Unit Affiliation: Marine and Polar Geophysics, Lamont-Doherty Earth Observatory (LDEO)
Understanding the reaction of large ice sheets to climate change is critical for predicting future sea levels. However, many details of past and future ice sheet behavior are still uncertain. Significant progress has been made with recent studies of the West Antarctic Ice Sheet (WAIS) that revealed the intrusion of warm ocean water onto the continental shelf as the main cause for present thinning of major ice streams.
Although the East Antarctic Ice Sheet (EAIS) is generally considered more stable than the WAIS, recent satellite data indicate that some East Antarctic ice streams are thinning. Hence, parts of the EAIS might be more vulnerable to future ice loss than previously thought. Similar to the WAIS the main thread to the EAIS is probably warm water intrusion onto the continental shelf.
Until recently there have been only few direct bathymetry and oceanographic observations from the East Antarctic continental margin that could shed light onto these questions and provide the basis for more reliable ice sheet models.
OUTCOMES: As part of this NSF-funded project we acquired new data during the expedition NBP1503 in 2015 with the US icebreaker NB Palmer in a section of East Antarctica between 116°E and 134°E. The goal of this expedition was to investigate the continental ice shelf in front of different glaciers along the margin and to determine vulnerable those glaciers might be to warm water intrusion. As result, we acquired over 5100 km (3170 miles) of new high-resolution bathymetry data, which are represent the first accurate seafloor depth measurements for much of the study area. In addition, we measured water properties including temperature, salinity and oxygen content at 42 stations along and across the continental shelf break and slope.
North of the Totten Glacier we found a relative warm water mass, so called modified Circumpolar Deep Water (mCDW) near and on the shelf break. This water had a temperature of >0.7°C and a high salinity. This seems cold, but is warmer than typical water on the Antarctic continental shelf, which can have temperatures between -1°C and -2°C. The 0.7°C is above the melting point of glaciers and this water mass has enough heat to cause significant melting of the Totten Ice Shelf and Glacier as it is observed by satellite measurement. Once this water passed over the shelf break the high density caused by the salinity allows the water to flow towards the glacier where the seafloor is deeper.
We found that the mCDW only crosses the shelf break where the water depth is deeper than 450-500 m. At another section of the continental margin, where the shelf break is only 400 m deep, the mCDW reaches the shelf break, but is not crossing it. This shows that the actual shelf break depth can be a critical control for warm water to access the continental shelf in this part of East Antarctica. Our survey found that the seafloor depths along the continental shelf of our study area varies significantly, with depths ranging between less than 300 m to over 500m. However, there are still large sections of the East Antarctic margin, for which no detailed bathymetric information is available.
At the eastern end of our study area we acquired multibeam bathymetry data from the Dibble polynya. There we discovered up to 1000m deep basin on the continental shelf. Detailed images of the seafloor show features that were created by fast flowing ice streams in the past when the entire continental shelf was covered by grounded ice. We also identified several subglacial meltwater channels. This indicates that there was a significant amount of melt water under the ice, which might have contributed to streaming ice flow. Together with sediment deposits on the deeper continental slope and rise these data suggest that this was a very active ice stream in the past.
Overall this study showed that the East Antarctic margin is a complex system and that the ice sheet has been more dynamic in the past than previously thought. The response of individual glaciers to warm water will depend on details of the local bathymetry. And in some cases, such as the Totten Glacier, warm ocean water might already be related to an increase of melting.
Images from the expedition
DATASETS: NBP1503 expedition data: http://www.marine-geo.org/tools/entry/NBP1503 (doi: 10.7284/901478)
USAP-DC project page with data links: https://www.usap-dc.org/view/project/p0000394 .
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