Project D.M. ones', W. W. Shilts, and R. W. Terrain Sciences Division

Transcription

1 HEAVY METAL CONTENT OF TUNDRA PLANT SPECIES Project D.M. ones', W. W. Shilts, and R. W. Terrain Sciences Division Introduction It has long been known that certain plant species may be associated with soils that are characterized by high heavy metal levels. Some plants also preferentially concentrate certain heavy metals. The genus Astragalus, for instance, is a good indicator for selenium and uranium in soil. Heavy metal uptake by plants has important implications to mineral prospecting and to animals harvesting the plants. Little detailed work on heavy metal uptake has been conducted in southern Canada and even less has been done in the subarctic and arctic areas. During the summer of 1975 a pilot study examining the uptake of copper, lead, zinc, cobalt, nickel, silver, cadmium, and uranium by arctic plants was carried out in the central District of Keewatin. The objective of this pilot project was to determine to what extent characteristic arctic species concentrated these metals in their tissues in areas of strongly contrasting geology and soil metal levels. Of the seven sites chosen for the study (Table 48.1) two were intended to be background areas, two were located on or near known uranium mineralization, one was located on a nickel-copper-rich gossan (only one species collected), one was located adjacent to zinclead mineralization with some copper, and one was located on a glacial indicator train about 2 km down-ice from zinc-lead mineralization. Methods At each site collections of the major plant species (Table 48.2) and peat were made within a 100 m-diameter area. Glacial sediments were sampled in detail around all but one area. Tissue from shrubby species consisted of growth from the last two years, which'was collected from several individual plants at each site. From other plants that have annual aerial parts, only the annual growth from several individuals was collected. Tissue was cut with shears or a knife and was put into a plastic bag until it could be taken to camp. The tissue then was transferred to a paper bag and dried in a field oven at 60' to 80 C. Peat samples also were collected and dried, as above. In areas of mud boils peat was taken from the rim of a mud boil. In the laboratory the plant and peat samples were redried and ground to fragments passing through a Table 48.1 Characteristics of sites sampled Collection site No. Geological characteristics Dominant vegetation 1 Yandle Lake background mud boils (control) 2 Spi Lake gossan 3 Spi Lake Island rock outcrop (Cu, Zn, Pb), mud boils on till and gossan, mixed till covered, mud boils, (Zn, Pb, Cu) indicator train on island 4 15 km southwest of rock outcrop (U, Cu); thin till over Yandle Lake (dry site) bedrock, only site above marine limit 4a 15 km southwest of mud boils (U, Cu) Yandle Lake (wet site) 5 Pitz Lake background (control) mud boils, till. marine sediment (sand, silty clay) 6 Kazan River uranium (u, Cu) mud boils, frost-heaved showing bedrock, thin till over bedrock 7 Ferguson Lake (Cu, Ni), gossan sedges to Betula sedges to Betula, some snowbed species heath species scattered heath species sedges in wet sites and Betula wide range of habitat from sedges to Betulaheaths only Ledum l~epartment of Biology, University of New Brunswick, Fredericton, New Brunswick Geol. Surv. Can., Paper 76-1B

2 Rankln H U D S O I V Kominok Whale Cove -62O00' L. 62 O 00'- B A Y 92O00' Km Biogeochemlcol collrcl ion site Townsite ' Figure Location of sites sampled.

