2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 4
Presentation Time: 3:30 PM


ANDREWS, John T., INSTAAR & Dept. Geological Sciences, Univ of Colorado, Box 450, Boulder, CO 80309, andrewsj@colorado.edu

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