Seventh Annual V. M. Goldschmidt Conference 2033.pdf Chemical Weathering Yields and Sr Isotope Systematics from Major Siberian Rivers. Youngsook Huh and John. M. Edmond, MIT/WHOI Joint Program in Oceanography (Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 42-44 Carleton Street, E34-166, Cambridge, MA 02139; [email protected], [email protected]). The extent to which chemical weathering yields The headwater streams of the Vitim, Chara, are influenced by climate (temperature/runoff) is a Olekma, and Aldan, the major Lena tributaries, subject of some controversy. Reported data from the drain the Archean to Proterozoic Aldan Shield and continental arc of the western Americas cast doubt the Proterozoic Trans Baikal Highlands (TBH). Sr upon the existence of a simple relationship; the isotope ratios are .709 - .740, in agreement with weathering yields do not vary systematically from their Rb poor granulite facies rocks. (The one the Amazonian Andes to the Yukon. The situation exception is the Kurkulik, a small stream draining for the basement terrain is less well understood since from an extension of the TBH into Lake Baikal most detailed studies have been in the tropics. which at .860 is the most extreme of the data set Systematic measurements of major elements and analyzed to date). The rivers are relatively dilute Sr isotopic ratios have been made in the basins of the (alkalinity between 200 and 700 ueq/L); the ratio of major rivers of Eastern Siberia (Lena, ~120 stations, Si to total cations is low (.05-.37). Yana, 22; Indigirka, 21; Kolyma, 22). These are in The Sr isotopes are very useful at diagnosing peri-glacial, Sub-Arctic to Arctic terrains that, due to geological terrains but are not noticeably influenced the aridity of the extreme continental climate, have by the severe climates. The Sr fluxes are generally never experienced extensive glaciation. They higher but much less radiogenic than in comparable provide a basis for comparison with the much better low latitude terrains. studied systems at lower latitudes in regimes of A comparison can be made between the areal similar geology and topography. consumption rates of carbon dioxide due to fixation The left bank tributaries of the Lena drain the by aluminosilicate weathering (in thousands of vast sedimentary basin of the Siberian Shield. The mol/km2/yr) from the exposures in Southeastern Sr isotope ratios range from .708 to .710, as is Siberia with those from the Guayana Shield in the expected from Cambrian to Ordovician limestones drainage of the Orinoco. The values are comparable, and dolomites with higher values from sandstones from 29 to 179 in the Sub-Arctic and 16 to 205 in and shales. The chemical composition is dominantly the Tropics. There is no indication of a climatic Ca-HCO3, with some rivers draining interbedded influence on this uptake rate although the chemical evaporites showing high concentrations of NaCl and systematics are quite different. In the Tropics CaSO4. weathering is complete to kaolinite and gibbsite but The right bank tributaries of the Lower Lena that occurs very slowly under a lateritic seal. In the peri- are fed from the Verkhoyansk collisional mountains glacial environments frost action prevents the are more radiogenic, .709 to .714, as are the left accumulation of a weathered mantle and thus bank tributaries of the Yana, east of the range. The exposure is maintained. This appears to counteract right bank tributaries of the Yana, the Indigirka, and the climatic effects of low temperatures and the Kolyma drain the complex collisional unit of the precipitation. Mesozoic. The Sr isotope ratios range from .706 to .714, lower ratios from the arc intrusions.