Geochemistry of Landscapes Covered by Glacially Crushed Debris William W. Shilts
Comminution (crushing) processes dominate glacial sediment production and deposition, trumping chemical weathering, hydrology, and gravity, which dominate in other sedimentary environments. Comminution processes create terminal grades, typical for every rock and mineral. Chemical partitioning in glacial sediments is directly related to physical partitioning of minerals. Glaciers carry a dense load of debris at their base, some debris in the englacial position and some on their surface. Depending on the topography of the glacial bed, any of these compositionally distinct primary glacial sediment facies may be deposited one on the other. The proglacial sedimentary environment may consist of several distinctly different sedimentary environments that follow the retreating glacier front as a glacier retreats. Though all sediments in these environments are ultimately derived from the glacial load, they vary chemically, particularly through diagenesis and weathering.
Crushed, unweathered bedrock provides abundant labile minerals which are easily weathered. Once crushed debris is entrained in ice, it is transported with little lateral or vertical mixing. Preexisting sediment, soil, or bedrock may be entrained by freezing on at the glacier s base and transported with little deformation or degradation. Entrained debris is diluted down-ice exponentially, by constant basal entrainment or deposition of debris along flow lines.
1138 ppm Cr; 2300 ppm Ni <100 ppm Cr; <60 ppm Ni
1250 ppm Ni 110 ppm Ni
Nickel Concentrations (ppm) in < 63 um fraction of till
SCALES OF GLACIAL DISPERSAL Continental---1000 S KM Regional-------100 s KM Local------------10 s KM Detailed-------- <1 KM
The passage of a continental glacier over any landscape causes widespread natural pollution while providing easily released nutrients; glacial dispersal is independent of drainage divides as basal debris is transported up and down gradients.
Map of natural As concentrations in till and other glacial sediments (<2 um fraction). The overall pattern reflects the distribution pattern of As sulphide-rich metasedimentary and metavolcanic bedrock in the area. This area was extensively mined for gold which when processed, produced arsenical tailings that have contaminated many drainage basins.
Programme and Abstracts 25th International Applied Geochemistry Symposium 2011 22-26 August 2011 Rovaniemi, Finland Pertti Sarala, V. Juhani Ojala and Marja-Leena Porsanger Publisher: Vuorimiesyhdistys - Finnish Association of Mining and Metallurgical Engineers, Serie B, Nro B92-1, Rovaniemi 2011
Sarala, P., Ojala, V.J. and Porsanger, M.-L. 2011. Programme and abstracts. The 25th International Applied Geochemistry Symposium 2011, 22-26 August 2011, Rovaniemi, Finland. Vuorimiesyhdistys - Finnish Association of Mining and Metallurgical Engineers, Serie B 92-1, 192 pages. Layout: Irma Varrio ISBN 978-952-9618-63-7 (printed) ISBN 978-952-9618-64-4 (CD) ISBN 978-952-9618-65-1 (PDF) ISSN 0783-1331 Vuorimiesyhdistys This volume is available from: Vuorimiesyhdistys ry. Kaskilaaksontie 3 D 108 02360 ESPOO Electronic version: http://www.iags2011.fi or http://www.vuorimiesyhdistys.fi/julkaisut.php Printed in: Painatuskeskus Finland Oy, Rovaniemi
52 A need to harmonize methodologies was detected and international co-operation was getting stronger. The role of Finnish geochemists in the activities of IGCP 259 (International Geochemical Mapping) and IGCP 360 (Global Geochemical Baselines) was remarkable. These projects were conducted by A. Darnley. A. Björklund and N. Gustavsson from GTK are among the authors of the Blue book (Darnley et al. 1995), which is today the basic document when geochemical mapping projects are planned in Africa, Asia, America or Europe. One of the main results of the international geochemical mapping projects until now is the Foregs Geochemical Atlas of Europe (Salminen et al. 2005). This project was initiated by the IGCP 360. Future of the geochemistry has already started. In the 2000s, new ideas and applications came to exploration geochemistry. Spatial data analysis which uses geochemical mapping data as one element together with other geological information was taken into use. The younger generation started to develop and apply new methods and technologies in exploration and in medical geology. Applications of stable isotopes will be one of the key issues in the future. In environmental geochemistry, risk analysis in the connection of contaminated land sites will need more detailed information from speciation of elements and their bioavailability. Geochemists expertise in developing methods for cleaning contaminated land is needed. The data and information which geochemists have demands them also to participate actively in social debate and preparation of legislation. References: Darnley, et al., 1995. Earth Science 19, UNESCO Publishing Salminen et al. 2005. Geochemical Atlas of Europe. Geological Survey of Finland Keynote 5 Geochemistry of Landscapes Covered by Glacially Crushed Debris William W. Shilts Prairie Research Institute, University of Illinois-Urbana, Champaign, USA Sediment carried by modern glaciers and sediment covering formerly glaciated terrain is created largely by comminution (crushing) processes that dominate sediment production and generate virtually unweathered, fresh bedrock detritus (till). Entrained debris is and was diluted exponentially down ice by constant incorporation of debris at the base of a glacier. Because glaciers flow up and down slopes, gravity plays a secondary role in determining distribution of sediment components, and the geochemical signature of glacial sediment can be draped over the landscape, irrespective of topography, except in areas of mountain glaciation. Conversely, in areas unaffected by glaciations, soils and sediments are produced largely through the processes of chemical and physical weathering of source rocks, and gravity or chemical processes dominate the depositional patterns of sediments dispersed by wind or water. Glacial comminution processes create terminal grades which are defined as the sizes to which rocks and their constituent mineral phases can be reduced, given the energy available at the base of a glacier (Driemanis and Vagners, 1971). This physical partitioning of minerals into specific size grades based on their physical properties and on their size distribution in source rocks results in chemical partitioning that is related to the mix of mineral phases derived from the various bedrock lithologies traversed by the depositing glacier. For example, chromium can be concentrated in Chromite or Uvarovite, which are hard, uncleavable minerals with terminal grades in the sand-sizes, or in Fuchsite, a chromium-bearing mica, which has a terminal grade in the clay or <4 micron size fraction. Thus, if an ultramafic source rock is rich in Chromite, its geochemical signature will be most prominent in the sand fraction of a glacial sediment, whereas glacial abrasion of Fuchsite-rich ultramafic rocks will produce a glacial sediment in which chrome is concentrated in the <4 micron sizes. From this example it can be seen that in order to interpret geochemical analyses of glacial sediment samples, the concept of terminal grades must be carefully considered. Another result of the crushing process is that as minerals are reduced to progressively finer sizes, the reaction surface area of the sediment increases, enhancing the release or adsorption of chemical components, such as nutrients and (natural) pollutants. Finally, till or water and wind-deposited sediment derived from till, contain easily weathered mineral phases (carbonates, sulphides, some clays, etc.) that can be altered to significant depths, with accompanying release of chemical components. Consequently, sampling and interpretation strategies for weathered samples, even those collected below the true solum, must take into account these weathering characteristics. Reference: Driemanis, A. and Vagners, U. J. (1971). Bimodal distribution of rock and mineral fragments in basal tills. In: Till: A Symposium, R. P. Goldthwait, ed., pp. 237-250. Ohio State University Press, Columbus.