3 Table 48.2 Summary of samples collected in the pilot study Collection site Sample a Mineral soil x x x x x x x x Peat x x x x x x x Arctostaphy 10s alpina Astragalus alpinus Betula glandulosa Calamagrostis canadensis Carex Bigelowii Cassiope tetragona Dryas integrifolia Empetrum nigrum Eriophorum angustifolium Eriophorum vaginatum Hedysarium Mackenzii Ledum palustre Oxytropis Maydelliana Oxytropis Bellii Rubus Chamaemorus Salix alaxensis Salix planifolia Vaccinium uliginosum x x x x x x Vaccinium Vitis-idaea x x x x x x 1 mm sieve: The samples were ashed and analyzed by Bondar-Clegg and Co. Ltd. The ash, obtained by slowly increasing temperatures to ignition, was dissolved in aqua regia and analyzed by atomic absorption (Cu, Zn, Pb, Co, Ni, Ag, Cd) and fluorimetric (U) techniques. Results are reported in ppm of the ash. Mineral Prospecting Previous Research Canadian,pioneers in the study of heavy metal uptake have been H. V. Warren and his co-workers who have conducted research since 1944 at the University of British Columbia. Their biogeochemical work has included studies on a large number of heavy metals (e. g., Warren and Delavault, 1948, 1949, 1955a, b; Warren et al., 1955). A progress report on Canadian biogeochemical research has been published recently (Fortescue and Hornbrook, 1967, 1969). The question of whether arctic plants can assimilate anomalous amounts of metal on sites overlying metal deposits or can act as indicator species has never been resolved adequately. Most of the experimental work has taken place in Russia, and much of this material is relatively unknown in the West. One of the main problems in the Arctic is that permafrost prevents deep root penetration and in some areas may inhibit roots from penetrating into the mineral layer. Malyuga (1964) reported that the penetration of root system is not always limited to the active layer but, for some plants (wood reed, rose, sedge, cloudberry, and horsetail), also extends into the permafrost. In Alaska, researchers in biogeochemistry have had limited success in mineral prospecting in perennially frozen terrain, possibly because of inhibition of root growth by permafrost. Although no flowering plants were found as indicators, several species of mosses and liverworts were found to be associated with mineralization (Chapman and Shacklette, 1960). Shacklette (1965, 1967) reported that a copper moss (Mielichhoferia macrocarpa (Drumm. ) Br. & S. ) was found to have an obligative relation to Alaskan copper deposits. In the forests of Yukon Territory and Northwest Territories dwarf birch (Betula glandulosa), Labrador tea (Ledum groenlandicum), and willow (Salix sp. ) have been found to concentrate zinc and copper from underlying glacial sediment or bedrock (Warren and Delavault, 1955b). In Finland, workers have found that leaves of Vaccinium Vitis-idaea concentrate copper and that Ledum palustre assimilates abnormal concentrations of copper and molybdenum over mineral deposits (Marmo, 1953). Despite the fact that this work was conducted in a nonpermafrost area, it has particular significance to the present study because these two species commonly are foupd in the eastern Canadian Arctic. Environmental Effects There is little information in the literature concerning the effects that the uptake of heavy metals by plants may have on the remainder of the biological system. Shacklette (1962) suggested, in a hypothetical model, that heavy metals ingested annually in browsing can have an effect on the distribution of large herbivores. Results Metal in the Mineral Soil Substrate Table lists typical metal content values for glacial soils and gossans on or near which the plants were growing at each sample site. Presumably the plants have concentrated metal from soils with these or similar compositions. It should be noted that these are considered to be typical values for the (250-mesh (<64p) portion of the soil within each 100 m-diameter collection area but that values can vary radically due to periglacial mixing of gossanous and true glacial soils near the sites of mineralization. Several samples from

