Carbonate preservation records of the past 3 Myr in the Norwegian–Greenland Sea and the northern North Atlantic: implications for the history of NADW production

Carbonate preservation records from a number of drill sites in the North Atlantic and adjacent Norwegian–Greenland Sea (NGS) are used to reconstruct variations in North Atlantic Deep Water (NADW) production over the past 3 Myr. Before the initiation of major Northern Hemisphere glaciation, good carbonate preservation was recorded in the North Atlantic and the NGS supporting the superconveyor model of accelerated NADW formation in the late Pliocene. After the inception of main Northern Hemisphere glaciation, NADW formation in the NGS was blocked-off during the period 2.8–1.9 Ma. Carbonate was only badly preserved due to (1) low production of carbonate shells in surface waters, (2) sluggish renewal of deep waters induced by a rather stable sea-ice cover, and/or (3) production of carbonate-corrosive dense brines during sea-ice formation (e.g. sea ice dissolution mode). In contrast, contemporaneous good preservation in the adjacent North Atlantic indicates efficient NADW production. During the middle Matuyama (1.9–1.4 Ma), the first intrusions of the Proto-Norwegian Current into a narrow corridor in the southeastern NGS are evidenced by much better carbonate preservation. A decrease both in carbonate content and preservation towards the north and west of this corridor indicates the proximity of the polar front and gives evidence that NADW production was efficiently triggered by the Atlantic water entrainment mode. During the past 1.2 Myr carbonate preservation patterns in the NGS clearly reflect major global events like the Mid-Pleistocene Transition and the mid-Brunhes dissolution event. The onset of the Mid-Pleistocene Transition at 1.2 Ma is characterized by a complete shift to higher carbonate contents in the southeastern NGS and Labrador Sea. Overall, good preservation during both interglacials and glacials is only interrupted by high-frequency, short-term dissolution spikes, which were induced by ice sheet collapse and development of extensive meltwater lids. As a consequence, NADW was produced nearly continuously during glacials in the Nordic Seas. However, due to its lower density it was entrained into intermediate water levels in the North Atlantic and, thus, enforced the intermediate water circulation loop, whereas a decrease in lower-NADW production is observed contemporaneously.