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SEDIMENT AND MASS ACCUMULATION RATES ON THE ICELAND MARGIN OVER THE LAST 12,000 CALENDAR YEARS

ANDREWS, JOHN T  University of Colorado.

Because Iceland was finally deglaciated only 8-9 14C ka, has abundant precipitation in the form of both rain and snow, has high relief, and easily erodable soils, it is reasonable to suggest a priori that the rates of late glacial/Holocene sediment transfer from land to sea would be efficient and high. Furthermore, observations from other formerly glaciated continental margins, plus theory of sediment transport, predicts that the rates of sediment accumulation will decrease in a non-linear fashion away from the coastline. The Iceland shelf is crossed by 18 large troughs with depths between 200 and 600 m. The intervening banks are 100 m or so. The shelf area ≤ 200 m water depth is similar to the area of Iceland, or around 110,000 km2. The troughs represent about 20% of the shelf area and are the main sites of Holocene sediment accumulation. However, numerous fjords provide efficient sediment traps for glacial and fluvially transported sediments. Data on sediment accumulation rates (SAR, cm/ky) and total mass accumulation rates (MAR, g/cm2.ky) from 40 sites around the Iceland margin (Fig. 1) between 18° and 29°W and 63° and 67°N are presented for the last 10,000 radiocarbon years. The 9,000 14C yrs Saksunarvatn tephra provides a regional isochron for deposition especially on the N-central and NW margins of Iceland. Dates from core tops vary in age from modern to as old as 1500 yrs, probably associated in many cases with sediment retrieval during the coring process. The sites vary in location from being within fjords, to those lying within large cross-shelf troughs, and finally to sites at the base of the NW Iceland continental slope (Denmark Strait). Data on average dry sediment density and carbonate weight percentage are available for 33 cores, as are data on the consolidation of sediment with depth. The MAR carbonate represents a measure of marine productivity and/or sediment transport and is subtracted from the MARtotal to obtain MARsediment. The contribution of carbonate to the sediment pile varies spatially, and temporally, but can be as high, on average, as ~40%. Over the last 10 14C ka SAR has varied from ≥650 to 3 cm/ky with a median value of 44 cm/ky; thus “on average” 22 yrs is represented in one centimeter of sediment whereas at some sites 1 cm of sediment represents only ~2 years of accumulation (Andrews et al., 2003a) Therefore, the Iceland sediment packages will archive multi-year to multidecadal records of paleo-environmental conditions around the Iceland margin. There is an extremely strong predictive association (r2=0.9) between estimates of MAR derived from integrating level by level in a core and those determined by simply using the average sediment density times core length. For the 33 cores where we have sufficient data the median MAR is 222 with a variation between 19 and 3150 g/cm2.ky. The amount of sediment accumulated between 10 and 9 14C ka B.P., at a time when deglaciation was still underway, divided by the average SAR/ky for the last 9 14C ka B.P. varies from around 2 to >20, indicating that at many sites the rate of sediment accumulation over the last 10 14C ka B.P. was not monotonic but decreased significantly after the retreat of the Iceland Ice Sheet. Spatially the smallest difference between sediment accumulation 9-10 14C ka B.P. /SAR <9/ky is in the north-central troughs, and the largest difference occurs in outer Djupall (NW Iceland).

    Although we do not have adequate, nor representative, coverage of Icelandic fjords in the sites from Vestfirdir the sediment accumulation within this fjords is not substantially high than sites from the inner shelf. Indeed, the sites with the highest rates of Holocene sediment accumulation are the troughs which lie 10’s km outside the coastline and the sites where maximum sediment accumulation might be predicted based on fluvially and glacially transported sediments to fjord heads. The disconnect in SAR and MAR values from the fjords/inner shelf troughs, to the mid/outer shelf troughs suggests extensive sediment reworking and transport on the shallow shelf banks, leading to the accumulation of “drift-like” sediment bodies in select locations (Andrews et al., 2003b).

    In 1997 the 3.5 kHz system was employed in several of the troughs off N and NW Iceland. A prominent acoustic reflector, subsequently identified as the basaltic Saksunarvatn tephra, allows us to track variations in SAR along the troughs. This exercise indicates significant spatial variations in SAR that cannot be explained by a simple land‡sea sediment transfer, but rather points to the accumulation in certain troughs of what might be termed “shelf sediment drifts” with maximum accumulations over the last 10 cal ka of 50 m or so of sediment.

REFERENCES
ANDREWS, J.T., HARDARDOTTIR, J., STONER, J.S., MANN, M.E., KRISTJANSDOTTIR, G.B., and KOC, N., 2003a, Decadal to millennial-scale periodicities in North Iceland shelf sediments over the last 12,000 cal yrs: long-term North Atlantic oceanographic variability and Solar forcing: Earth and Planetary Science Letters, v. 210, p. 453-465.



ANDREWS, J.T., HARDARDOTTIR, J., KRISTJANSDOTTIR, G.B., GRONVALD, K., and STONER, J., 2003, A high resolution Holocene sediment record from Húnflóaáll, N Iceland margin: Century to millenial-scale variability since the Vedde tephra: The Holocene, v. 13, p. 625-638.



Figure 1. Location of core sites around N, NW, and SW Iceland


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