A recent study, using remote sensing, provided evidence that a seafloor shoal
influenced the 2010 calving event of the Mertz Ice Tongue (MIT), by partially
grounding the MIT several years earlier. In this paper, we start by proposing
a method to calculate firn air content (FAC) around Mertz from
seafloor-touching icebergs. Our calculations indicate the FAC around Mertz
region as
Surface-warming induced calving or disintegration of floating
ice has occurred in Antarctica, such as the Larsen B ice shelf (Scambos et
al., 2000, 2003; Domack et al., 2005; Shepherd et al., 2003). While surface
or sub-surface melting has largely been recognized to contribute to floating
ice loss in Antarctica (Depoorter et al., 2013), calving caused by
interaction with the seafloor has not been widely considered. The Mertz Ice
Tongue (MIT) was reported to have calved in 2010, subsequent to being rammed
by a large iceberg, B-9B (Legresy et al., 2010). After the calving, the areal
coverage of Mertz polynya, sea ice production and dense, shelf water
formation in the region changed (Kusahara et al., 2011; Tamura et al., 2012).
However, the iceberg collision may have only been an apparent cause of the
calving as other factors had not been fully considered such as seafloor
interactions (Massom et al., 2015; Wang, 2014). By comparing inverted ice
thickness to surrounding bathymetry, and combining remote sensing analysis,
Massom et al. (2015) considered that the seabed contact may have held the
glacier tongue in place to delay calving by
The MIT (66–68
Mertz Ice Tongue (MIT), East Antarctica. Landfast sea ice is
attached to the east flank of the MIT and the Mertz Polynya is to the west.
The background image corresponds to band 4 Landsat 7, captured on 2 February
2003. The green square found in the upper left inset indicates the location
of the MIT in East Antarctica. A polar stereographic projection with
Spatial distribution of the ICESat/GLAS data from 2003 to 2009
covering the Mertz region. Ground tracks of ICESat/GLAS are indicated with
gray lines. Track 1289 (T1289) is highlighted in red as is used in Fig. 4.
The background image corresponds to band 4 Landsat 7, captured on 2 February
2003. A polar stereographic projection with
Freeboard extracted from Track T1289, ICESat/GLAS, the location of
which can be found from Figs. 2 and 3b. (
The primary data used to investigate grounding of the MIT in this study are elevation data from Geoscience Laser Altimeter System (GLAS) onboard the Ice, Cloud and land Elevation Satellite (ICESat) and the seafloor bathymetry data mentioned above. In this section, the ICESat/GLAS and bathymetry data, as well as some preprocessing are introduced.
ICESat is the first spacebone laser altimetry satellite orbiting the Earth,
launched by the National Aeronautics and Space Administration (NASA) in 2003
(Zwally et al., 2002) with GLAS as the primary payload onboard. ICESat/GLAS
was operated in an orbit of
Detailed bathymetry maps are fundamental spatial data for marine science
studies (Beaman and Harris, 2003; Beaman et al., 2011) and crucially needed
in the data-sparse Antarctic coastal region (Massom et al., 2015).
Regionally, around Mertz, a large archive of ship track single-beam and
multi-beam bathymetry data from 2000 to 2008 were used to generate a
high-resolution digital elevation model (DEM) for which the spatial coverage
can be found in Fig. 3b and c. The DEM product was reported to have a
vertical accuracy of approximately 11.5
Around Antarctica, the seafloor topography data from Bedmap-2 was produced by
Fretwell et al. (2013) which adopted the DEM from Beaman et al. (2011). In
this study, Bedmap-2 seafloor topography data (BAS, 2016) covering Mertz is
employed to detect the contact between seafloor and the MIT. Because of
inconsistent elevation systems for ICESat/GLAS and the seafloor topography
data, the Earth Gravitational Model 2008 (EGM08) geoid (Pavlis et al., 2012)
with respect to World Geodetic System 1984 (WGS-84) ellipsoid is taken as
reference. Since the seafloor topography from Bedmap-2 is referenced to the
so-called g104c geoid, an elevation transformation is required and can be
implemented through the following:
ICESat/GLAS data has been widely used to determine ice freeboard, or ice thickness, since its launch in 2003 (Kwok et al., 2007; Wang et al., 2011, 2014; Yi et al., 2011; Zwally et al., 2002, 2008). The methods we designed for grounding detection of the MIT using the ICESat/GLAS data are introduced here. First, assuming a floating MIT, based on freeboard data extracted in different observation dates, ice draft of the MIT is inverted. Next, ice bottom elevation is calculated based on the inverted ice draft and the lowest sea-surface height. Finally, the ice bottom is compared with seafloor bathymetry to detect ice grounding. The underlying logic for grounding detection is that if the inverted ice bottom is lower than seafloor, we can draw a conclusion that the ice tongue is grounding rather than floating.
