Hydromagnesite precipitation in the Alkaline Lake Dujiali, central Qinghai-Tibetan Plateau: Constraints on hydromagnesite precipitation from hydrochemistry and stable isotopes

Abstract

The mineral hydromagnesite, Mg 5 (CO 3) 4 (OH) 2 •4H 2 O, is a common form of hydrated Mg-carbonate in alkaline lakes, yet the processes involved in its formation are not well understood. This study focuses on Dujiali Lake, in the central Qinghai-Tibetan Plateau (QTP), which is one of the few environments on the earth's surface with extensive Holocene precipitation of hydromagnesite. The hydrogeochemistry of surface waters, and the mineralogical, stable isotope (δ 13 C and δ 18 O), and radiogenic isotope content of hydromagnesite deposits were analyzed to investigate formation mechanisms. The chemical composition of surface water around Dujiali Lake evolved from the rock-weathering-type waters of T1 (Ca-Mg-HCO 3 water type) to more concentrated sodic waters of T2 (Na-SO 4-Cl water type) due to evaporation. XRD results show that the mineralogical composition of samples is pure hydromagnesite. Analysis of oxygen isotopes in the hydromagnesite indicates that supergene formation with authigenic carbonate crystallization from evaporation water is the dominant precipitation process. Combined carbon-oxygen isotope analysis suggests atmospheric CO 2 provided a carbon source for the precipitation of hydromagnesite. These findings suggest that hydromagnesite precipitation at Lake Dujiali is mainly inorganic in nature, and the greenhouse gas, CO 2 , is trapped and stored in the hydromagnesite directly from the atmosphere. AMS radiocarbon dating of samples indicates CO 2 was sequestered between 5845±30 to 6090±25 cal a BP in the Dujiali Lake hydromagnesite deposit. The study contributes to improved understanding of hydromagnesite formation in modern and ancient playas.

Solubility of the hydrated Mg-carbonates nesquehonite and dypingite from 5 to 35 °C: Implications for CO2 storage and the relative stability of Mg-carbonates

“…Dypingite and nesquehonite are common weathering products of ultramafic rocks (Wilson et al, 2006(Wilson et al, , 2014Power et al, 2007;Beinlich and Austrheim, 2012;Garcia del Real et al, 2016;Lin et al, 2017), and are documented CO 2 sinks that can be used to store CO 2 (Power et al, 2007(Power et al, , 2013a(Power et al, , b, c, 2016Pronost et al, 2011;Bea et al, 2012;499 Assima et al, 2012499 Assima et al, , 2014cHarrison et al, 2013aHarrison et al, , 2015Harrison et al, , 2016Harrison et al, , 2017Wilson et al, 2014;McCutcheon et al, 2014;Morgan et al, 2015;Hamilton et al, 2016Hamilton et al, , 2018Gras et al, 2017). 501…”

“…For example, Gautier et al (2014) 59demonstrate a 2.5-fold greater growth rate constant for hydromagnesite 60 [Mg 5 (CO 3 ) 4 (OH) 2 •4H 2 O] compared to magnesite at 90°C. Thus, ultramafic weathering at the Earth's near-surface is dominated by hydrated Mg-carbonate formation (Shirokova et al, 62 2013;Power et al, 2014b;Lin et al, 2017), and engineered CO 2 sequestration efforts at near-Earth's surface temperatures will also tend to form hydrated Mg-carbonates. However, observed mineral assemblages in the field and in laboratory experiments reveal that multiple hydrated magnesium carbonate phases often co-exist, with the more-hydrated phases 66 frequently acting as transient precursors to the more stable, less-hydrated phases (Davies and 67 Bubela, 1973;Hopkinson et al, 2008Hopkinson et al, , 2012Ballirano et al, 2013;Power 68 et al, 2014b;Harrison et al, 2015Harrison et al, , 2016.…”

Magnesium Isotope Constraints on the Holocene Hydromagnesite Formation in Alkaline Lake Dujiali, Central Qinghai‐Tibetan Plateau

“…Hydromagnesite is common in (a) altered ultramafic rocks (Oskierski et al., 2021; Wilson et al., 2006), (b) as an authigenic mineral in alkaline lakes and playas (Braithwaite & Zedef, 1996; Chagas et al., 2016; Power et al., 2009; Zeyen et al., 2021), and (c) in karstic cave systems as speleothems and “moonmilk” (Broughton, 1972; Canaveras et al., 1999). Hydromagnesite is typically associated with high aqueous Mg/Ca ratios and high alkalinity ($${\mathsf{H}\mathsf{C}\mathsf{O}}_{3}^{-}$$ + $${\mathsf{C}\mathsf{O}}_{3}^{2-}$$) (Braithwaite & Zedef, 1996; Chagas et al., 2016; Lin et al., 2017; Power et al., 2009; Zeyen et al., 2021). Hydromagnesite formation in a natural setting has been directly linked to the Mg/Ca ratios of formation waters (Chagas et al., 2016; Fischbeck & Müller, 1971; Zeyen et al., 2021).…”

“…Mg isotopes are an emerging geochemical tracer that are known to fractionate during precipitation of carbonates and silicates from aqueous fluids, and are thus useful for quantifying carbonate precipitation processes (Harrison et al., 2021; Hindshaw et al., 2020; Li et al., 2012; Mavromatis et al., 2021; Oelkers et al., 2018; Tipper, 2022; Tipper et al., 2006a, 2008). In this study, we present Mg isotope data ($$\delta {}^{26}\mathsf{M}\mathsf{g}/{}^{24}\mathsf{M}\mathsf{g}$$ expressed as $$\delta {}^{26}\mathsf{M}\mathsf{g}$$ in per mil units) and Mg/Ca data on lake waters, incoming stream and groundwaters, lake sediments, and related hydromagnesite in the well‐studied alkaline Dujiali Lake (DL) in Tibet (Lin et al., 2017, 2019a, 2019b), and compare these to global databases of waters and other alkaline lakes where hydromagnesite forms. DL is undersaturated with respect to hydromagnesite in the present day, but in the recent past (Holocene), substantial volumes of hydromagnesite have been deposited from DL.…”

Mg-Rich Authigenic Carbonates in Coastal Facies of the Vtoroe Zasechnoe Lake (Southwest Siberia): First Assessment and Possible Mechanisms of Formation

“…Numerous examples of modern Mg-carbonate formation under conditions of playas and saline continental lakes were reported for various arid and semi-arid regions of the world [12,14,15,[31][32][33][34][35][36][37][38][39][40][41][42][43][44][45]. The majority of such environments are evaporitic and/or alkaline [12], which is most often related to specific climatic conditions and the specific geology of territories where such phenomenon appear.…”