ROCK, SEDIMENT AND SOIL GEOCHEMISTRY OF AN AFFORESTATION SITE, REYKJANES PENINSULA, ICELAND

For many years, the Icelandic Forest Service (IFS) has been promoting afforestation at various sites around the country, originally as a way to produce commercial lumber, and more recently as a means of carbon sequestration to help counter global climate change. The IFS has found with successful plantings that required macro- and micronutrients are available in the soil, with the exception of nitrogen. (Lupine is planted to fix nitrogen.) The purpose of this study is to determine the geochemical composition of the soils of an afforestation site at the eastern end of the Reykjanes Peninsula. It is part of a larger study that is examining the nutrient profiles of Icelandic soils and the extent to which symbiotic mycorrhizae are making the nutrients available to the trees. The study site is underlain by Holocene basaltic lava flows. Sand, assumed to be eroded from local bedrock, was deposited back over the area at a time of higher sea level. The result is that depressions in the irregular lava flow surface are filled with sand (up to 1 m deep). These depressions are where the trees are being planted. Below the sand is a red-brown silty clay layer that contains rounded sand-size grains, suggesting that sediment or volcanic ash from an earlier depositional event was weathered before being covered by the later deposition of sand. Fourteen sand samples, 11 clay samples, 5 basalt flow samples and 3 basaltic scoria samples were analyzed for major and trace elements, as well as one ash sample from a higher elevation site approximately 25 km to the west. The composition of the sand is similar to the basalt flow, but with variability greater than one standard deviation. The ash sample is very similar in major-element composition to the lava flow, reflecting the consistency of eruptions over time. The clay samples are depleted in mobile elements compared to the ash, indicating that the clay layer could indeed be weathered ash. Of the tree macronutrients, potassium and phosphorus are present in sufficient quantities (average K is 0.35% in sand and 0.30% in clay; average P is 0.11% in sand and 0.15% in clay). Petrographic analysis indicates that apatite is the likely source of the phosphorus, although a marine component may be present.