Periodic Reporting for period 4 - WACSWAIN (WArm Climate Stability of the West Antarctic ice sheet in the last INterglacial (WACSWAIN))
Okres sprawozdawczy: 2021-08-01 do 2023-01-31
Prominent papers predict the loss of most of the West Antarctic Ice Sheet (WAIS) by 2500 if CO2 emissions and rising global temperatures are not controlled. Is this realistic? We address this by investigating the last interglacial (LIG, 130,000-115,000 years ago) to assess the response of the WAIS to comparable warmth.
Evidence suggests that in the LIG, sea level reached 6-9 m higher than today. It is inferred that Antarctic ice sheets (most likely WAIS) contributed several metres of sea level rise. Models that predict large ice loss in the future also produce a very significant retreat of the WAIS and loss of the huge Ross and Ronne ice shelves under LIG conditions.
WACSWAIN aims to establish whether the WAIS and Ronne Ice Shelf were indeed significantly smaller in the LIG. If so, when did they shrink and regrow? The objective of the project was to remedy the surprising lack of direct evidence about these questions, through these (summarised) objectives:
1. Obtain an ice core to bedrock, reaching through the LIG, from Skytrain Ice Rise in Antarctica, a location sensitive to changes in the size of WAIS or the Ronne Ice Shelf. Analyse the ice to assess the climate and environment of the last glacial and interglacial.
2. Obtain a sequence of ice chips from Sherman Island, Antarctica, near to the most sensitive part of WAIS. Analyse the samples to assess its potential as a further old ice site.
3. Develop and improve several ice core analytical methods.
4. Assemble model predictions for different WAIS scenarios so that we can interpret especially the water isotope record from Skytrain Ice Rise.
5. Assess the state of the WAIS and Ronne Ice Shelf in the LIG, and provide the timing of any changes.
The findings of this project should allow us to support or question the findings of models to predict future change in the WAIS.
The project successfully obtained a bedrock core from Skytrain Ice Rise (reaching the LIG) and a 320 m chips core from Sherman Island covering the last 1000 years. The conclusions of the action, currently being prepared for publication, are:
A. The Ronne Ice Shelf did not retreat significantly from its present position during the bulk of the LIG, though our data are consistent with limited retreat of the WAIS. This has implications for models used to predict past and future changes in WAIS.
B. The Ronne Ice Shelf did retreat abruptly at about 8000 years ago from its position in the last glacial maximum, leading to fast drawdown of ice at Skytrain Ice Rise. This is a clear demonstration that rapid drawdown is possible if grounded ice restraint is lost.
C. The climate of Sherman Island has been rather stable for the last 1000 years, with no sign of unusual warming in recent decades. This suggests that recent retreat of Thwaites and Pine Island Glaciers is not due mainly to atmospheric change.
Evidence suggests that in the LIG, sea level reached 6-9 m higher than today. It is inferred that Antarctic ice sheets (most likely WAIS) contributed several metres of sea level rise. Models that predict large ice loss in the future also produce a very significant retreat of the WAIS and loss of the huge Ross and Ronne ice shelves under LIG conditions.
WACSWAIN aims to establish whether the WAIS and Ronne Ice Shelf were indeed significantly smaller in the LIG. If so, when did they shrink and regrow? The objective of the project was to remedy the surprising lack of direct evidence about these questions, through these (summarised) objectives:
1. Obtain an ice core to bedrock, reaching through the LIG, from Skytrain Ice Rise in Antarctica, a location sensitive to changes in the size of WAIS or the Ronne Ice Shelf. Analyse the ice to assess the climate and environment of the last glacial and interglacial.
2. Obtain a sequence of ice chips from Sherman Island, Antarctica, near to the most sensitive part of WAIS. Analyse the samples to assess its potential as a further old ice site.
3. Develop and improve several ice core analytical methods.
4. Assemble model predictions for different WAIS scenarios so that we can interpret especially the water isotope record from Skytrain Ice Rise.
5. Assess the state of the WAIS and Ronne Ice Shelf in the LIG, and provide the timing of any changes.
The findings of this project should allow us to support or question the findings of models to predict future change in the WAIS.
