T86 Paleoenvironmental Reconstructions from Biogenic Carbonates I

Monday, 5 November 2018
8:00 AM - 12:00 PM
Room 140

CONSTRAINING THE OXYGEN ISOTOPIC COMPOSITION VALUES OF THE LATE CRETACEOUS WESTERN INTERIOR SEAWAY USING BENTHIC BIVALVES

Paper number: T86-14
Program: Technical Sessions
Time: 11:15 AM – 11:30 AM

Authors

Abstract

We present data of the oxygen isotopic composition (δ18O) of benthic bivalves from the epicontinental Western Interior Seaway (WIS) that connected the northern Boreal Sea and the southern Tethys Sea across central North America during the Late Cretaceous. In the late Campanian (75-72 Ma), the WIS began to recede and was completely dried out by the early Paleogene (~65 Ma). Models suggest that the WIS should have δ18O values that are lower than the open ocean normal marine settings due to significant continental freshwater input and the limited restricted connection to the open ocean. We measured δ18O of a variety of well-preserved epifaunal (Anisomyon, Endocostea, Inoceramus, Ostrea, and Pteria) and infaunal (Cucullaea, Cymbophora, Geltena, Lucina, Nucula, and Tenuiptera) bivalves from the upper Campanian and lower Maastrichtian (72-69 Ma) WIS. We compare our WIS measured bivalves’ δ18O values to previous other WIS studies’ δ18O values of bivalves and ammonites in the WIS, and the δ18O values of those of foraminifera, bivalves, and ammonites obtained from a literature survey of the open ocean environments. Overall, the δ18O values of WIS organisms ranged from -14‰ to +1‰. The lowest WIS δ18O values reported here most likely result from diagenetic alteration. However, most WIS δ18O values range from -6‰ to 0‰, which largely overlaps with those of open ocean studies (which ranged from -5‰ to +2‰). A relationship between δ18O values and proximity to the WIS’s shoreline was observed, with lower values closer to the shoreline and higher values towards the WIS’s interior. Notably, even when only considering the best-preserved samples, the WIS demonstrates a larger spread of δ18O values than the open ocean dataset. This spread may be due to (1) freshwater input into the WIS and/or (2) enhanced evaporation in a shallow epicontinental sea. These data support the hypothesis that WIS waters were not significantly different isotopically from the open ocean at this time. Further, even as the WIS began to recede in the late Campanian, WIS δ18O values remain similar in the early Maastrichtian. This suggests that the diminished connection to the open ocean did not substantially modify δ18O values in the WIS.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-322982

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