Ice-core studies in the western Weddell Sea (Nathaniel B. Palmer 92-2)

In this report, we give some examples of the physical properties of the ice cores obtained in the western Weddell Sea and compare them to previous work in the area (Cow et al. 1987). Ice stations were occupied while transiting from the ice edge to the Ice Station Weddell #1 and during the return trip to the ice edge (21 May to 22 June 1992). A total of 15 stations were made, including two stations where first-year lead ice was sampled in addition to the thicker ice beside the ship. The other studies performed are outlined in Ackley etal. 1992 and described more fully in other accompanying articles. Locations of sampling sites are shown in figure 1 of Ackley et al. 1992. All sampling was conducted in the immediate vicinity of the ship, generally within 100 meters of the starboard side of the vessel. On the inbound leg ice stations were made either before or immediately following the oceanographic conductivity-temperature-depth (CTD) casts (Gordon et al. 1992). On the return trip ice sampling was conducted simultaneously with the CTD measurements. Ice sampling was accomplished by core drilling, using both 7.5- and 10.2-centimeter diameter coring augers mounted on Jiffy gasoline engines. Four cores were taken through the entire thickness of ice at each station, including a single 7.5-centimeter core dedicated to temperature, salinity, and biochemical profile measurements. The three additional 10.2-centimeter cores were

Ice-core studies in the western Weddell Sea (Nathaniel B. Palmer 92-2) A.J. Gow AND S.F. ACKLEY USA CRREL Hanover, New Hampshire 03755

V.I. LYTLE

Dartmouth College Hanover, New Hampshire 03755

D. BELL

Naval Research Lab Hanover, New Hampshire 03755

0

Salinity (%o) 4 8

17

17 cm Snow

0.2 Site 1 0.4

0.6 Temperature (°C) I

0

Aligned C Axes

Salinity (%o) 4 8 12

43 cm Snow

I I I

0.80 0.84 0.88 0.92 Density (Mg/M3) Salinity (%) 0 4 8 12

Snow

0. Brown Algae

E Cl) C

Site 11

0

10.2

Site 1 .2 o. Site (D

CO C,)

a

C 0

.c Fa 0

0.4 0. 0.6 -16 -8 0 Temperature (°C) 0.82 0.86 0.90 0.94 Density (Mg/m3)

-ID

gned C Axes

-0

V

Temperature (°C) I I I I I I 0.84 0.88 0.92 0.96 Density (Mg/M3)

Figure 1. Physical property profiles [temperature (T), salinity (S), density (D), and structure] vs. depth of Ice cores from sites 1, 3, and 11. The symbols C and G beside each structure profile denote columnar and granular ice respectively.

1992 REVIEW

89



used as follows: one core for a detailed physical and structural property study conducted aboard ship; a second core dedicated specifically to biological analyses; and a third core that was returned to the Cold Regions Research and Engineering laboratory (CRREL) for dielectric and related physical property measurements. A suite of meltwater samples from each station was also retrieved and returned to CRREL for detailed stable isotope and sea salt composition studies. Structural studies conducted aboard ship included vertical sectioning of each core to determine the major ice crystal textures present, horizontal thin section examination of crystalline structure and c-axis alignment in the ice, and density measurements for evaluating air porosities in conjunction with brine volumes. Examples of such profile studies of representative ice types from three sampling sites (sites 1, 3, and 11) are given in figure 1. Ice at site 1 consisted almost entirely of columnar-congelation ice overlaid by 17 centimeters of snow. Such a structural make-up is consistent with ice growth under relatively quiet conditions. Columnar growth had proceeded to the point where a strong alignment of c-axes had developed, most probably in response to persistent strong relative motion at the ice-water interface. Ice from site 3 also consisted mainly of columnar-type ice with aligned c-axes but it also contained three layers of granular frazil ice sandwiched between thin layers of columnar ice indicative of at least two separate rafting events prior to stabilization of the ice cover, followed by continued growth of congelation type ice. Site 11 consisted almost entirely of granular ice overlaid by 43 centimeters of snow. At midlevel this ice contained a prominent layer of brown algae. The inclined nature of this layer together with the abundance of frazil ice would suggest that the formation of ice at this particular site was associated with pressure ridging. Substantially linear temperature profiles were observed at the three sites; bulk salinities all exceeded 5 ppt. Ice crystal texture relationships at the 15 sampling sites are shown in figure 2. Twelve of the 17 ice types sampled contained columnar ice in excess of 50 percent of the total ice thickness with the highest columnar ice percentages generally occurring in the thinner ice. On the basis of total length of ice core recovered, however, granular frazil was present in about equal amounts with columnar-congelation ice. This ratio is very similar to that observed by Gow et al. 1987 in pack ice sampled during February and March 1980 at nearly the same geographical locations we sampled for this study. These observations again reinforce arguments that frazil ice, indicative of turbulent growth conditions, is much more prevalent in Weddell Sea pack ice than in the Arctic Ocean (Gow and Tucker 1991). A plot of bulk salinities versus ice thickness at the 15 sampling sites is shown in figure 3. In general, the salinities are highest in the thinner ice types. On the basis of observations made so far it would appear that we encountered three main kinds of ice: 1) young ice (less than 60 centimeters thick) that had formed during the current winter season; 2) ice that had formed in the 1991 winter, and had survived the 1991-1992 summer; and 3) older, thicker ice that had persisted through two separate winters and was entering a third winter.

90

0- 100 S •5

20- 80

• •

60

•s

co

Co

•

80— 20 I • I I I. 8 12 16 20 24 Ice Thickness (m)

100—

Figure 2. Percent of columnar or granular structure vs. ice thickness for the cores from the 15 sampling sites. 10 S

- 8-

• •.

Cs CI)

a) c,) 4a) >

S..

.

•

.

. .

S

I I I

0 4 8 12 16 20 Ice Thickness (m) Figure 3. Average or bulk salinities vs. Ice thickness for the 1 sampling sites.

This third type of ice was encountered near the ice edge an4 would most likely be advected out and melted before the end the current winter. This research was supported by National Science Foundatior grant DPP 90-24809. We are indebted to the crew of Nathaniel B4 Palmer for their support of this project. We also gratefully ac knowledge the untiring support of Antarctic Support Associates in all aspects of this research. References

Ackley, S. F., A. J . Cow, V. I. Lytle, M. N. Darling, and N. E. Yankie1un

1992. Sea-ice investigations on Nathaniel B. Palmer: Cruise 92-2.Antarc1 tic Journal of the U.S., this issue. Gordon, A., B. Huber, and D. L. Martinson. 1992. Physical oceanography studies on Akademic Federov and on Nathaniel B. Palmer, 92-1 an 92-2. Antarctic Journal of the U.S., this issue. I Cow, A. J. and W. B. Tucker 111.1991. Physical and dynamic properties o sea ice in the Polar oceans. CRREL Monograph 91-1. Cow, A. J., S. F. Ackley, K. R. Buck, and K. M. Golden. 1987. Physical an

structural characteristics of Weddell sea pack ice. CREEL Report 87-14

Arrr.&iciic JOURNAL