Figure 10 . Stress-flow angles for non-rift scenario (a ), rift-scenario 1 (b ), and rift-scenario 2 (c , enhanced weakening). First principal stress field for non-rift scenario (d ), and differences between rift-scenario 1 and non-rift scenario (e ), and between rift-scenario 2 and rift-scenario 1 (f ). The extent of R1 is shown (dashed line) with the rift tip locations between 2006 and 2017.
6 Discussion
The multidecadal satellite data and modeling experiments indicate the primary role of rifting processes in controlling the retreat of Larsen C over the past six decades. The two retreat cycles were characterized by tabular iceberg calving due to rift propagation. The rifts leading to calving developed near the Bawden Ice Rise in the north and the Gipps Ice Rise in the south. The time series of satellite data allowed us to analyze the rifting processes causing the 2017 calving event in a high level of detail. The 2017 calving event resulted from the propagation of two rifts, including a transverse rift (R1) formed downstream of Kenyon Peninsula and an oblique rift (R2) formed near Gipps Ice Rise. The observed flow accelerations were mainly related to the propagation of these two rifts. Enhanced weakening due to R2 near Gipps Ice Rise increased both longitudinal and first principal stresses around R1, and modified the stress-flow angles, creating favorable conditions for the northward propagation of R1. This indicates the importance of considering feedbacks between different rifts. Several studies (e.g., Borstad et al., 2017; Jansen et al., 2015) discussed the rift development causing the 2017 calving event, but treated R1 and R2 as one single rift and mainly focused on the northward propagation. Our results suggest that the southward propagation of both R1 and R2 preconditioned the northward propagation.
According to linear elastic fracture mechanics (LEFM), a fracture propagates when the stress intensity factor at a crack tip exceeds the material fracture toughness (Hulbe et al., 2010; Lipovsky, 2020). At the fracture tip, the stress intensity factor is determined by fracture geometry, fracture length, and stress field. The stress intensity factor has three modes: Mode I–opening (tensile), Mode II–sliding (in-plane shear), and Mode III–tearing (antiplane shear). The propagation of R1 was dominated by Mode I. Fracture propagation continues until the stress intensity factor is less than the fracture toughness. Suture zones are suggested to have resistive effect on rift propagation because of the high fracture toughness of marine ice underneath (Borstad et al., 2017; Kulessa et al., 2019; McGrath et al., 2014). Marine ice has different material properties from meteoric ice originating from glacial ice, and was detected underneath the Larsen C suture zones (Holland et al., 2009; Jansen et al., 2013). Assuming that the spatial distribution of fracture toughness is fixed, the increased tensile stresses at the crack tips of R1 are necessary conditions to activate the propagation of R1. The modeling results indicate the R2-induced weakening near Gipps Ice Rise caused the enhanced tensile stresses around R1, highlighting the importance of rift features around ice rises. Meanwhile, reduced fracture toughness of suture zones due to ice shelf thinning should also be considered. Widespread thinning has been observed over Antarctic ice shelves from satellite altimetry data (Adusumilli et al., 2020), including Larsen C (Adusumilli et al., 2018; Shepherd et al., 2003). The surface lowering of Larsen C has been attributed to multiple factors, including firn compaction, ice divergence due to flow acceleration, and basal melt (Holland et al., 2015; Shepherd et al., 2003). A high basal melt rate was observed near Bawden Ice Rise (Adusumilli et al., 2018). Ocean-driven melting underneath Larsen C can enhance the rifting processes by 1) reducing marine ice underneath the suture zones, decreasing the fracture toughness of suture zones; 2) destabilizing the ice rises and reducing the buttressing force from ice rises (Borstad et al., 2013); and 3) thinning and weakening shear margins (Alley et al., 2019; Dow et al., 2018; Lhermitte et al., 2020).
