Diagenesis of fossil ostracods: Implications for stable isotope based palaeoenvironmental reconstruction

A protocol for assessing the diagenetic alteration of ostracod carapaces for use in palaeoenvironmental reconstruction is proposed, based on comparison between fossil (Carboniferous) and recent specimens. Recent ostracods have a carapace ultrastructure of fine (< 0.5 μm diameter) crystals of primary low-magnesium calcite. The degree of recrystallisation in fossil ostracods is assessed based on the carapace ultrastructure and geochemistry. Six diagenetic stages are identified: 1, replacement of the original finely crystalline calcite carapace with neomorphic calcite; 2, framboidal and euhedral pyrite growth; 3, ferroan calcite carapace replacement and cavity (void) filling cement; 4, ferroan dolomite carapace replacement and void fills; 5, siderite growth; and 6, sphalerite and barite mineralisation. Stage 1 occurs during early shallow burial, stages 2–3 soon after, and stages 4–6 during much later burial (possibly hydrothermal) alteration. Ostracods and macrofossils exhibiting diagenetic stages 1–5 were analysed for oxygen and carbon isotope ratios. The oxygen isotope compositions clearly reflect the diagenetic stages. The most negative oxygen isotopes (δ18O − 12.9‰) represent the precipitation of ferroan dolomite (Stage 4), and the most positive values of the neomorphic calcite (δ18O − 0.9‰) represent Stage 1, and are within the predicted range of Carboniferous seawater (δ18O − 4‰ to 0‰). The diagenetic protocol established here is specific to Palaeozoic fossil ostracods from carbonate and clastic sediments that were subject to shallow burial diagenesis and later hydrothermal alteration. Nevertheless, the approach could be used to assess the utility of fossil ostracods for palaeoenvironmental interpretation from a wide range of geological settings.

Research Highlights
► The geochemistry of Carboniferous and Recent ostracods is compared.
► A protocol for the diagenetic assessment of fossil ostracods is demonstrated.
► Protocol relevant to the isotopic palaeoenvironmental analysis of fossils.
► The extent of diagenesis affects the oxygen isotopic signature of the fossils.
► Pristine fossils retain an isotopic signal similar to Carboniferous seawater.