The method for extracting a freeboard map using ICESat/GLAS from multiple campaigns over the MIT was described in Wang et al. (2014). Without providing details, here we only introduce it schematically. Four steps are included in freeboard map production for each of the data sets from 14 November 2002, 8 March 2004, 27 December 2006 and 31 January 2008.
The first step involves data preprocessing, saturation correction, data
quality control, and tidal correction removal. The magnitude of the
ICESat/GLAS waveform can become saturated because of different gain setting,
or high reflection from natural surfaces. Thus, saturated waveforms with
i_satElevCorr (i.e. an attribute from GLA12 data record) greater than or
equal to 0.50
The second step is to derive sea-surface height according to each track and
to calculate freeboard for each campaign. Because of tidal variations near
the MIT, surface elevations of the MIT can vary as well. To derive
sea-surface height from ICESat/GLAS and provide a reference for freeboard
calculation for different campaigns, the ICESat/GLAS data over the MIT within
a buffer region (with 10
The third step is to relocate footprints using estimated ice velocity. ICESat
observed the MIT almost repeatedly along different tracks in different
campaigns (Fig. 2). However, observations from only one campaign cannot
provide good coverage of the MIT. All observations from 2003 to 2009 are
combined together to produce a freeboard map of the MIT. Figure 2 shows the
spatial coverage of ICESat/GLAS from 2003 to 2009 over Mertz, but the
geometric relation between tracks is not correct over the MIT because the
tongue was fast moving and observed in different years by ICESat. Regions
observed in an earlier campaign would move downstream later (Wang et
al., 2014). For example, consider ICESat data from track T31 from 22 March
2003 and T165 (Fig. 2) from 1 November 2003, respectively. Figure 2 shows that
the distance between track T165 and T31 is
Freeboard changes with time should be considered as well, but it is neglected
because comparison of freeboard from crossing tracks showed a slightly
decreasing trend of
The forth step is to interpolate the freeboard map using the relocated freeboard data from the third step. Kriging interpolation in ArcGIS is selected in this study to produce freeboard maps of the MIT because it can provide an optimal interpolation estimate for a given coordinate location by considering the spatial relationships of a data set. With this method, freeboard maps of the MIT are produced for 14 November 2002, 8 March 2004, 27 December 2006, and 31 January 2008, respectively, when the ice tongue outline can be delineated from Landsat images.
Ice draft is calculated with Eq. (4) assuming hydrostatic equilibrium and
using the lowest sea-surface height
The sea surface is taken as the lowest sea surface height
(
The elevation of the underside (bottom) of the tongue
The Antarctic ice sheet is covered by a dry, thick firn layer which
represents an intermediate stage between fresh snow and glacial ice, having
varying density from Antarctic inland to the coast (van den Broeke, 2008).
The density and depth of the Antarctic firn layer has been modeled (e.g., van
den Broeke, 2008) using a combination of regional climate model output and a
steady-state firn compaction model. However, for ice thickness inversion,
firn air content (FAC) is usually used to make the calculation convenient
(Rignot and Jacobs, 2002). FAC is defined as the decrease in thickness (in
meters) that occurs when the firn column is compressed to the density of
glacier ice (Holland et al., 2011). Time-dependent FAC has also been modeled
by considering the physical process of the firn layer (e.g., Ligtenberg et
al., 2014). For the MIT, there are some in situ measurements of snow
thickness available from Massom et al. (2010) who used a snow layer depth of
1
Statistics of icebergs used to invert FAC with a least-square method and validation of grounding iceberg detection using this FAC. Icebergs A, B, and C are the same as what are used in Figs. 4 and S1. The measurements from icebergs A and C in February 2006 are used to derive the FAC with a least-squares method. However, the measurements from Icebergs A and B in 2008 are used for validation.
