DOM-Mn redox interactions promote metastable kutnahorite-dolomite carbonate frameworks

The intersection of organic geochemistry and mineralogy offers a critical research niche for understanding the preservation of dissolved organic matter (DOM) in marine depositional systems. While reactive metal oxides are recognized for stabilizing organic carbon against remineralization, the mechanisms by which ligands template the conversion of this organic matter into carbonate minerals remain elusive. While pH and redox coupling govern metal speciation and ligand availability, the specific role of carboxyl-rich polysaccharides in catalyzing manganese-mediated carbonate mineralization remains under-constrained. Here, we isolate the role of alginate—a model for carboxylated EPS. To simulate diagenetic redox oscillations, cyclic voltammetry was employed to target the Mn(III)/Mn(II) couple within alginate-bearing Mn-Mg-Ca electrolytes. This electrochemical framework evaluated manganese-driven proton exchange as a mechanism to lower kinetic barriers via stereochemical templating. Rather than functioning as a passive substrate, alginate actively directs a heterogeneous mineralization pathway: it promotes the crystallization of metastable magnesian kutnahorite, bypassing the high kinetic barriers of direct dolomite precipitation. Microstructural analysis (STEM-HAADF/EDS, SAED) reveals that organic-mediated Mn-rich cores template the subsequent epitaxial growth of disordered Mg-Ca carbonate (protodolomite) cortices within just 20 minutes. This "electrochemical Mn-pump" mechanism relies heavily on the specific coordination chemistry of the alginate’s carboxyl groups, which effectively shed the rigid hydration shell of metal cations (specifically Mg2+) via ligand-mineral surface proton exchange. These findings delineate a critical mechanism of organic-mineral interaction, showing that specific (carboxylated) DOM fractions can dictate mineralogical outcomes in low-temperature systems. This work specifically highlights how organic templates may serve as archives of paleo-environmental conditions by locking biogeochemical signatures into fabric-preserving carbonate mineral phases. By establishing a reproducible protocol for generating synthetic organic-carbonate frameworks, this study provides a baseline for future investigations into the stable isotope fractionation that occurs during ligand-mineral interactions in Mn-enriched precipitation environments supersaturated with respect to dolomite and metastable Mn-Ca carbonates, akin to the episodic precipitation events in the Baltic Sea deeps.