MgSCALE

Dolomitization is an important sink for seawater Mg in geological history and the process of dolomite formation is marked by a significant Mg isotope fractionation between dolomite and aqueous Mg²⁺. Stable isotope geochemistry of Mg in dolomite is a new and potentially powerful tool to investigate dolomitization in geological history, as well as to constrain Mg isotope composition of seawater, and Mg global cycling. This is, as is the isotopic composition of Mg (δ²⁶Mg) in seawater, controlled by a balance between the sources and sinks of Mg and their isotopic compositions.

The intensity of dolomitization in sedimentary records varied significantly throughout the geological history, and could have changed the Mg isotope balance for seawater, making it possible for tracing global Mg cycling using Mg isotopes in dolomites. However, concerns regarding the applicability of Mg isotopes in dolomites to study the seawater Mg isotope composition still remain unsolved. This is because dolomites can have diverse origins and their massive formation is commonly thought to have diagenetic origins, such as replacement of calcium carbonate by Mg-bearing brine water or seawater. Thus, Mg isotope spatial patterns must be evaluated in light of the interaction between depositional and diagenetic settings, to further understand the relation between deposition, diagenesis and Mg isotopes. It is a requirement for future utilization of Mg isotopes as a proxy of both oceanic Mg and within dolomite domains.

Despite the fact that there are numerous different dolomitization models that have been previously proposed, it is unclear whether there is an adequate isotope exchange model for Mg cations between dolomite and seawater during dolomitization processes. Little is known about the variability of Mg isotope composition across dolomitization fronts within carbonate formations, its controlling factors and further influence on the petrophysical properties of the strata, as well as to the heterogeneity of Mg isotope composition of dolomites at regional and global scales.

In this project, we questioned if Mg isotopes in dolomite formations are a diagenetic signature that reflects local processes or a global signature that represents contemporary seawater chemistry. To answer this query, we proposed to investigate the variability of Mg isotope composition of dolomite, and their accompanied sedimentary and petrophysical parameters at three different scales: across a dolomitization front (meter scale), across a sedimentary basin (kilometer scale), and between coeval sedimentary basins (global scale). Three sets of near coeval Cretaceous Albian-Cenomanian (~93-113 Ma) dolomite formations from Israel, USA, and offshore SW Africa were selected in order to investigate multiple-scale Mg isotope variations.

The project is funded by the ISF-NSFC. Lead PI: Dr. Weiqiang Li (University of Nanjing) and Dr. Nicolas Waldmann (University of Haifa).