Interstadial and stadial conditions over Reykjanes Ridge

In nearly all D-O events, maximum temperature occurs in the early part of the interstadials (Fig. 4g). The peaks are marked by low percentages of the polar planktonic foraminiferal species Neogloboquadrina pachyderma s, high percentages of subpolar species and high abundance of planktonic foraminifera (Fig. 4a–d). The interstadial SST between ~6.5 °C and ~8.5° are close to interstadial temperatures estimated from transfer functions and Mg/Ca ratios for nearby cores SO82-05GGC29, LO09-18 and DS97-2P31 and only slightly lower than the present temperatures in the area (Figs 2f and 4g). The low concentration of IRD indicates an almost total lack of sea ice and icebergs in the area (Fig. 4e,f).

Halfway through most interstadials, the planktonic faunas change rapidly reflecting a significant temperature drop and the later part of the interstadials (interstadial cooling phase2) were generally very cold with low abundances of planktonic foraminifera (Fig. 4a–d,g). Yet, the concentration of IRD remains low indicating that the number of icebergs did not increase. The cooling is in accordance with the δ18O record, which shows increasing values coinciding with the decreasing temperatures (Fig. 2d). Jonkers et al. (ref. 31) noticed a similar rapid cooling during interstadial GI 8 in nearby cores LO09-18 and DS97-2P. In fact, a prolonged period with cold surface water and slow convection before the arrival of IRD has been documented in several cores from the North Atlantic north of the IRD-belt10,13,29.

The abrupt increase in IRD at the beginning of the stadials indicates a rapid growth in sea ice cover and in the number of melting icebergs29,30,31,33 (Figs 2h and 4e,f). According to ref. 29, most of the IRD on the Reykjanes Ridge are from Eastern Greenland and delivered by the East Greenland Current. Sea surface temperatures, as estimated from transfer functions, remained very low and often reached a minimum at the interstadial-stadial transitions. From this point we see a decrease in N. pachyderma s and an increase in G. bulloides and T. quinqueloba indicating a warming of the surface water and the transfer functions calculate an increase in summer SST from 3.4 °C at the beginning of the stadials to about 7.0 °C at the transition to the next interstadial. Several studies have shown that the sudden decline in IRD at the end of the stadials signifies the disappearance of sea ice and icebergs from the Nordic seas and northernmost Atlantic and coincides with the resumption of convection in the Nordic seas and the abrupt temperature increase in the Greenland ice cores2,22,23,24,40.

However, in SO2, sea surface temperatures continued to increase and peak warmth were, on average, first reached about 200 years into the interstadials (Fig. 4g; Table 2). The average duration of the total interstadial warming period is close to 800 years as compared to 40 years for the atmospheric shifts in the NGRIP ice core (Table 2). Gradual surface and subsurface warming during stadials/Heinrich events over the Reykjanes Ridge has previously been demonstrated for H4 based on Mg/Ca31. The results from SO2 indicate that gradual surface warming occurred during all stadials/Heinrich events between c. 65 and 25 ka (Figs 4g and 5b,c). The relatively low δ13C values suggest poorer subsurface ventilation as compared to the interstadials (Fig. 2e).

The fluctuations in bottom water temperatures are an almost exact repetition of the fluctuations in the surface water, showing the same pattern of gradual warmings and coolings (Fig. 5b,c; Table 2). The similarity indicates a close coupling over the Reykjanes Ridge between surface water and intermediate water and a homogenous water column down to a depth of at least 1730 m.

The stadial-interstadial transitions in the Atlantic Ocean

Comparing the development of the D-O events over the Reykjanes Ridge with other records from the North Atlantic and Nordic seas it appears that the warming of the intermediate water was gradual throughout the North Atlantic and Nordic seas during all stadials and Heinrich events. Gradual warming of the intermediate water during stadials/Heinrich events and a slowdown of the AMOC was first proposed in 1996 for a core from the southern Nordic seas40 and it has later been corroborated in numerous studies from the North Atlantic and also in model experiments22,23,41,42,43,44,45. In contrast to the ubiquitously occurring gradual warming of the intermediate water, the warming of the surface and subsurface water shows significant local differences. A survey of previous studies from the North Atlantic realm indicates that the large central part of the northern Atlantic between the IRD-belt and the Greenland-Scotland Ridge experienced gradual warming similar to the warming pattern over the Reykjanes Ridge, while abrupt warming was limited to the Nordic seas, the IRD-belt and land-near areas farthest to the northeast and northwest (Fig. 1; Table 3).

