburst flood path from Lake Agassiz to the Arctic Ocean. Nature, 464, 740–743.

[4] Muschitiello, F., Pausata, F. S. R., Watson, J. E., Smittenberg, R. H., Salih, A. A. M., Brooks, S. J., Whitehouse, N. J., Karlatou-Charalampopoulou, A., Wohlfarth, B. (2015a). Fennoscandian freshwater control on Greenland hydroclimate shifts at the onset of the Younger Dryas. Nature Communications, 6, 1-8.

[5] Muschitiello, F., Lea, J. M., Greenwood, S. L., Nick, F. M., Brunnberg, L., MacLeod, A., Wohlfarth, B. (2015b). Timing of the first drainage of the Baltic Ice Lake synchronous with the onset of Greenland Stadial 1. Boreas, 1-13.

[6] Andrén, T., Lindeberg, G., Andrén, E. (2002). Evidence of the final drainage of the Baltic Ice Lake and the brackish phase of theYoldia Sea in glacial varves from the Baltic Sea. Boreas, 31, 226–238.

[7] Swärd, H., O'Regan, M., Ampel, L., Ananyev, R., Chernykh, D., Floden, T.,Jakobsson, M. (2015). Regional deglaciation and postglacial lake development as reflected in a 74 m sedimentary record from Lake Vättern, southern Sweden. GFF, 1-19.