Plateleticeandtheland-fastseaiceofMcMurdoSound, Antarctica IngaJ.Smith, 1 PatriciaJ.Langhorne, 1 TimothyG.Haskell, 2 H.JoeTrodahl, 3 RussellFrew, 4 M.RossVennell 5 1 DepartmentofPhysics,UniversityofOtago,Dunedin,NewZealand 2 IndustrialResearchLimited,P.O.Box31-310,LowerHutt,NewZealand 3 SchoolofPhysicsandChemistry,VictoriaUniversity,Wellington,NewZealand 4 DepartmentofChemistry,UniversityofOtago,Dunedin,NewZealand 5 DepartmentofMarineScience,UniversityofOtago,Dunedin,NewZealand ABSTRACT . Dendritic crystals of platelet ice appear beneath the columnar land-fast sea ice of McMurdo Sound, Antarctica. These leaf-like crystals are frozen into place by the advancing columnar growth. The platelets most probably begin to appear during July, although in some parts of the Sound they may not appear at all. In addition, the amount and extent of platelet ice within the Sound varies from year to year. Previous authors have suggested that the formation of platelet ice is linked to the presence of the nearby ice shelf. It is a matter of debate whether these platelets form at depth and then float upwards or whether they grow in slightly supercooled water at the ice/water interface.The phenomenon is similar to that observed in the Weddell Sea region, but previous authors have suggested the two regions may experience different processes.This paper presents the results of field- work conducted in McMurdo Sound in 1999. Ice-structure analysis, isotopic analysis and salinity and temperature measurements near the ice/water interface are presented. Freezing points are calculated, and the possible existence of supercooling is discussed in relation to existing conjectures about the origin of platelets. INTRODUCTION The term platelet ice is used by different authors to mean either the loose platelets underlying a sea-ice sheet, but not yet frozen into place e.g. Dayton,1989), or the incorporated layer consisting of platelets and columnar ice formed between the platelets e.g. Jeffries and others, 1993; Gow and others,1998). In this paper we will discuss both loose and incorporated platelet ice, and have attempted to make this distinction clear in the terminology used if it is not clear from the context. There is strong evidence that platelet-ice formation is linked to the presence of ice shelves. Moretskiy 1965) found that loose billows of platelet ice beneath the fast ice off Mirny station, Antarctica, in late November 1958 were thickest adjacent to the ice shelf. Gow and others 1998) saw ``no systematic variation of [incorporated] platelet thicknesses with location in McMurdo Sound''. Crocker and Wadhams 1989) noted the thickest billows of loose platelets beneath the fast ice of McMurdo Sound in 1986 occurred in regions where Lewis and Perkin 1985) had observed maximal supercooling in 1982. The generally accepted explanation for the supercooling observed is that sea water near its surface freezing point has been trans- ported by currents beneath the ice shelf and then has been cooledbycontact withthe ice shelf Lewis and Perkin,1985). When this water rises to the surface along the underside of the ice shelf, it becomes supercooled due to the change in freezing point with depth Foldvik and Kvinge,1974; Robin, 1979; Crocker and Wadhams, 1989). Of some debate is whether the platelets then form at considerable depths or at the underside of the sea-ice cover.There is evidence for both hypotheses: Dieckmann and others 1986) observed a large cloud of platelets at 250m depth in the Weddell Sea, while Gow and others 1998) state that the orientation of the platelet crystals beneath McMurdo Sound fast ice indicates that the platelets grow attached to the ice above, rather than form- ing at depth. It has been acknowledged Eicken and Lange, 1989) that different processes may cause platelet-ice forma- tion in different regions of Antarctica. Smetacek and others 1992) suggested that platelet ice observed under pack ice could be caused by different processes than those that cause platelet ice observed under fast ice.This possibility was also alluded to by Moretskiy 1965). In this paper the results of field measurements made in McMurdo Sound in 1999 are presented and discussed in relation to consistency with the conjecture that platelet ice in McMurdo Sound forms in situ. LOCATIONS Three sites in McMurdo Sound were studied see Fig.1). 1) Site AS array site) was on the sea ice off Arrival Heights. This site was the location of a thermistor temperature sensor) array, part of an investigation of the thermal con- ductivity of sea ice by one of us H.J.T.). The thermistor- array global positioning system GPS) coordinates were 77³50.197' S, 166³36.764' E. Temperatures were automati- cally logged simultaneously at each depth every 30 min. Thermistors were spaced 100mm apart along the 2m length of the probe.Water salinity andtemperature meas- AnnalsofGlaciology 33 2001 # International Glaciological Society 21