A two-millennium dinoflagellate cyst record from Gullmar Fjord, a Swedish Skagerrak sill fjord

Abstract

Gullmar Fjord, a sill fjord on the Skagerrak coast of western Sweden, contains a valuable sedimentary archive for the last 2500years. This archive encompasses a temporal record from the Roman Warm Period, the Dark Ages, the Medieval Warm Period, the Little Ice Age and into the modern warm period. A high resolution dinoflagellate cyst analysis has been completed on this archive using material from two cores taken from the deepest part of the fjord, Alsbäck Deep. The recovered dinoflagellate cysts have provided a quantitative temporal record that has been used to construct a dinoflagellate cyst spectrum and has been subjected to both Q-mode cluster analysis and CABFAC factor analysis with varimax rotation. In addition the heterotrophic ratio and both the thermophilic and cryophilic ratios have been calculated to assist with the interpretation of the results. Well preserved and diverse dinoflagellate cyst assemblages have been recovered throughout the sedimentary sequence and have been used to explore surface water conditions within the fjord over this time interval. Although a clear link is observed between the cyst assemblages and the climate phases of the Subatlantic, established from previous stable isotope work, there was little change in the cyst populations and indeed they reflect the known modern cyst floras. However a major change was seen in the incoming of Gymnodinium nolleri during the Dark Age and its reduction to a relict species towards the end of the Little Ice Age. Comparisons to other published work confirm the regional nature of this dinoflagellate cyst event but its ecological preferences remain enigmatic. A second major change was also recognised around the late 1960s/early 1970s and was associated with differences in nutrient availability from either a diminution in upwelling, as the NAO changed from a negative phase to a positive, or from marine pollution or a combination of both. Otherwise the dinoflagellate cyst assemblages can be grouped, using the factor analysis, into F1 Protoperidinium spp. indet. (round, brown cysts); F2 G. nolleri and F3 Lingulodinium polyedrum and Protoceratium reticulatum. The cluster analysis shows a similar subdivision into units closely associated with the recognised climate phases of the Subatlantic. Possible climate environments have been explored, but because of the relatively minor variations in the assemblages and the lack of autecological information, only relatively small scale changes were recognised except for the clear, but complex nature of the recent warm period.