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CURRENTS AND CLIMATE ON THE NORTHWEST ICELAND SHELF IN THE LATE QUATERNARY (10-30 KA CAL BP): RECONSTRUCTION BASED ON FORAMINIFERAL AND SEDIMENTOLOGICAL RESEARCH
OLAFSDOTTIR, SAEDIS Dept. of Geosciences, University of Iceland, 101 Reykjavik.
Aslaug, Geirsdottir Dept. of GEosciences, University of Iceland, 101 Reykjavik.
Anne, Jennings E INSTAAR and Dept. of Geol. Sciences, University of Colorado, Boulder.
John, Andrews T INSTAAR and Dept. of Geol. Sciences, University of Colorado, Boulder.
The shelf area northwest off Iceland has gained a significant attention for oceanographic studies because of its unique location. The area is close to the boundary (the Polar Frontal zone) between the cold and fresh southward flowing East Greenland Current and the warm and saline northward flowing Irminger Current. A 38 m long core (MD99-2264) spanning the last ca. 30 cal kyrs BP was obtained at 230 m water depth in the Djúpáll trough on the Vestfirdir shelf. The core location is bathed in relatively warm waters today, but the boundary zone has migrated through time leading to exchange between warm and chilled water masses.
The aim of this research is to obtain a marine based deglaciation history for the northwest Iceland shelf. We focus on the following questions: 1) Did the LGM ice sheet extend beyond the modern shelf break, 2) when was the first incursion of warm and saline N-Atlantic water, 3) how do the environmental conditions changes during the deglaciation, and 4) are there marked changes between the bottom and surface flow during the deglaciation.
To approach these questions multiple proxy records were established; sedimentological properties (visual description, IRD (>2mm on X-radiographs), total carbon, and magnetic susceptibility), foraminiferal assemblages and foraminiferal stable isotopes studies. The chronological model is constrained by 13 14C AMS dates and two well dated tephra layers (Saksunarvatn and Vedde tephra).
The results indicate 6 main sediment and foraminiferal faunal units.
Unit 1 ~ Glacimarine diamict (>30,000 cal yr BP)
This unit is composed of stiff mud loaded with IRD and hardly any bivalves. The foraminiferal assemblages is composed of a mixture of fresh looking and reworked foraminiferal tests, and a mixture of boreal and arctic species. This unit indicates that the main ice stream from the Vestfirdir ice sheet was most likely calving out to the Djúpáll region, with its margin close to the MD99-2264 core location.
Unit 2 ~ Turbidites (>15,300 cal yr BP)
This unit is characterized by several rhythmic sequences, each showing a gradual decrease in grain size toward the top. The sedimentation accumulation rate is high and the foraminiferal assemblages look reworked. This can be interpreted as the break up of the main Vestfirdir ice sheet.
Unit 3 ~ Glacimarine diamict (15,300-14,000 cal yr BP)
This unit is composed of fine sediment, with occasional sulfide streaks and frequent IRD. The foraminiferal fauna is composed of arctic species (Cassidulina reniforme and Elphidium excavatum) with peak abundance of Nonion labradoricum. The bottom δ13C signal is very low at the same time which might indicate renewing of the bottom water flow with increased nutrient supplies and high productivity, possibly, in some relation to the relocation of the Polar Front zone or fluctuation of the local ice-front margin.
Unit 4 ~ Marine sedimentation (14,000-13,800 cal yr BP)
This unit is composed of sandy mud with no IRD and high content of total carbon. The foraminiferal fauna has higher diversity and an increase in boreal species is noticeable. The δ18O value in the boreal benthic species Cassidulina laevigata and the planktic species Neoglobigerina pachyderma (sinstral) is light. This might indicate a fresh water spike or the first incursion of warm and saline Atlantic water after the LGM.
Unit 5 ~ Glacimarine (13,800-11,500 cal yr BP)
This unit shows high sedimentation rate, with frequent sandlayers (often with high content of bivalve fragments). IRD is abundant and total carbonate content is low. This unit includes the Vedde tephra layer. The foraminiferal fauna has low diversity and the most abundant species are C. reniforme and E. excavatum. The benthic and planktic δ18O signal is high. This indicates that the fauna and sedimentation is influenced by cold conditions in front of a calving glacier, sea-ice and presumably saline conditions and stagnant bottom water.
Unit 6 ~ Marine sedimentation (11,500-10,000 cal yr BP)
This unit is composed of muddy sand, with no IRD, high total carbonate content, and frequent bivalves. The foraminiferal fauna is composed of a high percentage of boreal species. The benthic δ18O signal decreases gradually reaching the lightest value (2.8 ‰) in the record, around 10,000 cal yr BP. The surface water isotope signal also shows lighter values (2.4 ‰). These data suggest that the shelf is ice free and that the Irminger Current has attained its modern path and strength.
Figure 1. Map of Iceland, red dot showing the MD99-2264 core location.
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