To better predict ice shelf retreat due to rifts, we not only need to know whether a rift would propagate until calving occurs, but also where rifts tend to initiate. Rifts initiate at preexisting cracks or flaws (Hulbe et al., 2010). A recent study based on ICESat-2 data (Wang et al., 2021) found that the vertical depth of preexisting fracture features is critical for predicting rift locations. Theoretical modeling studies (e.g., Lipovsky 2020) suggest that the loss of marginal strength, when ice flows out of an embayment, can trigger rift growth. Our modeling results (Figure 8) suggest that the transitional area from a compressive (longitudinal) condition to a tensile (longitudinal) condition resulting from ice shelf geometry conditions is also a potential location for rift initiation and opening. There were three such transitional areas at the downstream portion of Larsen C, including the area near Bawden Ice Rise, the area near Gipps Ice Rise, and the area downstream of Kenyon Peninsula. Satellite images showed that the rifts causing the northern and southern calving events were initiated in these areas. The area downstream of Kenyon Peninsula is a typical transitional area from strong to weak shear margins, consistent with the modeling study of Lipovsky (2020). The transitional areas can be identified through diagnostic modeling given a front geometry condition without considering ice shelf damage. Once a rift is initiated, the rift-induced weakening must be incorporated to examine whether the rift will propagate in response to the modification of stress field. The weakening is enhanced when the rift growth continues, further altering the stress field. Therefore, it is important to capture the time-varying effect of rifts on ice rigidity in ice flow models. Capturing such feedbacks appears to be essential for modeling flow velocity fields and subsequent rift propagation. Continuum damage mechanics (Borstad et al., 2012, 2013; Krug et al., 2014) have been recently utilized to incorporate fracture-induced weakening into a viscous continuum damage model. The damage factor is a scalar variable that is quantified by an inversion (ice rigidity) approach. In this case, damage-related flow velocities are required to invert for the damage factor. Further investigation into the temporal evolution of rift length and associated damage is needed to enable parameterization of these processes within a modeling framework.
The velocity data indicate that the impact of the 2017 calving event is limited on the ice shelf flow dynamics. The calved portion was located in the “passive ice” zone (Fürst et al., 2016; Reese et al., 2017), where the front retreat has little effect on ice shelf buttressing. We observed flow accelerations in the central ice shelf after 2000, which might be a velocity adjustment to the front geometry change resulting from the calving events before 1990. The modeling experiments (Figures 7c, 7k) show the front geometry change due to the 2017 calving would increase the velocity (although at a low magnitude) and decrease the backstress in the central ice shelf, suggesting a potential time-lag between front retreat and ice shelf flow acceleration. If the retreat continues and the ice front reaches the compressive arch, widespread flow accelerations will be triggered due to front retreat. The observed flow accelerations were mainly caused by the rift development in the south. To predict future ice shelf flow dynamics due to front retreat, rift initiation and propagation and associated weakening effect on ice rigidity should be considered for modeling flow velocity fields.
Following the disintegration of Larsen A and Larsen B, one pressing question is whether Larsen C will disintegrate in the same manner. The collapses of Larsen A and Larsen B were preceded by extensive surface melt, and hydrofracturing has been the prevailing hypothesis for explaining the ice shelf collapse (e.g., van den Broeke, 2005; MacAyeal et al., 2003). Abundant meltwater, surface fractures, and favorable stress conditions are the necessary elements of hydrofracturing. Lai et al. (2020) applied LEFM and assessed the vulnerability to hydrofracturing across all Antarctic ice shelves, in which Larsen C was classified as vulnerable to hydrofracturing. However, it is still questionable whether there will be sufficient meltwater to fill the crevasses on Larsen C. Although several studies indicated that foehn wind enhances the surface melt rate over Larsen C (Datta et al., 2019; Elvidge et al., 2020; King et al., 2017), the extent of ponded water is limited in space, mostly occurring at the northern upstream portion at the Cabinet Inlet (C3 unit). Besides excessive melt occurred on Larsen A and Larsen B, their pre-collapse front geometries were already in unstable conditions after the compressive arch was reached (Doake et al., 1998). Larsen C will resemble the pre-collapse Larsen B if the front geometry reaches the position of H3 and the compressive arch is absent (Figure 6). The disintegration of Larsen C is less likely to occur in the current condition. However, given the importance of rifting processes to the stability of Larsen C, it is critical to monitor how atmospheric warming and oceanic forcing will affect rifting processes in the future.
7 Conclusions
We combined multi-source satellite data over the past six decades to investigate changes occuring over the Larsen C ice shelf. The total ice shelf area was reduced by more than 20% from 1963 to 2020, as a result of two large retreat cycles. Each cycle was characterized by a northern retreat near Bawden Ice Rise followed by a southern retreat near Gipps Ice Rise, in which rifting-induced tabular iceberg calving was the dominant mechanism. We analyzed the temporal evolutions of two rifts that caused the 2017 calving event in the south, and found their close interactions in weakening the ice shelf. In particular, the weakening due to R2 near Gipps Ice Rise preconditioned the rapid propagation of the major rift R1. We observed a lagged response of flow acceleration to front retreat, which leads to our conjecture that there will be a gradual velocity increase in response to the 2017 calving event, despite relatively small recent changes. The ice shelf retreat away from pinning points (ice rises) to the location of compressive arch will reduce the ice shelf backstress and lead to widespread flow accelerations. When the compressive arch ceases to exist, small retreat events can trigger large-scale backstress loss and flow accelerations extending to the upstream glaciers.
