David J. R. Thornalley

Understanding the evolution of deep ocean circulation and chemistry over the last glacial cycle is key for elucidating the ocean’s role in modulating atmospheric CO₂ changes on millennial and orbital timescales. Marine Isotope Stage (MIS) 4 is a key paleoclimatic interval of the last glacial inception for assessing the role of the deep-ocean carbon storage in driving atmospheric CO₂ levels, because it is characterized by a large decrease of air temperature and a rapid atmospheric CO₂ drop of ~40 ppmv, likely linked to changes in ocean circulation, and includes several millennial climatic events, for example Heinrich Stadial (HS) 6. Although previous proxy-based studies have suggested a weakened Atlantic overturning during MIS 4, and particularly HS 6, basin wide changes in circulation remain poorly constrained. Here, we present high-resolution deep-water hydrography reconstructions from the mid-depth Iberian Margin (core MD01-2444, ~2.65 km water depth), based on benthic foraminiferal Mg/Ca-derived deep-water temperature (T_dw), benthic δ¹⁸O, and associated δ¹⁸O_dw records, alongside SS - based flow-speed record. Our data reveal three distinct deep-water hydrographic ‘regimes’ at the Iberian Margin: 1) warm “interglacial-like” mode during MIS 5a (including Greenland stadials C19 and C20); 2) a colder glacial circulation mode during early MIS 4 (pre- HS 6); and 3) a “Heinrich” circulation mode during HS 6. A stronger influence of colder southern-sourced waters is inferred at the Iberian Margin throughout MIS 4 and intensifies during HS 6, whereas MIS 5a stadials are characterised by a pronounced subsurface warming. The warm stadials C19 and C20 were likely driven by a southward displacement of convection sites in the North Atlantic and appear to be unique to MIS 5a as they are not observed during MIS 3 Dansgaard-Oeschger events at this site. We also find evidence of “millennial-type” variability and, similar to recent studies for MIS 2, a persistent contribution of northern sourced waters during MIS 4 (pre- HS 6). The contrasting impacts of similar millennial-scale DO-type variability on North Atlantic deep-water hydrography and circulation strength during MIS 3, 4, and 5 highlight the strong dependence on the background climate state.

Understanding the evolution of deep ocean circulation and chemistry over the last glacial cycle is key for elucidating the ocean’s role in modulating atmospheric CO₂ changes on millennial and orbital timescales. Marine Isotope Stage (MIS) 4 is a key paleoclimatic interval of the last glacial inception for assessing the role of the deep-ocean carbon storage in driving atmospheric CO₂ levels, because it is characterized by a large decrease of air temperature and a rapid atmospheric CO₂ drop of ~40 ppmv, likely linked to changes in ocean circulation, and includes several millennial climatic events, for example Heinrich Stadial (HS) 6. Although previous proxy-based studies have suggested a weakened Atlantic overturning during MIS 4, and particularly HS 6, basin wide changes in circulation remain poorly constrained. Here, we present high-resolution deep-water hydrography reconstructions from the mid-depth Iberian Margin (core MD01-2444, ~2.65 km water depth), based on benthic foraminiferal Mg/Ca-derived deep-water temperature (T_dw), benthic δ¹⁸O, and associated δ¹⁸O_dw records, alongside SS - based flow-speed record. Our data reveal three distinct deep-water hydrographic ‘regimes’ at the Iberian Margin: 1) warm “interglacial-like” mode during MIS 5a (including Greenland stadials C19 and C20); 2) a colder glacial circulation mode during early MIS 4 (pre- HS 6); and 3) a “Heinrich” circulation mode during HS 6. A stronger influence of colder southern-sourced waters is inferred at the Iberian Margin throughout MIS 4 and intensifies during HS 6, whereas MIS 5a stadials are characterised by a pronounced subsurface warming. The warm stadials C19 and C20 were likely driven by a southward displacement of convection sites in the North Atlantic and appear to be unique to MIS 5a as they are not observed during MIS 3 Dansgaard-Oeschger events at this site. We also find evidence of “millennial-type” variability and, similar to recent studies for MIS 2, a persistent contribution of northern sourced waters during MIS 4 (pre- HS 6). The contrasting impacts of similar millennial-scale DO-type variability on North Atlantic deep-water hydrography and circulation strength during MIS 3, 4, and 5 highlight the strong dependence on the background climate state.

