Was the Western Interior Seaway Stratified?: Evidence from Late Campanian and Early Maastrichtian (Late Cretaceous) Shipworms

A spectrum of oceanographic models – developed based on a range of different approaches – have been presented to describe the nature of circulation and stratification within the Western Interior Seaway. Inherent to these models, though rarely directly presented, are a range of predictions about the values of and variation in light stable isotopic signatures both horizontally across the seaway and vertically through the water column. Here, we present evidence from the isotopic analyses of exceptionally well-preserved, wood-boring bivalves (Pholadidae) from the Fox Hills Formation (Lower Maastrichtian) and from the upper part of the Pierre Shale (Upper Campanian-Lower Maastrichtian) that precipitated their valves in the uppermost portion of the water column. These specimens were collected from preserved driftwood found sporadically through these units. As a control and also to constrain the potential influence of the wood on isotopic values, modern specimens were also examined. Furthermore, the surrounding matrix and, where present, cements were analyzed to ensure that there was no sample contamination by these components. Within the samples from the Pierre and Fox Hills, the δ¹⁸O and δ¹³C values are -5.77 ± 2.83‰ and -5.88 ± 4.79‰ as well as -4.11 ± 1.57‰ and -9.75 ± 3.19‰, respectively. Even taking into account a -1‰ decrease in oceanic values for a greenhouse interval with no ice sheets, these values are considerably more depleted than a limited analysis of modern specimens (currently n=2) with average δ¹⁸O values of -1.03 and d13C values of -0.54. The δ¹⁸O values are also depleted relative to other benthic bivalves and from normal(?) marine, nektic ammonoids from the same settings and suggest that there was a freshwater cap across at least a portion of the seaway.