Because of different density and thickness of the firn layer on the top of an ice tongue, it is challenging to simulate the density profile of the MIT without in situ measurements as control points. In this study, we use FAC extracted from adjacent seafloor-touching icebergs rather than that from modeling to investigate the grounding of the MIT. The MIT may be composed of pure ice, water, air, firn or snow that will influence the density of the ice tongue. However, if assuming a pure ice density only to calculate ice mass, the thickness of MIT must be corrected by the FAC. The FAC can be inferred from surrounding icebergs that are slightly grounded under the assumption of hydrostatic equilibrium and known ice draft and freeboard. It is, however, critical to target and use icebergs that fulfil the condition of slight grounding. From Smith (2011), icebergs can be divided into three categories based on bathymetry and seasonal pack ice distributions: grounded, constrained, and free-drifting icebergs. Without pack ice, an iceberg can be free-drifting or grounded. Free-drifting icebergs can move several tens of kilometers a day, such as iceberg A-52 (Smith et al., 2007). Grounded icebergs can be heavily or lightly anchored. Heavily grounded icebergs have firm contact with the seafloor and can be kept stationary for a long time, such as iceberg B-9B (Massom. 2003). However, slightly grounded icebergs may have less contact with the seafloor and can possibly move slowly under the influence of ocean tide, ocean currents, or winds, but much slower than free-drifting icebergs. The relation of grounded iceberg to the drifting velocity is not well-known. However, slowly drifting or nearly stationary icebergs in open water are good indicators for slight grounding and therefore are used to infer FAC.
Because of the heavily grounded iceberg B-9B to the east of the MIT blocking the drifting of pack ice or icebergs from the east, icebergs located between B-9B and the MIT are most likely generated from the Mertz or Ninnis glaciers. Some icebergs may be slightly grounded as can be detected from remote sensing. We calculate the FAC from these slightly grounded icebergs and later apply it to grounding event detection of the MIT. Around the MIT, the locations of three icebergs (A, B, and C) were investigated using MODIS and Landsat images in the austral summers of 2006 and 2008, respectively, and shown in Fig. 4. Fortunately, ICESat/GLAS observed these icebergs on 23 February 2006 (54th day of 2006) and 18 February 2008 (49th day of 2008) which allows us to analyze the behavior of these icebergs three-dimensionally. Figure 4a shows that icebergs A, B, and C were almost stagnant and only slightly changed their positions and orientation over 2 months (from 28 to 85 day of 2006). Thus we can consider these icebergs slightly grounded. For these slightly grounded icebergs, hydrostatic equilibrium should still apply, so the ice draft inverted from freeboard measurement assuming hydrostatic equilibrium should be equal to the water depth. Based on this analysis, we can take water depth as the draft to calculate the FAC.
Because only icebergs A and C were observed by track T1289 of ICESat/GLAS in
2006, the FAC is inverted using freeboard and water depth from bathymetry
from both icebergs (Figs. 3b, c, 4, Table 1). However, the icebergs were not
stationary, which indicates that only some parts were slightly grounded.
Therefore, only the top two largest freeboard measurements of icebergs A
and C from T1289 in 2006 are used to calculate the FAC with Eq. (7) with a
least-squares method under hydrostatic equilibrium.
Table 1 shows the freeboard of iceberg A and C from 2006 and seafloor
bathymetry for FAC inversion and grounding detection of icebergs A and B in
2008 (detailed freeboard values for these icebergs can be found from Fig. S1
in the Supplement). With the freeboard from 2006 and seafloor bathymetry
(Table 1), the FAC is calculated as
FAC varies across the Antarctica ice sheet, usually decreasing from the
interior to the coast. For Mertz we obtain a FAC of
First, for FAC calculation, icebergs just touching the seafloor should be used in which case the FAC calculated assuming hydrostatic equilibrium is the same as its actual value. However, it is difficult to ascertain whether an iceberg is just touching the seafloor from remote sensing images. The near stationary or slowly rotating icebergs detected with remote sensing may be grounded more than just touching the seafloor, which may result in a inverted FAC theoretically greater than its actual value. Thus, using this FAC value to detect grounding can potentially lead to smaller grounding results. However, once a grounded iceberg or ice tongue is detected using this FAC, the result is more convincing.
