Ross Ice Shelf Sea Temperatures

Abstract

Two temperature profiles recorded by a sensitive bathythermograph at the Ross Ice Shelf Project site (82 degrees 22.5'S, 168 degrees 37.5'W) are presented. From the shape of the profiles it is concluded that an inflow of water at intermediate depths provides a source of heat to drive a regime in which ice is melted from the interface at a depth of 360 meters. Melting maintains the temperature of a thick layer under the ice at about -2.14 degrees C, close to the ambient freezing temperature. A very well mixed layer about 35 meters thick was found at the seabed.

Primary production under the Ross Ice Shelf, Antarctica1

“…The profiles are consistent with the hypothesis that bicarbonate fixation occurs uniformly over the "homogeneous layer" (Gilmour 1979), if bicarbonate fixation is due to the activity of nitrifying bacteria. A sink for nitrate must be present above the homogeneous layer; Gilmour (1979) postulated a current at middepth (above 195 m) which could act as a sink for NO,-. The levels of NO,-are low (~0.02 lugatoms * liter-') and constant through the water column.…”

“…In addition, this model is consistent with the possibility of temperature limitation of nitrifying bacteria; the temperature profile (Gilmour 1979) shows an inverse thermocline with temperatures immediately below the ice of -2.13" to -2.16"C, close to the freezing point of seawater. Detailed temperature studies would be needed to determine if the 0.03"C temperature difference between the water below the ice and that in the homogeneous layer affects the activity of nitrifiers.…”

“…Bicarbonate was fixed in samples taken from 200 m. The average of the triplicate incubations from the van Dorn bottle samples was 0.11 mg Cm rnp3 * d-l; that from the pumped samples was 0.09 (Table 1). Gilmour (1979), using temperaturedepth profile data, found a layer of homogeneous water below 195 m which he suggested is exceptionally well mixed. As a starting point, I assumed that bicarbonate fixation occurs uniformly over the homogeneous layer defined by Gilmour, from 195 m below the ice to the sediments at 237 m. Thus I can calculate an estimate of primary production in the water column: Nutrients were determined in 250-ml or about 1.5 g C*m-'* yr-l, if production aliquots of the sample, filtered through is constant throughout the year.…”

Estimation of Ice Shelf Melt Rate in the Presence of a Thermohaline Staircase

“…Previous direct oceanographic measurements beneath ice shelves suggest that the flow in the outer layer is fully turbulent as a result of the large-scale circulation, modulated by tidal motion (e.g., Gilmour 1979;Jacobs et al 1979;Nicholls et al 1991;Nicholls and Jenkins 1993). When the far field is sufficiently energetic, a parameterization based on Kader and Yaglom (1972) is commonly used to represent the heat and salt fluxes within the viscous and inertial sublayers to estimate the melt rate (e.g., Holland and Jenkins 1999;McPhee 2008).…”

“…The present George VI Ice Shelf may thus represent an ice shelf that is in the process of disintegrating from the surface (Vaughan and Doake 1996) as well as from rapid basal melting by the intrusion of CDW (Bishop and Walton 1981;Talbot 1988). The majority of measurements beneath ice shelves have been made in the ice shelf cavities flooded with a cold water mass (temperature near the surface freezing point), for example, the Filchner-Ronne (Nicholls et al 1991;Nicholls and Jenkins 1993;Nicholls et al 1997), Fimbul (Orheim et al 1990;Hattermann et al 2012), Larsen C (Nicholls et al 2012), and Ross (Gilmour 1979;Jacobs et al 1979;Arzeno et al 2014;Robinson et al 2014) ice shelves. Robin (1979) argues that thermohaline forcing, rooted in the depth dependence of the freezing point of seawater coupled with the exchange of heat and salt at the ice shelf-ocean interface, generates thermohaline convection within these colder subice shelf cavities.…”