SEDIMENT RECORDS OF MODERN AND PAST CLIMATE AND LAND USE IN NORTHWEST ICELAND

Climate and land use factors are changing the character of high latitude watersheds at a scale and pace that rivals that of glacial-interglacial transitions. Monitoring efforts aim to identify factors most responsible for ongoing changes, yet are hampered by the transitional nature of the modern environment. One solution is to combine monitoring with high-resolution paleolimnology, through sediment traps, surface cores, hydrological and soil analyses. Here, we describe collaborative efforts to characterize specific watershed sources for lake inputs, in the northwest region of Iceland, in order to relate them to modern climate and land use factors, including the North Atlantic Oscillation (NAO). We use tracers for organic carbon (TOC) with C:N ratios, branched glycerol dialkyl glycerol tetraethers (brGDGTs), and ¹⁴C dating of soil fractions. We identify transport paths for trace minerals using XRF geochemistry of soils, soil water, input streams and lake sediments.

We find the lake sediment organic carbon deposits to be mostly authigenic, with intermittent inputs of terrestrial carbon associated with significantly higher levels of TOC. Lake deposit mineral concentrations closely resemble those from lower A and B watershed soil horizons. Intervals of heavy spring rain or high snowmelt may create gullies that tap and erode deep soil layers. Alternatively, years with extended summer drought, followed average autumn rains may create conditions for erosion of deeper soils, by saturating cracked soils with dead root zones. We witnessed three such years in 2011 -2013.

To link watershed events to synoptic climate patterns, we use the NASA Modern Era-Retrospective Analysis for Research and Applications (MERRA) atmospheric reanalysis for different areas of Iceland and compare seasonal precipitation with NAO index time series, A.D. 1979-2010. The strongest relations between NAO indices and precipitation occur in winter with significant correlations for western Iceland in winter, spring and summer. In strong positive NAO years, northwest Iceland has increased precipitation in winter and spring, while strong negative NAO years tend to have less. Changes in the NAO pattern can most likely affect lake sediment composition in NW Iceland through soil erosion associated with increased winter and spring precipitation.