148-3 Investigating Offsets in the Stable Isotope Composition of Co-Occurring Deep-Sea Inoceramid Bivalves and Benthic Foraminifera from the Middle Cretaceous Southern High Latitudes

Session: Insights from Microfossils and Their Modern Analogs: From Traditional to Emerging Approaches (Posters)

Presenting Author:

Authors:

Abstract:

Deep-sea cores drilled offshore southern and southwestern Australia at high paleolatitudes during International Ocean Discovery Program (IODP) Expedition 369 recovered well-preserved Cretaceous inoceramid bivalves and foraminifera that were analyzed for their stable oxygen (d¹⁸O) and carbon (d¹³C) isotopic ratios. The stable isotope values of all the benthic fossils measured from the same samples should be similar if they grew their shells in equilibrium with the same water mass. However, Turonian (~92-90 Ma) inoceramid d¹⁸O values from the south-central margin of Australia (IODP Site 1512) are much lower than values in co-occurring benthic foraminifera. One possible explanation for this offset is that the clams and foraminifera grew in different water masses with the clams favoring lower salinity water than the benthic foraminifera.

To test this hypothesis, we compare the stable isotope values of Albian-Cenomanian (~110-95 Ma) inoceramids to values in four species of benthic foraminifera (Lenticulina, Gavelinellinae, Gyroidinoides globosus, and Osangularia schloenbachi) from three deep sea sites (IODP sites 1513, 1514, and 1516) in Mentelle Basin (MB) that overlap in age and were also located at ~60°S paleolatitude but represent a more open ocean setting than Site U1512. Site U1513 inoceramids yield d¹³C values that are higher than benthic foraminifera and nearly the same as planktic foraminifera, and d¹⁸O values that are similar to or much lower than benthic and planktonic foraminifera. Inoceramids from the same Albian-Cenomanian age interval but deeper water at Site U1514 yield stable isotope values that overlap with benthic foraminifera values, with no samples yielding extremely low d¹⁸O values as at Site 1513. Invoking warmer temperatures or growth in lower salinity water to explain low d¹⁸O values of inoceramid at Site 1513 are difficult to reconcile with the oceanographic setting. Alternative explanations include differential diagenesis and disequilibrium isotopic fractionation due to vital effects. Additional stable isotope measurements from middle Cretaceous benthic foraminifera and inoceramid prisms at IODP sites 1513, 1514, and 1516 may help discriminate among hypotheses and will also provide new constraints on the stable isotope systematics of the four calcareous benthic foraminifera species.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7605

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.