The effects of diagenesis and dolomitization on Ca and Mg isotopes in marine platform carbonates: Implications for the geochemical cycles of Ca and Mg

The Ca, Mg, O, and C isotopic and trace elemental compositions of marine limestones and dolostones from ODP Site 1196A, which range in depth (~58 to 627. mbsf) and in depositional age (~5 and 23. Ma), are presented. The objectives of the study are to explore the potential for non-traditional isotope systems to fingerprint diagenesis, to quantify the extent to which geochemical proxies are altered during diagenesis, and to investigate the importance of diagenesis within the global Ca and Mg geochemical cycles. The data suggest that Ca, which has a relatively high solid to fluid mass ratio, can be isotopically altered during diagenesis. In addition, the alteration of Ca correlates with the alteration of Mg in such a way that both can serve as useful tools for deciphering diagenesis in ancient rocks.Bulk carbonate δ⁴⁴Ca values vary between 0.60 and 1.31‰ (SRM-915a scale); the average limestone δ⁴⁴Ca is 0.97±0.24‰ (1SD), identical within error to the average dolostone (1.03±0.15 1SD ‰). Magnesium isotopic compositions (δ²⁶Mg, DSM-3 scale) range between -2.59‰ and -3.91‰, and limestones (-3.60±0.25‰) and dolostones (-2.68±0.07‰) are isotopically distinct. Carbon isotopic compositions (δ¹³C, PDB scale) vary between 0.86‰ and 2.47‰, with average limestone (1.96±0.31‰) marginally offset relative to average dolostone (1.68±0.57‰). The oxygen isotopic compositions (δ¹⁸O, PDB scale) of limestones (-1.22±0.94‰) are substantially lower than the dolostones measured (2.72±1.07‰).The isotopic data from 1196A suggest distinct and coherent trends in isotopic and elemental compositions that are interpreted in terms of diagenetic trajectories. Numerical modeling supports the contention that such trends can be interpreted as diagenetic, and suggests that the appropriate distribution coefficient (K_Mg) associated with limestone diagenesis is ~1 to 5×10^-3, distinctly lower than those values (>0.015) reported in laboratory studies. With respect to Mg isotopes, the modeling also suggest that diagenetic fractionation factors of ~0.9955 (-4.5‰) and 0.9980 (-2‰) are appropriate for limestone diagenesis and dolomitization, respectively.