Constraints on dolomite formation in a Late Palaeozoic saline alkaline lake deposit, Junggar Basin, north‐west China

Abstract

Alkaline lakes (pH > 9) are among the few modern sedimentary environments that are hydrochemically favourable for low‐temperature dolomite formation. While Mg‐clays and Mg‐evaporites also form more easily in alkaline environments, few studies have focused on how the kinetically inhibited dolomite wins the competition for Mg2+. Here, a basin‐wide survey of the distribution, paragenesis and stable C, O and Mg isotopes of main Mg‐rich minerals in the Late Palaeozoic saline alkaline lake deposit of the north‐west Junggar Basin, north‐west China, is conducted to study the influence of the formation and diagenesis of eitelite, northupite and Mg‐clays on dolomite formation. Large, isolated dolomite crystals (20 to 70 μm in diameter), show positive δ13C values (ranging from +1 to +7‰) and a restricted distribution in the mudstones of the lake‐transitional zone. These crystals have been interpreted as organogenic dolomite driven by methanogenesis via fermentation of organic substrates. The δ18O values of dolomitic mudstones (from −7.4 to +3.4‰), calcitic mudstones (from −15.1 to −3.3‰) and bedded Na‐carbonate evaporites (from +0.08 to +3.7‰), together with their Mg isotopic compositions, suggest that dolomite was not enriched in the most concentrated environments or during stages with most Mg sources, but in the organic‐rich deposits containing few other authigenic Mg‐rich minerals. Dolomite is at a competitive disadvantage for Mg2+ ions compared to Mg‐evaporite and Mg‐clay minerals due to its slow crystallization rates and the deficiency of micritic calcium carbonate precursors. However, it can nucleate and progressively grow into large crystals (>20 μm) if bacterial methanogenesis could effectively lower porewater pH (<8.5) and induce the dissolution of generated eitelite, northupite or Mg‐clays. These findings suggest that high salinity and/or high alkalinity are not always favourable conditions for dolomite formation and highlight the active role of pH fluctuations in inducing low‐temperature dolomite formation.

Searching for microbial contribution to micritization of shallow marine sediments

“…Different bacterial facies can induce opposite processes depending on the environmental conditions (Dupraz et al, 2009; Dupraz & Visscher, 2005; Fernández‐Remolar et al, 2012). For instance, as anaerobic methane oxidation (AMO) can induce the precipitation of carbonates (Guo, Wen, Li, et al, 2023; Guo, Wen, & Sánchez‐Román, 2023; Michaelis et al, 2002), aerobic methane oxidation favours carbonate dissolution by producing CO 2 and acidifying the local pH (Krause et al, 2014). These pathways are performed by different microorganisms (e.g., Archaea of the cluster ANME‐1/2 and bacteria like Methylosinus trichosporium , respectively), and the outcome in terms of carbonate alteration depends on their coupling and balance, as well as on their synergies with other components of the microbial consortia (e.g., sulfate reducers as Desulfosarcina/Desulfococcus spp.…”

“…Such a rational approach has been used to discover ammonia oxidation metabolisms alternative to aerobic oxidation (Strous et al, 2002; van de Leemput et al, 2011) and, in the field of carbonate dissolution/precipitation, to identify cyanobacteria microborers (Garcia‐Pichel, 2006; Garcia‐Pichel et al, 2010). Other studies have reviewed the current knowledge of metabolic processes that may be implicated in carbonate turnover, including dissimilatory sulfate reduction, AMO and ureolysis (Dupraz et al, 2009; Guo, Wen, Li, et al, 2023; Guo, Wen, & Sánchez‐Román, 2023; Visscher & Stolz, 2005). Fewer efforts have been devoted to exploring the role of the overall microbial community in the micritization process, except in the microbialite community, where multiple taxonomic and even functional studies of the microbial diversity involved in carbonate precipitation/dissolution have been performed (Ruvindy et al, 2016; Saghaï et al, 2016).…”

Lacustrine dolomite in deep time: What really matters in early dolomite formation and accumulation?

“…2A-F), poorly ordered and slightly Ca-rich, and display bright red under cathode luminescence light (Table 1). (Wu et al, 2016;Liu, 2022); 2) diagenetic dolomite by alteration of tuffs (Wang et al, 2014c); 3) hydrothermal dolomite (Yu et al, 2021); 4) methanogenesis-induced formation (Guo et al, 2023) Note: BBB = Bohai Bay Basin; Fm. = Formation; SHJ = Shahejie.…”

“…Alkaline saline lakes are referred to as "soda lakes", which are characterized by the extensive deposition of bedded Na-carbonates in lake centers. Ancient alkaline saline lakes in China include the examples of the Biyang Sag in the Nanxiang Basin (during deposition of Hetaoyuan Formation) (Yang et al, 2015) and the Mahu Sag in the Junggar Basin (during deposition of Fengcheng Formation) (Guo et al, 2019(Guo et al, , 2023 (Fig. 1) (Table 2).…”