Sr and Mg isotope geochemistry of the basal Ediacaran cap limestone sequence of Mongolia: Implications for carbonate diagenesis, mixing of glacial meltwaters, and seawater chemistry in the aftermath of Snowball Earth

摘要

The seawater chemistry and oceanographic information associated with Snowball Earth are commonly inferred from the geochemistry of cap carbonates deposited on continental margins during and after deglaciation. However, interpretation of such records can be complicated by carbonate diagenesis and contamination from siliciclastic components. In an attempt to disentangle these effects, we studied the geochemistry of the post-Marinoan cap carbonate sequence from Mongolia using a step-leaching procedure, which revealed that most samples are heterogeneous with respect to multiple geochemical signatures, including trace element concentrations, Sr, Mg, C and O isotopic signatures, raising questions to previous studies applying carbonate bulkrock geochemistry for paleoenvironment reconstructions. Such sample heterogeneity can be explained by contamination from non-carbonate phases and carbonate alteration. After stepped leaching, the least-altered/contaminated geochemical signatures for each sample were identified and the influences of carbonate diagenesis were evaluated. Our data indicate that mixing of glacial meltwater persisted to the maximum flooding surface within the cap carbonate sequence, below which carbonates record significant Mg and Sr isotope fluxuations that are most readily interpreted in the context of the mixing of water masses having distinct isotopic compositions. Only limestones deposited above the maximum flooding surface formed in a well-mixed ocean and exhibit Mg and Sr isotope values that record the integrated effects of Snowball Earth on ocean chemistry. Our study cautions against interpreting the geochemistry of cap carbonates in terms of whole ocean geochemical cycles.