Evidence from Mg isotopes indicating reverse weathering as a significant Mg sink in Tibetan Plateau lakes

Abstract

The magnesium (Mg) isotopic compositions of Tibetan Plateau rivers have been recently used to quantify silicate weathering fluxes and associated CO₂ consumption. In this work, we present a systematic study on the element and Mg isotope geochemistry of rivers and lakes in the interior of the Tibetan Plateau, to improve our understanding of the magnesium-carbon (C) cycle. δ²⁶Mg values of river waters range from −2.00 ‰ to −0.60 ‰, with an average of −1.12 ± 0.60 ‰. The observed δ²⁶Mg variability of the rivers reflects overall changes in the proportion of silicate over dolostone weathering fluxes, while the effects of geothermal fluid and rainfall are rendered insignificant. The restricted lakes fed by these rivers have low Mg/Na, Al/Na and Si/Na ratios but are considerably enriched in ²⁶Mg, with an average δ²⁶Mg value of −0.61 ± 0.65 ‰, presumably controlled by the in-situ formation of authigenic Mg-bearing minerals. Element and isotope mass balance modeling indicates that nearly 90 % of the riverine Mg²⁺ is consumed by the formation of authigenic Mg-bearing minerals in the lakes, and the Mg isotope fractionation factor between the minerals and water (Δ²⁶Mg_{mineral-water}) is −0.09 ‰ to −0.06 ‰. The best explanation to the contrast between the large Mg sink and a small Δ²⁶Mg_{mineral-water} value is the co-precipitation of Mg-bearing carbonate minerals and phyllosilicates in the lakes. The microscopic observation and EDS analyses of paleo-lake sediments shows that the laminae of carbonate and authigenic Mg-rich clay minerals alternatively overlapped, and elemental and δ²⁶Mg values also suggested that the mixture of Mg-bearing carbonate and authigenic phyllosilicates dominated the sediment geochemistry. An estimation yields that the precipitation of authigenic phyllosilicates due to reverse weathering accounts for nearly 80 % of the total Mg sink in these restricted lakes, from which a considerable amount of CO₂ had been released into the atmosphere. Our results provide new insights into the long-term Mg-C cycle on the Tibetan Plateau.