Effects of early diagenesis on Mg isotopes in dolomite: The roles of Mn(IV)-reduction and recrystallization

Sedimentary dolomite plays an important role in global Mg cycling, and Mg isotopes in massive dolostones may be used to infer secular changes in seawater chemistry through geological history. However, sedimentary dolomite is generally regarded as a diagenetic product, and many details about the effects of early diagenesis on the Mg isotope composition of dolomite remain unclear. The mid-Cretaceous (Albian) Soreq and Givat Ye'arim formations near Jerusalem, Israel, contain exceptionally well-preserved massive dolostones, which provide an ideal opportunity to investigate the Mg isotope responses to early diagenesis. Dolomite samples from this section show large variations in δ ¹³ C values and Mn contents that are negatively correlated, reflecting degradation of organic matter and mineralization of organic carbon in the Mn(IV) reduction zone within soft sediment during dolomite formation. This is a rare example of a clear link between Mn(IV) reduction and dolomite precipitation based on geochemical evidence in the rock record. The dolomite samples also exhibit large variations in d(104) values and the degree of cation ordering. The latter is negatively correlated with Sr contents, implying that variable degrees of dolomite recrystallization occurred during diagenesis. δ ²⁶ Mg values of >50 dolomite samples from this section vary from −2.28‰ to −1.78‰ but do not correlate with indicators of organic matter degradation (δ ¹³ C values and Mn contents) or dolomite recrystallization (e.g., Sr contents), suggesting that Mg isotopes behave conservatively after initial dolomite (or proto-dolomite) precipitation during the very early stages of diagenesis. We propose that the Mg isotope composition of dolomite formed due to Mn(IV) reduction is buffered by seawater due to the shallowness of the Mn(IV) reduction zone in platform sediments, which is different from the dolomitization associated with bacterial sulfate reduction or methanogenesis, where Mg supply can be diffusion-limited. Furthermore, Mg isotopes in dolomite are robust to resetting by recrystallization during burial. Magnesium isotope compositions of platform dolomite that show variable and negatively correlated δ ¹³ C values and Mn contents can, therefore, be considered robust archives for reconstructing paleo-seawater Mg isotope compositions. The variation in δ ²⁶ Mg values of the dolostones in the Soreq and Givat Ye'arim formations is interpreted to reflect Rayleigh fractionation in response to dolomitization in a restricted water body. Therefore, the lowest δ ²⁶ Mg value is considered to be the Mg isotope composition of dolomite that was in equilibrium with coeval seawater in the open ocean, and thus the δ ²⁶ Mg value of Albian seawater was around −0.4‰.