The giant inoceramid Platyceramus platinus as a high-resolution paleoclimate archive for the Late Cretaceous of the Western Interior Seaway

Platyceramus platinus was a giant inoceramid bivalve that inhabited the outer shelf environments of the Western Interior Seaway (WIS) in North America. With axial heights typically exceeding 1 m, the shells of this species potentially serve as a unique high-resolution geochemical proxy archive for Late Cretaceous paleoclimate. Here we present the first sclerochronological investigation of P. platinus shells to evaluate the usefulness of this species as an archive of short-term (e.g., seasonal to inter-annual) paleoenvironmental variability. We analyzed the growth patterns, the stable oxygen (δ18O) and carbon (δ13C) isotope values of well-preserved P. platinus shell fragments from the Santonian Niobrara Formation at Monument Rocks (Kansas, USA), a National Natural Landmark. A series of diagenetic tests, including cathodoluminescence (CL), scanning electron microscopy (SEM), and geochemical (LA-ICP-MS) analysis, confirmed the good state of preservation of the material. Shell microgrowth patterns suggested lunar daily (circalunidian) growth and that P. platinus grew nearly uninterruptedly throughout the year. Assuming a δ18Ow value of −3.45 ± 0.26‰, reconstructions based on shell δ18O data suggest average seasonal temperature variations between 12.5 ± 3.0 and 25.5 ± 1.1 °C and a mean annual temperature of 17.0 ± 4.1 °C for the outer shelf environment of the WIS. Repeated sudden negative δ13C shifts of up to 2.00‰ and Mn-rich shell growth bands (Mn/Ca ratios up to 90.21 μmol/mol) suggest that P. platinus filter-fed on suspended organic detritus which sank from the upper water column during episodic events. The availability of large amounts of suspended food, however, slowed shell accretion rates of P. platinus. This shell growth behavior combined with the positive δ13C values (0.03-3.96‰) possibly indicate a chemosymbiotic lifestyle that allowed P. platinus to survive under oxygen-depleted conditions at the seafloor.