The application of Mg isotopes in carbonate rocks to reconstruct environmental records

Abstract:

Previous studies showed that Mg isotope fractionation between carbonate and solution depends on several parameters including 1) temperature (with a sensitivity of ~0.01‰/^oC); 2) precipitation rate; 3) aqueous Mg concentration and pH; 4) mineralogy: All else being equal, the enrichment of ²⁶Mg in carbonate precipitated from solutions with the same δ²⁶Mg values follow the sequence: aragonite > dolomite > magnesite > calcite; and 5) biogenic effect [1]. Despite the complexity, the weak temperature sensitivity and strong mineralogical control offer Mg isotope compositions of carbonate rocks as a paleoproxy to provide constraints on environmental records, and an index for diagenesis and low degree metamorphism.

Here, we present four cases describing 1) how Mg isotope varied during the mineralogical transition from aragonite (corals) to calcite; 2) how δ²⁶Mg, ⁸⁷Sr/⁸⁶Sr ratios of a Cambrian limestone changed due to the formation of secondary high-Mg calcite; 3) how δ²⁶Mg and trace metal compositions of limestones can be used to reconstruct ocean chemistry, and 4) how δ²⁶Mg, ⁸⁷Sr/⁸⁶Sr ratios of dolomite can be used to infer ocean chemistry in the deep time.