The results of this study in combination with observations of the pre-collapsed Larsen B suggest a chronology of ice shelf destabilization processes for embayment-confined ice shelves. Rifts initiate at preexisting cracks and/or areas with favorable stress conditions, such as the transitional area from compressive to tensile longitudinal conditions. Ice rises and locations with a transition from strong lateral shear margins to weak margins (e.g., occurring near an island or peninsula) generally have such characteristic stress patterns. Rifts would propagate at these transition areas, if the stress intensity at the crack tips exceeds the fracture toughness. Rift growth leads to mechanical weakening of ice and a change in ice rigidity property. The weakening, particularly near ice rises and/or shear margins, results in flow accelerations, further modifies the stress field by increasing longitudinal stretching and decreasing stress-flow angles, hence creates more favorable conditions for rift propagation, and eventually causes ice front retreat. If the front retreat occurs within the “passive ice” area and does not extend to the compressive arch, a limited or lagged response in flow velocity and stress field would be triggered. If the front retreats behind the compressive arch, widespread flow accelerations and backstress loss would be triggered, causing dynamic thinning and stress field change across the ice shelf. The compressive arch location may move further upstream or become fully absent. The absence of a compressive arch would create an unstable front condition, followed by persistent front retreat. If surface melting and ponding are prevalent due to atmospheric warming, hydrofracturing can destabilize the ice shelf rapidly. Oceanic forcing can affect this process by thinning shear margins and reducing marine ice underneath suture zones, both of which lower the fracture toughness and therefore enhance weakening and fracturing. Our results emphasize the need for capturing key processes in ice shelf evolution models. In particular, the response of the ice shelf mechanical strength to the presence of rifts and the feedbacks that can accelerate ice shelf retreat need to be incorporated in the models.
Acknowledgments
This research was supported in part by a Seed Grant award from the Institute for Computational and Data Sciences at the Pennsylvania State University. Part of the work was supported by the University of Cincinnati Graduate School Dean’s Fellowship When S. Wang was a graduate student. The authors acknowledge the United States Geological Survey (USGS) for the Declassified Intelligence Satellite Photography and Landsat images, the Alaska Satellite Facility for the ALOS PALSAR images, the European Space Agency for the ERS-1/2 and Envisat SAR images, the Byrd Polar and Climate Research Center at Ohio State University for the orthorectified Radarsat-1 SAR images and other RAMP data products, the National Snow and Ice Data Center for the Antarctic DEM data and the MEaSUREs phase-based Antarctica ice velocity data, and the Jet Propulsion Laboratory and University of California at Irvine for the Ice-sheet and Sea-level System Model (ISSM).
There are no restrictions to access the data used for this study. The DISP imagery (https://doi.org/10.5066/F78P5XZM), Landsat-4/5 TM (https://doi.org/10.5066/F7N015TQ), Landsat-7 ETM+ (https://doi.org/10.5066/F7WH2P8G), and Landsat-8 OLI (https://doi.org/10.5066/F71835S6) images were downloaded from the USGS EarthExplorer data portal (https://earthexplorer.usgs.gov/), courtesy of the U.S. Geological Survey. The ALOS PALSAR images (© JAXA/METI ALOS PALSAR L1.5 2006-2010. Accessed through ASF DAAC 1 February 2014) were downloaded from the Alaska Satellite Facility data portal (https://asf.alaska.edu/). The Radarsat Antarctic Mapping Project (RAMP) data (https://doi.org/10.5067/HHK2QT3LQMEL) are availabe at https://asf.alaska.edu/data-sets/derived-data-sets/ramp/ramp-get-ramp-data/. ERS-1/2 SAR images were assessed from https://earth.esa.int/eogateway/catalog/ers-1-2-sar-im-l0-sar_im__0p- , and Envisat ASAR images were accessed from https://earth.esa.int/eogateway/catalog/envisat-asar-im-precision-l1-asa_imp_1p-, courtesy of European Space Agency. The DEM data were accessed from the National Snow and Ice Data Center (https://nsidc.org/data/nsidc-0082 and https://nsidc.org/data/NSIDC-0422/versions/1) (Liu et al., 2001; Bamber et al., 2009). The MEaSUREs phase-based Antarctica ice velocity data are availabe at https://nsidc.org/data/NSIDC-0754/versions/1 (Mouginot et al., 2019b). The Ice-sheet and Sea-level System Model (ISSM) is open source and publicly available at https://issm.jpl.nasa.gov/ (Larour et al., 2012).
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