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

Abstract. Obtaining temperature data from the mid-Piacenzian warm period (ca. 3 Ma, Pliocene epoch) is a key factor in outlining the impact of projected anthropogenic climate change. The mid-Piacenzian warm period was a high-CO2 world with a paleogeography similar to modern times. The time interval has been used to validate and improve climate model retrodictions, which in turn enables assessing the predictive strength of these models. Validating climate models requires a large array of robust proxy data. Here, we increase the potential of this proxy database by showing that the extinct tellinid bivalve Angulus benedeni benedeni can be used for stable isotope-based temperature reconstructions. This species is found in the Pliocene sediments of the southern North Sea basin. Oxygen isotope and carbonate clumped isotope measurements on the shell of A. benedeni benedeni resulted in a mean annual temperature reconstruction of 13.5±3.8 °C. This is 2.5 °C warmer than today, 3.5 °C warmer than the pre-industrial North Sea, and in line with global Pliocene temperature estimates of +2–4 °C compared to the pre-industrial climate. Limited amounts of clumped isotope data hindered determining summer and winter temperatures, but the oxygen isotope record shows that the growth band spacing of A. benedeni benedeni allows for sampling at a resolution of 2–3 months. The species could live for up to a decade, and therefore has the potential to be used for multiannual seasonality reconstructions. The pristine nature of the aragonitic shell material was verified through electron backscatter diffraction analysis (EBSD), and backed by light microscopy, X-ray diffraction, and X-ray fluorescence. The various microstructures as obtained from the EBSD maps have been described, and they provide a template of pristine A. benedeni benedeni material to which potentially altered shells may be compared. The bivalve A. benedeni benedeni is suitable for high resolution isotope-based paleoclimatic reconstruction and it can be used to unravel the marine conditions in the Pliocene southern North Sea basin at a seasonal scale, yielding enhanced insights into imminent western European climate conditions.