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Identifying the seasonal variability of the isotopic composition of seawater by combined stable oxygen and dual clumped isotopes in marine bivalves

Reconstructing sea water temperatures from carbonate derived δ18OC is a widely used approach in paleo-environmental studies. However, converting δ18OC to water temperatures requires information about the isotopic composition of the sea water (δ18OSW), which usually can only be estimated for paleo-environments. Especially in shallow marine settings δ18OSW can potentially be altered by a strong seasonal variability of fresh water supply. Dual clumped isotopes (Δ4748) can be employed to determine seawater temperatures independent of δ18OSW, as isotopic clumping is independent of the bulk isotopic composition of seawater.

In the current study we aim to resolve the seasonal variability of δ18OSW by the combined measurement of δ18OC and dual clumped isotopes (Δ4748) in Eocene bivalve shells (Venericor planicosta) from the Paris Basin. To determine seasonal variations and to detect annual extrema, δ18OC was measured along the shell. Based on the δ18OC record, annual extrema were resampled for dual clumped analysis.

The analysed bivalve revealed a pronounced seven-year seasonal cycle in δ18OC, yielding an average annual amplitude of 2.5‰. Translating this seasonal range to sea water temperatures, applying a constant δ18OSW, results in an annual temperature amplitude of ~11°C. The dual clumped isotope measurements, however, point to a more damped seasonal temperature range (~4°C) and a variable δ18OSW with a seasonal difference of ~1‰.

The Δ4748-based seasonal temperature amplitude agrees with an Eocene warm-house climate and a considered weaker latitudinal thermal gradient. Reconstructed δ18OSW exhibits high seasonal variability, indicating the periodic influx of isotopically lighter fresh water into the Paris Basin.


Jorit F. Kniest1, Amelia Davies1, Jonathan A. Todd2, Jens Fiebig1, Jacek Raddatz1
1Goethe-University, Frankfurt a.M., Germany; 2The Natural History Museum, London, UK
GeoBerlin 2023