The project successfully obtained a bedrock core from Skytrain Ice Rise (reaching the LIG) and a 320 m chips core from Sherman Island covering the last 1000 years. The conclusions of the action, currently being prepared for publication, are:
A. The Ronne Ice Shelf did not retreat significantly from its present position during the bulk of the LIG, though our data are consistent with limited retreat of the WAIS. This has implications for models used to predict past and future changes in WAIS.
B. The Ronne Ice Shelf did retreat abruptly at about 8000 years ago from its position in the last glacial maximum, leading to fast drawdown of ice at Skytrain Ice Rise. This is a clear demonstration that rapid drawdown is possible if grounded ice restraint is lost.
C. The climate of Sherman Island has been rather stable for the last 1000 years, with no sign of unusual warming in recent decades. This suggests that recent retreat of Thwaites and Pine Island Glaciers is not due mainly to atmospheric change.
We collected a bedrock core from Skytrain Ice Rise.
In 2018/19 a team of 6-8 people spent 8 weeks in a tented camp on Skytrain Ice Rise (79.74 S, 78.54 W). Drilling reached the bed at 651.04 metres depth. Drilling a core of this length to bedrock in a single field season is an exceptional achievement. Disseminated in a paper (Mulvaney et al 2021), and a Royal Society video.
We drilled with rapid access through Sherman Island.
In 2019/20 a small team used a rapid access drill to collect ice chips to 320 m, although bedrock was not attained. We established that the drill provides ice that can be analysed for isotopes and chemistry. Disseminated in two papers (Mulvaney et al., 2021; Rowell et al., 2022).
The data from Sherman Island provide unique climate records of the past millennium in a region where there is concern about the retreat of the ice sheet.
We completed a high depth-resolution record of the Holocene, last glacial and last interglacial at Skytrain Ice Rise.
After long hours in the cold room cutting the Skytrain ice core for analysis, we improved the continuous analysis system. In 2020 and 2021 we produced continuous records of water isotopes, methane and ionic impurities at resolution of order 4 cm along the entire core. Disseminated in a paper (Grieman et al 2022).
Several new methods were instituted and applied: total air content of the ice (providing evidence about past changes of elevation); laser ablation ICPMS, used on discrete sections of the core; sulfur isotope analysis, applied along the entire core (3 papers in preparation).
Skytrain shows a very interesting feature in the early Holocene, allowing us to date a period of rapid retreat of the ice shelf and consequent loss of inland ice. Major paper to be submitted soon.
Skytrain shows the expected pattern of climate change through the glacial period, but with isotopic change amplified by elevation changes at our site. Two papers are planned on this.
We dated the ice at both sites, showing the presence of last interglacial ice at Skytrain, and of ice 1000 years old at Sherman Island.
Both cores were dated near the top using volcanic markers (confirmed with sulfur isotope analysis) and annual layer counting. Beyond 2000 years the Skytrain core was dated by an inverse method using particularly the pattern of change in methane to tie our ice to previously dated cores. In the LIG there is some disturbance of the ice leading to a non-linear age scale and some folding. We were able to confirm that ice from ~117-126 ka (thousand years), in the LIG, is in good order, and to create a robust age model. Disseminated in a paper currently in review.
We used model output to investigate atmospheric (including water isotope) change under a range of WAIS scenarios.
Disseminated in a paper (Goursaud et al 2021).
We are completing an assessment of the timing and rate of changes in the Ronne Ice Shelf and WAIS between 140 and 100 ka (as well as at later times).
Although we are missing the very warmest section at the very start of the LIG, we can see from our data that by 126000 years ago, the Ronne Ice Shelf was at about its present position (based on sea salt), while the water isotopes suggest some retreat of WAIS. While it is possible there was greater retreat before 126 ka, this seems unlikely as ice would have had to regrow under warmest conditions. This important result suggests that the ice shelf is not as sensitive as some models suggested. This is being prepared for a high impact publication to be submitted in spring 2023.
In 2018/19 a team of 6-8 people spent 8 weeks in a tented camp on Skytrain Ice Rise (79.74 S, 78.54 W). Drilling reached the bed at 651.04 metres depth. Drilling a core of this length to bedrock in a single field season is an exceptional achievement. Disseminated in a paper (Mulvaney et al 2021), and a Royal Society video.
