Temperatures Of Late Cretaceous (Campanian) Methane-Derived Authigenic Carbonates From The Western Interior Seaway, South Dakota, Usa, Using Clumped Isotopes

Methane Seep Deposits, Comprising Large, Carbonate-Rich Mounds Formed From Hy-Drocarbon Seepage, Were Widely Distributed In The Late Cretaceous Western Interior Seaway (Wis) Of North America. Well-Pre-Served, Methane-Derived Authigenic Carbon-Ates (Mdacs) From These Deposits Have Been Shown To Retain Petrological, Paleontological, And Geochemical Imprints Of Their Ancient Depositional Setting, All Of Which Are Impor-Tant For Understanding The Dynamics And Evo-Lution Of The Shallow, Epeiric Wis. To Better Characterize The Environmental Conditions Of Wis Seeps, We Applied Clumped Isotope Pa-Leothermometry To Magnesium Calcite Mdac Samples From Five Seep Localities In The Up-Per Campanian Pierre Shale, South Dakota, Usa. We Measured 21 Subsamples, Including 18 Micritic Carbonates And Demonstrated Ap-Parent Clumped Isotope Equilibrium Between Mdacs And Wis Bottom Waters. Extreme 13c Depletion In Most Samples (Δ13c Rang-Ing To −45.44‰) Indicates They Were Pre-Cipitated With Oxidized Methane As A Major Source Of Dissolved Inorganic Carbon, Which Itself Implies A Close Association With Ancient Methanotrophic Metabolism. The Average Clumped Isotope Paleotemperature From The Micritic Carbonates Is 23 ± 7 °C (1σ Standard Deviation), Which Agrees With Bottom Water Paleotemperatures Inferred From Δ18o Mea-Surements Of Mdacs And Well-Preserved Mollusk Shells At Similar Localities In The Wis. The Calculated Average Δ18ow Value For These Samples Is −0.5 ± 1.7‰ (1σ Sd), Which Is In-Distinguishable From Previously Reported Calculation On Campanian Seawater Δ18ow From Fossil Mollusk Shells, But Elevated Above Younger Fossils Collected From Other Locations In The Wis.