Marine ice formation and deformation at the Southern McMurdo Ice Shelf, Antarctica

Abstract

Marine ice accretes at the base of ice shelves often infilling open structural weaknesses and is thus thought to increase ice shelf stability. However, marine ice formation and deformation processes still remain poorly understood. Through measurements of marine ice properties, this study indirectly infers processes that occur during ice shelf flow and in the ice shelf cavity. Marine ice water isotope and solute chemistry are examined in ice cores from the Southern McMurdo Ice Shelf (SMIS) to derive marine ice source water composition and its origin. Marine ice microstructure (ice fabric, crystals size and shape) is also investigated in ice samples collected along an ice shelf flowline of increasing total strain to establish marine ice deformation in situ and compare it to deformation of ice formed from solid precipitation (meteoric ice).

The measured marine ice water isotope composition together with the output of a boundary-layer freezing model indicate a spatio-temporally varying water source of sea water and relatively fresher water, such as melted meteoric or marine ice. This is in agreement with the occurrence of primarily banded and granular ice crystal facies typical for frazil ice crystals that nucleate in a supercooled mixture of water masses. It is proposed that marine ice exposed at the surface of SMIS, which experiences summer melt, is routed to the ice shelf base via the tide crack. Here frazil crystals nucleate in a double diffusion mechanism of heat and salt between two water masses at their salinity-dependent freezing point and accrete at the ice shelf where they consolidate to marine ice. Recycling of previously formed marine ice facilitates ice shelf selfsustenance with increasing air temperatures.

Marine ice microstructure dynamically recrystallizes as a response to 20 - 25% total shear strain and vertical extension/horizontal compression. The marine ice extracted closer to shore develops a slightly less pointed anisotropic fabric, loses some of its horizontal shape preferred orientations (SPO) (with reference to vertical thin sections). Marine ice also adjusts its microstructure differentially downcore, indicating that it does not deform uniformly but shears in distinct planes. However, there is no evidence that SMIS marine ice deforms more easily than meteoric ice. Even though total strains at the meteoric and marine ice core sites are not equal, annual strain rates are in the order of x10-4 and the different ice types have similar minimum ages (of several thousand years). This makes their microstructural response to strain comparable. Meteoric ice shows stronger circle girdle fabrics, development of a vertical SPO and a decrease in its mean grain size with increasing total vertical extension and shear strain to 20% and 60% respectively downflow. The development of a circle girdle fabric and large increase in total strain downflow at the meteoric ice sites suggests that meteoric ice microstructure is preferentially oriented for horizontal compression as SMIS flows against shore. In contrast the marine ice microstructure is harder to deform in the ambient strain setting. The presence of marine ice thus could thus slow ice shelf dynamics and hence contribute to prolonging ice shelf life.

This study relates ice shelf surface melting to basal marine ice accretion in a coldbased ice shelf cavity and the presence of marine ice to decelerated shelf ice deformation. Thus, knowledge gained in this study contributes to a better assessment of the behaviour of heterogenic ice shelves. In a changing climate, ocean circulation patterns and atmospheric conditions will change and it is important to understand current ice shelf behaviour in order to make sound predictions of their future buffering capability of land ice.