The fossil bivalve Angulus benedeni benedeni: a potential seasonally resolved stable-isotope-based climate archive to investigate Pliocene temperatures in the southern North Sea basin

Bivalves record seasonal environmental changes in their shells, making them excellent climate archives. However, not every bivalve can be used for this end. The shells have to grow fast enough so that micrometre- to millimetre-sampling can resolve sub-annual changes. Here, we investigate whether the bivalve Angulus benedeni benedeni is suitable as a climate archive. For this, we use ca. 3-million-year-old specimens from the Piacenzian collected from a temporary outcrop in the Port of Antwerp area (Belgium). The subspecies is common in Pliocene North Sea basin deposits, but its lineage dates back to the late Oligocene and has therefore great potential as a high-resolution archive. A detailed assessment of the preservation of the shell material by micro-X-ray fluorescence, X-ray diffraction, and electron backscatter diffraction reveals that it is pristine and not affected by diagenetic processes. Oxygen isotope analysis and microscopy indicate that the species had a longevity of up to a decade or more and, importantly, that it grew fast and large enough so that seasonally resolved records across multiple years were obtainable from it. Clumped isotope analysis revealed a mean annual temperature of 13.5 ± 3.8 ∘C. The subspecies likely experienced slower growth during winter and thus may not have recorded temperatures year-round. This reconstructed mean annual temperature is 3.5 ∘C warmer than the pre-industrial North Sea and in line with proxy and modelling data for this stratigraphic interval, further solidifying A. benedeni benedeni's use as a climate recorder. Our exploratory study thus reveals that Angulus benedeni benedeni fossils are indeed excellent climate archives, holding the potential to provide insight into the seasonality of several major climate events of the past ∼ 25 million years in northwestern Europe.