Diatom-based Reconstruction Of Palaeoclimatic Changes And Sea-ice Concentration Off West Greenland During The Last1200Years

Being located in the climatically extremely sensitive Arctic part of the North Atlantic region, West Greenland is an area which is highly suitable for conducting studies on regional climate changes. Palaeoclimatic data from this region are particularly important because of the proximity to both the Greenland ice sheet and the Davis Strait, which play important roles in North Atlantic climate dynamics, including both the oceanic and the atmospheric circulation patterns in the region. Furthermore, sea ice is a major component of the climate system, because it influences the planetary albedo and the exchanges of heat, moisture and gases between the ocean and the atmosphere. However, observed reliable data, such as satellite data, are only available as far back as30years with the exception of records from ship logs, which are sporadic and irregular. Only a limited amount of data have been published on marine Holocene sediments and palaeoenvironments from West Greenland, and most of these studies have focused on northernmost Baffin Bay and the Disko Bugt area.This project attempts to study the relationship between surface sediment diatoms and sea-ice concentration (SIC) off West Greenland and to build up a diatom based SIC transfer function, as well as to reconstruct palaeoclimatic changes and SIC during the last1200years in high time resolution based on a precisely dated diatom record (GA306-GC4/BC4).A diatom-based sea ice transfer function is developed using72surface samples from west of Greenland and Iceland, and the associated modern SIC. Canonical correspondence analysis (CCA) is applied to the species assemblages of the surface sediment samples and the association of the species with average monthly SIC is assessed. The results of CCA reveal that April SIC is the most important environmental factor controlling the distribution of diatoms off West Greenland. Consequently, April SIC has the potential to be reconstructed in the sedimentary record. A diatom-based transfer function for April SIC has been built up using the computer program C2and six numerical reconstruction methods were tested. The results show that the WA-PLS using2components should be employed to obtain the most reliable diatom-based SIC transfer function in the area, which has the lowest RMSEP (Jack)(11.568) and the highest R2Jack (0.90033).A high degree of consistency between changes in frequency of selected diatom species and instrumental data from the same area during the last70years confirms the reliability of diatoms (particularly sea-ice species and warm-water species) for the study of palaeoceanographic changes in this area. A general cooling trend with some fluctuations is marked by an increase in sea-ice species throughout the last1200years. A relatively warm period with increased influence of Atlantic water masses of the Irminger Current (IC) is found at AD750-1300, although with some oceanographic variability after AD1000. A pronounced oceanographic shift occurred at AD1300, corresponding in time to the transition from the so-called Medieval Warm Period (MWP) to the Little Ice Age (LIA). The LIA cold episode is characterized by three intervals with particularly cold sea-surface conditions at AD1300-1350, AD1400-1575and AD1660-1710as a result of variable influence of Polar waters in the area. During the last70years, two relatively warm periods and one cold period (the early1960s to mid-1990s) are indicated by changes in the diatom components.Based on the diatom record from the GC4and BC4cores, a April SIC reconstruction is presented that shows a slightly increasing trend during the time period AD715-1760, but with some fluctuations superimposed on this general trend. Relatively low April SIC during the interval AD750-1300suggests a warm period, corresponding to the MWP. A marked increase in April SIC indicates an abrupt change to colder conditions after AD1300, particularly at AD1330-1400, AD1450-1580and AD1650-1760. A general declining trend during the last70years is interrupted by peak values around AD1950and1975, as well as in the early1980s, early1990s and early2000s. The present study demonstrates that the sedimentary records on the West Greenland shelf provide valuable palaeoclimatic and SIC data that confirm a linkage between local and large scale North Atlantic oceanographic and atmospheric oscillations.