by Karol Tylmann
“I don’t desire to await at stones anymore, they don’t tell anything” [1]...
...but what if they gain accept something of import to tell to us?
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| Fig. 1. Erratic boulder inwards Pomerania, Poland |
Glaciers as well as H2O ice sheets grew as well as shrunk many times inwards response to past times climate changes. It is of import to know when as well as how fast it happened, to live able to predict time to come climate behaviour. But how tin nosotros appointment as well as reconstruct fluctuations of the Pleistocene H2O ice sheets which disappeared completely ca 10 000 years ago? In fact, stones may live the answer!
Surface exposure dating amongst cosmogenic nuclides
Today, dating erratic boulders amongst cosmogenic nuclides is i of the best techniques for tracking the retreat of paleo H2O ice sheets [2]. Cosmogenic nuclides (e.g. 10Be, 36Cl, 26Al, 14C) are formed when high-energy cosmic rays collide amongst atoms inwards terrestrial surface rocks as well as build upwardly predictably amongst fourth dimension [3]. Therefore, erratics exposed afterward deglaciation may live a skillful target for surface exposure dating amongst these nuclides as well as reconstruction of the decay of paleo H2O ice sheets (Fig. 2). It is the alone geochronological technique which offers straightaway dating of the most obvious geomorphological tape of paleo H2O ice sail activeness – moraines.
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| Fig. 2. Mechanism of surface exposure dating amongst cosmogenic nuclides. (A) Ice sail retreat as well as deposition of erratic boulder; start of the exposure time. (B) Production of cosmogenic 10Be inwards quartz; “a cosmogenic clock” is ticking. (C) Cosmogenic 10Be production due to transformation of oxygen-16. Amount of 10Be inwards the stone sample increases amongst the exposure time. (D) Sampling of the scope of erratic amongst saw – checking “a cosmogenic clock”. A as well as B modified afterward [4]. C modified afterward [5] as well as D modified afterward [6]. |
However, non all erratic boulders are “good” plenty to live sampled for this form of dating. The “good” boulders should live intact as well as stable inwards the surroundings since deglaciation, preferably located on the moraine crests. Unfortunately, this is non ever the case, specially inwards regions amongst a long history of human activeness [7].
The “DatErr” project
In 2015 together amongst my 2 colleagues, nosotros started the projection “Retreat of the in conclusion Scandinavian Ice Sheet inwards Poland inferred from cosmogenic 10Be dating of erratic boulders (DatErr)”. Our destination is to reconstruct the chronology as well as dynamics of the in conclusion H2O ice sail recession inwards due north Poland based on cosmogenic dating of erratics. The persuasion of this projection was born at the conference inwards Russian Federation inwards 2012 during the loooong eventide party… every bit nosotros all know, non alone sessions are of import during conferences!
The “DatErr” project
In 2015 together amongst my 2 colleagues, nosotros started the projection “Retreat of the in conclusion Scandinavian Ice Sheet inwards Poland inferred from cosmogenic 10Be dating of erratic boulders (DatErr)”. Our destination is to reconstruct the chronology as well as dynamics of the in conclusion H2O ice sail recession inwards due north Poland based on cosmogenic dating of erratics. The persuasion of this projection was born at the conference inwards Russian Federation inwards 2012 during the loooong eventide party… every bit nosotros all know, non alone sessions are of import during conferences!
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| Fig. 3. Boulder hunters. (A) Hunters on the way. (B) We finally got it! |
We chose cosmogenic glucinium (10Be) for surface exposure dating, because it is produced inwards i of the most widespread minerals on basis – quartz. Therefore, nosotros needed quartz-rich erratics, such every bit granites, gneisses or quartzites. Our kickoff work was to discovery appropriate boulders. We knew that granitic erratic boulders are i of the most characteristic remnants of the Pleistocene H2O ice sail inwards due north Poland. However, to discovery a meaning issue of “good” boulders, nosotros decided to apply a systematic physical care for of searching for as well as selecting them – nosotros accept gradually transformed into boulder hunters (Fig. 3)!
Hunting for boulders
At first, nosotros collected all available information virtually large erratics located inwards the written report area, which were non dated. We used the next sources of information:
To maximize the direct chances that our boulders are intact as well as stable inwards the surroundings since the in conclusion deglaciation nosotros decided that the “ideal” candidate needs to accept the next features:
The side past times side measuring was selection inwards GIS based on an analysis of boulder dimensions; boulders smaller than v x 0.5 m were filtered out. We institute 452 erratics of sufficiently large size. By comparison the terrain relief to the surface sediments from the geology layer nosotros determined the geomorphological context of private boulders (i.e. the type of landform on which a given erratic is located). We chose landforms related to glacial accumulation (moraines as well as moraine plateaux, kames, kame terraces, eskers, outwash plains as well as fans) every bit a geomorphological context of the correct boulders (as they volition accept been exposed since the in conclusion H2O ice sail retreat). Finally, nosotros institute 135 boulders large plenty as well as located on glacial accumulation landforms (Fig. 4).
