Stable-isotope paleoecology of Campanian–Maastrichtian planktonic foraminifera from the equatorial Pacific Ocean (ODP Leg 198, Hole 1210B, Shatsky Rise)

Several recent stable-isotope studies on Cretaceous planktonic foraminifera found a number of exceptions to the morphology-based scheme in analogy with modern species suggesting that small-sized globigeriniform taxa inhabited shallow/warm layers of the water column while large-sized keeled taxa were deep/cold dwellers. These findings may have deep implications on the paleoceanographic and paleoenvironmental reconstructions based on the abundances of planktonic foraminiferal species, because traditional Cretaceous morphogroups (shallow/deep dwellers, oligotrophic/eutrophic taxa) need to be revised according to stable-isotope data. Here we present about 500 δ18O and δ13C species-specific analyses on planktonic foraminifera from a complete Campanian–Maastrichtian (Late Cretaceous) sequence recovered at Shatsky Rise during ODP Leg 198, Hole 1210B (Pacific Ocean) with the aim to reconstruct species paleoecological preferences, life strategies and depth distribution in the surface water column. Results indicate the development of a thermally-stratified water column only in the mid Maastrichtian, likely mirroring the northward migration of the Shatsky Rise out from the equatorial upwelling area. In this phase, species within most genera yield a tightly clustered isotopic signature suggesting that ecological niches were sharply differentiated. Our data indicate that the large-size double-keeled contusotruncanids inhabited the shallowest/warmest layers of the water column together with the biserial Pseudoguembelina species. Double-keeled globotruncanids mostly inhabited the lower mixed layer to upper thermocline, while the deepest/coldest component of the assemblages were represented by species of the genera Planoglobulina, Globotruncanella and Racemiguembelina that were likely adapted to the thermocline. Moreover, Pseudoguembelina yield particularly depleted δ13C values indicating a preference toward more eutrophic conditions and/or a very high metabolic rate. In addition, we observed a stepwise enrichment in the δ13C and δ18O values of specimens along the phyletic lineage from Pseudotextularia nuttalli to Racemiguembelina fructicosa, possibly indicating a progressive migration toward deeper waters that argues against the presence of photosymbionts in the end-members Racemiguembelina powelli and R. fructicosa. Finally, our results provide new insights on latest Cretaceous paleoenvironments and will contribute to a better understanding of the effects of the K/Pg boundary mass extinction on sea-surface ecosystems.