THE RECENT SEDIMENTATION HISTORY OF THINGVALLAVATN, ICELAND

The sediments in the Thingvellir lake basin have been successfully dated by tephra layers back to ca. AD 900. the time of Nordic Settlement in Iceland. Nine of the twelve tephra layers known to have been deposited within or near the Thingvallavatn area since that time have been identified in sediment cores, providing an excellent source of information on the rate of sedimentation in the lake basin. The rate of net sedimentation increases proportionally with water depth below a critical limit of 30 m water depth, indicating a hydrologically controlled sedimentation. Accordingly, the Thingvellir lake basin can be divided into three sedimentation zones, a zone of erosion and transportation (ca. 48%), a zone of accumulation and transportation (ca. 4%) and a zone of net accumulation (ca. 48%). The sedimentation rate increases with time everywhere in the lake basin. This increase is related to increased fertility or productivity of the lake. It coincides with increased soil erosion and deterioration of the vegetation cover in Iceland. As ground water constitutes 90% of the total inflow in the lake, minor variations in the nutrient content of the ground water can cause large variations in productivity and sedimentation rate in the lake. A hypothesis is launched that nutrient elements, leached from organic soil laid bare when vegetation cover deteriorated within the drainage area, are released and transferred into the ground water system of Thingvallavatn.