Second, limited observations from ICESat/GLAS may not catch the same and the thickest section of a slight grounding iceberg. Because ICESat/GLAS observed only several times a year on repeat tracks and icebergs were rotating slowly, the elevation profile in 2006 and 2008 along the same track T1289 may not refer to the same ground surface. Figure S1 shows the freeboard of icebergs A, B, and C derived from ICESat/GLAS from 2006 and 2008, respectively. By comparing the freeboard of iceberg A in 2006 (Fig. S1a), and 2008 (Fig. S1c), we find the larger freeboard and the longer freeboard profile in 2006. Comparatively, the smaller freeboard in 2008 may be caused by basal melting or observing a different portion of iceberg A by ICESat. Since the larger freeboard measured in 2006 indicates a high possibility of capturing the thickest portion, it is reasonable to use it to invert the FAC. Additionally, icebergs A and C did show a similar maximum freeboard (Table 1), which is another important reason to select the measurements of 2006 for the inversion.
The accuracy of
Evaluation of kriging interpolation method over the MIT using
freeboard data derived from the ICESat/GLAS data.
The influence of elevation system transformation on final elevation
difference can be neglected. Based on the error propagation, the uncertainty
of elevation difference
Statistics of grounding grids inside the MIT or grounding potentials
outside of the MIT (I: inside the thick black line, Fig. 6; number in
brackets indicates how many grids are located inside the 2000 Mertz boundary;
O: between the black and gray lines, Fig. 6) from 14 November 2002, 8 March
2004, 27 December 2006 and 31 January 2008, respectively. Each grid covers an
area of 1
Elevation difference of Mertz ice bottom and seafloor topography.
Figure 5a shows the spatial distribution of freeboard data over the MIT used
for grounding detection from 14 November 2002. The spatial difference of the
ICESat/GLAS data between Figs. 2 and 5 is caused by the footprint relocation,
after which the spatial geometry between different tracks is reasonably
correct. In the lower right of the Mertz ice front (Fig. 5a), the
crossing-track distance between T1289 and T165 is approximately 7
To investigate the uncertainty of kriging interpolation method, freeboard
measurements from ICESat/GLAS should be compared with the interpolated
freeboard estimates. A testing region with freeboard measurements is selected
(dashed blue square in Fig. 5a,
The freeboard measurement varies from 31.6 to 40.0
Since the sea surface height is extracted from the ICESat/GLAS data track by
track, we use
From the calculations above, a less than
The spatial distribution of the elevation difference
As illustrated in Table 2 and Fig. 6, the minimum
Statistics of grounding outlines of the MIT as shown with thick polylines in Fig. 7 from 14 November 2002, 8 March 2004, 27 December 2006 and 31 January 2008, respectively.
Based on the calculated elevation difference, the grounding outlines of the
MIT are delineated for 14 November 2002, 8 March 2004, 27 December 2006 and
31 January 2008, respectively (Fig. 7). For the grounded part of the outlines
in different years, the starting and ending location and the perimeter are
also extracted (Table 3), from which we conclude that the length of the
grounding outline on the Mertz Bank was only limited to a few kilometers. We
find that the lower right (northwest) section of the MIT was always grounded
and grounding did not occur in other regions (Fig. 6). The shallowest
seafloor that the Mertz ice front touched was
Using Landsat TM/ETM
The average area-change trend of the MIT from 1989 to 2007 was also obtained
using a least-squares method, corresponding to 35.3
The surface dynamics of the MIT such as ice flow direction changes and middle rift changes caused by grounding was analyzed by Massom et al. (2015). In the history of the MIT, one or two large calving events were suspected to have happened between 1912 and 1956 (Frezzotti et al., 1998). Based on the interactions between the MIT and Mertz Bank suggested by our observations and description below, it is likely that only one large calving event occurred between 1912 and 1956. When the MIT touched Mertz Bank, the bank started to affect its stability by bending it clockwise to the east, as can be found from velocity changes from Massom et al. (2015). With continuous advection of the ice and flux input from the upstream, a large rift from the west flank of the tongue would ultimately have to occur and could potentially calve the MIT. A sudden length shortening of the MIT can be caused by such ice tongue calving as that which happened in February 2010. We also consider that even without a sudden collision of iceberg B-9B in 2010, the MIT would eventually have calved because of the effect of the shallow Mertz Bank.
