| Summary: | The Dry Valley region of Victoria Land is the largest ice-free area in Antarctica. Within the frigid arid environment of the Dry Valleys, where the mean annual air temperature is -20℃, there are several permanently ice-covered, amictic, saline lakes occupying undrained bedrock depressions. Three of these lakes, namely Lakes Vanda, Bonney and Joyce, have been studied with the aim of determining the nature of their bottom sediments and relating the stratigraphy of bottom sediment cores to Holocene climatic fluctuations. These lakes have an area of 0.8 to 5.2 km², are from 35 to 68m in maximum depth and have an ice-cover 3 to 4m thick. The lakes are chemically and thermally stratified and receive their water from meltwater streams draining the local glaciers. The bottom sediments consist of detrital sands and silts, chemical precipitates and organic material. The detrital sediments consist of feldspar and quartz with smaller quantities of hornblende, augite, hypersthene and mica that are derived locally from the rocks exposed on the adjacent valley sides. These sediments are mainly wind-transported or, to a lesser extent, river-transported into the lakes. The wind derived sediments are either blown onto the lake-ice, where they eventually sink to the lake-floor, or they are blown into the moat developed about the shores of the lake in summer. The chemical precipitates consist mainly of gypsum, halite, aragonite and calcite whose constituent elements were derived from meltwater streams discharging into the lakes; however Lake Bonney also received dissolved solids of marine origin about 300,000 years B.P. and 1,200 years B.P. From the sequence of chemical precipitate - rich bands and grain-size cycles in cores from Lakes Vanda and Joyce, together with the stratigraphy in the Lake Bonney cores, a sequence of climatic fluctuations is inferred. Colder climatic phases are most probably associated with periods of low lake-level during which chemical precipitates formed following the concentration of brines under frigid evaporitic conditions. At these times the sediment input from meltwater streams was low because of the locking-up of water in valley glaciers, and the main source of sediment would probably then be wind-derived. U/Th dating of the chemical precipitates has provided an absolute record of past climatic changes which indicate that the major glacier systems in the Dry Valley region were nonsynchronous. Low lake levels occurred in Lake Vanda some 2,000 and 5,500 years B.P. and on at least four earlier occasions. Sediment cores from Lake Joyce indicate a period of low lake-level about 3,000 years B.P. The Lake Bonney cores suggest periods of low lake-level occurred following each of the marine incursions into the valley (300,000 and 1,200 years B.P.) and that the advance of the Taylor Glacier into the Bonney Basin at least 10,000 years B.P. probably coincided with the Taylor I Glaciation. Frigid evaporitic conditions have continued to operate in Lake Bonney since 1,200 years B.P. with halite crystals forming on the lake-floor. However, the lake-level has been steadily rising over the last 500 years.
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