Tide-induced microseismicity in the Mertz glacier grounding area, East Antarctica

International audienceThe deployment of a seismic network along the Adélie and George V coasts in East Antarctica during the period 2009–2012 provides the opportunity to monitor cryoseismic activity and to obtain new insights on the relationship between tidal cycles and coastal glacier dynamics. Here we focus on records from a seismometer located on a rocky outcrop in the vicinity of the grounding line of the 35 km broad Mertz glacier, a major outflow of this region. We detect numerous icequakes (50,000 events within 10 months and up to 100 events/h) and demonstrate their clear tidal modulation. We suggest that they result from ice friction and fracturing around the rocky peak and from the glacier flexure in response to the falling and rising tides at its grounding area. We propose that such icequake monitoring could be used as a climate proxy since grounding lines are subject to migrate with sea level changes

In situ measurements of snow surface roughness using a laser profiler

International audienceThe snow surface roughness at centimetre and millimetre scales is an important parameter related to wind transport, snowdrifts, snowfall, snowmelt and snow grain size. Knowledge of the snow surface roughness is also of high interest for analyzing the signal from radar sensors such as SAR, altimeters and scatterometers. Unfortunately, this parameter has seldom been measured over snow surfaces. The techniques used to measure the roughness of other surfaces, such as agricultural or sand soils, are difficult to implement in polar regions because of the harsh climatic conditions. In this paper we develop a device based on a laser profiler coupled with a GPS receiver on board a snowmobile. This instrumentation was tested successfully in midre Lovénbreen, Svalbard, in April 2006. It allowed us to generate profiles of 3 km sections of the snow-covered glacier surface. Because of the motion of the snowmobile, the roughness signal is mixed with the snowmobile signal. We use a distance/frequency analysis (the empirical mode decomposition) to filter the signal. This method allows us to recover the snow surface structures of wavelengths between 4 and 50 cm with amplitudes of >1 mm. Finally, the roughness parameters of snow surfaces are retrieved. The snow surface roughness is found to be dependent on the scales of the observations. The retrieved RMS of the height distribution is found to vary between 0.5 and 9.2 mm, and the correlation length is found to be between 0.6 and 46 cm. This range of measurements is particularly well adapted to the analysis of GHz radar response on snow surfaces

External influences on the Mertz Glacier Tongue (East Antarctica) in the decade leading up to its calving in 2010

The Mertz Glacier Tongue (MGT) in East Antarctica lost ~55% of its floating length in February 2010, when it calved large tabular iceberg C28 (78 × 35 km). We analyze the behavior of the MGT over the preceding 12 years using a variety of satellite data (synthetic aperture radar and Landsat imagery and Ice, Cloud, and land Elevation Satellite laser altimetry). Contact of its northwestern tip with the eastern flank of shoals from 2002/2003 caused eastward deflection of the ice flow by up to ~47°. This change contributed to opening of a major rift system ~80 km to the south, along which iceberg C28 eventually calved. Paradoxically, the seabed contact may have also held the glacier tongue in place to delay calving by ~8 years. Our study also reveals the effects of other, more localized external influences on the MGT prior to calving. These include an abrupt sideways displacement of the glacier tongue front by at least ~145 m following an apparent collision with iceberg C08 in early 2002 and calving of numerous small icebergs from the advancing northwestern front due to the "chiseling" action of small grounded icebergs and seabed contact, resulting in the loss of ~36 km2 of ice from 2001 to 2006. The example of the MGT confirms the need for accurate bathymetry in the vicinity of ice shelves and glacier tongues and suggests that the cumulative effect of external factors might be critical to understanding and modeling calving events and ice shelf stability, necessarily on a case-specific basis.This work was supported by the Australian Government’s Cooperative Research Centre (CRC) program through the Antarctic Climate & Ecosystems CRC, and Australian Antarctic Science Projects 3024 and 4116 and contributes to WCRP Climate and Cryosphere (CliC) project Targeted Activity Interactions Between Cryosphere Elements

Examining the interaction between multi-year landfast sea ice and the Mertz Glacier Tongue, East Antarctica: Another factor in ice sheet stability?

International audienceThe Mertz Glacier tongue (MGT), East Antarctica, has a large area of multi-year fast sea ice (MYFI) attached to its eastern edge. We use various satellite data sets to study the extent, age, and thickness of the MYFI and how it interacts with the MGT. We estimate its age to be at least 25 years and its thickness to be 10-55 m; this is an order of magnitude thicker than the average regional sea-ice thickness and too thick to be formed through sea-ice growth alone. We speculate that the most plausible process for its growth after initial formation is marine (frazil) ice accretion. The satellite data provide two types of evidence for strong mechanical coupling between the two types of ice: The MYFI moves with the MGT, and persistent rifts that originate in the MGT continue to propagate for large distances into the MYFI. The area of MYFI decreased by 50% following the departure of two large tabular icebergs that acted as pinning points and protective barriers. Future MYFI extent will be affected by subsequent icebergs from the Ninnis Glacier and the imminent calving of the MGT. Fast ice is vulnerable to changing atmospheric and oceanic conditions, and its disappearance may have an influence on ice tongue/ice shelf stability. Understanding the influence of thick MYFI on floating ice tongues/ice shelves may be significant to understanding the processes that control their evolution and how these respond to climate change, and thus to predicting the future of the Antarctic Ice Sheet

A 4-decade record of elevation change of the Amery Ice Shelf, East Antarctica

0148-0227We report on long-term surface elevation changes of the central Amery Ice Shelf (AIS) by comparing elevation records spanning 4 decades (1968-2007). We use elevation records acquired with the following methods: optical leveling (1968-1969); ERS radar altimetry (1992-2003); GPS (1995-2006); and Ice, Cloud, and land Elevation Satellite (ICESat) laser altimetry (2003-2007). We compute multidecadal elevation trend (dh/dt) values at crossovers between the leveling route and each of the GPS and ICESat tracks as well as shorter-period dh/dt at ERS-ERS, GPS-GPS, and ICESat-ICESat crossovers. At GPS-leveling crossovers the mean long-term dh/dt is -0.003 m a(-1), and at ICESat-leveling crossovers the mean dh/dt is +0.013 m a(-1); neither trend is significantly different from zero. The data do, however, exhibit variable trends: near-zero change between 1991 and mid-1996, then thickening to similar to 2003, followed by thinning similar to 2003-2007, with 5 year dh/dt averages exceeding similar to +/- 0.1 m a(-1). The changes in dh/dt pattern in mid-1996 and again in 2003 occur with unexpected speed. The ice shelf exhibits different dh/dt patterns than does the surrounding grounded ice, suggesting that surface mass balance variations or longer-term variations in firn densification processes are unlikely to be major causes. We conclude that these observed multiyear elevation changes must be due to currently unexplained or presently poorly quantified phenomena involving surface or basal processes and/or ice dynamics. With the multidecadal stability of the AIS established, the short-term fluctuations that we observe suggests that for other ice shelves, observed strong dh/dt signals over short time periods do not necessarily indicate ice shelf instability