The Homerian (late Wenlock, Silurian) carbon isotope excursion from Perunica: Does dolomite control the magnitude of the carbon isotope excursion?

Abstract

The δ13Ccarb records from two geographically close sections of the shallow-water Kozel Limestone Member (late Wenlock Motol Formation; Perunica microplate) significantly differ in the magnitude of the Homerian carbon isotope excursion as well as in their dolomite content. The present paper tests a hypothesis as to whether a difference of about 2‰ in the magnitude of the δ13Ccarb anomaly is caused by the different content of dolomite, which could be enriched in both 13C and 18O relative to coexisting calcite, as has been suggested by experimental data. The new data obtained reveal that the δ18O composition of calcite and dolomite was probably controlled by the pore-fluid composition during limestone diagenesis and that both carbonates seem to be close to equilibrium in oxygen isotope composition. On the other hand, the δ13C values of dolomite are similar to those of calcite, and thus the carbon isotope composition of both carbonates was probably determined by the precursor carbonate composition. Moreover, the values of δ13Cdolomite/δ13Ccalcite ratios as well as their variability suggest that both calcite and dolomite did not reach equilibrium in their carbon isotope composition. Whole-rock, mineralogical, and C and O isotope data clearly show that dolomite is not the cause for the differences in magnitudes of the δ13C records observed between dolomite-bearing and dolomite-lacking shallow-water limestone successions. The question as to whether the observed differences in the δ13C records of the studied sections across the Homerian carbon isotope excursion were controlled by the dependence of seawater composition on water depth and (or) proximity to shoreline or if the δ13C values were later affected by secondary processes during limestone diagenesis is still unsolved.