The Holocene Paleoclimate And Paleoceanography In The Northern North Atlantic

A high-resolution diatom record and selected foraminiferal and magnectic data from core MD99-2275 reveal a series of palaeoceanographic change on the North Icelandic shelf since about 9000 cal.yr BP.The influence of relatively warm high-salinity Atlantic water masses of the Irminger Current was strong in the study area during the Holocene Climate Optimum(HCO,8760(base of record)-c.5300 cal.yr BP).The relatively warm and stable period in the early Holocene was interrupted by remarkably enhanced influence of the East Greenland Current and East Icelandic Current during about 8120-8000 cal.yr BP.After that,the warmest conditon was reached between c.8000 and 7000 cal.yr BP on the North Icelandic shelf.There is a general cooling trend from c.7000 cal.yr BP to the present,which is indicated by an increase in the influence of cold Polar water masses from the East Greenland Current and the East Icelandic Current and a decrease in the influence of the Irminger Current.A major change in diatom assemblages at around 5300 cal.yr BP indicates a significant palaeoceanographic shift on the North Icelandic shelf in the mid-Holocene,which is also demonstrated by an abrupt change in the magnectic properties at c.5500 cal.yr BP. There should be an important change in the provenance and dynamic of the water mass in the mid-Holocene on the North Icelandic shelf,which is followed by gradual cooling and unstable condition during the remainder of the Holocene.The influence of the Norwegian-Atlantic Water might have increased on the North Icelandic shelf since this mid-Holocene palaeoceanogrphic change.Stepwise increases in the influence of Polar and Arctic water masses are indicated by the benthic and planktonic foraminiferal data at c.4200 cal.yr BP and c.3000 cal.yr BP.In addition,several abrupt palaeoceanographic changes are distinguished.Enhanced influence of Polar waters is reflected during the intervals 8120-8000,6650-6100,4300-4100,3000-2700,1300-1100 and 600-200 cal.yr BP.Apparent regional differences are indicated on the North Icelandic shelf by comparing the floral and faunal indications from core MD99-2275 and the other Holocene records on the North Icelandic shelf.The Kolbeinsey Ridge influences the Holocene palaeoceanography,especially the middle and deep water mass,as an oceanographic barrier on the North Icelandic shelf. The Holocene Palaeoclimatic and Palaeoceanographic changes in the northern North Atlantic are discussed in this thesis.There is a general cooling trend from the early Holocene to present in the northern North Atlantic,forced by gradual decreasing Nothern Hemisphere solar insolation.The HCO in the whole northern North Atlantic occurs later than the Holocene insolation maximum at～11,000 cal.yr BP.The lag is suggested to be the result of the remnants of the last Glacial ice sheets,as the atmospheric jet stream is sensitive to the size and geometry of the ice sheet,and the North Atlantic Deep Water(NADW)is susceptible to the melting water from the ice sheet.In addition,Regional differences in the onset and duration of the HCO are also shown in the northern North Atlantic.The HCO in the area strongly influenced by the North Atlantic water,such as the North Icelandic shelf and the southern Norwegian Sea, started at～10,000-9000 cal.yr BP,while HCO in the area mainly influenced by the Polar water mass(eg.The East Greenland shelf)occured at～8500 cal.yr BP.This HCO delay is suggested to be results of transgressively influence of the North Atlantic water in the Nordic Seas and the westwardly movement of the Polar and Arctic Front as the the initial Holocene warming took place.In the subpolar Atlantic to the south Iceland, the HCO occurred at～7500-7000 cal.yr BP,which is～3500-4000 years later than the Holocene Northern Hemisphere insolation maximum.This long-term lag is closely related to the strong influence of the Canadian Arctic Water and the melting water from the Laurentide ice sheet in the early Holocene.An abrupt severe cooling around 8200 cal.yr BP interrupted the early Holocene warming in the northern North Atlantic.Regional difference is also indicated in the extent and timing of this 8200 cal.yr BP cooling event.For example,this early Holocene cooling event is distinctly revealed on the eastern and northern North Icelandic shelf,but is shown as an only minor change on the western North Icelandic shelf.The 8200 cal.yr BP cooling event is correlated with the final collapse of the Laurentide ice sheet and the resulting drainage of glacial lakes Agassiz and Ojibway.A particularly profound change is indicated in the mid-Holocene(～6000-5000 cal.yr BP),which is followed by general cooling trend in the northern North Atlantic. The Neoglacial in the northern North Atlantic and adjacent areas is mainly forced by the markedly decreasing of the North Hemisphere solar insolation since the mid Holocene and may be also triggered by remarkable weakening of the NADW in the mid-Holocene.In addition,periodically climatic and oceanographic change is revealed in the northern North Atlantic during the Holocene.It is apparently reflected by recurrence of the abrupt cooling events,occurring at～-400,～1400,～2800,～4200,～5800,～8200,～9300 and～10,300 cal.yr BF.A diatom record from Multicore B05-MC04 reveals the oceanographic change on the North Icelandic shelf during the last 100 years.A general warming is indicated by gradually decreasing sea ice and Arctic diatom species and reconstructed summer sea surface water temperature(SSTs)based on diatom,which is coincident with the general global warming since the industrial revolution.Three short-term cooling events are shown superimposed on the general warming trend,occurring at～1920 A.D., 1960-1975 A.D.and 1995-2002 A.D.All of the three oceanographic coolings fell into the times of declined or negative North Atlantic Oscillation index,showing close correlation with this decadal timescale changes in atmospheric pattern.The cooling during 1960-1975 A.D is also related with the Great Salinity Anomaly(GSA)in the North Atlantic.The quantitatively reconstructed SSTs based on the diatom record from core B05-MC04 is consistent with the instrumental data at the adjacent observation sites in the north Iceland,indicating that it is feasible to quatitatively reconstruct the Falaeoclimatic and Falaeoceanographic changes on the North Icelandic shelf based on sediment diatom.