RECONSTRUCTING ENVIRONMENTAL CHANGE IN ICELAND OVER THE PAST 2000 YEARS FROM LAKE SEDIMENTS

Deep lakes with high sedimentation rates (1 to 2 m / ka) offer the potential for high-resolution reconstructions of past environmental change. We recently recovered long sediment cores from Haukadalsvatn (37 m a.s.l.) a large (3.28 km2) deep (42 m) lake located at the neck of the Vestfirdir peninsula, NW Iceland, using the DOSECC GLAD-200 core rig. The sediment cores are very finely laminated with numerous thin tephra layers that will aid in the correlation between Haukadalsvatn and other Icelandic lakes. The sediment fill in Haukadalsvatn consists of 16 m of rapidly deposited ice-proximal to ice-distal deglacial marine sediment overlain by an upper lacustrine section of approximately 14 m thickness. A radiocarbon-based age model for the lacustrine portion indicates that sedimentation rates in the early Holocene were less than half the late Holocene sedimentation rates, with a substantial increase around 5.5 ka. We interpret the increased sedimentation rate to signal the onset of increased hillslope erosion and landscape instability, broadly consistent with the beginning of Neoglaciation. Even though there are no glaciers in the Haukadalsvatn catchment, increased periglacial activity and decreased vegetation cover as summer temperatures dropped is likely to produce increased hillslope erosion.

The upper 4.5 m represents the last 2000 years of sedimentation. This interval includes the Medieval Warm Period, the colonization of Iceland (ca. 1100 BP), the Little Ice Age, and 20th century warming. Historical records suggest the Little Ice Age may have resulted in summer temperature reductions of as much as 4 °C, half the global ice-age difference. We have measured total carbon continuously at 45-year intervals for the past 2000 years. Bulk sediment δ¹³C and C:N from sediment deposited over the past 1000 years confirm that the majority of organic matter preserved in the lake sediment is derived from terrestrial sources, as opposed to primary productivity within the lake. This offers a potential measure of the intensity of hillslope erosion. Contrasts in δ¹³C and C:N between the warm early Holocene and the Little Ice Age aid in quantifying the evolution of hillslope instability through the Holocene.