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OCEANOGRAPHIC CHANGES AROUND 8 KYR. BP ON THE NORTHERN ICELANDIC SHELF INTERPRETED FROM SEDIMENTOLOGICAL AND FORAMINIFERAL FAUNAL ANALYSES IN CORE MD992275
SøNDERGAARD, METTE K.B. Department of Earth Sciences, University of Aarhus, DK-8000 Aarhus C, Denmark.
Eiríksson, Jón Science Institute, University of Iceland, IS-101 Reykjavík, Iceland.
Knudsen, Karen Luise Department of Earth Sciences, University of Aarhus, DK-8000 Aarhus C, Denmark.
During the past few years the North Icelandic shelf area has been subject to several studies. Many of these investigations deal with oceanographic changes from the Late Glacial and throughout the Holocene. This talk will concentrate on changes, which have been observed in a short time interval around 8 kyr. BP in a core a high sedimentation rate area off North Iceland.
The investigated core MD992275 is located at app. 440 m water depth in the Tjörnes Fracture Zone east of the Kolbeinsey Ridge in the oceanographically sensitive area of the Northern Icelandic shelf. Here the warm Irminger Current, running clockwise along the southern and western part of Iceland, meets a modified branch of the cold East Greenland Current, the East Icelandic Current. The easternmost part of the shelf is possibly also affected by the Jan Mayen Gyre carrying cooled Atlantic water. The core contains sediment deposited during approximately the last 14 kyr. with a mean sedimentation rate of 2.6 m/kyr. The high sedimentation rate enables a high time resolution, and the identification of known tephra layers e.g. the Vedde Ash, the Saksunarvatn ash, the Hekla 5 and the Hekla 3, combined with radiocarbon datings provide a robust fundament for construction of an age model and for correlation to other localities.
A series of sedimentological analyses have been performed on the early Holocene part of core MD992275. They include water content, solid density, organic and inorganic carbon content as well as grain size analyses and mineralogical counts. A prominent change in the grain size distribution and total inorganic carbon content of the sediment is observed around 8 kyr BP. The same interval is characterized by major changes in the foraminiferal faunal composition of both the benthic and the planktonic assemblages. For instance, the benthic species Cibicides lobatulus shows major oscillations and the planktonic Neogloboquadrina pachyderma sinistral also varies in abundance.
Investigations by Knudsen et al. (in press) of other cores from the North Icelandic shelf area west of the Kolbeinsey Ridge also clearly show an oceanographic event. This is seen by increased amounts of Neogloboquadrina pachyderma (sinistral), lighter 18O stable isotope values and changes in the grain size distribution occurring at approximately the same time. This oceanographic event, which has been found in several North Icelandic cores probably correlates with the well-known, so-called “8.2 kyr event”.
Evidence of the contemporaneous climatic event at approximately 8.2 kyr BP has been observed in many different records throughout the North Atlantic Ocean and the surrounding landmasses. It is seen as a notable change in 18O stable isotope values compared to the otherwise relatively stable Holocene values in the Greenland ice cores (e.g. Johnsen et al., 1992) as well as in a number of proxies in marine sediment cores e.g. changes in foraminiferal and coccolithic assemblage compositions, 18O stable isotope values and sediment grain size distribution (e.g. Andrews and Giraudeau, 2003; Klitgaard-Kristensen et al., 1998) and in European terrestrial records such at tree-ring width (Klitgaard-Kristensen et al., 1998). Different suggestions of the forcing mechanism behind this event have been put forward. Among those is affection of the North Atlantic thermohaline circulation by changes in the freshwater flux to the area (e.g. Clark et al., 2001; Klitgaard-Kristensen et al., 1998).
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D. Klitgaard-Kristensen, H.P. Sejrup, H. Haflidason, S. Johnsen and M. Spurk, 1998, A regional 8200 cal. yr BP cooling event in northwest Europe, induced by final stages of the Laurentide ice-sheet deglaciation?: Journal of Quaternary Science, v. 13, p. 165-169.
K.L. Knudsen, H. Jiang, E. Jansen, J. Eiríksson, J. Heinemeier and M.-S. Seidenkrantz, in press, Environmental changes off North Iceland during the deglaciation and the Holocene: foraminifera, diatoms and stable isotopes: Marine Micropaleontology.
Figure 1. Present day oceanic surface circulation pattern in the North Atlantic. Light grey arrows mark cold water mass currents and dark grey arrows mark warm water mass currents. The Polar Front is the boundary zone between the warm and the cold water masses.
Figure 2. Bathymetric map of the shelf and upper slope area around Iceland. Location of the studied sediment core MD992275 is marked with a black dot. Modified after Knudsen et al. (in press).
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