The marine environmental evolution in the northern Norwegian Sea revealed by foraminifera during the last 60 ka

Both planktonic and benthic foraminifera were identified in a sediment core collected from the northern Norwegian Sea to reconstruct the paleoceanographic evolution since the last glaciation.The assemblages and distribution patterns of dominant foraminiferal species with special habitat preferences indicated that three marine environments occurred in the northern Norwegian Sea since 62 ka BP:(1)an environment controlled by the circulation of the North Atlantic Current(NAC);(2)by polynya-related sinking of brines and upwelling of intermediate water surrounding the polynya;(3)by melt-water from Barents Sea Ice Sheet(BSIS).At 62-52.5 ka BP,a period with the highest summer insolation during the last glaciatial period,intensification of the NAC led to higher absolute abundances and higher diversity of foraminiferal faunas.The higher abundance of benthic species Cibicidoides wuellerstorfi indicates bottom water conditions that were well-ventilated with an adequate food supply;however,higher abundances of polar planktonic foraminiferal species Neogloboquadrina pachyderma(sin.)indicate that the near-surface temperatures were still low.During mid-late Marine Isotope Stage(MIS)3(52.5-29 ka BP),the marine environment of the northern Norwegian Sea alternately changed among the above mentioned three environments.At 29-17ka BP during the last glacial maximum,the dominant benthic species Bolivina arctica from the Arctic Ocean indicates an extreme cold bottom environment.The BSIS expanded to its maximum extent during this period,and vast polynya formed at the edge of the ice sheet.The sinking of brines from the formation of sea ice in the polynyas caused upwelling,indicated by the upwelling adapted planktonic species Globigerinita glutinata.At 17-10 ka BP,the northern Norwegian Sea was controlled by melt-water.With the ablation of BSIS,massive amounts of melt water discharged into the Norwegian Sea,resulting in strong water column stratification,poor ventilation,and an oligotrophic bottom condition,which ledto a drastic decline in the abundance and diversity of foraminifera.At 10-0 ka BP,the marine environment was transformed again by the control of the NAC,which continues to modern day.The abrupt decrease in relative abundance of Neogloboquadrina pachyderma(sin.)indicates a rise in near-surface temperature with the strengthening of the NAC and without the influence of the BSIS.