In early June, 2026, a workshop was convened to address the topic of climatic changes in the Atlantic Ocean circulation – specifically, a weakening or collapse of the large-scale circulation in response to anthropogenic global warming. The gathering brought together 61 experts from across the fields of physical oceanography, climate dynamics, paleoceanography, and dynamical systems, and involved participation from 11 countries. This document reports on the workshop’s aims, structure, and themes, and provides a summary of key discussions. The information comes from notes and recordings made during the workshop itself, as well as a summary survey conducted immediately following the event. The above listed authors constituted the workshop’s organising committee. All participants were invited to contribute to and comment on this report. It should thus be considered a shared effort from that wider group. A full list of workshop participants can be found in the acknowledgements.

Understanding the evolution of deep ocean circulation and chemistry over the last glacial cycle is key for elucidating the ocean’s role in modulating atmospheric CO₂ changes on millennial and orbital timescales. Marine Isotope Stage (MIS) 4 is a key paleoclimatic interval of the last glacial inception for assessing the role of the deep-ocean carbon storage in driving atmospheric CO₂ levels, because it is characterized by a large decrease of air temperature and a rapid atmospheric CO₂ drop of ~40 ppmv, likely linked to changes in ocean circulation, and includes several millennial climatic events, for example Heinrich Stadial (HS) 6. Although previous proxy-based studies have suggested a weakened Atlantic overturning during MIS 4, and particularly HS 6, basin wide changes in circulation remain poorly constrained. Here, we present high-resolution deep-water hydrography reconstructions from the mid-depth Iberian Margin (core MD01-2444, ~2.65 km water depth), based on benthic foraminiferal Mg/Ca-derived deep-water temperature (T_dw), benthic δ¹⁸O, and associated δ¹⁸O_dw records, alongside SS - based flow-speed record. Our data reveal three distinct deep-water hydrographic ‘regimes’ at the Iberian Margin: 1) warm “interglacial-like” mode during MIS 5a (including Greenland stadials C19 and C20); 2) a colder glacial circulation mode during early MIS 4 (pre- HS 6); and 3) a “Heinrich” circulation mode during HS 6. A stronger influence of colder southern-sourced waters is inferred at the Iberian Margin throughout MIS 4 and intensifies during HS 6, whereas MIS 5a stadials are characterised by a pronounced subsurface warming. The warm stadials C19 and C20 were likely driven by a southward displacement of convection sites in the North Atlantic and appear to be unique to MIS 5a as they are not observed during MIS 3 Dansgaard-Oeschger events at this site. We also find evidence of “millennial-type” variability and, similar to recent studies for MIS 2, a persistent contribution of northern sourced waters during MIS 4 (pre- HS 6). The contrasting impacts of similar millennial-scale DO-type variability on North Atlantic deep-water hydrography and circulation strength during MIS 3, 4, and 5 highlight the strong dependence on the background climate state.

Understanding the evolution of deep ocean circulation and chemistry over the last glacial cycle is key for elucidating the ocean’s role in modulating atmospheric CO₂ changes on millennial and orbital timescales. Marine Isotope Stage (MIS) 4 is a key paleoclimatic interval of the last glacial inception for assessing the role of the deep-ocean carbon storage in driving atmospheric CO₂ levels, because it is characterized by a large decrease of air temperature and a rapid atmospheric CO₂ drop of ~40 ppmv, likely linked to changes in ocean circulation, and includes several millennial climatic events, for example Heinrich Stadial (HS) 6. Although previous proxy-based studies have suggested a weakened Atlantic overturning during MIS 4, and particularly HS 6, basin wide changes in circulation remain poorly constrained. Here, we present high-resolution deep-water hydrography reconstructions from the mid-depth Iberian Margin (core MD01-2444, ~2.65 km water depth), based on benthic foraminiferal Mg/Ca-derived deep-water temperature (T_dw), benthic δ¹⁸O, and associated δ¹⁸O_dw records, alongside SS - based flow-speed record. Our data reveal three distinct deep-water hydrographic ‘regimes’ at the Iberian Margin: 1) warm “interglacial-like” mode during MIS 5a (including Greenland stadials C19 and C20); 2) a colder glacial circulation mode during early MIS 4 (pre- HS 6); and 3) a “Heinrich” circulation mode during HS 6. A stronger influence of colder southern-sourced waters is inferred at the Iberian Margin throughout MIS 4 and intensifies during HS 6, whereas MIS 5a stadials are characterised by a pronounced subsurface warming. The warm stadials C19 and C20 were likely driven by a southward displacement of convection sites in the North Atlantic and appear to be unique to MIS 5a as they are not observed during MIS 3 Dansgaard-Oeschger events at this site. We also find evidence of “millennial-type” variability and, similar to recent studies for MIS 2, a persistent contribution of northern sourced waters during MIS 4 (pre- HS 6). The contrasting impacts of similar millennial-scale DO-type variability on North Atlantic deep-water hydrography and circulation strength during MIS 3, 4, and 5 highlight the strong dependence on the background climate state.

