Co-variation of Mg and C isotopes in late Precambrian carbonates of the Siberian Platform: A new tool for tracing the change in weathering regime?

摘要

This study reports significant Mg isotopic variations in 32 samples of late Precambrian dolomites from various parts of the Siberian Platform exhibiting a range of δ~(13)C from -10‰ to +6‰ (V-PDB). In the Ediacaran dolomites from the N and NE part of the Patom Paleobasin, a systematic increase of δ~(26)Mg from -2.3±0.2‰ to -0.8±0.3‰ (DSM3) along with a δ~(13)C decrease from +6‰ to -10‰ (δ~(26)Mg=-0.072×δ~(13)C-1.83) with a correlation coefficient of R~2=0.73 (n=20) are observed. Within the stratigraphic sequence of Ediacaran and Ediacaran-Lower Cambrian transition successions, all positive C-isotope excursions co-vary with negative Mg-isotope shifts and vice versa. Other samples from the Siberian Platform lay on this regression with R~2=0.66 (n=28) except for 4 samples, probably diagenetic dolostones, that exhibit a positive δ~(26)Mg-δ~(13)C correlation. The enrichment of sedimentary dolomite in heavy carbon and light Mg isotopes may be interpreted as a result of a complex interplay of contrasting sedimentation conditions (sea regression versus sea transgression regime) yielding different weathering regimes and relative contributions of carbonate versus silicate rocks. The enrichment of both oceanic water and carbonate sediments by ~(26)Mg could occur during transgression and would coincide with sedimentation of low-~(13)C carbonates, whereas sedimentation of the depleted in ~(26)Mg, and enriched in ~(13)C carbonates could happen under regression regime when exposed carbonate platforms and coastal planes are subjected to intense weathering.