CARBONATE CLUMPED ISOTOPES OF ARAGONITIC FOSSILS FROM THE LATE CRETACEOUS WESTERN INTERIOR SEAWAY AND THE ROLE OF BOND REORDERING IN SIGNAL PRESERVATION (Invited Presentation)

Over the past decade, carbonate clumped isotopes have been effectively applied to both modern and fossilized mollusk shells as a solution to the shortcomings of carbonate oxygen isotope thermometry. Despite this clear promise, previous studies have appealed to clumped isotope disequilibrium, cryptic diagenesis, and solid-state resetting of clumped isotope bonds (“bond reordering”) when interpreting clumped isotope values (Δ₄₇) of aragonitic fossil shells that disagree with informed priors (e.g., paleotemperatures that are too warm or too cool). An example of these challenges is the well-preserved shells of Cretaceous ammonites. These are common and widely distributed cephalopod fossils that occur from the Devonian to the Cretaceous.

In North America, well-preserved ammonite fossils are commonly found in Upper Cretaceous sediments from the Western Interior Seaway (WIS). New measurements of δ¹³C, δ¹⁸O, and Δ₄₇ from 12 exceptionally well-preserved ammonite fossils from WIS sediments in South Dakota yield unrealistically warm clumped isotope paleotemperatures (T(Δ₄₇)), some as high as 45 °C. In contrast, published fossil T(Δ₄₇) from contemporaneous WIS sediments infer water temperatures of 20-25 °C. Most of the shells we have analyzed are petrographically pristine and 100% aragonite, ruling out cryptic diagenesis, excluding the conversion of some aragonite to calcite, but leaving open the possibility of clumped isotope bond reordering.

To derive the kinetics of aragonite clumped isotope bond reordering, we show the results of aragonite heated to 250-350 °C for different time intervals. Our results show that at lower temperatures, aragonite bond reordering can lead to clumped isotope alteration without aragonite converting to calcite, implying kinetics of aragonite bond reordering are faster than aragonite-to-calcite conversion. These new experimental results demonstrate that aragonite more rapidly breaks ¹³C-¹⁸O bonds than does calcite, putting its geologic thermal stability below the canonical ~100°C threshold for millions of years of burial heating. Ultimately this may affect the reliability of implementing clumped isotopes on aragonitic fossils for paleotemperature reconstruction.