Variations in Surface Water Mass Conditions in the Norwegian-Greenland Sea: Evidence from Pliocene/Pleistocene Calcareous Plankton Records (Sites 644, 907, 909)

Abstract

Records of bulk carbonate content, calcareous plankton assemblages, as well as sedimentological data from Ocean Drilling Program Sites 644, 907, and 909 are used to examine past variations in the surface circulation system of the Norwegian-Greenland Sea. This study includes investigations of the species abundance as well as the preservation of planktonic foraminifer species Neogloboquadrina pachyderma. In general, the Sites 644 and 907 data indicate an intensification of glaciation 3.0 m.y. (Iceland Sea) and 2.75 m.y. (Norwegian Sea) ago. The interval until ~l.l Ma is marked by a series of changes in surface water environments. Weak but still recognizable northward penetrations of relatively warm Atlantic Ocean water reached the Norwegian Sea (Site 644) several times, although glacial conditions dominated. However, the interval 1.65 to 1.4 Ma at Site 644 is characterized by a first considerable warming of the surface water conditions. Relatively high amplitudes in the biogenic calcareous records and abundant warmadapted species indicate the presence of relatively warm surface waters during interglacials, whereas low numbers of microfossils are related to conditions dominated by sea-ice cover during glacials. In addition, the nannoplankton and foraminifer assemblages are characterized by well-preserved subpolar species during climatic optima, although cold-adapted species dominated throughout this interval. However, the Iceland Sea and the Fram Strait were uninfluenced by the relatively warm precursor of the Norwegian Current. A significant decrease in the numbers of calcareous nannofossils and planktonic foraminifers at 1.4 Ma is related to another cooling that lasted until ~l.l Ma. The last 1.1 m.y. records a continuous intensification of the glacial to interglacial contrasts, reflecting a strengthening of the Norwegian Current. Warm surface water intruded into the Iceland Sea now, which is indicated by the relatively high abundances of calcareous plankton assemblages at Site 907. In the Fram Strait, planktonic foraminifer assemblages first appeared after 0.8 Ma. The amplitude of glacial to interglacial conditions further increased at -0.65 Ma. In general, the highest amplitude shifts in the biogenic calcareous records are observed during this interval. Carbonate preservation generally is in good accordance with the overall trend toward amplified glacial to interglacial contrasts throughout the last 3.1 m.y. In the late Pliocene carbonate is well preserved. The period from -2.75 to 1.65 Ma is characterized by widespread strong dissolution. A considerable improvement in carbonate preservation at -1.65 to 1.4 Ma is coupled with a first increased northward heat transport, possibly indicating increased deep-water production in the Norwegian Sea. Strong carbonate dissolution indicates a major reduction in deep-water formation during the interval 1.4 to 1.1 Ma. With increased advection of temperate Atlantic surface water, the magnitude of deep-water production in the Norwegian-Greenland Sea increased during the past 1.1 m.y.