The Scotian Shelf lies at the confluence of warm Gulf Stream (GS) waters and the cold Labrador Current (LC), making it highly sensitive to large- and small-scale climate variability. Modern observations show rapid regional warming accelerated by episodic GS-derived intrusions, yet Holocene paleoceanographic reconstructions from this margin are sparse and often conflicting with respect to the frequency and extent of intrusion events. Here, we present high-resolution Mg/Ca-derived sea-surface temperature (SST) and planktonic δ¹⁸O records from St. Anns Basin on the north-eastern Scotian Shelf that provide new insights into the hydrographic surface-ocean variability of the past 8.5 ka calibrated Before Present (cal BP). While the SST record does not capture the 8.2 ka event, this event is evident in the δ¹⁸O and Ca/Sr records, indicating that its freshwater signal reached the Scotian Shelf. Reconstructed SSTs are generally cold from ~8.5 to ~6.2 cal ka BP, followed by a gradual increase in mean SSTs punctuated by multiple short-lived warm and saline events beginning around 6 cal ka BP, at 6.0–5.8, 5.5–5.4, 5.1–4.9, 3.2–3.1, 2.5–2.2 and 1.05–0.8 cal ka BP, which we interpret as intrusions of GS-sourced slope waters. We attribute these events to basin-scale reorganizations of the GS-LC system, consistent with the minimum/maximum modal state framework of Pickart et al. (1999). Minimum modal state circulation, characterized by a strong onshore LC and an intensified Deep Western Boundary Current (DWBC), which is dominated by Denmark Strait Overflow water, creates a sharp front which restricts intrusions of warm water onto the Scotian Shelf. Maximum modal state conditions feature a weakened LC and increased Labrador Sea Water (LSW) contribution to the DWBC, and reduce cross-slope temperature and salinity gradients that permit GS-derived waters to penetrate the shelf. Overall, our results indicate that warm-water intrusions occurred regularly throughout the past 6.5 ka BP with magnitudes of 6.7 °C and 1.5 psu comparable to those observed today.

Understanding the evolution of deep ocean circulation and chemistry over the last glacial cycle is key for elucidating the ocean’s role in modulating atmospheric CO₂ changes on millennial and orbital timescales. Marine Isotope Stage (MIS) 4 is a key paleoclimatic interval of the last glacial inception for assessing the role of the deep-ocean carbon storage in driving atmospheric CO₂ levels, because it is characterized by a large decrease of air temperature and a rapid atmospheric CO₂ drop of ~40 ppmv, likely linked to changes in ocean circulation, and includes several millennial climatic events, for example Heinrich Stadial (HS) 6. Although previous proxy-based studies have suggested a weakened Atlantic overturning during MIS 4, and particularly HS 6, basin wide changes in circulation remain poorly constrained. Here, we present high-resolution deep-water hydrography reconstructions from the mid-depth Iberian Margin (core MD01-2444, ~2.65 km water depth), based on benthic foraminiferal Mg/Ca-derived deep-water temperature (T_dw), benthic δ¹⁸O, and associated δ¹⁸O_dw records, alongside SS - based flow-speed record. Our data reveal three distinct deep-water hydrographic ‘regimes’ at the Iberian Margin: 1) warm “interglacial-like” mode during MIS 5a (including Greenland stadials C19 and C20); 2) a colder glacial circulation mode during early MIS 4 (pre- HS 6); and 3) a “Heinrich” circulation mode during HS 6. A stronger influence of colder southern-sourced waters is inferred at the Iberian Margin throughout MIS 4 and intensifies during HS 6, whereas MIS 5a stadials are characterised by a pronounced subsurface warming. The warm stadials C19 and C20 were likely driven by a southward displacement of convection sites in the North Atlantic and appear to be unique to MIS 5a as they are not observed during MIS 3 Dansgaard-Oeschger events at this site. We also find evidence of “millennial-type” variability and, similar to recent studies for MIS 2, a persistent contribution of northern sourced waters during MIS 4 (pre- HS 6). The contrasting impacts of similar millennial-scale DO-type variability on North Atlantic deep-water hydrography and circulation strength during MIS 3, 4, and 5 highlight the strong dependence on the background climate state.