4 Table * Typical Metal Content, Mineral Soils Site Cu Pb Zn Ni U PPm Yandle background (No. 1) or preferential concentration of certain elements derived from background materials higher up the slope behind the site. However, peat, which forms the substrate on which sedges, in particular, grow, is impoverished in all metals except uranium. Alternatively, these metals may be derived from mineralization in the concealed, underlying bedrock, but such mineralizaton without associated uranium would be unusual in the Dubawnt Group. Spi showing (No. 2) Spi Lake, Zn-Pb-Cu Showing Spi train (No. 3) Samples were collected from mud boils developed Kinga River (No. 4, 4a) 68 l on a mixture of till and gossanous material within Pitz background (No. 5) m downslope from a well known outcrop of massive u sphalerite, galena, and minor chalcopyrite on Spi Kazan River (No. 6) 24 lo l3 5' Lake. A related suite of samples was collected from Ferguson Lake (No. 7) the middle of a small island in Spi Lake, about 2 km * (< 250 mesh, -64p) down-ice from the above mentioned mineralization. The island is underlain by Archean pyroclastics and volcanic rocks and covered by till with abundant mud boils. The till is enriched in zinc and includes an the Spi Lake showing, for instance, have zinc variations from less than 100 pprn to more than 2000 pprn over the space of a few metres. indicator train with several sphalerite-galenachalcopyrite-bearing boulders (Shilts, 1974, p. 259). At both sites several species are enriched in copper, zinc, lead, and cadmium. Birch (Betula), Background Areas willow (Salix), blueberry (Vaccinium), and bearberry (Arctostaphylos) show the greatest values, but the sedges on Spi Island are notably enriched in copper, zinc, and lead. Sedges were not found at the Spi Lake mineralization. Birch and willow seem to be the best indicators of this type of mineralization, but sedges, where present, also may be a useful group of plants to indicate copper-lead-zinc mineralization. Most species of plants collected from the background areas have lower concentrations of the metals tested than those from sites where either uranium or base metals have influenced the soil. Yandle Lake samples were collected from an island where a thin mantle of till covers interbeds of Archean felsic to basic metavolcanic bedrock, which contains numerous 1 to 2 m-diameter pods of sulphide (mostly pyrite) mineralization. As at all sites, Yandle Lake Betula contains more than 4000 pprn zinc. The single peat sample collected from that site contains abnormal amounts of copper and lead of unknown origin. At Pitz Lake the vegetation collected was growing on a wet, complex terrain comprising till, marine silty clay, and marine nearshore sand, which form a continuous cover over sandstone, agglomerate, or porphyritic volcanic rocks of the Proterozoic Dubawnt Group. The collection site was at the base of a gentle slope, several kilometres long, which rises above the level of postglacial marine inundation. The samples from this site are slightly elevated in cadmium and uranium; the latter element is an economically important one which has spurred much ongoing exploration in the areas of Dubawnt outcrop. Concentrations of most other metals are relatively low except in two sample of sedges and the willow (Salix) sample. Ash from these two sedge samples contains over pprn and over pprn (6 per cent) copper, with abnormally high concentrations of lead, zinc, nickel, and silver; Salix contains over pprn zinc and has high cadmium values but moderate to low amounts of other elements. The source of these high concentrations is not known, but the chemical conditions in the generally poorly drained terrain may have caused some sort of deposition Uranium Showings Samples were collected from mud boils and from a marshy area on a till plain above marine limit near Kinga River, 15 km southwest of Padlei. The local bedrock is a basal conglomerate of the Montgomery Lake Sediments (Bell, 1971) in which uranium mineralization occurs. Drift sampling around the collection site reveals locally anomalous copper-uranium concentrations in the soils, and uranium exploration was carried out in the early sixties by drilling at the collection site. Elevated uranium values, relative to nearby sites at Spi Lake and Yandle background, were found in several species at this site. Relatively high copper values were found in peat and willow samples but, unfortunately, no birch was collected here. Sedges show a slight tendency to concentrate copper, but are impoverished in other metals. It is noteworthy that the uranium values, while elevated with respect to other sites near Kinga River, are approximately equivalent to the background values obtained for similar species at Pitz Lake. The Kazan River site is a commercially attractive site of copper-uranium mineralization in a Dubawnt Group sandstone host rock. Birch, Dryas, vetches, and peat have elevated uranium contents compared to

