Periodic Reporting for period 1 - IceLab (Ice-ocean interactions during Heinrich Events in the Labrador Sea)
Reporting period: 2020-10-01 to 2022-09-30
Under the influence of ongoing climate warming, melting of the Greenland ice sheet is accelerating. The release of freshwater from the ice sheet into the surrounding ocean has important implications for the global sea level and the ocean circulation, driven by the balance of heat and salt. Climate model projections for future sea level rise under different warming scenarios, however, are associated with significant uncertainty, partly due to the lack of understanding of the interactions of the ice sheet with the surrounding ocean and other components of the climate system, including sea-ice.
The objective of the Marie Skłodowska-Curie Action Fellowship IceLab was to improve our understanding of sea-ice/ocean interactions and their implications for the melting of ice sheets by investigating periods in the geological past characterized by the collapse of an ice sheet or rapid ice sheet retreat. One specific objective was to study sea-ice extent and subsurface temperatures in the outer Labrador Sea during so-called Heinrich events between 30 and 60 thousand years before present (ka BP). These are events of ice-sheet collapse during the last ice age, associated with the release of vast amounts of icebergs from Hudson Strait into the Labrador Sea and adjacent North Atlantic. One possible mechanism that might have triggered the collapse of the Hudson Strait ice sheet is melting induced by warmer ocean temperatures. A warmer ocean in turn might have been caused by the formation of a ‘sea-ice lid’ (an extensive sea-ice cover) in the Labrador Sea, which prevented the release of heat from the surface ocean to the atmosphere. Sea-ice and subsurface temperatures can be reconstructed using different geochemical methods applied to marine sediment cores. Sediment core U1302/03 from the outer Labrador Sea, investigated during IceLab, was taken as part of the International Ocean Discovery Program (IODP).
The original research plan for IceLab also included the reconstruction of sea-ice extent in the northern Baffin Bay and on the Faroe Banks for the same interval from 30-60 ka BP. This would allow to understand the variability of sea-ice extent in both space and time. Different challenges with the available sediment cores, however, meant that this objective could not be implemented. Instead, sea-ice conditions were reconstructed from the northern Baffin Bay and the southern Lincoln Sea, based on four sediment cores (two from each region), covering approximately 20 and 12 ka BP, respectively. These time periods cover the transition from the last ice age to the current warm period and the onset of the current warm period, characterized by pronounced ice sheet retreat. Thus, although not part of the original research plan of IceLab, the sediment cores from the northern Baffin Bay and southern Lincoln Sea are able to inform the main objective of IceLab; to understand the interactions of sea-ice and the ocean with ice sheets in the past.
The objective of the Marie Skłodowska-Curie Action Fellowship IceLab was to improve our understanding of sea-ice/ocean interactions and their implications for the melting of ice sheets by investigating periods in the geological past characterized by the collapse of an ice sheet or rapid ice sheet retreat. One specific objective was to study sea-ice extent and subsurface temperatures in the outer Labrador Sea during so-called Heinrich events between 30 and 60 thousand years before present (ka BP). These are events of ice-sheet collapse during the last ice age, associated with the release of vast amounts of icebergs from Hudson Strait into the Labrador Sea and adjacent North Atlantic. One possible mechanism that might have triggered the collapse of the Hudson Strait ice sheet is melting induced by warmer ocean temperatures. A warmer ocean in turn might have been caused by the formation of a ‘sea-ice lid’ (an extensive sea-ice cover) in the Labrador Sea, which prevented the release of heat from the surface ocean to the atmosphere. Sea-ice and subsurface temperatures can be reconstructed using different geochemical methods applied to marine sediment cores. Sediment core U1302/03 from the outer Labrador Sea, investigated during IceLab, was taken as part of the International Ocean Discovery Program (IODP).
The original research plan for IceLab also included the reconstruction of sea-ice extent in the northern Baffin Bay and on the Faroe Banks for the same interval from 30-60 ka BP. This would allow to understand the variability of sea-ice extent in both space and time. Different challenges with the available sediment cores, however, meant that this objective could not be implemented. Instead, sea-ice conditions were reconstructed from the northern Baffin Bay and the southern Lincoln Sea, based on four sediment cores (two from each region), covering approximately 20 and 12 ka BP, respectively. These time periods cover the transition from the last ice age to the current warm period and the onset of the current warm period, characterized by pronounced ice sheet retreat. Thus, although not part of the original research plan of IceLab, the sediment cores from the northern Baffin Bay and southern Lincoln Sea are able to inform the main objective of IceLab; to understand the interactions of sea-ice and the ocean with ice sheets in the past.
Past sea-ice conditions were reconstructed by means of sea-ice biomarkers. These include a suite of molecules produced by different algae living in and around sea-ice. Their relative concentrations can be used to make inferences about past sea-ice extent. Subsurface temperatures are reconstructed by means of the magnesium calcium (Mg/Ca) ratio of microscopic shells, which are produced by unicellular organisms living in the surface ocean. The Mg/Ca ratio preserves the temperature of the surrounding water masses during the life of the organism.
At IODP Site U1302/03, the sea-ice biomarker results show a clear pattern with rapidly increasing sea-ice extent a couple of hundred to a thousand years prior to the distinctive sedimentary layers associated with Heinrich events. These new sea-ice results thus support a potential role of sea-ice in causing ice-sheet collapse and the release of icebergs during Heinrich events. Reconstructions of subsurface temperature are still in progress but will be available in early 2023. These will be able to confirm if an expanded sea-ice cover caused the subsurface ocean to warm, which in turn might have caused the ice in Hudson Strait to melt.
