First workshop of the PAGES Varves Working Group,
Palmse, Estonia, 7-9 April 2010.
Full Text:
The interest in climatic records has increased markedly due to the
concern for global warming. A major aim of natural scientists is to
improve the predictive power of climate simulations. The instrumental
record of the impact of climate change on the environment is too short
to capture the whole range of climatic variability. Therefore geological
records are investigated and different proxy data are produced to
reliably reconstruct past shifts in the climate beyond the historical
record. PAst Global changES (PAGES), a core project of the International
Geosphere and Biosphere Programme (IGBP), has stressed the need to
quantify the natural range of climatic variability that has occurred in
the recent geological past. Research priority has been given to
continuous high-resolution natural records with decadal to annual
resolution. Excellent examples that can be achieved include the studies
of the Greenland ice cores and the construction of regional
dendro-climatological time-series from tree-ring data.
Lake sediment records are potentially one of the most useful
sources of palaeoclimatic and palaeoenvironmental proxy data. However,
the construction of an accurate geochronology on sediment sequences is
often a complicated task. Radiocarbon dating has so far been the most
widely used method for dating and correlation of different sediment
sequences. However, the technique has a number of limitations, including
hard-water errors, reservoir effects and calibration on the 14C
plateaux. Annually laminated or varved lake sediments provide
independent dating tools that may contain records with calendar year
chronology and deliver high-resolution proxy data showing changes in the
lake ecosystem and climate at annual to seasonal resolution (Fig. 1).
The Swedish word 'varv', meaning cycle or layer, was
originally used by Gerard De Geer, a Swedish geologist and the father of
varvochronology, for limnoglacial clays with annual couplets of
light-coloured silt and dark-coloured clay. In our days, when annually
laminated sediments have been discovered in different sedimentological
environments, varve has become a common word for a sediment structure
representing the deposition during a single year, including several
seasonal sublayers of different composition and colour.
Eleven years ago, in 1999, the last specific meeting for the
'varve community' was held in the Lammi Biological Station in
Finland. Since then, different research groups have worked with varved
sediment sequences worldwide and developed new advanced tools for
studying annual sediment records (e.g. X-ray fluorescence scanning,
image analysis), hence numerous publications have appeared on varved
sediments. It was time to take a step forward and therefore the Varves
Working Group (VWG), a new community of scientists, was established
under the frame of the IGBP-PAGES Cross Cutting Theme 1
'Chronology'. The purpose of the VWG is to gather the varve
community together and to bridge the gaps between people working with
sedimentary varves and other communities dealing with annually resolved
records (tree-rings, ice cores, corals, speleothems).
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The workshop in Palmse, northern Estonia, was the first in the
series of workshops that has been planned. The workshop was organized by
Dr Antti Ojala and MSc. Emilia Kosonen, Geological Survey of Finland,
and hosted by the Institute of Geology at Tallinn University of
Technology and locally organized by Dr Atko Heinsalu. More than 40
participants attended the workshop, primarily from European countries,
including Russia, and from the USA and Canada (Fig. 2). Oral
presentations were made in four sessions, grouped according to the
themes of varved sites, environments, sedimentology and geochemistry;
chronology; analytical and numerical methods and tools; and
environmental and climate history case studies. They were complemented
by a poster session.
Generally, there were many attractive oral and poster
presentations, especially those introducing new analytical and software
tools for the varve studies. The first session keynote speaker W.
Tylmann talked about the systematic survey to find annually laminated
lake sediments and geological, geomorphological and limnological
settings that control the varve deposition in lakes of northern Poland.
The next session keynote speaker I. Snowball presented an original talk
about validating varve chronologies with independent complementary
dating techniques (historically dated marker layers, palaeomagnetic
secular variations, tephrochronology, atmospheric lead pollution
isochrones and radiocarbon wiggle matching) to reduce chronological
uncertainties. The last keynote speaker A. Brauer presented a new
approach to studying annually laminated sediments, by combining
micro-facies analyses on thin sections with high-resolution micro-XRF
element scanning on impregnated sediment blocks, and discussed its
potential for improving varve counting and interpreting seasonal
palaeoclimatic signals.
The scientific presentations were followed by discussions of the
objectives, expectations and the future of the VWG. The group decided to
implement a website with information related to the tools and
methodologies useful for the study of varves. Moreover, the group agreed
that some technical guidelines and quality banners needed to be fixed to
facilitate the assessment of varved records for the use of climate
modellers and decision-makers. These criteria will be agreed upon after
learning from the other scientific communities dealing with annually
resolved records during the next VWG workshop to be held in North
America in 2011. Further information on the VWG activities is available
on the webpage http://www.pages-igbp.org/science/varves/ varves2010.html