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

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 (Δ26Mgmineral-water) is −0.09 ‰ to −0.06 ‰. The best explanation to the contrast between the large Mg sink and a small Δ26Mgmineral-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. •The majority of Mg in the rivers are sourced from the weathering of dolostone and silicate.•Nearly 80% of Mg from riverine influx into the restricted lake systems were consumed by the reverse weathering.•A considerable amount of CO2 consumed by chemical weathering on the Tibetan Plateau has been counterbalanced by the reverse weathering.