We refined the age model of MD99-2284 previously presented by Dokken et al. (14) for the studied interval. Synchronization with the Greenland ice core is based on tuning of coherent signals in both cores, supported by geomagnetic events and tephrochronology as follows:

1) We tuned the MD99-2284 depth scale to the GICC05 (b2k) ice core chronology (4, 51, 52), using distinct transitions in the sedimentary records of ARM and the near-surface temperature overshoot, as well as pertinent transitions in the NGRIP δ18O record (fig. S1 and table S1) (2). It has been shown that ARM variations in Nordic Seas cores appear to be synchronous with the D-O cycles in Greenland ice core δ18O, resulting from changes in the efficiency of magnetic particle transport linked to deep- and intermediate-ocean currents (31, 53). Increased deposition of magnetic particles at core site MD99-2284 during GI may be linked to enhanced deep convection and bottom currents recirculating within the Nordic Seas (31, 53). We note that the erosional center of the deep overflow streaming southward into the North Atlantic would be located in the narrow and shallow parts of the central Faroe-Shetland Channel, away from site MD99-2284 situated at 1500 m water depth north of the sill. We speculate that increased transport of magnetic particles to core site MD99-2284 in the northeastern exit of the Faroe-Shetland Channel might also be driven by enhanced northward inflow of intermediate Atlantic waters. The inflow currents would need to extend to a water depth of a few hundreds of meters to erode magnetite-containing sediments from the eastern flank of the Faroe-Shetland Channel with its volcanic bedrocks, a situation comparable to today (54). Unfortunately, we cannot disentangle whether increased ARM signals in the glacial section of core MD99-2284 are linked to enhanced deep-water circulation, intermediate inflow, or both. However, since inflow and deep overflow dynamics across the Greenland-Scotland ridge are compensating each other (54), we argue that the sedimentary ARM signals at core site MD99-2284 generally reflect the meridional ocean circulation strength in the Faroe-Shetland Channel.

The steep ARM increases parallel near-surface temperature overshoots, which provides further evidence of warm Atlantic waters and oceanic reorganizations that we tied to atmospheric warming of the D-O events (fig. S1). We applied a change point analysis (55) to objectively derive change point probabilities for all records used for the tuning. The final age model was based on 11 tie points and linear interpolation between them (table S1).

2) The tuning-based age model is further supported by the positions of the Laschamp and Mono Lake geomagnetic excursions (fig. S1), which derive from minima in natural remanent magnetization (NRM)/ARM (56) in MD99-2284 and maxima in ice core cosmogenic nuclides (57).

3) Moreover, we identified a pronounced basaltic cryptotephra layer at 3040 to 3041 cm in core MD99-2284 (fig. S2A). Both its stratigraphic position (peak GI8) and its geochemical composition (i.e., Grimsvötn source) suggest that this tephra layer falls within the Faroe Marine Ash Zone III previously found in other marine cores (58) and the high-resolution NGRIP ice core (fig. S2B) (42, 43). If compared with geochemical signatures of several well-investigated tephra layers from the GS9-GI8 transition in the NGRIP ice core (42, 43), the homogeneous geochemical composition of the MD99-2284 tephra layer at 3040 to 3041 cm is statistically most similar to that of the NGRIP tephra layer at 2065.65 m (38.081 ka b2k) (SC = 0.98, D2 = 3.57). The tuning-based age model of MD99-2284 results in an age of 37.955 ka b2k for the tephra layer at 3040 to 3041 cm, which is in good agreement with the age of 38.081 ka b2k given for the pertinent NGRIP tephra layer (42, 43), thus independently validating our age model. In essence, this distinct tephra layer forms the ultimate support that the parallel near-surface temperature overshoot and final ARM rise are most likely closely tied to the abrupt Greenland warming (fig. S2).

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