Abstract
AbstractThe crystal orientation fabric of ice reflects its flow history, information which is required to better constrain projections of future ice sheet behavior. Here we present a novel combination of polarimetric phase‐sensitive radar and seismic anisotropy measurements to provide independent and consistent constraints on ice fabric at Korff Ice Rise, within the Weddell Sea sector of West Antarctica. The nature and depth distribution of fabric in the ice column is constrained using the azimuthal variation in (1) the received power anomaly and phase difference of polarimetric vertical radar soundings and (2) seismic velocities and shear wave splitting measurements. Radar and seismic observations are modeled separately to determine the nature and strength of fabric within the ice column. Both methods indicate ice fabric above 200‐m depth which is consistent with present‐day ice‐divide flow. However, both measurements also indicate an oblique girdle fabric below 230‐m depth within the ice column, inconsistent with steady state divide flow. Our interpretation is that this deeper fabric is a remnant fabric from a previous episode of flow, which is currently being overwritten by ongoing fabric development associated with the present‐day flow regime. The preexisting fabric is consistent with ice flow from the south prior to ice‐divide formation, in agreement with models of Holocene ice sheet evolution. These findings apply new constraints to the flow history at Korff Ice Rise prior to divide formation and demonstrate the capacity of radar and seismic measurements to map fabric and thus constrain past ice flow.
Highlights
Knowledge of the past behavior of the West Antarctic Ice Sheet is key to evaluating future projections of sea level rise with predictive ice sheet models (e.g., DeConto & Pollard, 2016)
A number of qualitative observations indicate the presence of fabric within the ice column: azimuthal variation in radar return power and phase difference, misfit between isotropic synthetic seismic travel times and observations at long offsets (i.e., nonhyperbolic moveout (Baan & Kendall, 2002)), significant energy on the transverse component of the converted P to S phase, a delayed arrival on the transverse component of the S phase, and azimuthal variation in travel times of both direct and reflected seismic energy
A more complex relationship is observed in the phase difference (Figure 4b), making variation with depth and azimuth of phase difference diagnostic of both fabric orientation and strength, especially when the power anomaly signal from scattering anisotropy dominates and masks the signal resulting from birefringence.Following the matrix‐based method of Fujita et al (2006), we explored a range of fabric models to determine the configuration which fits the a phase‐sensitive frequency modulated continuous‐wave radar (ApRES) data
Summary
Knowledge of the past behavior of the West Antarctic Ice Sheet is key to evaluating future projections of sea level rise with predictive ice sheet models (e.g., DeConto & Pollard, 2016). Post‐LGM retreat of the grounding line to inland of its current position, followed by more recent advance, has been proposed by a number of authors (Bradley et al, 2015; Kingslake et al, 2018; Siegert et al, 2013) This sequence, corroborated by ice sheet modeling (Kingslake et al, 2018), potentially explains the presence of ice streams currently resting on reverse bed slopes along the grounding line of the Weddell Sea, which otherwise raise questions of how the ice sheet retreat could have stabilized in this configuration (Bradley et al, 2015). These studies provide a data set in the Weddell Sea that, sparse, provides a glimpse of a flow history perhaps more complex than a simple monotonic retreat from LGM to present
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