Return to the Table of Contents for this chapter.
One of the unusual aspects of the McMurdo Dry Valley lakes, relative to lakes in other regions of the Earth, is their perennial ice covers. This ice cover is responsible for many of the interesting properties of these lakes (Wharton et al. 1993). One of the obvious effects is the impact on lake dynamics. Simply stated, the ice cover minimizes temperature-driven turnover and wind-driven mixing. Therefore, it has been thought that the McMurdo Dry Valley lakes are characterized by permanent stratification and little to no advective movement. This paradigm has recently been challenged for Lake Fryxell by Miller and Aiken (1996). They have argued that downward mixing of denser water does occur at certain times in Lake Fryxell as evidenced by the presence of tritium at depth. This mixing is thought to occur due to water exclusion from ice formation during fall moat freezing and the sinking of these denser water masses (Miller and Aiken 1996).
As part of a large investigation to understand more fully the source of solutes to the McMurdo Dry Valley lakes, we measured chlorine-36 to chlorine (36Cl/Cl) ratios in the surface and deep waters of all the Taylor Valley lakes (Lyons, Welch, and Sharma in preparation). These measurements were made, in part, to compare data from the 1990s to the earlier work of Carlson et al. (1990) from samples collected in the mid-1980s. These investigators had observed an extremely high value (36Cl:Clx10-15= 1,660±180) in Lake Hoare surface waters that they attributed to glacial melt input from the atmospheric hydrogen bomb testing spike of the late 1950s (Carlson et al. 1990). Our surface water (approximately 5-meter) value collected 10 years later indicated a 36Cl:Clx10-15 ratio of 226±9 (Lyons et al. in preparation). During the 1995-1996 field season, samples were collected at 10 and 12 meters in Lake Hoare. These 36Cl:Clx10-15 values were 262±12 and 226±9, respectively. These more recent data indicate that the bomb spike observed by Carlson et al. (1990) during the 1984-1985 season has since "disappeared."
If we assume a mean increase of water-level rise of approximately 10 centimeters per year over this period (Chinn 1993, p. 1-51), an ice ablation rate of 35 centimeters per year (Clow et al. 1988) over the same period, and no mixing, the bomb spike observed in 1984-1985 should have been displaced from 4-5 meters depth to approximately 9-10 meters depth. (This shift was calculated simply by assuming a 45-centimeter inflow per year; this water displaces downward the "older" water.) Although the 10-meter 36Cl:Cl ratio is higher than water that is currently above it and below it, it is not close to the values observed in 1984-1985.
Although we certainly cannot prove or disprove it with the data presented here, one possibility accounting for the bomb peak's demise is that it has been lost due to mixing and/or sinking. Again, we can only speculate at this time, but density-driven downwelling through moat refreezing as described by Miller and Aiken (1996) for Lake Fryxell may be important for Lake Hoare as well. Recent measurements (S. Tyler and P. Cook, unpublished data) indicate relatively modern chlorofluorocarbon concentrations at depth in Lake Hoare. This finding also suggests "mixing" has occurred. If it turns out that density-driven mixing is a major process in Lakes Fryxell and Hoare, the entire biogeochemical dynamics of these lakes will have to be rethought.
This work was supported by National Science Foundation grant OPP 92-11773.
References
Carlson, C.A., F.M. Phillips, D. Elmore, and H.W. Bently. 1990. Chlorine-36 tracing of salinity sources in Dry Valley of Victoria Land, Antarctica. Geochimica et Cosmochimica Acta, 54, 311-318.
Chinn, T.J. 1993. Physical hydrology of the Dry Valley lakes. In W.J. Green and E.I. Friedmann (Eds.), Physical and biogeochemical processes in antarctic lakes (Antarctic Research Series, Vol. 59). Washington, D.C.: American Geophysical Union.
Clow, G.D., C.P. McKay, G.M. Simmons, Jr., and R.A. Wharton, Jr. 1988. Climatological observations and predicted sublimation rates at Lake Hoare, Antarctica. Journal of Climatology, 1, 715-728.
Lyons, W.B., K.A. Welch, and P. Sharma. In preparation. Chlorine-36 in the waters of the McMurdo Dry Valley lakes, Southern Victoria Land, Antarctica: Revisited. Geochimica et Cosmochimica Acta.
Miller, L.G., and G.R. Aiken. 1996. Effects of glacial meltwater inflows and moat freezing on mixing in an ice-covered antarctic lake as interpreted from stable isotope and tritium distributions. Limnology and Oceanography, 41, 966-976.
Wharton, R.A., Jr., C.P. McKay, G.D. Clow, and D.T. Andersen. 1993. Perennial ice covers and their influence on antarctic lake ecosystems. In W.J. Green and E.I. Friedmann (Eds.), Physical and biogeochemical processes in antarctic lakes (Antarctic Research Series, Vol. 59). Washington, D.C.: American Geophysical Union.