5 all other sites. Cadmium levels are high in birch and blueberry, and copper is elevated in cranberry and willow. The very high lead concentration in peat may be related to a spill of lead-rich gasoline, for numerous barrels indicate that drilling and aircraft activity has been carried on at the site over a period of several years. Ferguson Lake Nickel Not shown in Figure are results of sampling at Ferguson Lake, a nickel sulphide prospect that was explored extensively in the early 1960's. Only one species, Labrador tea (Ledum palustre), was growing on the extensive gossan associated with the sulphide mineralization. Ash from this sample contained 550 pprn nickel and about 2000 pprn copper on a gossanous soil that contained (-80 mesh) 54 pprn nickel and 1140 pprn copper. Average Ledum values for the rest of the sites are 71 pprn nickel (range 36 to 137) and 427 pprn copper (range from 174 to 1075). Threshold Metal Levels Discussion There tended to be stable threshold concentrations of some metals in the plant tissues although the metal content in the soil or peat varied radically from site to site. Thus, in general, metal concentration in the plant tissues was much less variable than metal content in the soil or peat samples. This threshold for species tended to be most pronounced with silver and cobalt (i. e., for silver: Betula glandulosa, Empetrum nigrum L., Vaccinium Vitis-idaea, V, uliginosum L., Cassiope tetragona (L. ), D. Don, the various vetch species, Arctostaphylos alpina, and Calamagrostis sp. ; for cobalt: Ledum palustre, E, nigrum, V. uliginosum, C. tetragona, Rubus Chamaemorus, the various vetch species, A. alpina, and Calamagrostis sp. ). There also tended to-be a threshold level of zinc in L. palustre, of cadmium in the various vetch species, and of copper in R. Chamaemorus. The reason for the apparent threshold of heavymetal concentrations in these plants is open to speculation It could mean that plants have homeostatic control and absorb the metals up to a certain point (somewhere below the level of toxicity) after which they do not absorb further quantities. Epstein (1972), however, believed that metal tolerance in plants is not necessaniiy based upon restricting or controlling uptake of large amounts of toxic ions. The only vascular species found on Ferguson Lake gossan was Ledum palustre, accompanied by a lichen species. Possibly the levels of metals in the soils were above the toxicity levels for other plant species. L. palustre possibly is an example of a tolerant plant (proposed by Epstein (1972)), which can synthesize chelating compounds that form complexes with the heavy metal ions and thereby render them harmless. There are probably a number of such complexing agents, each more or less specific for a certain toxic metal. In addition to heavy metals, colonization of other plant species also may be inhibited by the low ph of the gossan (<2. 0). Uranium Accumulator Plants The four highest uranium concentrations in plant tissue came from the Kazan River site: Salix planifolia Pursh, 65.0 ppm; Dryas integrifolia M. Vahl, 14.0 pprn Oxytropis Maydelliana Trautv., 10.2 ppm; and Hedysarium Mackenzii Richards., 9.5 pprn (Fig. 48.2). The average uranium content in ash from the plants collected from the uranium showing was 10.1 pprn while the average uranium concentration in plants collected from the other six areas was only 1.0 ppm. The vetch species 0. Maydelliana and H. Mackenzii are particularly important as they are closely related to the genus Astragalus which is one of the better known examples of an accumulator plant (Cannon, 1960; Epstein, 1972). The concentration of uranium in the tissues of 0. Maydelliana was 10.2 pprn whereas uranium was not even detectable in the same species from the two other areas where it was collected. Uranium was not detectable in any of the Calamagrostis sp. samples except the sample collected from the Kazan River area which showed an uranium concentration of 2.4 ppm. Peat - There has been some research in biogeochemical peat prospecting (Brooks, 1972). Peat biogeochemical analysis, however, is complex because many factors influence the chelating ability of humus matter, such as ph and the atomic weight and the valence of the element. For instance, humic acid loses its ability to fix metals with decreasing ph to the extent that lead, copper, zinc, nickel, and cobalt are released almost completely from humic matter once the ph is lowered to 1.0 (Chowdhury and Bose, 1971). Mineral Exploration Preliminary Conclusions This preliminary study has demonstrated a number of aspects of trace metal fixation by plants that may warrant further work. In relationship to mineral exploration, the study shows that dwarf birch and dwarf willow species are the most consistent concentrators for the elements studied - at least in close proximity to mineralization. Willow particularly seems to concentrate uranium and cadmium, whereas birch is very rich, even in background areas, in zinc. Both tree species concentrate cobalt to levels of two or more times those of other plants sampled. Sedges show some tendency to reflect copper, lead, and zinc mineralization, but the very high values for copper and zinc in two of the five sedge samples from Pitz Lake background site suggest that caution should be exercised in interpreting sedge values. Also, sedges are more restricted in their occurrence than some of the other species..

