A HIGH-RESOLUTION RECORD FROM SEDIMENT ANALYSES AND SEISMIC DATA FROM THE NW SHELF OF ICELAND, DURING THE TIME PERIOD 9.600 - 12.800 14C YEARS BP.
LAFSDžTTIR, SDżS . Dept. of Geosciences, University of Iceland.
Geirsdžttir, slaug . Dept. of Geosciences, University of Iceland.
Andrews, John T. INSTAAR and Dept. of Geol. Sciences, University of Colorado.
The NW shelf of Iceland is an extremely sensitive area where the East Greenland current of polar origin and the warmer and saline North Atlantic current meet. In 1999, a core was collected aboard the IMAGES cruise (MD99-2264) on the Marion Dufresne from the Djupall trough system (66° 40.74 N; 24° 11.76 W). The core is 38m long and numerous AMS 14C dates show that it contains sediment that has been accumulating over the last 35 ky.
In the summer of 2001, a new seismic reflection profile was obtained from the MD99-2264 core site. The data was obtained with the WHOI SUBSCAN Chirp Sonar, which is a shallow water, high-resolution acoustic imaging system. The system operates at frequencies between 0.5-16 kHz and is therefore able to provide very high-resolution (10s of cm resolution) subbottom profiling data up to 80 m thick. The Djupall data clearly shows five different sedimentation units, erosional structures and tephra layers in the trough. Comparison between the seismic reflection profile and the lithofacies from the MD99-2264 core agree perfectly.
In this study we are focusing on a 7 m long part of the core, from 8-15 m. In this succession we have three AMS 14C dating and also the Vedde tephra, which serves as a useful time marker. According to the radiocarbon dates this interval corresponds to sedimentation during deglaciation (~12.800-9.600 14C years BP). The mean sedimentation accumulation rate during this period is 220cm/1000 years. However, in the beginning of the period (12.600 14C BP) the rate increases from 9 years/cm to 1 year/cm at the end of the interval. This implies fluctuations during this interval with extremely high sedimentation rate around 9800 14C years BP. Our interests in this study were to see if 1) data from grain size analyses would reflect environmental changes during this unstable period and, 2) to see if the grain size data would correlate to the IRD, total carbon and magnetic susceptibility data.
Our results show considerable changes in the grain size, MS and total carbon, reflecting large fluctuations in sediment deposition. In addition, pikes of ice rafted debris were followed by coarser sedimentation indicating a high-energy environment during the retreat periods of the glacier. At the same depth interval in the Chirp profile, a strong seismic reflector is detected coinciding with Vedde tephra. It also reflects an erosional boundary indicating changes in the paleobottom current system, which corresponds in time with the Younger Dryas chronozone. Above the erosional boundary finer sediments accumulate indicating the beginning of Holocene sedimentation with stable bottom currents similar to the ones controlling the current system in Djupall trough today.
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Copyright © 2001 INSTAAR, Univ. of Colorado