Geochemical fingerprints of early diagenesis in shallow-water marine carbonates: Insights from paired δ^44/40Ca and δ²⁶Mg values

We present a suite of major element stable isotope (δ¹³C, δ¹⁸O, δ^44/40Ca, δ²⁶Mg), and selected trace element (Sr/Ca and Mg/Ca) data from Pleistocene sediments from the Great Barrier Reef (IODP Expedition 325), as well as Holocene surface sediments from the Bahamas (Triple Goose Creek, Andros Island) to identify geochemical fingerprints associated with early marine and meteoric diagenesis. Sediments from both sites exhibit co-variation in δ¹³C and δ¹⁸O values, depletion in trace elements, and distinct geochemical trends in δ²⁶Mg and δ^44/40Ca values that reflect differences between diagenetic alteration in marine and meteoric fluids. While marine diagenesis results in lower Sr/Ca ratios, higher δ^44/40Ca values, and little effect on bulk sediment δ²⁶Mg values, meteoric diagenesis leads to lower Sr/Ca ratios, lower δ^44/40Ca values, and lower δ²⁶Mg values. Using a numerical model of diagenesis, we show how diagenetic alteration by meteoric fluids must occur after an initial period of diagenetic alteration by marine fluids, a two-stepped diagenetic history that complicates the interpretation of geochemical data in meteorically altered marine carbonate sediments. Finally, we discuss how paired metal isotopes may serve as a robust indicator of meteoric alteration in ancient shallow-water marine carbonate sediments.