In late MIS 3 to early MIS 2 (c. 35–22 kyr ago), the interstadials of D/O events 6–2 are short-lasting, and because of the relatively low sampling resolution in JM-FI-19PC in this time interval (>300 years per sample) not all the interstadials are equally well resolved (Fig. 5; see also ref. 30). In the following discussion we therefore focus on the time interval 90–35 kyr ago comprising the longer-lasting D/O events 21–7 with the highest resolution, including H-events H8–H4 (Fig. 5). A total of three different phases within each D/O event could be determined; insterstadial conditions, insterstadial transitional cooling conditions and stadial conditions (Fig. 5). A fourth phase describes the particular conditions during some larger H-events (H6, part of H4 and H1; Fig. 5).

Interstadials are characterized by absence or near absence of IP 25 , maximum content of brassicasterol and dinosterol, and low P B IP 25 and P D IP 25 values (Fig. 5). The (near) absence of IP 25 indicates absence of sea ice-related diatoms, while high brassicasterol and dinosterol values indicate favourable conditions for phytoplankton growth in general and higher primary productivity at the surface ocean. Thalassiosira oestrupii (Ostenfeld) Hasle, an indicator of relatively warm temperatures32,33 is present in higher amounts in IS21 (MIS 5a) and in the Holocene, coinciding with high values of brassicasterol and dinosterol (Fig. 5, see Methods for details on diatoms). The species occurs sporadic or is absent in interstadials of MIS 3 and 2 (IS17–IS3) indicating colder than modern conditions. Altogether, the biomarker signals indicate open-ocean conditions and relatively high surface water temperatures over the northern Faroe Islands margin, with the Arctic Front located north of the core position (Fig. 6a). Planktic foraminiferal species from nearby core ENAM93-21/MD95-2009 indicated inflow of Atlantic Water during interstadials of MIS 3 and MIS 23 in support of the interpretation based on diatom floras and organic biomarkers. Furthermore, the δ13C org values are higher in interstadials, which point to a (relative) decrease of terrigeneous organic matter supply and/or an increase in marine organic matter input28. The latter option, that is, an increased influence of marine organic matter at times of higher water temperatures due to higher marine productivity at the core location, is supported by higher flux of diatoms and higher concentrations of brassicasterol and dinosterol (Figs 5 and 6a).

Figure 6: Schematic reconstruction of sea ice conditions at the northern Faroe slope. (a) Interstadial. (b) Interstadial transitional cooling phase. (c) Stadial. (d) H-events H6, H4 and H1. Black horizontal arrows indicate location of the Arctic Front (AF) and Polar Front (PF). Green colour (including arrows) indicates degree of phytoplankton productivity inferred from concentrations of brassicasterol, dinosterol and flux of diatoms. Productivity levels are indicated in the lowermost part of each figure: Magenta coloured line, no or very little phytoplankton productivity; yellow line, medium/variable productivity; green line, high or very high productivity. Grey vertical bars on the right indicate strength of deep convection. Full size image

The optimum conditions are followed by a cooling phase defined by gradual changes to medium or higher values of IP 25 , brassicasterol, and dinosterol, P B IP 25 and P D IP 25 (Fig. 5). The relatively higher P B IP 25 and P D IP 25 values together with the increase in brassicasterol and dinosterol values (most clearly seen in IS21, IS20, IS13–10 and IS8; marked by arrows in Fig. 5) indicate increasing seasonal sea ice cover and elevated surface productivity, respectively, typical of the marginal ice zone11,12 (Fig. 6b). These findings suggest that the IFF had moved to a probably variable position south-east of the core location and that the area was in the zone of Arctic surface water, likely resulting in decrease of both atmospheric and surface-water temperatures (Fig. 6b). This phase correlates with an increase in ice rafting and an increase in cold-water planktic foraminiferal species as seen in nearby core ENAM93-21/MD95-2009 (ref. 3).

The cooling phase terminated in a phase of extended sea ice cover. This is associated with a maximum in IP 25 values (higher than the calculated mean value), an abrupt decrease in the concentration of brassicasterol, dinosterol and high P B IP 25 and P D IP 25 values (Fig. 5, light-blue colour bars). Generally supporting this, the sea ice-associated Fragilariopsis oceanica (Cleve) Hasle33,34,35 is present in higher relative abundance in H-event 8, S16 and S1 (Fig. 5, see Methods for details on diatoms). The lack of diatoms in most of the identified phases of extended sea ice cover (Fig. 5) is in accordance with the low abundances of diatoms found in areas with near-perennial sea ice cover today16,20 (for diatom preservation, see Methods). With cold surface conditions and extended sea ice cover as far south as the core location, the Polar Front migrated to a position close to or probably south of the core site (Fig. 6c). Conditions were probably similar to conditions currently observed in areas governed by the East Greenland Current with (very) low primary productivity36,37 and a dense pack-ice cover. Rather low δ13C org values might indicate decreased marine organic carbon flux due to the low productivity and/or an increased influence of terrigenous organic matter, probably being ice-rafted to the core site. The presence of near-perennial sea ice prevented heat exchange and the atmosphere was probably very cold. These last phases of the D/O events represent the stadial conditions of the Greenland ice cores.

The sea ice proxy data for the H-events 6, 4 and 1 (Figs 4 and 5) show a very prominent period of perennial or near-perennial sea ice cover (absent or medium IP 25 , respectively, absent or minimal brassicasterol and dinosterol, and maximum P B IP 25 and P D IP 25 values). In these intervals, the IP 25 values of zero cannot be interpreted as absence of sea ice. Instead, the combined signals likely represent a thick permanent sea ice cover and very cold temperatures15,26 (see also discussion in ref. 17). As long as the sea ice cover is thin enough for sunlight to penetrate, sea ice diatoms synthesizing IP 25 can grow attached beneath the ice14,16,20, which was probably the case at the beginning and end of some of the stadial intervals, where high peaks in P B IP 25 and P D IP 25 can occur (Fig. 5). When the sea ice becomes permanent and too thick for sunlight to penetrate, the signal of IP 25 will drop to zero (Fig. 5). To allow for the perennial sea ice and extreme cold temperatures, Polar surface water most likely spread over the area north of the Faroe Islands and the Polar Front was located far more southerly than today and during the smaller stadials (Fig. 6d).