We drilled with rapid access through Sherman Island.
In 2019/20 a small team used a rapid access drill to collect ice chips to 320 m, although bedrock was not attained. We established that the drill provides ice that can be analysed for isotopes and chemistry. Disseminated in two papers (Mulvaney et al., 2021; Rowell et al., 2022).
The data from Sherman Island provide unique climate records of the past millennium in a region where there is concern about the retreat of the ice sheet.
We completed a high depth-resolution record of the Holocene, last glacial and last interglacial at Skytrain Ice Rise.
After long hours in the cold room cutting the Skytrain ice core for analysis, we improved the continuous analysis system. In 2020 and 2021 we produced continuous records of water isotopes, methane and ionic impurities at resolution of order 4 cm along the entire core. Disseminated in a paper (Grieman et al 2022).
Several new methods were instituted and applied: total air content of the ice (providing evidence about past changes of elevation); laser ablation ICPMS, used on discrete sections of the core; sulfur isotope analysis, applied along the entire core (3 papers in preparation).
Skytrain shows a very interesting feature in the early Holocene, allowing us to date a period of rapid retreat of the ice shelf and consequent loss of inland ice. Major paper to be submitted soon.
Skytrain shows the expected pattern of climate change through the glacial period, but with isotopic change amplified by elevation changes at our site. Two papers are planned on this.
We dated the ice at both sites, showing the presence of last interglacial ice at Skytrain, and of ice 1000 years old at Sherman Island.
Both cores were dated near the top using volcanic markers (confirmed with sulfur isotope analysis) and annual layer counting. Beyond 2000 years the Skytrain core was dated by an inverse method using particularly the pattern of change in methane to tie our ice to previously dated cores. In the LIG there is some disturbance of the ice leading to a non-linear age scale and some folding. We were able to confirm that ice from ~117-126 ka (thousand years), in the LIG, is in good order, and to create a robust age model. Disseminated in a paper currently in review.
We used model output to investigate atmospheric (including water isotope) change under a range of WAIS scenarios.
Disseminated in a paper (Goursaud et al 2021).
We are completing an assessment of the timing and rate of changes in the Ronne Ice Shelf and WAIS between 140 and 100 ka (as well as at later times).
Although we are missing the very warmest section at the very start of the LIG, we can see from our data that by 126000 years ago, the Ronne Ice Shelf was at about its present position (based on sea salt), while the water isotopes suggest some retreat of WAIS. While it is possible there was greater retreat before 126 ka, this seems unlikely as ice would have had to regrow under warmest conditions. This important result suggests that the ice shelf is not as sensitive as some models suggested. This is being prepared for a high impact publication to be submitted in spring 2023.
Our Skytrain Ice Rise core is the only vertical core in West Antarctica that has demonstrated presence of dated last interglacial ice. The site is uniquely suitable for assessing the past state of the WAIS and the Ronne Ice Shelf.
The ability to fast drill a core of ice chips, and our demonstration that they can be used for chemistry as well as isotopic measurements, opens up new prospects for rapidly obtaining climate information across a region.
The 1000 year record from Sherman Island, though less than we hoped for, is the longest record from the region and therefore provides a new view of the climate context of the vulnerable outlet glaciers there.
We have for the first time shown the effect of ice shelf retreat in the early Holocene, and the speed with which inland ice is drawn down as a result.
We have shown that the Ronne Ice Shelf was present during the LIG, a result which immediately differentiates between more and less aggressive models of ice sheet retreat under past and future warming.
The ability to fast drill a core of ice chips, and our demonstration that they can be used for chemistry as well as isotopic measurements, opens up new prospects for rapidly obtaining climate information across a region.
The 1000 year record from Sherman Island, though less than we hoped for, is the longest record from the region and therefore provides a new view of the climate context of the vulnerable outlet glaciers there.
We have for the first time shown the effect of ice shelf retreat in the early Holocene, and the speed with which inland ice is drawn down as a result.
We have shown that the Ronne Ice Shelf was present during the LIG, a result which immediately differentiates between more and less aggressive models of ice sheet retreat under past and future warming.