THE HOLOCENE DENUDATION OF ICELAND BASED ON THE ACCUMULATION OF MARINE SEDIMENTS

Estimates from the sediments load in rivers and from direct measurements of glacial erosion, primarily from southern Iceland, indicate a present-day denudation of Iceland in the range of 1 to 3 mm/yr. On Holocene time-scales, most of this sediment is transported offshore as river plumes and bedload, and accumulates in the 17 or so troughs that cut across the continental shelf---our data indicate that the fjords do not appear as major sediment sinks. The area of the troughs is approximately 20% of the land area, hence allowing for this and for the density of the marine sediments (average ~830 ± kg/m3), then the average Holocene sediment accumulation rate (SAR) should be of the order of 15 m/ky or a mass accumulation rate (MAR) of 14,600 kg/m2.ky. In the troughs of NW and N Iceland a major seismic reflector in the trough sediment packages is the 10,200±60 cal yr Saksunarvatn tephra. This tephra can be traced over 100’s km and verified by its presence in marine sediment cores where it is between 2 and 10 cm thick. The distribution of sediment thicknesses since this event is bimodal with the main mode at 5-6 m of accumulation but with a smaller mode at 25 m of sediment. Based on this marker bed and data from 30 marine cores, the SAR over the last 10,200± cal yrs averaged 60 ± cm/ky and an MAR of 2500 kg/m2.ky. These estimates differ from the estimated present denudation rate on Iceland by a factor of 5 or so. This large discrepancy might be associated with the different geographic and geomorphic-process areas from which the two estimates are derived, as N and NW Iceland is not characterized by large glaciers and ice caps nor by large, glacially fed rivers. Interestingly, the expected pattern of a decrease in SAR/MAR with distance from the present coast is not always followed and the thickest sediment packages overlying the Saksunarvatn tephra are located in sediment drifts on the mid-shelf region