Geochemical mass balances of major elements in Lake Baikal

Major element mass balances for Lake Baikal are calculated with mostly previously published data for soluble fluxes and new, unpublished data for riverine suspended particulate matter chemistry. Physical transport seems to be the most important riverine process. The elements Ca, Mg, and Na seem to be very mobile in the weathering mantle and K and Si seem to be relatively mobile. A comparison of elemental input-output budgets and mass accumulation rates (MAR) in bottom sediments shows that most major elements, except Ca, Si, and Mn, have comparable riverine particulate matter fluxes and MARS. The addition of wet atmospheric deposition fluxes results in an excess of Ca, Mg, and Na entering the lake. The additive effect of these excess inputs during a 40-year period amounts to undetectable concentration increases in the water column. If erosion of weathered bedrock is the source of most dissolved and all particulate matter transported to the lake, theoretical elemental fluxes can be calculated with Al as the conservative element. Flux ratios (observed/theoretical) range from 0.7 to 2.2, but most fall within the acceptable range of 0.7-l .5. Major rock-forming elements are carried by rivers as weathering products and there are minimal biogeochemical processes that modify these inputs as suspended particulate matter accumulates in the bottom sediments of the lake. In terms of continental paleoclimatic reconstruction, Lake Baikal represents a unique opportunity in that its long, continuous history of sedimentation in a northern latitude continental setting affords scientists the ability to compare continental and marine records for the last 30X 10” yr. Environmental change, as documented from the sedimentary record, is assumed to result from changes in the concentrations of various sedimentary components owing to external forces that either act on the drainage basin directly or cause changes in the lake’s properties. For example, a drop in temperature can cause a decline in chemical and mechanical weathering as well as a lowering and species shift in primary productivity within the lake. Thus, it is important to determine whether the sedimentary record reflects climatic “imprints” that can be distinguished from diagenetic “overprints.” This paper is part of a series describing the geochemical mass balances of major, minor, and nutrient elements in Lake Baikal in order to ascertain whether this important lake is at steady state with respect to chemical inputs and whether diagenesis plays a significant role in determining the biogeochemical cycle of these elements. Additionally, such massbalance calculations may provide a foundation for assessing the impact of human influences on the Baikal basin. This paper deals with the rock-forming elements (Ca, Mg, Na, K, Al, Si, Fe, Mn) that constitute the major mass of Acknowledgments We thank many colleagues at the Limnological Institute, Irkutsk, Russia, for their help in retrieving data and tributary water samples. In particular, N. Granin was instrumental in collecting many tributary water samples for suspended particulate matter. Brenda Coleman of the U.S. Geological Survey conducted the chemical analyses of the suspended sediment samples and R. Seal of the U.S. Geological Survey, Reston, Virginia, provided a helpful review of the manuscript. sediments in the Lake Baikal basin and are delivered primarily as waterborne particles. Thus, mass-balance calculations should show whether the concentration of each of these elements results primarily from mechanical erosion, chemical weathering, or both. Lake Baikal is known for its great depth (1,642 m), ancient age (30X IO” yr), and the presence of many unique plants and animals. It is located in the tectonically active Baikal Rift Zone and is separated geomorphically and structurally into three interconnected basins (Mats 1993; Fig. 1). The Baikal Rift Zone extends more than 2,500 km from northwestern Mongolia to southeastern Siberia (Logatchev et al. 1974) and separates the tectonically stable Siberian Craton from the seismically active tectonic belts in the Trans-baikal region and Mongolia (Solonenko 1978). The Siberian Platform, west of Lake Baikal, is underlain by Precambrian crystalline rocks that are overlain by Cambrian and Ordovician terrigenous rocks and horizontally bedded carbonate and evaporite deposits. Near Lake Baikal’s western shore, these deposits are folded and separated from Proterozoic crystalline rocks by a major fault system. The Lake Baikal basin is surrounded by several mountain ranges composed of Precambrian crystalline rocks, mostly granites and granodiorites. The Selenga River basin is underlain by bedrock composed mainly of Proterozoic granites, although there are many depressions filled with Mesozoic sediments, and Tertiary basalts crop out on the tops and crests of mountains (Pampoura et al. 1993; Litvinovskii et al. 1985). The geology of the northeastern shore is characterized by granites with outliers of Cambrian carbonates. On the west side of the lake, where the mountains are steepest and close: to the lake, the bedrock is composed of metasandstones, metaconglomerates, tuffs, gneisses, and granites (Stanevich and Nemerov 1993; Bukharov et al. 1985). The most basic constituents of a nonvolatile geochemical