Hot on the trail of the causes of rapid ice sheet instabilities in climate history


Share post:

Heinrich Events or, more accurately, Heinrich Layers, are recurrent conspicuous sediment layers, usually ten to 15 centimeters thick, with very coarse rock components that interrupt the otherwise fine-grained oceanic deposits in the North Atlantic. Discovered and first described in the 1980s by the geologist Hartmut Heinrich, U.S. geochemist Wally Broecker later officially named them Heinrich Layers, which has become a standard term in paleoceanography.

The presence of Heinrich Layers has been established throughout the North Atlantic, from off Iceland, southward to a line running from New York to North Africa. Such coarse rock debris could only have been transported such a great distance from its point of origin in the Hudson Bay by icebergs.

The research vessel MARIA S. MERIAN leaving the harbour of St. John’s (Canada). As a participant on Expedition MSM 39 (2014), Lars Max, along with other researchers, obtained the sample material for this study [Credit: MARUM – Center for Marine Environmental Sciences, University of Bremen; D. Kieke]

“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 freshwater were introduced into the North Atlantic,” says Lars Max, paleoceanographer at MARUM — Center for Marine Environmental Sciences at the University of Bremen and first author of the study. As part of their work, he and his co-authors reconfigure the interrelationships among Heinrich layers, freshwater supply, and changes in the ocean circulation. A thin freshwater lens lying atop millions of cubic kilometers of water during the Heinrich events is presently considered to be the cause of the disruption of the Atlantic Meridional Overturning Circulation (AMOC), or its complete shutdown, with profound regional and global climatic consequences. The AMOC is just one segment of the global conveyor belt of ocean currents that is driven by temperature and salinity and plays a significant role in the climate system.

“Originally the disruption was considered to be the result of internal instabilities of the ice sheet itself. Our study, however, provides evidence that changes in the ocean had a destabilizing impact on the ice sheet on the North American continent,” says Lars Max. The study of a sediment core obtained by the research vessel MARIA S. MERIAN at the outlet to the Labrador Sea in the North Atlantic provides the first solid evidence of recurrent, massive accumulations of ocean heat in the deeper layers of the subpolar North Atlantic. This facilitated the melting of the polar ice sheets from below.

Deep sea sediment core with coarse ice-transported lithogenic components (Heinrich layer) [Credit: Lars Max]

“Using trace-element and isotopic analytical methods, we were, in fact, able to reconstruct temperature and salinity increases at around 150 meters 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,” explains Dirk Nurnberg of the GEOMAR Helmholtz Centre for Ocean Research in Kiel, who is responsible for the laboratory analyses.

This suggests that changes in ocean circulation triggered the ice-sheet instabilities. A continuous warming of the ocean at this depth was critical for destabilizing the ice shelf from below, and eventually led to the accelerated shedding of icebergs — the Heinrich Events.

Planktonic microfossils such as the species Neogloboquadrina pachyderma sinistral carry the isotope geochemical information used to perform oceanographic and climatic reconstructions [Credit: Antonov/WikiCommons]

Understanding the processes from Earth’s history also enables us to better predict changes that can be expected to accompany the current global warming. “If the overturning circulation should weaken in the future due to anthropogenic climate change,” suggests Cristiano Chiessi of the University of Sao Paulo, “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) concludes that, with continued warming of the climate, there could be a weakening of the overturning circulation in the Atlantic Ocean within this century. Intensified warming of the deeper subpolar North Atlantic and more rapid melting of the Arctic glacial masses could also have the result of further accelerating the global rise in sea level. As Lars Max also points out, however, we can expect that the stability of the Antarctic Ice Sheet will play a significant role in the course of sea-level rise. Further studies are crucially needed in order to better predict to what extent the future deceleration of overturning circulation and possible warming of the deeper ocean could have on the future stability of the Antarctic Ice Sheet.

The findings are published in Nature Communications.

Source: MARUM – Center for Marine Environmental Sciences, University of Bremen [July 26, 2022]



Related articles

First direct proof of ozone hole recovery due to chemicals ban

For the first time, scientists have shown through direct satellite observations of the ozone hole that levels of...

Combined Arctic ice observations show decades of loss

It's no surprise that Arctic sea ice is thinning. What is new is just how long, how steadily,...

Hidden losses deep in the Amazon rainforest

Few places on Earth are as rich in biodiversity and removed from human influence as the world's largest...

Understanding tropical rainfall, both past and present

A drop of rainwater that falls on a cassava field in Uganda takes a different path than one...

Study shows acceleration of global mean ocean circulation since 1990s

A study published in the journal Science Advances, suggests global ocean circulation has accelerated during the past two...

Warming ocean water undercuts Antarctic ice shelves

"Upside-down rivers" of warm ocean water threaten the stability of floating ice shelves in Antarctica, according to a...

Undersea topography generates hot spots of ocean mixing

Using underwater robots in the waters surrounding Antarctica, scientists at Caltech have shown that the intersection of strong...

Excavations for Copenhagen Metro dig up evidence of interglacial period

Work on the new Metro station revealed traces of a hitherto unknown interglacial period in Denmark, science media...