At first, nosotros collected all available information virtually large erratics located inwards the written report area, which were non dated. We used the next sources of information:
- lists of natural monuments available inwards the Regional Directorates for Environmental Protection inwards Poland
- database of geosites (object amongst outstanding geological values) available inwards the Central Registry of the Geosites of Poland
- any useful information virtually large erratics nosotros could discovery inwards catalogues, books, as well as articles
To maximize the direct chances that our boulders are intact as well as stable inwards the surroundings since the in conclusion deglaciation nosotros decided that the “ideal” candidate needs to accept the next features:
- large surface (the wider, the better) as well as meaning tiptop (the taller, the better)
- located on a flat, stable surface
- embedded inwards the basis (indicating that it was non moved)
- traces of glacial carry as well as erosion (e.g. striations, crescentic gouges)
- a apartment upper surface amongst no marks of human activeness (e.g. carvings, worshipping marks)
The side past times side measuring was selection inwards GIS based on an analysis of boulder dimensions; boulders smaller than v x 0.5 m were filtered out. We institute 452 erratics of sufficiently large size. By comparison the terrain relief to the surface sediments from the geology layer nosotros determined the geomorphological context of private boulders (i.e. the type of landform on which a given erratic is located). We chose landforms related to glacial accumulation (moraines as well as moraine plateaux, kames, kame terraces, eskers, outwash plains as well as fans) every bit a geomorphological context of the correct boulders (as they volition accept been exposed since the in conclusion H2O ice sail retreat). Finally, nosotros institute 135 boulders large plenty as well as located on glacial accumulation landforms (Fig. 4).
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| Fig. 4. Selection of erratic boulders. (A) The GIS database of non-dated large erratics inwards the written report area. White dots – all identified boulders; night dots – boulders visited inwards the field; reddish triangles – erratics suitable for surface exposure dating. (B) One of the suitable boulders (perimeter 20.5 m; tiptop 1.1 m) as well as boulder hunters. |
The in conclusion measuring was to see these 135 erratics inwards the acre as well as to verify their suitability for exposure dating. We visited them during 2 acre seasons of 2015 as well as 2016. We spent almost 2 months inwards the acre as well as travelled over four thousand kilometres across due north Poland, driving from boulder to boulder. Although it was difficult sometimes (especially inwards wood areas amongst various topography), nosotros institute almost all of the selected erratics! We were unable to discovery seven boulders because their coordinates or descriptive locations were incorrect or peradventure because they gain non be anymore – belike at that spot are other boulder hunters (real hunters) treating boulders every bit a source of stone material. After acre inspection of 128 boulders, nosotros excluded 65 erratics every bit non suitable for surface exposure dating (26 amongst incorrect geomorphological context; xv excavated, moved, or rotated past times humans; xiv likewise small-scale or likewise low, 10 amongst traces of anthropogenic activity). Finally, 63 boulders (granites, granitic gneisses as well as gneisses) were selected every bit our targets for sampling (Fig. 4).
So, our hunt was to a greater extent than or less successful. We obtained a population of over sixty intact as well as stable erratic boulders, which nosotros believe are skillful candidates for surface exposure dating. You may await frontwards to novel publications virtually the chronology of the in conclusion Scandinavian Ice Sheet retreat inwards due north Poland inwards journals of glacial geomorphology as well as geochronology soon…
…because: “These stones are your friends” [1].
Karol Tylmann, PhD
Department of Marine Geology, Institute of Oceanography, University of Gdańsk, Poland
(with back upwardly from Piotr P. Woźniak1 and Vincent R. Rinterknecht2)
(1) Department of Geomorphology as well as Quaternary Geology, Institute of Geography, University of Gdańsk, Poland (2) Laboratoire de Géographie Physique, Université Paris 1, Panthéon-Sorbonne, CNRS, UMR 8591, France
UPDATE - Editor's note: Karol et al.'s newspaper on erratics alternative for cosmogenic nuclide exposure dating has at nowadays been published inwards Baltica. Please larn discovery the newspaper here to read to a greater extent than virtually the hunt for the perfect stone.
If y'all accept questions or comments concerning Karol's post, delight larn out a comment below, or ship him an email. You tin also connect amongst him on ResearchGate. ______________________________________________________
References:
[2] Ivy-Ochs S., Briner J.P., 2014. Dating Disappearing Ice. Elements 10: 351-356.
[3] Ivy-Ochs S., Kober F. 2008. Surface exposure dating amongst cosmogenic nuclides. Eiszeitalter und Gegenwart Quaternary Science Journal 57: 179-209.
[4] https://www.youtube.com/watch?v=48ikKLGUhNs [access: 04.07.2018.]
[5] http://www.cosmogenic-nuclides.de [access: 04.07.2018.]
[6] http://www.antarcticglaciers.org [access: 04.07.2018.]
[7] Akçar, N., Ivy-Ochs, S., Kubik, P.W., Schlüchter, P., 2011. Post-depositional impacts on ‘Findlinge’ (erratic boulders) as well as their implications for surface-exposure dating. Swiss Journal of Geosciences 104: 445-453.