Mixed marine and meteoric early diagenetic alteration of shallow water carbonate sediments: insights from paired δ44/40Ca and δ26Mg values

Shallow-water carbonate sediments are an important and widely used archive of the history of the global carbon cycle and the chemical composition of seawater. However, the interpretation of chemical and isotopic signals in ancient carbonate sediments is complicated by the potential for syn- and post-depositional alteration by marine, meteoric, and burial fluids. Here we present stable isotope (δ¹³C, δ¹⁸O, δ^44/40Ca, δ²⁶Mg) and trace element (e.g. Sr/Ca, Mg/Ca) data from Pleistocene sediments from the Great Barrier Reef (IODP Expedition 325), as well as surface sediments from the Bahamas (Triple Goose Creek, Andros Island). Sediments from both locations are characterized by a positive correlation between δ¹³C, δ¹⁸O, and δ²⁶Mg values where δ²⁶Mg values as low as -5‰ are observed in sediments that exhibit clear petrographic and geochemical signatures (low δ¹³C and δ¹⁸O values) of meteoric alteration. In contrast, relationships between bulk sediment δ^44/40Ca values and trace elements (e.g. Sr/Ca) are more complex and suggest a two-stage diagenetic history for samples from both the Bahamas and Great Barrier Reef. Cross-plots of δ^44/40Ca values and Sr/Ca ratios suggest that initial Sr loss during diagenesis is associated with an increase in bulk sediment δ^44/40Ca values, a geochemical signature that is characteristic of marine diagenesis (Higgins et al., 2018). This correlation between Sr/Ca ratios and δ^44/40Ca values appears to be reversed - lower Sr/Ca ratios are associated with lower δ^44/40Ca values - in samples with clear petrographic and geochemical evidence for meteoric alteration. These results highlight the utility of paired Ca and Mg isotope measurements in identifying and distinguishing both early marine and meteoric diagenesis