TRACING FABRIC-SELECTIVE DOLOMITIZATION PROCESS IN BIOTURBATED CARBONATE BY A MULTI-TRACER GEOCHEMICAL APPROACH

In certain cases, dolomite distribution clearly indicates selectivity with respect to primary fabrics and textural properties on macroscopic or microscopic scale. One type of fabric-selective dolomitization, characterized by dolomitic burrows and micrite matrix, is remarkable in bioturbated carbonate, which has been world widely preserved since the Cambrian Explosion and The Great Ordovician Biodiversification Event (GOBE). Although interactions between macro- and microorganisms involving biogeochemical consequences of macro- and microbiological activities in early diagenetic sediments have been proved essential for the burrow associated dolomitization, the feedback mechanisms and physicochemical forces controlling the selective dolomitization process still elude definitive analysis. Considering the advantages of Mg isotopes in tracing Mg sources and dolomitization processes, we applied Mg isotopes coupled with carbon, oxygen isotopes and trace elements analyses to semi-quantitatively simulate the selective dolomitization process of the bioturbated carbonate from the Ordovician Majiagou (MJG) Formation, North China. Petrographic analysis suggests two burrow filling types developed in the MJG bioturbated carbonate with similar geochemical compositions. We propose a same dolomitization process proceed by the replacement of ¹³C-depleted calcite precursors, which precipitated near the sediment-water interface and inherited lower δ¹³C values of organic matters. Bioturbation-associated physical and chemical disruption facilitated the dissolution of calcite precursor, dehydration of Mg²⁺, nucleation and growth of dolomite crystal through biomixing, bioirrigation and the associated mediation of bacteria and organic matters within the burrows. Spatially homogeneous δ²⁶Mg values of burrow-filled dolomite implies Mg²⁺was supplied by seawater via diffusion during burrow ventilation, which can be well simulated by DAR model. Our study highlighted the effects of syn-depositional metazoan bioturbation in selective dolomitization, and established a selective dolomitization model of bioturbated carbonate, which further promoting the theoretical understanding of the early diagenesis process of marine carbonate from a biogeochemical perspective.