Biogeochemical reappraisal of the freshwater–seawater mixing‐zone diagenetic model

Abstract

First proposed nearly half a century ago, the mixing-zone model of dolomitization enjoyed a brief stay in the limelight before falling out of favour. Despite extended past criticism, arguments that build on its current validity are presented and discussed. The coastal mixing zone can be seen as an aquifer system exhibiting marked physicochemical gradients, reflective of the admixture of low salinity freshwater and seawater sources with variable redox potentials. This perspective requires a more holistic look at the mixing zone, not only as a gradient of major element concentrations, but also as the locus of enhanced subsurface redox sensitive reactions that occur at the pore-space scale within a moveable diagenetic front. Combined genomic and isotopic data indicate that an active subsurface biosphere thrives in the mixing zone. This could facilitate Mg 2+ dehydration, generate alkalinity, consume protons and mobilize potentially catalyzing ions (i.e. Mn and Zn), which are all low temperature factors thought to promote dolomite formation from soluble precursors. In the updated model, the advective mix of fluids with contrasting composition modulate a range of biogeochemically induced mineral dissolution and reprecipitation reactions. Biotic and abiotic interactions between these fluids affect carbonate equilibrium and result in dissolution of soluble aragonitic and calcitic phases, while dolomite precipitates (as cement) and neomorphic replacement. The secondary dolomite often exhibits compositional heterogeneity and contentious δ 18 O signatures indicative of re-equilibration. The role of manganese, zinc, intermediate sulphur species and ammonia are far from being fully understood, nor is their fingerprint in ancient deposits. Application of in situ spectroscopic imaging techniques, clumped and metal isotope analyses, as well as a more extended use of traditional approaches, such as sulphur isotopes, are poised to open many opportunities to further explore the biogeochemistry of this diagenetic environment and how it relates to platform dolomitization.

Aqueous system-level processes and prokaryote assemblages in the ferruginous and sulfate-rich bottom waters of a post-mining lake

“…Together these factors would have enabled more widespread ferruginous conditions (Reershemius and Planavsky, 2021) that transiently encompassed the water column of Mesozoic epicontinental seas (Petrash et al, 2016;Bauer et al, 2022). Therefore, the link between the biogeochemical controls operating in the water column of our study site and the mineral equilibrium conditions prevailing near and at its anoxic SWI may be relevant for studying elusive shallow burial diagenetic signals developed in fluidbuffered sediments and also for unravelling overprinting of redox proxies in carbonates altered in redox-stratified coastal aquifers (Petrash et al, 2021).…”

Systematic laboratory approach to produce Mg-rich carbonates at low temperature

“…, manganese-, iron-, and sulfate-reduction coupled to organic matter oxidation) that generates high rate of alkalinity in pore waters rich in organic molecules. 67 Similarly, in supratidal organic-rich pore waters of modern sabkhas 30,50 dolomite vs. aragonite formation may be determined not only by the presence of organic molecules ( i.e. , living and degrading microbial cells and extracellular polymeric substances) 22 but also by cyclic mixing of seawater brought by the tide with more evaporated seawater present in the sediment pores, in which the presence of hotspots of alkalinity production associated to microbial respiration of organic matter.…”

“…2,65,66 In mixing zones, not only there is a rapid mixing of solutions characterized by different chemical compositions, but conditions are particularly favorable for sustaining microbial metabolisms (e.g., manganese-, iron-, and sulfate-reduction coupled to organic matter oxidation) that generates high rate of alkalinity in pore waters rich in organic molecules. 67 Similarly, in supratidal organic-rich pore waters of modern sabkhas 30,50 dolomite vs. aragonite formation may be determined not only by the presence of organic molecules (i.e., living and degrading microbial cells and extracellular polymeric substances) 22 but also by cyclic mixing of seawater brought by the tide with more evaporated seawater present in the sediment pores, in which the presence of hotspots of alkalinity production associated to microbial respiration of organic matter. Future studies measuring alkalinity production-rate and daily uctuations in sabkhas, and other modern dolomite-forming environments will be essential to test the abovementioned hypothesis.…”

Aqueous system-level processes and prokaryote assemblages in the ferruginous and sulfate-rich bottom waters of a post-mining lake

“…The link between the biogeochemical controls operating in the water column of our study site and the mineral equilibrium conditions prevailing near and at its anoxic SWI may also be relevant for studying elusive shallow burial diagenetic signals developed in fluid-buffered sediments. Also, to unravel long-term overprinting of redox proxies in carbonates altered in movable redox stratified coastal aquifers (Petrash et al, 2021).…”