Eocene seasonality resolved by coupled Ba/Ca and stable oxygen isotope ratios in bivalve shells

The Eocene, as the warmest epoch during the Cenozoic, has received much attention as it can inform us about the features of global warmth, highly relevant to a “high-CO₂” future. However, there is still a lack of knowledge regarding some key features of global warm climates, such as how higher global temperatures might have affected the duration and intensity of seasonality.  Furthermore, recognizing seasonal cycles is essential when interpreting proxy data and reconstructing paleo climate, e.g. in order to understand inter-annual bias between proxies. 

In the current study the seasonal variations in sea surface temperature (SST) and fresh water input into the Anglo-Paris Basin (subjacent areas of the Paleo-North Sea) was investigated. Marginal seas, like the Paleo North Sea, are an important intersection between the continental and marine realm, and are especially sensitive to short-term climate variations.

In order to resolve seasonal and perennial changes in SST and freshwater balance, we measured Ba/Ca, δ¹⁸O, and the clumped isotopic composition (∆₄₇) of exceptionally well-preserved fossil molluscs. Although δ¹⁸O is commonly used for the reconstruction of temperature, its calculation often assumes a constant δ¹⁸O value of seawater, which might not be true on seasonal scales and/or within swallow marine basins. In this context, ∆₄₇ was employed to determine the average temperature amplitude, due to its independence from δ¹⁸O_seawater. Additionally, Ba/Ca was used to account for periods with enhanced fresh water input, because barium mostly enters the oceans via fluvial systems and could therefor indicate seasonally enhanced and isotopic lighter fresh water input.            

The bivalve species Venericor planicosta was employed as proxy archive, due to its long life span (10-20 years) and its wide distribution in the Anglo-Paris Basin during the Eocene. The pristinely preserved, aragonitic bivalve shells were sampled by micro-milling (δ¹⁸O, ∆₄₇), as well as, laser ablation (Ba/Ca), to generate proxy records with high temporal resolution.

The isotopic data reveal well pronounced seasonal oscillation with a sinusoidal shape and a maximum difference of 2‰, from -3,5‰ to -5,5‰. On average, the inter-annual variation of the δ¹⁸O record is around 1‰. The Ba/Ca record, on the other hand, shows a flat background with recurring large and sharp peaks. While the baseline Ba/Ca values are around 20 µmol/mol, the peaks can reach up to 300 µmol/mol. The peaks largely fall together with periods of depleted δ¹⁸O values. These results hint to a possible seasonal bias of temperature records in the Anglo-Paris basin based purely on δ¹⁸O, due to variable δ¹⁸O of seawater. This is further implied by the back-calculation of δ¹⁸O_seawater from ∆₄₇ measurements, revealing a range from 2‰ to -4‰.