Biogeochemical Cycles and Mixing in Lake Baikal

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As the most voluminous, deepest and longest continually existing freshwater lake on the surface of the earth, Baikal presents unparalleled opportunity for paleo-climate reconstruction at a key continental site in southeastern Siberia. Here, it is proposed to complete our investigations of the fundamental mixing and biogeochemical characteristics of the modern lake and its tributaries. This study will provide groundwork essential for deciphering Baikal sedimentary paleo-climate records. In addition, baseline data needed to establish the nature and magnitude of perturbations caused by expanding human activities in the Baikal watershed will be contributed and the chemical conditioning of the source waters of the Yenisei, one of the most important rivers emptying into the Arctic, will be defined. Findings from our June, 1988 expedition include: 1) Deduction of a unique mechanism, related to thermobaric instabilities, by which mixing of this deepest of lakes occurs ıCarmack and Weiss, 1991!; 2) Determination, based on CFC, O2 and nutrient distributions, of an 8 to 10 year average rate of ventilation for the lake and confirmation that Baikal is highly oligotrophic with nitrate as the limiting nutrient ıWeiss et al., 1991!; 3) Cycling of major elements in the lake is dominated by riverine throughout; hydrothermal activity known by temperature anomalies and subsequently observed by submersible, does not impact the chemistry of the modern lake; lake waters are undersaturated with respect to calcite phases, consistent with lack of their occurrence in lake sediments; lake waters are also undersaturated with respect to opal, however, high diatom productivity assures a sufficient rain rate to result in the preservation of siliceous materials in sediments which constitute a sink for Si; of the minor elements, only Ba showed non-conservative behavior in the lake ıFalkner et al, 1991!. A second expedition to the lake was undertaken in July, 1991 during which a method by which to use deep water temperature and oxygen profiles to adequately constrain the extent of ventilation was established. Sampling, by trace-metal clean techniques was extended to rivers and land-based hot-springs surrounding the lake in order to better constrain elemental budgets and to allow analyses of trace elements which tend to be more sensitive indicators of impact by human activities. Funds are being requested from NSF to complete chemical and isotopic analyses of this sample suite and to extend our observations through intercalibration and coordination with ongoing efforts by Russian scientist. In addition, a workshop to collate, discuss and publish the findings of our interdisciplinary group will be held in order to provide the most complete understanding of mixing and biogeochemistry of the modern lake possible.