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PACLIVA Résumé de rapport

Project ID: EVK2-CT-2002-00143
Financé au titre de: FP5-EESD
Pays: Denmark

Surface water palaeoreconstructions from the N. Icelandic Shelf

Sea surface conditions in the time interval 0-2ka and 6-8ka have been reconstructed from two North Iceland cores (MD99-2275 and MD99-2271. Due to the present position of the Polar Front on the North Icelandic shelf, this is a very sensitive area for reconstruction of climate change through the Holocene. In this region, the relatively warm Irminger Current, bringing Atlantic Water masses to the shelf, meets the cold, low-salinity East Icelandic Current (Arctic Water), which partly derives from the East Greenland Current (Polar Water) and partly from westerly eddies of the Norwegian Atlantic Current (Atlantic Water).

Dating of the North Icelandic shelf records has been achieved by a combination of AMS radiocarbon dates and tephrochronology. Tephra samples from marine sediment cores and from soil sections in Iceland have been geochemically analysed for major elements at microprobe facilities at the University of Bergen, Norway. This has enabled the construction of age models that are independent of marine reservoir age uncertainties hampering reliable radiocarbon dating of marine samples. Mineralogical analyses have been employed to investigate deposition of sand-sized sediments released from melting sea-ice and icebergs. These records have been compared with the sea-ice index record from Iceland back to AD 1600. Methods have been developed to differentiate primary and secondary deposits of volcanic tephra, a key issue in tephrochronological work in settings proximal to volcanoes. Domains for primary versus secondary volcanics have been established employing morphometric techniques.

Several different proxies are used to reconstruct high resolution sea surface temperature (SST): 1) diatom-based transfer function, 2) transfer functions based on planktonic foraminiferal assemblages (Maximum Likelihood, ML) and 3) planktonic oxygen isotopes (sinistrally coiled Neogloboquadrina pachyderma, NPS).
The diatom-based transfer function uses a modern diatom-environmental variable dataset from around Iceland covering areas with sufficient environmental gradients for the deduction of quantitative relationship between diatom species and environmental variables. A Monte Carlo permutation test with forward selection shows that diatom distribution in the area is primarily controlled by summer and winter sea surface temperatures (SSTs, SSTw). WAPLS transfer functions for SSTw and SSTs have uncertainties of less than 1°C.

The reconstructed temperature for the time interval 2-0 cal kyr BP on the North Icelandic shelf shows an overall decreasing trend through the last two millennia, with generally higher temperatures during the Roman Warm Period than during the Medieval Warm Period. There is a sharp temperature drop between AD 1250 and 1350, at the initiation of the Little Ice Age. There is a high correlation between the reconstructed marine temperature and historical records. The absolute level of reconstructed temperatures is almost similar for the diatom-based reconstruction and for the isotopic-based one, while the ML reconstruction results in generally lower values (a difference of about 4°C).

The temperature reconstruction for the time interval 8-6 cal kyr BP show a clear climatic optimum between 8.0 and 7.3 cal kyr BP off North Iceland. After that, there is a generally decreasing trend in sea surface temperature. The 8.2 cooling event is clearly expressed in all the MD99-2275 data. This core site was located close to the Polar Front area at that time, an area which was therefore very sensitive to minor changes in the distribution of the surface water masses. The 8.2 event has, however, not been recorded at core site MD99-2271, which is located in the central part of the Atlantic Water masses of Irminger Current. This site must have been located to the south of the frontal area at that time. As seen for the 2-0 cal kyr interval, the ML-based temperature reconstruction gives temperature values, which are a few degrees lower than those based on isotopes and diatoms. A comparison between the two time intervals shows that the values are close to 2°C higher in the 8-6 cal kyr interval than in the 2-0 cal kyr time interval.


Karen Luise KNUDSEN, (Associate Professor)
Tél.: +45-89423557
Fax: +45-86183936