Table 3 List of cores indicated in Fig. 1 showing core names, temperature proxy used in the evaluation of D-O configurations, authors and reference numbers. Full size table

Paleoceanographically, it appears that gradual surface warming occurred in open marine areas with low influx of meltwater and modest amounts of sea ice and icebergs, while abrupt warming occurred in areas with a large influx of meltwater, numerous icebergs and an extensive ice cover. The conditions promoting abrupt warmings have been examined in detail in several studies from the Nordic seas. During stadials, large numbers of melting icebergs40 and sea ice created a stratified water column composed of a relatively thin layer of cold, low saline surface water overlying a denser intermediate water mass, which was gradually warming22,40. Similar conditions probably existed in the IRD belt during Heinrich events6,21. In the Nordic seas, the abrupt warming has been attributed to a rapid surfacing of the warm intermediate water, which broke the stratification and restored convection22.

The results from SO2 add some significant details to this scenario. In SO2, the gradual warming begins simultaneously with or slightly before the abrupt rise in ice rafting (Fig. 4f,g). This indicates to us that the increased melting of icebergs was caused by the warming. Warming in connection with stadials/Heinrich events has previously been suggested to cause ice melting and increased discharges of icebergs43,46. The melting lead to a higher input of meltwater and in the Nordic seas and IRD-belt, where icebergs were more numerous, the upper ocean became stratified.

The average temperature for the bottom water in SO2 (Fig. 5c) follows roughly previous estimates for intermediate-water temperatures in the North Atlantic and southern Nordic seas22,41,42,43. This suggests that the temperature fluctuations in SO2 reflect the general temperatures of the Gulf Stream system18, which again is controlled by the temperatures of the central and southern Atlantic (Fig. 5f). This implies that the similarity between the D-O events in SO2 and the events in the southern Atlantic and in the Antarctic ice cores is the result of a direct southern influence on the paleoceanography of the northern Atlantic during D-O events23,24,33,47.

Furthermore, new evidence from Antarctic ice core WDC indicates that the interstadial warmings in SO2 and in the Antarctic ice core probably were synchronous (Fig. 5b,d,e). Precise correlation between core WDC and the Greenland NGRIP ice core indicate that the maximum interstadial temperature in WDC on average occurred 218 years after the abrupt warming over Greenland16. Our calculations indicate that the maximum temperature in SO2 on average occurred 211 years after the start the interstadial (Table 2). The similarity of these figures strongly indicates that maximum interstadial warmth was reached practically simultaneously in core WDC and core SO2. The synchronicity of maximum interstadial warmth combined with the overall similarity of the D-O events in WDC and SO2 (Fig. 5b,d) indicate further that the gradual warmings at the stadial-interstadial transitions occurred synchronously throughout the Atlantic Ocean. Only the subsurface penetration into the Nordic seas was possibly slightly delayed.

Implications

The results of this study indicate that the D-O warmings in the open North Atlantic were gradual and in phase with the gradual warmings in the Antarctic ice cores and in the southern and central Atlantic. They also indicate that the warmings were out of phase with the abrupt warmings in the Greenland ice cores, in the Nordic seas and areas in the North Atlantic strongly affected by meltwater during stadials. This implies that the hinge line between areas showing gradual warming and areas showing abrupt warming was displaced far to the north close to the Greenland-Scotland Ridge. Considering this geographical asymmetry, the term “bipolar seesaw” seems confusing with respect to marine conditions. This is underlined by the fact that the main cause for D-O events appears to be the ‘turn-on’ and ‘turn-off’ or a slow-down of convection in the Nordic seas4,10,11,12,13,25 with the southern Atlantic reacting mainly passively4. It would be more accurate to describe the system as a ‘push and pull’ system. ‘Pull’ during interstadials, when convection in the Nordic seas was active and the AMOC strong and ‘push’ during stadials, when convection stopped or slowed.

The driving forces for the interstadial Atlantic circulation system were undoubtedly the same as at present, when 75% of the inflow to the Nordic seas is returned to the Atlantic as cold deep water overflowing the Greenland-Scotland Ridge48. In the Atlantic, the overflow water enters the NADW and becomes a very important component of the AMOC49. The overflow creates a sea level gradient (barotropic pressure gradient) across the Greenland-Scotland Ridge pulling warm Atlantic water into the Nordic seas48. The gradient is an important part of the forcing for the inflow to the Nordic seas and a reduced overflow can be expected to create a corresponding reduction in the inflow48.

The stadial circulation system has no modern analogue, but it is generally agreed that the AMOC was weak and warm water from the central and southern Atlantic ‘pushed’ northwards gradually warming the North Atlantic3,4. This study shows that in the northernmost North Atlantic the warming coincides with sharp increase in deposition of IRD implying increased ice rafting, increased melting of icebergs and increased spreading of meltwater. In the Nordic seas and IRD-belt the result was a stratified ocean and the development of very cold stadial conditions.