Over the last 18,000 years, there have been phases of extreme bottom water temperatures reaching up to 6°C. The inflow of warmer Atlantic Waters onto the continental shelf during these phases has been associated with repeated changes in the stability of subsurface gas hydrates, facilitating the release of greenhouse gasses from the seabed, and triggering multiple collapses of the past ice sheet.
Text: Mariana Esteves
A new study by Dr Naima El bani Altuna and co-authors in Communications Earth & Environment reconstructed the bottom water temperature (BWT) changes throughout the last 18,000 years in the ‘Pingo Area’ located in outer Storfjordrenna, North Western Barents Sea. Findings from this study provide quantifiable evidence for bottom warming events in the North Western Barents Sea since the Last Glacial Maximum (ca. 21,000yrs ago), and its impact on the retreat of the former ice sheet and destabilisation of the sub-seafloor gas hydrates.
The BWT record for this study area was developed using Mg/Ca (Magnesium-Calcium ratios) in the benthic foraminiferal species well adapted to cooled Atlantic waters, Cassidulina neoteretis. These results were then coupled with an ice sheet hydrate stability model (see CAGE ToolBox), to investigate the past dynamics of the gas hydrate stability zone and constrain the history of methane release from the seafloor.
“In our study, we observed that bottom water temperature is one of the main drivers for changes in the stability of gas hydrates in this shallow area. Periods of persistent warming causes the thinning of the gas hydrate stability zone, which could facilitate the release of methane from the seabed.” Says Naima El bani Altuna, who conducted this study in as part of her PhD at CAGE.
Map showing the location of the study area which is south of Svalbard, in addition to the extent of the Barents Sea Ice Sheet during the Last Glacial Maximum (ca. 21,000 years ago; white dashed line) and the directions and locations of the main surface water currents (Atlantic water, WSC, red solid line; and Polar water, ESC, blue solid line). Modified from El bani Altuna et al., 2021.
Persistent warming caused the thinning of gas hydrate stability zone and ice stream retreat
The environmental changes occurring in the western Barents Sea margin throughout the last 18,000yrs are an important analogue for the present-day subsurface warming-induced retreat of the West Antarctic Ice Sheet and the marine-terminating glaciers in Greenland. Thus it is crucial to understand the impact of BTW changes on the retreating ice masses and the potential destabilisation of the gas hydrate stability zone.
El bani Altuna and co-authors observed that there were three major high BWT events, where the temperatures were ca. 4-5°C warmer than present, which correlated closely with the main retreat stages of the former Barents Sea Ice Sheet. As this large and heavy ice sheet retreated it relieved the pressure on the seafloor, causing a rapid reduction and thinning of the gas hydrate stability zone. Results from this study indicate that these persistent warming events of high BWT played an important role in driving the retreat of the ice sheet, and in turn promoting a thinning of the gas hydrate stability zone.
Cassidulina neoteretis, a benthic foraminifera species well adapted to cooled Atlantic Waters. Gif by Siri Ofstad, Przemek Domel, and Naima El bani Altuna.
Atlantification of the Barents Sea
It is undeniable that the Arctic is experiencing dramatic changes in response to climate change.
A major change that has been observed in recent decades is the ongoing ‘Atlantification’, which is an increased inflow of warm Atlantic Water into the Barents Sea leading to a reduction in sea ice. The inflow of these warmer Atlantic waters can destabilize the frozen gas hydrates, releasing vast quantities of greenhouse gases into the water column. These events of major methane release, could amplify the effects of ocean acidification and potentially escape into the atmosphere leading to further climatic warming.
“Our findings show that bottom water temperatures played a key role throughout the last 18,000 years, and we now need to reconstruct the BWT in other areas to better understand the full impact of Atlantification on the release of greenhouse gases from the seabed and the retreat of the former ice sheet. Connecting this information with an improved chronology and understanding of the ocean-ice interactions are essential.” Says El Bani Altuna.
By improving our understanding the records of past BTW in the Barents Sea at different time scales and their impact on the gas hydrate stability zones, we can better assess and evaluate the risk posed by the present ‘Atlantification’ process undergoing in the Barents Sea.
Reference: El bani Altuna, N., Rasmussen, T.L., Ezat, M.M. et al. Deglacial bottom water warming intensified Arctic methane seepage in the NW Barents Sea. Commun Earth Environ 2, 188 (2021). https://doi.org/10.1038/s43247-021-00264-x
For more information about ‘The CAGE Toolbox: From birds-eye view to deep sediment imaging’, click here.