7.1.2019

Science Blog: The distribution of elements in peat

Teksti:
Teuvo Herranen, Geologist

The Geological Survey of Finland (GTK) has thus far studied 2.27 million ha of Finnish peatland. The peat data generated in peat mapping has been saved in a peat database maintained by GTK. The database contains data on over 17 900 peatlands, totalling more than 1.75 million study and depth points. Samples have been taken for more detailed laboratory analyses from 9 700 peatlands. In these peatlands, there have been over 19 000 sampling points, over 1 600 of which have been elemental assay points.

In addition to recording the peat type and humification, the peat samples have also nearly always been assayed for their ash and water content. Over 214 000 ash content and 213 000 water content assays have been conducted. For those samples that have been precisely volume weighted (nearly 178 000), the dry bulk density has been assayed. Over 125 000 calorific value determinations have been performed. The peat sulphur content has been assayed for nearly 64 000 samples and the pH value for over 175 000 samples. In addition, the carbon and nitrogen content has been determined for over 8 000 samples.

Elemental assays have been carried out since the 1970s. The majority of the peat sample series taken in connection with peat studies have extended from the surface to near the bottom of the peat layer, with the samples being a continuum of 20-cm sub-samples. Most of the elemental assays in connection with peat mapping have been carried out so that in addition to samples taken from surface and bottom parts, assays have also been conducted on several samples between them. Elemental assays were formerly carried out in the chemistry laboratory of GTK, but following its incorporation in 2006, they have been carried out by Eurofins Labtium. The assay method used is that involving nitric acid dissolution of dried and comminuted peat (0.5 g) in a microwave oven (US EPA 3051A), and the element concentrations are determined by ICP-OES and ICP-MS. ICP-MS (method 503M of Labtium) can detect elements at very low concentrations.

A summary of the peatland, municipality and county-specific data is available via the Internet in the peat resource accounting of GTK (www.gtk.fi/turvevarat), in which the study data are regularly updated. The earlier published municipal reports can be found in the Hakku data service of GTK (https://hakku.gtk.fi/fi/reports). In the same service can also be found the newly published Turpeen alkuainemääritykset GTK:ssa Summary: The element assays of peat in the GTK (http://tupa.gtk.fi/raportti/arkisto/73_2018.pdf)

The contents of most elements in peat are lower or clearly lower than in mineral soil and are often at the same level as in the humus layer. The cadmium content of peat in the analysis results used in calculation is on average clearly higher than in mineral soil and also on average higher than in humus and the lithosphere. In reality, the difference is not so clear, because the element contents of the majority of the peat samples analysed have been under the assay limit. The molybdenum content of peat is on average higher than in humus, the lithosphere and mineral soil. The sulphur content of peat is nearly one-fifth higher than the corresponding value of humus and nearly five times higher than the average sulphur content of the lithosphere. Compared with mineral soil, the sulphur content of peat is approximately 8–28 times higher. Sulphur therefore clearly appears to be enriched in the peat and humus layer, which mostly results from the presence of black schist zones nearby or sulphide soil under peatland.

High element contents of peat often indicate the presence of black schist zones nearby, but high element contents can additionally be useful in ore prospecting, because the surrounding bedrock is also otherwise reflected in the element content of peat. In black schist areas the average Al, As, Ba, Cd, Co, CR, Cu, Fe, Mn, Ni, Pb, S, Ti, V, Zn and ash contents of peat are often higher than normal. Taking note of black schist zones and having a denser spacing of analysis points in the neighbourhood of these zones might be reasonable, if a peatland is found to be either a potential peat harvesting area or suitable for other use. The abundant occurrence of potential sulphide soil under the peat stratum also increases the need for denser placement of analysis points.

The results of the element assays of peat can be utilized regionally in the planning of land use and after-use of peat harvesting areas, for instance. In addition it is possible to gain a more exact understanding of regional geochemistry and the risk areas of heavy metals. The data have a very important role in developing new ways and possibilities to use peat and peatlands.

Fig. 1. The peat sampling points and element analysis points of the peatlands investigated by GTK. Older (blue) and 2018 (green) peat sampling points in the database of the Geological Survey of Finland are presented, representing the situation in October 2018. The peat sampling points for element analysis are marked with a red colour. There are altogether 19 117 peat sampling points and 1 649 element sampling points. (Basemap database © Maanmittauslaitos).
Fig. 2. The frequency distribution of titanium contents in peat mg/kg (N = 6 503).
Fig. 3. The distribution of aluminium, calcium, iron and sulphur in peat presented as a box plot.
Fig. 4a. The median element contents of peat, humus and soil.
Fig. 4b. The median element contents of peat, humus and soil.


References:

Herranen, T. 2009. Turpeen rikkipitoisuus Suomessa. Abstract: The sulfur concentration of peat in Finland. Geological Survey of Finland,
Report of Peat Investigation 398. 55 pages, 19 figures and 27 tables. Available at: http://tupa.gtk.fi/raportti/turve/ttr_398.pdf

Herranen, T. & Toivonen, T. 2018. Turpeen alkuainemääritykset GTK:ssa. Summary: The element assays of peat in the GTK. Geological Survey of Finland, Open File Research Report
73/2018. 45 pages, 37 figures and 3 tables. Available at: http://tupa.gtk.fi/raportti/arkisto/73_2018.pdf

Salmi, M. 1955. Prospecting for Bog covered ore by means of peat investigations. Bull. Comm.Geol. Finlande 169. 34 p. Available at:
http://tupa.gtk.fi/julkaisu/bulletin/bt_169.pdf

Virtanen, K. 1990. The Influence of Bedrock Belts on the Trace Elements of Mires in
Haapavesi, Central Finland. Peat 90. Int. Conf. on Peat Production and Use 11-15. 6. 1990. Jyväskylä, Finland, 462–474.

Virtanen, K. & Lerssi, J. 2008. The influence of metal bearing black schist bedrock to metal and sulphur contents of peat deposits. Farrell, C. & Feehan, J. (eds) After Wise Use − The Future of Peatlands. Proceedings of the 13th International Peat Congress, Tullamore, Ireland 8−13 June 2008. Volume 2, Poster presentations. International Peat Society, 55−59.

Virtanen, K. & Lerssi, J. 2006. Mustaliuskekivilajin vaikutus turpeen alkuainepitoisuuksiin. Influence of black schists to element content of peat. Geological Survey of Finland. 33 pages, 2 appendixes and 30 figures. Available at: http://tupa.gtk.fi/raportti/arkisto/s42_0000_2006_1.pdf

Teuvo Herranen

Teksti: Teuvo Herranen

Teuvo Herranen (M.Sc.) has four decades of experience in researching peatlands and peat. He works as a peat geologist in the Peat Resources Unit of the Geological Survey of Finland.