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7 Vetches and, possibly, Dryas might be investigated further for their uranium-concentrating tendencies. Trace Elements in the Environment Little can be said about the effects of metal levels in plants from these sites and their relationship to the food chain. The relatively high levels of cadmium in certain species from both background and mineralized sites in the Pitz Lake-Kazan River area might bear further investigation because of their proximity to the settlement of Baker Lake and to areas of intense herbivore browsing activity. Implications for Lacustrine Sedimentation The allochthonous organic components of lacustrine sediments tend to be important in this region because of the relatively low productivity of the lakes (Shilts et al., 1976). The chemistry of the organic component of lake sediments certainly will be influenced by whether the allochthonous component is largely derived from woody plants with their generally high metal levels or from grasses, sedges, or other annual plants with their low to variable metal levels. Since significant portions of lake shores or entire lakes tend to be dominated by one or the other type of vegetation, these are important parameters to evaluate when studying variations in lake-sediment chemistry. Because of the concentration of metal in the easily eroded ash left after a tundra fire (Shilts, 1975; Wein and Shilts, 1976), tundra fires could have a significant, albeit temporary, influence on lake-sediment and water chemistry within the drainage basin. References Bell, R. T. 1971: Geology of Henik Lakes (east half) and Ferguson Lake (east half) map-areas, District of Keewatin; Geol. Surv. Can., Paper 70-61, 31 p. Brooks, R. R. 1972: Geobotany and Biogeochemistry in Mineral Exploration; Harper and Row, New York, 290 p. Cannon, H. L. 1960: Botanical prospecting for ore deposits; Science, v. 132, p Chapman, R. M. and Shacklette, H. T. 1960: Geochemical exploration in Alaska; U. S. Geol. Surv., Prof., Paper 400-B, p Chowdhury, A. N. and Bose, B. B. 1971: Role of humus matter in the formation of geochemical anomalies; Can. Inst. Min. Met.,. v. 11, p Epstein, E. 1972: Mineral Nutrition of Plants: Principles and Perspectives; John Wiley and Sons Inc., Toronto,. 412 p. Fortescue, J. A. C. and Hornbrook, E. H. W. 1967: Progress report on biogeochemical research at the Geological Survey of Canada ; Geol. Surv. Can., Paper 67-23, Part I, p : Progress report on biogeochemical research at the Geological Survey of Canada ; Geol. Surv. Can., Paper 67-23, Part 11, 101 p. Malyuga, D. P. 1964: Biogeochemical Methods of Prospecting; Consultants Bureau Enterprises, New York, 205 p. Marmo, V. 1953: Biogeochemical investigations in Finland; Econ. Geol., v. 48, p Schacklette, H. T. 1962: Biotic implication of Alaskan biogeochemical distribution patterns; Ecology, v. 43, p : Bryophytes associated with mineral deposits and solutions in Alaska; U. S. Geol. Surv., Bull C, 18 p. 1967: Copper mosses as indicators of metal concentrations; U. S. Geol. Surv., Bull G, 18 p. Shilts, W. W. 1974: Drift prospecting in the Ennadai-Rankin Inlet greenstone belt, District of Keewatin; in Report of Activities, Part A; Geol. Surv. Can., Paper 74-lA, p : Tundra fires, southeast District of Keewatin; in Report of Activities, Part B; Geol. Surv. Can., Paper 75-lB, p Shilts, W. W., Dean, W. E., and Klassen, R.A. 1976: Physical, chemical, and stratigraphic aspects of sedimentation in lake basins of the eastern Arctic Shield; in Report of Activities, Part A; Geol. Surv. Can., Paper 76-lA, p Warren, H. V. and Delavault, R. E. 1948: Biogeochemical investigations in British Columbia; Geophysics, v. 13, p : Further studies in biogeochemistry; Geol. Soc. Am., Bull., v. 60, p a: Some biogeochemical investigations in Eastern Canada, I and 11; Can. Min. J., v. 76, p and b: Biogeochemical prospecting in northern latitudes; Roy. Soc. Can., Trans., v. 49, p Warren, H.V., Delavault, R.E., and Fortescue, J.A. C. 1955: Sampling in biogeochemistry; Geol. Soc. Am., Bull., v. 66, p Wein, R. W. and Shilts, W. W. 1976: Tundra fires in the District of Keewatin; in Report of Activities, Part A; Geol. Surv. Can., Paper 76-lA, p

8 GEOLOGICAL INFORMATION DIVISION 5; DlVlSlON DE L'INFORMATION G~OLOGIQUE GEOLOGICAL SURVEY PAPER 76-1 B REPORT OF ACTIVITIES PART B

9 O Minister of Supply and Services Canada 1976 Available by mail from Printing and Publishing Supply and Services Canada, Ottawa, Canada KIA OS9, from the Geological Survey of Canada 601 Booth St., Ottawa, KIA 0E8 and at Canadian Government Bookstores: Catalogue No. M B HALIFA 1683 Barrington Street MONTREAL 640 St. Catherine Street West OTTAWA 171 Slater Street TORONTO 221 Yonge Street WINNIPEG 393 Portage Avenue VANCOUVER 800 Granville Street or through your bookseller Price: Canada: $5. 00 Other countries: $6. 00 Price subject to change without notice