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 ∂¹³C (≤ + 3‰ to − 10‰VPDB) but always negative ∂¹⁸O (− 1 to − 6‰VPDB). Dolomite and reef limestone show the highest ∂¹⁸O and ∂¹³C values. The most negative ∂¹³C (≥ − 10‰) occurs mainly as cement in dolomite of a basinal, partly dolomitic, biostrome interval. Basinal limestone shows intermediate ∂¹³C 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 ∂¹³C ≥ 3‰ and a ∂¹⁸O 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 ∂¹⁸O was reset. Imported cement carbonate precipitated in equilibrium with the pore fluid with negative isotope values, whereas ∂¹³C of the in situ lime mud remained unmodified. The negative shift of the bulk ∂¹³C and ∂¹⁸O is variable and depends on pH and the contribution of ¹²C from anaerobic sulfate reduction in the zone of cement precipitation. This produced an ubiquitous covariance of ∂¹⁸O and ∂¹³C. Incorporation of seawater-derived Mg²⁺ during recrystallization of carbonate can account for the local dolomitization. Elevated ⁸⁷Sr/⁸⁶Sr 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 ∂¹⁸O and ∂¹³C of the bulk rock are always reset, and that carbonates can show, in the presence of clay minerals, elevated ⁸⁷Sr/⁸⁶Sr ratios even when the pore fluids were never exchanged.

Graphical Abstract