SEEING THROUGH DIAGENESIS TO RECONSTRUCT CEPHALOPOD HABITAT AND PALEOENVIRONMENT IN THE LATE CRETACEOUS WESTERN INTERIOR SEAWAY, USA USING TRIPLE OXYGEN ISOTOPES (Invited Presentation)

Triple oxygen isotope analysis is a novel method for understanding ancient ocean chemistry and the habitat of extinct cephalopod mollusks. Traditional δ¹⁸O values of carbonates are an underconstrained system when trying to reconstruct paleoenvironmental information because there are three unknowns: temperature, the δ¹⁸O value of the water, and the degree of carbonate alteration. By including the δ¹⁷O value of a carbonate, we can better constrain the system and calculate the oxygen isotopic value of the water in which it formed. Generally, the δ¹⁷O value covaries with δ¹⁸O value, however there are measurable deviations related to temperature. Triple oxygen isotope analyses reveal whether a carbonate formed in a marine or meteoric setting. During diagenesis, triple oxygen isotope values change in a predictable manner, allowing the estimation of the ‘pristine’ triple oxygen isotope composition of the carbonate. Using a simple fluid-rock interaction model, we can reconstruct the diagenetic fluid to ‘see through’ post-depositional diagenesis and gather paleoenvironmental information.

We analyzed four carbonate concretions that contain well-preserved fossil mollusks (including ammonites – Baculites and Placenticeras), semi-crystalline calcite infill, and carbonaceous matrix to reconstruct diagenetic fluid and estimate equilibrium triple oxygen isotope values for the Late Cretaceous Western Interior Seaway (WIS) of the USA. Two concretions are derived from the late Campanian Baculites compressus ammonite biozone and two from the early Maastrichtian Baculites clinolobatus biozone. All samples are from the offshore environment represented by the Pierre Shale of South Dakota. Between three and four phases in each concretion were analyzed for triple oxygen isotope compositions. Each set of concretions suggest different diagenetic histories and fluid compositions. Samples from the B. compressus zone experienced diagenetic fluids with a δ¹⁸O value of -6‰ while fluid interacting with B. clinolobatus concretions had a composition as low as -13 ‰. We estimate the WIS seawater δ¹⁸O values at ~-1 – 0‰ (VSMOW) and an equilibrium carbonate composition of ~-3.3‰ (PDB), corresponding to seawater temperatures of 28-30 °C. These data are consistent with previous oxygen isotope analysis of ammonites.