GEOCHEMICAL PROCESSES AFFECTING ELEMENT MOBILITY AND BIOAVAILABILITY IN BOREAL FOREST AND TUNDRA ECOSYSTEMS, ALASKA

We evaluate the geochemical processes that affect total and extractable element occurrences and their distribution patterns in "cold" soils (Inceptisols and Gelisols), and their subsequent influence on the biogeochemistry of boreal forest and tundra vegetation. These geochemical surveys are focused in the Big Delta B-1 and B-2 quadrangles, of the Yukon-Tanana Upland, east-central Alaska. Our studies are relevant to both mineral resources exploration and regional baseline geochemical surveys by increasing our understanding of landscape geochemical patterns.

Study area soils are developed from a wide variety of bedrock types. Crystalline rocks include biotite-sillimanite gneiss, quartzofeldspathic biotite gneiss, and metagraywacke. These metamorphic units were intruded by Devonian and Cretaceous plutonic rocks (the source of mineralization for the epigenetic, quartz-vein Pogo gold deposit) and subsequently cut by minor Tertiary basaltic dikes and rhyolite. Where available A-, B-, and C-horizon soils were collected over known parent bedrock in both mineralized and non-mineralized areas along with willow (Salix), alder (Alnus), and moss (Hylocomium). Organic-rich colluvial soils of mixed parent bedrock origin were also collected. Bulk plant and soil samples were analyzed as well as water extracts (1:5, soil sample:water) and soil sequential extraction fractions.

Of the elements examined, only Cu shows a strong relation between increasing concentration in vegetation and the relative increase of water extractable Cu to total soil Cu. Iron also shows a strong relation but only for the A horizon. In general, there is no relation between the relative increase of water extractable transition metal amounts to an increase in their plant tissue concentration for the B- and C-horizon soils. For a number of elements (Al, Fe, Ba, K, Mg and Na; and transition metals Cr, Ni, and Zn), an inverse relation exists between soil organic matter (OM) and element concentration. Ca, Hg, and S are the only elements tested that show a positive relation with OM. In these soils, where weathering processes are slow and the bulk composition of bedrock is similar, variation in soil geochemistry and element bioavailability is dominated by processes that affect OM accumulation, drainage, and depth to permafrost.