Some record it hot: Clumped isotope temperatures from Middle Jurassic molluscan aragonite

Clumped isotope (Δ₄₇ ) data from the shell aragonite of Praemytilus strathairdensis, a non-marine Middle Jurassic mytilid from the Inner Hebrides of Scotland, presents a rare geological context for better understanding the effects of diagenesis on Δ₄₇ in skeletal aragonite. Nineteen P. strathairdensis shells gave Δ₄₇ temperatures between 23 ± 5°C and 48 ± 9°C despite having an identical burial/thermal history. During the early Palaeocene these rocks were briefly buried to temperatures warm enough (∼50 to 80°C for <1 my) to sufficiently overcome the activation energy for bond breakage and diffusion in fossil molluscan aragonite. This caused mostly incomplete resetting of some Δ₄₇ derived temperatures to warmer values. As individual shells with the same thermal history record different Δ₄₇ an extrinsic defect regime associated with impurities is implied, possibly varying amounts of water, in the aragonite lattice. Seven P. strathairdensis Δ₄₇ derived temperatures, between 23°C and 30°C, overlap those of within-sequence, early diagenetic calcitic concretion bodies that are not reset. While it is not certain these mollusc anaragonite Δ₄₇ temperatures are wholly unaltered, they may represent a best estimate of Jurassic lagoon water temperatures.