The Larsen B ice shelf in the Northwest Weddell Sea experienced a catastrophic breakout in
early 2002, resulting in an open-water embayment in the region formerly covered by shelf ice,
with remnant ice fronts and exposed tidewater glacier regimes. Oceanographic station data
occupied at the Larsen B ice front in December 2001 and at the remnant fronts in February 2005
are examined for changes in water mass properties which might be related to the ice shelf
breakout of 2002.
Water mass characteristics within the Larsen B embayment are presented as a composite
potential temperature-salinity (θ/S) diagram. The December 2001 (NBP0107, n1-3) and
February 2005 (LMG0502, g1-9) CTD profiles share the common features of a near-freezing
point deep layer overlain by a warmer, fresher seasonal cap. However, there are notable
differences in the seasonal surface layer: in December 2001, the surface salinities were
approximately 34.5, nearly unchanged from the underlying Winter Water (WW) values. This
indicates that little, if any, surface melt water was present at the time, implying that the sea ice
had been advected out of the region prior to the onset of melting. The 2005 data, however,
display a much fresher, and somewhat cooler surface layer, indicative of a substantial
contribution from melting ice. The relative contributions of glacial and sea ice to the surface
melt component are not known.
The 2001 profiles were obtained near the shelf ice front present at the time. There is very little
evidence of significant Ice Shelf Water (ISW) in the 2001 θ/S properties, and no substantial
influence of Modified Warm Deep Water (MWDW, see Foster and Carmack, 1976, for
example).
The 2005 profiles were also obtained near ice fronts, some of which are remmant tidewater
glacial fronts remaining after the 2002 breakout of the Larsen B ice shelf. Station G1 was
occuppied in front of the Larsen C ice shelf, and displays a low salinity variety of ISW (S
=34.45) and some evidence of MWDW. This is consistent with previously reported data from
nearby (Nicholls, et al., 2004).
Stations G2-9 were obtained within the Larsen B embayment. G2 and G8 were obtained at
roughly the same location in front of the remnant of the Larsen B ice shelf lying to the north of
Cape Framnes. G2 displays a significant plume of ISW (S=34.58, σθ~ 27.85), but G8, taken only
6 days later, shows a much reduced ISW component. All of the 2005 stations in the embayment
show some evidence of ISW, but very little if any MWDW. The observed ISW is of
approximately the salinity range (34.55 – 34.60) suggested by Gordon et al. (1993) as a possible
precursor to the low-salinity variety of WSBW observed in the western Weddell Sea.
Several of the 2005 profiles containing evidence of ISW also display a corresponding decrease in
percent light transmission. Profiles from G5 and G6 are shown here as examples - the shaded
area represents that portion of the water column whose potential temperature is less than the
surface freezing point at S=34.5. The corresponding decrease in transmittance is thought to be
due to the presence of glacial debris carried out with the ISW plume. Station G2, which contains
the coldest ISW signal, does not have a corresponding transmittance minimum.
Three CTD stations obtained in front of the 2001 Larsen B ice shelf show little or no evidence of
ISW emanating from beneath the shelf ice, and no component of MWDW. Stations obtained
close to the remnant ice shelf and tidewater glacial fronts in 2005, after the major breakup of
2002, all contain some sub-freezing ISW to varying degrees. In one location where 2 stations
were occuppied 6 days appart, the ISW signal appears to be transitory.
Figure Captions
[θ/S] Potential temperature-salinity (θ/S) diagram for CTD stations obtained in December 2001
(cruise NBP0107, profiles n1-n3, gray) and February 2005 (cruise LMG0502, g1-9, colors).
Locations of the corresponding stations are indicated in the adjacent map. The freezing point
temperature at the surface is shown as a dashed line. Potential density anomaly (relative to the
surface) contours are shown for reference. Water masses are indicated by the abbreviations
defined in the text. The temperature and salinity ranges have been cropped to emphasize deep
water characteristics.
[profiles] Profiles of potential temperature, salinity, and percent light transmittance obtained
with a SBE9 CTD profiler in 2005 from LMG0502. The shaded area highlights that part of the
profile in which the potential temperature is less than the surface freezing point of seawater at
S=34.5, indicative of Ice Shelf Water (ISW).
Ref:
Foster, T.D., and E.C. Carmack, Frontal zone mixing and Antarctic Bottom Water formation in the
southern Weddell Sea, Deep-Sea Res., 23, 301-317, 1976.
Gordon, A.L., B.A. Huber, H.H. Hellmer, and A. Ffield, Deep and bottom water of the Weddell Sea's
western rim, Science, 262, 95-97, 1993.
Nicholls, K.W., C.J. Pudsey, and P. Morris, Summertime water masses off the northern Larsen C Ice
Shelf, Antarctica, Geophysical Research Letters, 31 (L09309), doi:10.1029/2004GL019924, 2004.