Radiogenic Halogen Isotopes in Cold Places: Deep Seeps or Recent Salt Dissolution?

Saline waters of the Dry Valley lakes in Antarctica yield ³⁶Cl and ¹²⁹I values that are similar to those of brines generally found in deep basins. In this investigation, we compare high-resolution isotopic depth profiles of lake waters in the McMurdo Dry Valleys with their potential brine sources. We focus on water sampled at two sites proximal to McMurdo Sound, Garwood Valley and Lake Fryxell, as well as two more distal sites situated at greater elevation, Lakes Joyce and Vanda. Deep waters in the lakes show co-produced radiogenic ⁴He, ³⁶Cl and ¹²⁹I. Shallow lake waters show varying degrees of active exchange between surficial and atmospheric sources. In some cases, dissolution of cryogenic salts by meltwaters, degassing of helium in the seasonally uncovered portions of the lakes, and redox reactions involving iodine effectively decouple these three isotopic systems in the shallow waters. At other study locations around the world, the fracture fluid brines of basement rocks--particularly those with low isotopic ratios of ³⁶Cl/Cl and ¹²⁹I/I -- have been conventionally attributed to subsurface seepage from adjacent basin waters. Consequently, it is implied that the residence time of these brines is on the order of tens of millions of years. We compare the data from the Antarctic Dry Valleys lakes, where residence times of the surface waters are likely much shorter and discuss the influence that the active formation of cryogenic brines and ongoing salt dissolution may have on the present halogen isotopic signatures elsewhere.