Excessive cooling events, defined as Heinrich Events, in the North Atlantic during the last glacial period are a perfect example of how local processes affect global climate. While the effects of Heinrich events on the global glacial environment have been well outlined in scientific papers, the cause of these events remains unknown.
Scientists from Bremen, Kiel, Köln, and So Paulo (Brazil) have now demonstrated that heat accumulation in the deeper Labrador Sea caused disruptions in the Laurentide Ice Sheet, which enclosed much of North America at the time. The Heinrich Events occurred as a result.
The scientists demonstrated this by reconstructing past temperatures and salinities in the North Atlantic. Their findings have now been published in the journal Nature Communications.
Heinrich Events, or Heinrich Layers, are recurring conspicuous sediment layers, typically 10 to 15 cm thick, with very coarse rock constituents that disrupt the otherwise fine-grained oceanic deposits in the North Atlantic.
Geologist Hartmut Heinrich observed and named them for the very first time in the 1980s, and US geochemist Wally Broecker later formally named them Heinrich Layers, which has become a common reference in paleoceanography.
The presence of Heinrich Layers’ has been identified throughout the North Atlantic, from off the coast of Iceland southward to a line running from New York to North Africa. This coarse rock debris could have only been traveled such a long distance from its origin in Hudson Bay via icebergs.
The actual significance of these Heinrich Events, however, lies in the fact that, along with the melting phase and release of icebergs, large quantities of fresh water were introduced into the North Atlantic.
Lars Max, Study First Author and Paleoceanographer, MARUM – Center for Marine Environmental Sciences, University of Bremen
As part of their research, he and his co-authors reconstructed the interconnections between Heinrich layers, freshwater supply, and changes in ocean circulation.
A thin freshwater lens resting on top of millions of cubic km of water during the Heinrich events is now thought to be the cause of the disturbance or comprehensive closure of the Atlantic Meridional Overturning Circulation (AMOC) with profound regional and global climatic consequences.
The AMOC is only one segment of the global conveyor belt of ocean currents, which is driven by temperature and salinity and plays a vital role in the earth’s climate.
“Originally the disruption was considered to be the result of internal instabilities of the ice sheet itself. Our study, however, provides evidence changes in the ocean had a destabilizing impact on the ice sheet on the North American continent,” says Lars Max.
The analysis of a sediment core acquired by the research vessel Maria S. Merian at the Labrador Sea outlet in the North Atlantic offers the first solid evidence of massive, recurrent accumulations of ocean heat in the deeper layers of the subpolar North Atlantic. This helped in the melting of the polar ice sheets from underneath.
Using trace-element and isotopic analytical methods, we were, in fact, able to reconstruct temperature and salinity increases at around 150 m of water depth that always systematically preceded the Heinrich Events in time, and that corresponded to times of an already weakened Atlantic Meridional Overturning Circulation.
Dirk Nürnberg, GEOMAR Helmholtz Center for Ocean Research
Nürnberg was also responsible for the laboratory analyses.
This suggests that changes in ocean circulation caused ice-sheet instability. Constant warming of the ocean at this depth was crucial for destabilizing the ice shelf from below and even resulted in accelerated iceberg shedding — the Heinrich Events.
Acknowledging the processes that have occurred throughout Earth’s history also allows researchers to predict the variations of current global warming.
Christiano Chiessi of the University of São Paulo, states, “If the overturning circulation should weaken in the future due to anthropogenic climate change, we would expect an accelerated warming of the deeper subpolar North Atlantic that could negatively impact both the stability of the present-day Arctic glaciers and the freshwater budget of the North Atlantic.”
The latest Intergovernmental Panel on Climate Change (IPCC) Assessment Report (2021) proposes that, with prolonged warming of the climate, there could be a diminishing of the Atlantic Ocean overturning circulation within this century.
Exacerbated warming of the profound sub-polar North Atlantic and faster melting of the Arctic glacial masses could also speed up the rate of global sea level rise. Moreover, as Lars Max states, researchers can expect the consistency of the Antarctic Ice Sheet to play an important role in the course of sea-level rise.
Further research is critical to forecast the impact of future deceleration of overturning circulation and potential warming of the deeper ocean on the long-term stability of the Antarctic Ice Sheet.
Max, L., et al. (2022) Subsurface Ocean warming preceded Heinrich Events. Nature Communications. doi.org/10.1038/s41467-022-31754-x.