Present and Future Stability of Larsen C ice shelf (SOLIS)

Abstract

In partial fulfilment of the NERC-AFI grant SOLIS (Present and Future Stability of the Larsen C Ice Shelf, NE/E012914/1) we conducted a major field season on the Larsen C ice shelf, Antarctic Peninsula, in the austral summer of 2008-09. This is a joint report for the three GEF loans that supported our fieldwork, which include ground-penetrating radar (GPR, Sensors & Software PE Pro), differential GPS (Leica Geosystems 1200) and passive seismic (SAQS) equipment. We are submitting a joint report for the three loans not only to avoid much repetition because all three types of data are intimately related to each other, but also because our time in the field was significantly curtailed (from ~ 2 months to ~ 3 weeks) due to logistical problems with BAS aircraft in that field season. We were therefore forced to place a focus on our planned multi-component seismic reflection surveys (not reported here since they used equipment owned by Swansea), and were only able to collect skeleton GPR (~ 50 km of skidoo-towed common-offset data, and multiazimuth (at 30º increments) common-midpoint (CMP) data in two locations), GPS (2 receivers) and passive seismic data (1 SAQS station). Nonetheless, these skeleton data proved to be highly valuable for our AFI project, revealing the presence of a large body of marine ice on the underside of a thin flow stripe in the southern part of the Larsen C ice shelf (GPR). Since the ice-flow rates of this thin stripe are statistically the same as those of the surrounding ice (GPS), the flow stripe is not mechanically decoupled from the surrounding ice. In conjunction with our seismic reflection data we can therefore confirm the key hypothesis of our AFI project, concluding that structurally and mechanically heterogeneous ice critically affects rates of rift propagation (observed to effectively halt when rift tips arrive at the thin flow stripe) on the Larsen C ice shelf, and thus ultimately also its stability (which is quantitatively supported by continuum-mechanical ice-flow and fracture mechanics modelling). Our SAQS passive seismic data are virtually devoid of noticeable events, which qualitatively agrees with the notion that rift propagation has virtually halted at the flow stripe owing to the presence of the marine ice body. Preliminary analyses tentatively suggest that crystal anisotropy, as inferred from our the seismic and radar CMP data, does not modulate ice mechanical heterogeneity. In contrast, we can reject the hypothesis that critical slowing down of rift propagation at the thin flow stripe is caused by mechanical decoupling between this stripe and surrounding ice. Our findings are therefore likely to have far-reaching consequences for Pan-Antarctic ice-shelf stabilities.