NORTH ICELANDIC SHELF SEDIMENTS: HOLOCENE AND LATEGLACIAL RECORD OF OCEANOGRAPHIC EVENTS
Eiriksson, Jon . Science Institute, University of Iceland, IS-101 Reykjavýk, Iceland.
Knudsen, Karen Luise . Department of Earth Science, University of Aarhus, DK-8000 Aarhus C, Denmark.
RYTTER, FRANK . Department of Earth Science, University of Aarhus, DK-8000 Aarhus C, Denmark.
Modern atmospheric and oceanographic processes in the Iceland region of the North Atlantic are characterized by strong gradients separating the Arctic and Atlantic realms. Geologically, the north Icelandic shelf is partly distinguished by the Tjörnes Fracture Zone featuring numerous active basins in a mud dominated shelf environment. Lateglacial and Holocene high-resolution sedimentary records from this area have been studied with tephrochronology as the main tool for correlation and for exact timing of palaeoceanographic events in the area.
Several larger palaeoceanographic shifts occurred in the area during the Lateglacial and the early Holocene. Benthic foraminiferal assemblages indicate a strong influence of Atlantic water masses (Alabaminella weddellensis, Cassidulina neoteretis, Miliolida) on the sea floor. Similar, but weaker, environmental signals are repeated during part of the Younger Dryas around the level of the Vedde tephra marker. An oceanographic cooling started at the beginning of both the Bölling-Alleröd and the Preboreal (Islandiella norcrossi, Nonionellina labradorica, Elphidium excavatum, Cassidulina reniforme), indicating an antiphase relationship between the climate signals in this area and those of the eastern North Atlantic. Foraminiferal assemblages of Holocene records demonstrate that relatively warm conditions prevailed between 10,000 and 6,000 cal. BP (the Holocene Climatic Optimum), interrupted by a cooling of the sea surface of about 3C at around 8,200 cal. BP.
Data from three new IMAGES piston cores recovered in the shelf area show how tephra markers can be used for the first evaluation of a chronology and correlation of the cores. The correlation is extended with lithological logs and with magnetic susceptibility records. Two different possible age models, one based on 14C dates combined with tephra markers and one based on tephra markers alone, are presented in context with the problem of different marine reservoir ages of the water masses in the area.
A detailed multidisciplinary study of a late Holocene record from the area demonstrates that marked variations in the distribution of water masses occurred repeatedly through the last 4500 cal. years. Of special interest is the exact timing of a marked drop in sea-surface temperature in the area, indicated by ice rafting debris concentration, to about 50 years before the Hekla 3 eruption, which occurred at 2980 cal. BP. This appears to predate most records of a general cooling event in NW Europe by a couple of centuries. Also the Medieval Warm Period and the Little Ice Age are expressed in the assemblages.
The present study emphasizes the importance of combining age models based on AMS 14C datings with tephrochronological studies, in order to obtain a better understanding of marine reservoir ages of the different water masses both in space and in time. This applies not only to stadial versus interstadial intervals with major shifts in the oceanic systems, but also to the Holocene time period, where variation in the influence of different water masses causes a change in reservoir age through time as well.
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