THE FLUVIAL GEOCHEMISTRY OF THE RIVERS OF EASTERN SIBERIA: II. TRIBUTARIES OF THE LENA, OMOLOY, YANA, INDIGIRKA, KOLYMA, AND ANADYR DRAINING THE COLLISIONAL/ACCRETIONARY ZONE OF THE VERKHOYANSK AND CHERSKIY RANGES

Abstract

Fundamental to the global carbon cycle over geologic time scales is the control of atmospheric CO2 by aluminosilicate weathering. Much of the information on the rates of this process comes from rivers in the tropics and subtropics. To understand the possible climatic influences systematic studies are needed for the arctic/subarctic regions. This is the second in a series of papers addressing this problem by systematic studies of the pristine rivers of the Russian Far East.The region to the east of the Siberian Platform (Huh et al., 1998) is a geologically complex terrain formed by the Mesozoic collision and accretion of the Siberian and Kolyma plates. Because of the arid continental climate, it has not been glaciated in the recent past. Thus, it is possible to study weathering processes in an arctic environment dominated solely by cryogenic interactions without contamination by heterogeneous components derived from scouring glaciers. All the major rivers and their tributaries in this area have been sampled on expeditions to individual basins (∼100 samples) on a reconnaissance basis at falling stage, usually in July and August. The total dissolved cation levels (TZ+) are moderate (up to ∼3,100 μEq), and the major ion chemistry is indicative of Ca-aluminosilicate and carbonate weathering with significant contributions from black shales in some tributaries. The Si/TZ+∗, Si/(Na∗ + K) and 87Sr/86Sr ratios indicate that the weathering is superficial, i.e., only to cation-rich secondary minerals. The areal total dissolved solid fluxes range from 0.04 to 0.39 × 106 mol/km2/yr, up to an order of magnitude lower than for the Amazon-Orinoco draining the Andes in the tropics (0.6–4.1 × 106 mol/km2/yr). The CO2 consumption by aluminosilicate weathering (18–230 × 103 mol/km2/yr) is also at the lower end of the range observed in the Amazon-Orinoco headwaters (143–1,000 × 103 mol/km2/yr). However, as the North American counterparts in similar latitudes and with comparable relief, the Mackenzie, Yukon, and Fraser draining the Rockies, also have high dissolved solids (0.2–2.9 × 106 mol/km2/yr) and CO2 (19–1,750 × 103 mol/km2/yr) fluxes, these low values seem to be more a function of lithology than simply climate. Ice action in cold environments appears to overcome the inhibiting effects of the decreased temperatures and lack of precipitation in producing a high chemical yield but results in superficial weathering in the case of aluminosilicates.