When considering 6127
After the MIT calved in February 2010, the Mertz polynya size, sea-ice
production, sea-ice coverage and high-salinity shelf water formation changed
as well. A sea-ice production decrease of approximately 14–20 % was
found by Tamura et al. (2012) using satellite data and the high-salinity
shelf water export was reported to reduce up to 23 % using a
state-of-the-art ice-ocean model (Kusahara et al., 2010). Recently, Campagne
et al. (2015) pointed out a
From these findings addressed above and the MIT calving cycle we find that
the calving cycle of the MIT leads to the
Digital elevation map (DEM) of seafloor around the Mertz and
grounding section of the boundaries extracted from 2002 to 2008. The
grounding sections of the MIT boundary from 2002, 2004, 2006, and 2008 are
marked with thick red, purple, green and blue polylines, respectively, and
the MIT boundaries are indicated with polygons with the same legend as in
Fig. 3a. Additionally, the MIT boundary from 2000 indicated with dash-dotted
yellow polygon is used to show the different quality of the seafloor DEM.
Inside this polygon no bathymetry data were collected or used. The dashed red
line indicates the “extension line” of the west flank of the MIT on
14 November 2002, passing the shallowest region of the Mertz Bank
(approximately
High accuracy seafloor is critical to the final success of the grounding
detection. According to our best knowledge, Beaman et al. (2011) provided the
most accurate seafloor DEM over the Mertz, so the seafloor DEM inside the
dash-dotted polygon (Fig. 7) was kept and the grounding detection was
conducted there (Fig. 6). Additionally, the ice tongue continued to advance
out into the ocean, where the bathymetry observation density is good. From
the results shown in Fig. 6 all grounding sections of the MIT boundary were
located outside of the 2000 boundary. Thus the analysis of the grounding
detection near the ice front in 2002, 2004, 2006, and 2008 is convincing.
Inside the 2000 boundary, most of the grounding detection results were above
100
Figure 7 shows the extension line of the west flank in November 2002, from
which we can find that if the MIT advected along the former direction, the
ice flow would be seriously obstructed when approaching the Mertz Bank. The
shallowest region of the Mertz Bank has an elevation of approximately
Average trend of the area change of the MIT. The area of the MIT is extracted from the Landsat images from 1988 to 2013.
In this study, a method of FAC calculation from
seafloor-touching icebergs around the Mertz region is presented as an
important element in understanding the MIT grounding. The FAC around the
Mertz is
From remote sensing images we are able to quantify the trend of area increase
of the MIT before and after the 2010 calving. While the area-increase trend
of the MIT after calving was slightly greater than that before, we use the
averaged trend to estimate a timescale required for the MIT to re-advance to
the area of the shoaling bathymetry from its retreated, calved position. Our
estimate is
We are grateful to the Chinese Arctic and Antarctic Administration, the
European Space Agency for free data supply under project C1F.18243, the
National Snow and Ice Data Center (NSIDC) for the availability of the
ICESat/GLAS data (
This research was supported by the Center for Global Sea Level Change (CSLC) of NYU Abu Dhabi (Grant: G1204), the Open Fund of State Key Laboratory of Remote Sensing Science (Grant: OFSLRSS201414), the National Natural Science Foundation of China (Grant: 41176163), the China Postdoctoral Science Foundation (Grant: 2012M520185, 2013T60077) and Fundamental Research Fund for the Central University. Fruitful discussions with M. Depoorter, P. Morin, T. Scambos and R. Warner, and constructive suggestions from Editor Andreas Vieli and two anonymous reviewers are acknowledged. Edited by: A. Vieli Reviewed by: two anonymous referees