Ice-shelf – ocean interactions at Fimbul Ice Shelf, Antarctica from oxygen isotope ratio measurements

K J Heywood,K W Nicholls,M R Price

doi:10.5194/os-4-89-2008

Abstract

Abstract. Melt water from the floating ice shelves at the margins of the southeastern Weddell Sea makes a significant contribution to the fresh water budget of the region. In February 2005 a multi-institution team conducted an oceanographic campaign at Fimbul Ice Shelf on the Greenwich Meridian as part of the Autosub Under Ice programme. This included a mission of the autonomous submarine Autosub 25 km into the cavity beneath Fimbul Ice Shelf, and a number of ship-based hydrographic sections on the continental shelf and adjacent to the ice shelf front. The measurements reveal two significant sources of glacial melt water at Fimbul Ice Shelf: the main cavity under the ice shelf and an ice tongue, Trolltunga, that protrudes from the main ice front and out over the continental slope into deep water. Glacial melt water is concentrated in a 200 m thick Ice Shelf Water (ISW) layer below the base of the ice shelf at 150–200 m, with a maximum glacial melt concentration of up to 1.16%. Some glacial melt is found throughout the water column, and much of this is from sources other than Fimbul Ice Shelf. However, at least 0.2% of the water in the ISW layer cannot be accounted for by other processes and must have been contributed by the ice shelf. Just downstream of Fimbul Ice Shelf we observe locally created ISW mixing out across the continental slope. The ISW formed here is much less dense than that formed in the southwest Weddell Sea, and will ultimately contribute a freshening (and reduction in δ18O) to the upper 100–150 m of the water column in the southeast Weddell Sea.

Highlights

Floating ice shelves at the periphery of Antarctica play an important role in the water mass transformations that take place there
The wide continental shelf provides a source of cold and saline High Salinity Shelf Water for the extensive under-ice cavity, where it is transformed into Ice Shelf Water (ISW)
The ISW forms an important component of the subsequent water mass mixture that is dense enough to descend the continental slope into the deep layers of the Weddell Sea

Summary

Introduction

Floating ice shelves at the periphery of Antarctica play an important role in the water mass transformations that take place there. They find a flow of relatively warm water (some warmer than 0◦C, though the average modelled cavity temperature is −1◦C) across the sill into the main cavity, driving a typical sub-ice-shelf thermohaline plume at the base of the ice shelf (Grosfeld et al, 1997) The ingress of such warm water gives a modelled maximum basal melt rate of greater than 10 m per year, with an average of 1.9 m per year. Waters warmer than 0◦C are found as shallow as 200 m less than 100 km offshore at the Greenwich meridian, isotherms deepen markedly as they approach the coast, reflecting the baroclinic westward flow associated with the Antarctic Slope Front/Coastal Current system (Heywood et al, 1998) By the time it intersects the bathymetry, the 0◦C isotherm in this area is typically found at a depth of around 700 m, below the sill at the entrance to the Fimbul cavity. The consequence is that high latitude meteoric water, and especially glacial ice, is strongly depleted in 18O, which makes oxygen isotopes an excellent tracer of water of glacial origin

Data and methods

Hydrographic properties

Oxygen isotope ratios

Findings

Discussion and conclusions