Theory and models suggest the Gulf Stream may shift northwards under projected Atlantic Meridional Overturning Circulation weakening. Yet Gulf Stream behaviour during past abrupt cold events remains poorly constrained. Here we present high-resolution paleoceanographic records from the Northwest Atlantic during the last deglaciation. During the Younger Dryas cold period, we document a northward Gulf Stream shift evidenced from coherent surface and subsurface warming. Our sortable silt data suggest a strengthening of upper North Atlantic Deep Water that opposes weakening lower North Atlantic Deep Water, consistent with a seesaw feedback between the Nordic overflows and subpolar gyre. Our results constrain a co-ordinated sequence at the Younger Dryas onset: initial lower North Atlantic Deep Water weakening and subpolar sea‑ice expansion, lagged (58 ± 38 yr) by an increase in upper North Atlantic Deep Water and an eventual atmospheric reorganization (84 ± 51 yr after onset). These findings provide empirical support for model projections of future Gulf Stream shifts.

The Scotian Shelf lies at the confluence of warm Gulf Stream (GS) waters and the cold Labrador Current (LC), making it highly sensitive to large- and small-scale climate variability. Modern observations show rapid regional warming accelerated by episodic GS-derived intrusions, yet Holocene paleoceanographic reconstructions from this margin are sparse and often conflicting with respect to the frequency and extent of intrusion events. Here, we present high-resolution Mg/Ca-derived sea-surface temperature (SST) and planktonic δ¹⁸O records from St. Anns Basin on the north-eastern Scotian Shelf that provide new insights into the hydrographic surface-ocean variability of the past 8.5 ka calibrated Before Present (cal BP). While the SST record does not capture the 8.2 ka event, this event is evident in the δ¹⁸O and Ca/Sr records, indicating that its freshwater signal reached the Scotian Shelf. Reconstructed SSTs are generally cold from ~8.5 to ~6.2 cal ka BP, followed by a gradual increase in mean SSTs punctuated by multiple short-lived warm and saline events beginning around 6 cal ka BP, at 6.0–5.8, 5.5–5.4, 5.1–4.9, 3.2–3.1, 2.5–2.2 and 1.05–0.8 cal ka BP, which we interpret as intrusions of GS-sourced slope waters. We attribute these events to basin-scale reorganizations of the GS-LC system, consistent with the minimum/maximum modal state framework of Pickart et al. (1999). Minimum modal state circulation, characterized by a strong onshore LC and an intensified Deep Western Boundary Current (DWBC), which is dominated by Denmark Strait Overflow water, creates a sharp front which restricts intrusions of warm water onto the Scotian Shelf. Maximum modal state conditions feature a weakened LC and increased Labrador Sea Water (LSW) contribution to the DWBC, and reduce cross-slope temperature and salinity gradients that permit GS-derived waters to penetrate the shelf. Overall, our results indicate that warm-water intrusions occurred regularly throughout the past 6.5 ka BP with magnitudes of 6.7 °C and 1.5 psu comparable to those observed today.

The Scotian Shelf lies at the confluence of warm Gulf Stream (GS) waters and the cold Labrador Current (LC), making it highly sensitive to large- and small-scale climate variability. Modern observations show rapid regional warming accelerated by episodic GS-derived intrusions, yet Holocene paleoceanographic reconstructions from this margin are sparse and often conflicting with respect to the frequency and extent of intrusion events. Here, we present high-resolution Mg/Ca-derived sea-surface temperature (SST) and planktonic δ¹⁸O records from St. Anns Basin on the north-eastern Scotian Shelf that provide new insights into the hydrographic surface-ocean variability of the past 8.5 ka calibrated Before Present (cal BP). While the SST record does not capture the 8.2 ka event, this event is evident in the δ¹⁸O and Ca/Sr records, indicating that its freshwater signal reached the Scotian Shelf. Reconstructed SSTs are generally cold from ~8.5 to ~6.2 cal ka BP, followed by a gradual increase in mean SSTs punctuated by multiple short-lived warm and saline events beginning around 6 cal ka BP, at 6.0–5.8, 5.5–5.4, 5.1–4.9, 3.2–3.1, 2.5–2.2 and 1.05–0.8 cal ka BP, which we interpret as intrusions of GS-sourced slope waters. We attribute these events to basin-scale reorganizations of the GS-LC system, consistent with the minimum/maximum modal state framework of Pickart et al. (1999). Minimum modal state circulation, characterized by a strong onshore LC and an intensified Deep Western Boundary Current (DWBC), which is dominated by Denmark Strait Overflow water, creates a sharp front which restricts intrusions of warm water onto the Scotian Shelf. Maximum modal state conditions feature a weakened LC and increased Labrador Sea Water (LSW) contribution to the DWBC, and reduce cross-slope temperature and salinity gradients that permit GS-derived waters to penetrate the shelf. Overall, our results indicate that warm-water intrusions occurred regularly throughout the past 6.5 ka BP with magnitudes of 6.7 °C and 1.5 psu comparable to those observed today.