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 δ¹⁸O_C is a widely used approach in paleo-environmental studies. However, converting δ¹⁸O_C to water temperatures requires information about the isotopic composition of the sea water (δ¹⁸O_SW), which usually can only be estimated for paleo-environments. Especially in shallow marine settings δ¹⁸O_SW can potentially be altered by a strong seasonal variability of fresh water supply. Dual clumped isotopes (Δ₄₇+Δ₄₈) can be employed to determine seawater temperatures independent of δ¹⁸O_SW, as isotopic clumping is independent of the bulk isotopic composition of seawater.

In the current study we aim to resolve the seasonal variability of δ¹⁸O_SW by the combined measurement of δ¹⁸O_C and dual clumped isotopes (Δ₄₇+Δ₄₈) in Eocene bivalve shells (Venericor planicosta) from the Paris Basin. To determine seasonal variations and to detect annual extrema, δ¹⁸O_C was measured along the shell. Based on the δ¹⁸O_C record, annual extrema were resampled for dual clumped analysis.

The analysed bivalve revealed a pronounced seven-year seasonal cycle in δ¹⁸O_C, yielding an average annual amplitude of 2.5‰. Translating this seasonal range to sea water temperatures, applying a constant δ¹⁸O_SW, 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 δ¹⁸O_SW with a seasonal difference of ~1‰.

The Δ₄₇+Δ₄₈-based seasonal temperature amplitude agrees with an Eocene warm-house climate and a considered weaker latitudinal thermal gradient. Reconstructed δ¹⁸O_SW exhibits high seasonal variability, indicating the periodic influx of isotopically lighter fresh water into the Paris Basin.