The transition from stadial and H-events to interstadials is rapid and interpreted as a sudden transition from extended sea ice cover to open-ocean conditions, seen as a decrease in values of IP 25 , P B IP 25 and P D IP 25 (with the exception of H6, H4 and H1; Fig. 5). The abrupt decrease in sea ice cover is in line with the abrupt increase in atmospheric temperatures at the beginning of D/O events1 and increase in sea surface temperatures as also seen in nearby record ENAM93-21/MD95-2009 (ref. 3).

Our data generally show a good correlation between climate and sea ice cover for MIS 3, as well as for the last 30,000 years (Figs 4 and 5). We demonstrate that the presence/absence of sea ice varies closely in pace with the different climatic phases of the D/O millennial-scale climate events (Fig. 5). The peak warm interstadials with no sea ice (Fig. 7a) (Supplementary Fig. 1 and Supplementary Table 1) resembled the modern conditions, which have been shown by numerous marine core studies from the northern North Atlantic and Nordic seas, and have generally been interpreted as a sign of strong flow of Atlantic surface water (Fig. 6a). The transitional cooling phase of the insterstadial with gradually expanding sea ice cover from northwest (Figs 6b and 7b) correlate with increase in ice rafting from icebergs, decreasing atmospheric temperatures1 and an increasing amount of meltwater over a larger area of the northern North Atlantic region and Nordic seas. In the following stadial events iceberg rafting reached a maximum (Figs 6c and 7c) as also seen in nearby core ENAM93-21/MD95-2009 and other records from the Nordic seas and North Atlantic (Supplementary Fig. 1 and Supplementary Table 1), and δ18O values reached very low values indicating presence of meltwater (Fig. 5). Sea ice advanced and the sea ice cover became near-perennial in the case of smaller stadial events (Figs 6c and 7c) or perennial as during the colder H-events H6, H4 and H1 (Figs 6d and 7d). The latter events are the three strongest events during the last 90 kyr, probably due to orbital forcing2,38. All stadial and H-events in the North Atlantic and Nordic seas show dominance by the polar planktic foraminifera Neogloboquadrina pachyderma sinistral (s) and cold polar conditions (see references in Supplementary Information; Supplementary Fig. 1 and Supplementary Table 1).

Figure 7: Reconstructed sea ice distribution during D/O events in the Nordic seas. The figure is based on published records with sufficient time resolution to identify individual D/O events of MIS 5 to MIS 2. We have included only records that contain counts of ice-rafted debris (IRD) in combination with planktic δ18O values and/or surface temperature proxies based on planktic foraminifera (transfer functions and/or % N. pachyderma). From areas that are well covered with a high number of records, we have selected a few considered as representative. For areas with sparse coverage and low resolution (such as central basins), we chose a few records, where the results may be based on other or fewer proxies (Supplementary Table 1). For locations of the published records and references, see Supplementary Fig. 1. (a) Interstadial with open-ocean conditions. The Arctic Front (AF), Iceland-Faroe Front (IFF) and Polar Front (PF) are located north of the core location. (b) Interstadial transitional cooling phase with variable sea ice cover and variable positions of AF (IFF) and PF, which is indicated by white arrows. The core location is always between AF and PF. (c) Stadial with extended sea ice cover. The core location is again between AF and PF, but markedly closer to PF (potentially PF can be south of the core location; see text). Both fronts are located further towards the south compared with interstadial and interstadial cooling conditions. (d) H-events (H6, H4 and H1) with perennial sea ice cover. Both Fronts, AF and PF, are now located south of the core location. Stars mark the location of the studied core JM11-FI-19PC, as well as the location of core MSM5/5-712-217 used for comparison; dashed white lines give approximate positions of fronts AF (IFF) and PF. The reconstructions of regional distribution of sea ice cover are based on published records together with the records of JM11-FI-19PC and MSM5/5-712-217 (Supplementary Fig. 1, Supplementary Table 1 and Supplementary References). Bathymetry from GEBCO 2014 grid (http://www.gebco.net/). NGRIP, North Grip ice core (orange triangle). Scale bar, 500 km. Full size image

Sea ice was an active player in millennial climate change, in most cases probably enforcing trends already caused by the predominantly cold, glacial atmospheric conditions during the last glacial period. The peak warmth of the interstadials lasted only shortly1 and was immediately followed by cooling and spreading of sea ice. The inflow of Atlantic surface Water to the core area diminished to the extent that deep-water formation became very slow and stopped and sea ice cover became perennial or near perennial. The ocean circulation in the Nordic seas was probably more similar to the system in the northern Fram Strait today, in our view the closest analogue to the situation during stadials and North Atlantic H-events38 and with similar circulation patterns of water masses with warmer Atlantic water flowing at intermediate depth below Polar surface Water38,39,40,41. In other words, the present-day conditions of the Fram Strait moved far south into the Atlantic Ocean (Fig. 7c,d). The remarkable abrupt disappearance of sea ice at the end of stadials/H-events correlates with sudden renewed inflow of Atlantic surface Water, peak insterstadial warmth and probably onset of deep-water formation. The distribution of sea ice thus correlated closely with variations in ocean circulation and expanding-retreating ice sheets and variations in flow of meltwater and stratification of the ocean surface.