Sea ice reconstructions and lithological investigations on the sediment cores from northern Baffin Bay covering the last approximately 20 ka BP suggest that an ice shelf and/or thick year-round sea-ice covered the northern Baffin Bay during large parts of the transition from the last ice age to the current warm period. At the onset of the current warm period, the sea-ice cover becomes more variable. The transition to reduced sea-ice likely preceded the retreat of the Smith Sound ice stream that flowed into the northern Baffin Bay during the last ice age. Additional investigations to precisely determine the age of the transition from a year-round to a variable sea-ice cover are needed.
The two cores from the southern Lincoln Sea cover the last ~12 ka BP. Sea ice reconstructions across this interval show a period of reduced sea-ice during the onset of the current warm period between ca. 9.7 and 11.3 ka BP. This is significant, as the southern Lincoln Sea lies within what is referred to as ‘the last ice are’, the area of the Arctic Ocean that is suggested to be the last stronghold of summer sea-ice under continued climate warming. The result from IceLab suggest that the reduced sea-ice extent in the Lincoln Sea was a response to warmer atmospheric temperatures and reduced net sea-ice transport to the Lincoln Sea from other areas of the Arctic Ocean. Additionally, Ryder glacier, draining parts of the Greenland ice sheet into the Lincoln Sea might have contributed to the reduced sea-ice extent by promoting the up-mixing of warmer bottom water masses of Atlantic origin.
Thus, all new IceLab records document the interactions between sea-ice, the ocean, and ice sheets during past periods of ice-sheet collapse or retreat. The results highlight the intricate relationships of different components of the climate system, important to understand past, present, and future climate change.
At IODP Site U1302/03, the sea-ice biomarker results show a clear pattern with rapidly increasing sea-ice extent a couple of hundred to a thousand years prior to the distinctive sedimentary layers associated with Heinrich events. These new sea-ice results thus support a potential role of sea-ice in causing ice-sheet collapse and the release of icebergs during Heinrich events. Reconstructions of subsurface temperature are still in progress but will be available in early 2023. These will be able to confirm if an expanded sea-ice cover caused the subsurface ocean to warm, which in turn might have caused the ice in Hudson Strait to melt.
Sea ice reconstructions and lithological investigations on the sediment cores from northern Baffin Bay covering the last approximately 20 ka BP suggest that an ice shelf and/or thick year-round sea-ice covered the northern Baffin Bay during large parts of the transition from the last ice age to the current warm period. At the onset of the current warm period, the sea-ice cover becomes more variable. The transition to reduced sea-ice likely preceded the retreat of the Smith Sound ice stream that flowed into the northern Baffin Bay during the last ice age. Additional investigations to precisely determine the age of the transition from a year-round to a variable sea-ice cover are needed.
The two cores from the southern Lincoln Sea cover the last ~12 ka BP. Sea ice reconstructions across this interval show a period of reduced sea-ice during the onset of the current warm period between ca. 9.7 and 11.3 ka BP. This is significant, as the southern Lincoln Sea lies within what is referred to as ‘the last ice are’, the area of the Arctic Ocean that is suggested to be the last stronghold of summer sea-ice under continued climate warming. The result from IceLab suggest that the reduced sea-ice extent in the Lincoln Sea was a response to warmer atmospheric temperatures and reduced net sea-ice transport to the Lincoln Sea from other areas of the Arctic Ocean. Additionally, Ryder glacier, draining parts of the Greenland ice sheet into the Lincoln Sea might have contributed to the reduced sea-ice extent by promoting the up-mixing of warmer bottom water masses of Atlantic origin.
Thus, all new IceLab records document the interactions between sea-ice, the ocean, and ice sheets during past periods of ice-sheet collapse or retreat. The results highlight the intricate relationships of different components of the climate system, important to understand past, present, and future climate change.
The sea-ice record at IODP Site U1302/02 produced as part of IceLab is the first direct reconstruction of sea-ice variability across Heinrich events from the Labrador Sea. Together with subsurface temperature reconstructions, it will be able to answer the question whether an expanded sea-ice cover caused subsurface warming prior to the collapse of the Hudson Strait ice sheet. This will be an important step forward in understanding the causes of Heinrich events and provide new insights to ocean/ice sheet interactions.
The new records of sea-ice variability from the northern Baffin Bay are the longest continuous records from the region as of yet. Thus, they will be able to provide new insights into the climate history of the northern Baffin Bay and the relationship of sea-ice and glacier retreat in the region.
While also documenting sea-ice/ice sheet interactions, IceLab results from the southern Lincoln Sea are additionally significant as they show that the year-round sea-ice cover in the Arctic’s last ice area may be less stable than previously assumed. The new sea-ice records suggest that the southern Lincoln Sea might experience short periods of no sea-ice during summer, even if the current global warming is halted at an average global temperature rise of 2 °C (The Paris Agreement).
The new records of sea-ice variability from the northern Baffin Bay are the longest continuous records from the region as of yet. Thus, they will be able to provide new insights into the climate history of the northern Baffin Bay and the relationship of sea-ice and glacier retreat in the region.
While also documenting sea-ice/ice sheet interactions, IceLab results from the southern Lincoln Sea are additionally significant as they show that the year-round sea-ice cover in the Arctic’s last ice area may be less stable than previously assumed. The new sea-ice records suggest that the southern Lincoln Sea might experience short periods of no sea-ice during summer, even if the current global warming is halted at an average global temperature rise of 2 °C (The Paris Agreement).