Biochemically induced diagenesis of Jurassic micrite: evidence from phase analysis, carbon, oxygen, and strontium isotopes (Franconian Alb, Germany)

Abstract The marine Upper Jurassic rocks of the Franconian Alb consist largely of micritic carbonate of partly dolomitized reef mounds and bedded basinal limestone. All carbonates were lithified in the shallow (centimeters, meters) subsurface and have a wide range of ∂ 13 C (≤ + 3‰ to − 10‰VPDB) but always negative ∂ 18 O (− 1 to − 6‰VPDB). Dolomite and reef limestone show the highest ∂ 18 O and ∂ 13 C values. The most negative ∂ 13 C (≥ − 10‰) occurs mainly as cement in dolomite of a basinal, partly dolomitic, biostrome interval. Basinal limestone shows intermediate ∂ 13 C values. Because freshwater diagenesis and elevated temperatures cannot explain the observed isotope values, pH is here considered a major factor influencing the isotope signal of micritic limestone. The bulk sediment isotope signal was reset to lower values, from an original lime mud with ∂ 13 C ≥ 3‰ and a ∂ 18 O of ≥ + 1‰, as a result of biochemically induced diagenesis. Carbonate, probably mostly aragonite but occasionally including dolomite, was dissolved in a zone where low pH developed as a result of organic matter degradation. Dissolved carbonate was translocated by diffusion and re-precipitated as cement (ca. 50vol%) in a zone with elevated pH where all in situ lime mud ∂ 18 O was reset. Imported cement carbonate precipitated in equilibrium with the pore fluid with negative isotope values, whereas ∂ 13 C of the in situ lime mud remained unmodified. The negative shift of the bulk ∂ 13 C and ∂ 18 O is variable and depends on pH and the contribution of 12 C from anaerobic sulfate reduction in the zone of cement precipitation. This produced an ubiquitous covariance of ∂ 18 O and ∂ 13 C. Incorporation of seawater-derived Mg 2+ during recrystallization of carbonate can account for the local dolomitization. Elevated 87 Sr/ 86 Sr ratios are explained as a result of interaction of clay minerals with the stationary pore fluids. This study shows that the isotopic signal produced by biochemically induced shallow submarine subsurface carbonate diagenesis can be indistinguishable from freshwater diagenesis, that ∂ 18 O and ∂ 13 C of the bulk rock are always reset, and that carbonates can show, in the presence of clay minerals, elevated 87 Sr/ 86 Sr ratios even when the pore fluids were never exchanged. Graphical Abstract