Abstract
Salt deposits are widely distributed across the Earth’s surface, and form via evaporation and freezing of surface or near surface brines. Accumulations of almost pure sodium sulphate deposits are located in the ice cored moraine between the terminus of Hobbs Glacier and McMurdo Sound, Antarctica. Although several investigations have attempted to determine brine origin and salt deposition, specific processes remain poorly understood. Understanding the origin and formation of salt deposits in this environment can reveal important information about past climate, hydrological and glaciological processes and environmental conditions, and form a foundation for understanding the interaction between polar glaciers and salt enriched substrate. This study has employed a multi-disciplinary approach to investigate the salt deposits and surrounding ice-cored moraine. Geochemical and geological analyses were conducted on ice and mineral samples extracted from a transect that extended through a deformed salt deposit near Hobbs Glacier terminus. XRD identified the primary mineral as sodium sulphate deca-hydrate (Na2SO4.10H2O; mirabilite), and solute analyses identified a distinct transition along the sampling transect between geochemical systems (A and B). A significant increase in SO42- and Na+ concentrations and significant decrease in Cl-, Mg2+, K+ and Br- concentration occurred at a transition point. Stable isotope analyses produced a co-isotopic relationship of δD = 8 δ18O - 2.1 (n=30, R2=0.99), which plots slightly below the standard meteoric water line (MWL) and close to the local MWL. Values of -67 ‰ to +15 ‰ (δD) and -8 ‰ to +2 ‰ (δ18O) were interpreted to originate from a marine source, whereas ice enclosed within the mirabilite outcrop (δD -249 ‰; δ18O -31 ‰) was interpreted to originate from a meteoric source. Interpretations of solute and isotopic data indicate a seawater source for ice surrounding the mirabilite deposit in comparison to involvement of meteoric water during salt deposit formation. Evaluation of previous research in conjunction with results from this investigation has allowed for development of an original depositional model. In this model, concentrated brines and mirabilite deposits formed through cryo-concentration in depressions and hydrological channels during development of the Ross Ice Shelf > 47,000 years ago. Following grounding of the RIS along the western McMurdo Sound c. 40,000 years ago, the stagnant ice has been reworked and ablated to reveal extensive deposits of mirabilite. The Hobbs Glacier began advancing around 12,000 years before present and currently overrides the region of salt enriched ice-cored moraine, interacting to deform the permafrost and mirabilite deposits.