Land snail Δ47 thermometry using cultured and European natural populations of Clausilia pumila, Succinella oblonga and Trochulus hispidus

Abstract

Clumped isotope composition (Δ₄₇) of aragonitic shells of terrestrial molluscs allow the quantitative reconstructions of continental paleotemperatures from loess-paleosol records of the last 2.5 million years, but it needs testing the method under controlled laboratory conditions and using natural populations. In this study the stable carbon, oxygen and clumped isotope compositions of cultured individuals and field-collected specimens of three species (Clausilia pumila, Succinella oblonga and Trochulus hispidus), widespread in Quaternary loess deposits in Europe, were measured in the Debrecen (ATOMKI) and ETH Zürich laboratories. Our data of cultured snails demonstrate that shells of the three species record Δ₄₇ signatures close to equilibrium conditions with mostly positive (C. pumila), both positive/negative (T. hispidus), and mostly negative (S. oblonga) offsets on the order of 0.0158 ± 0.0105 ‰ (mean offset±1 SD) compared to the Anderson et al. (2021) unified calibration. Calcification temperatures (T_Δ47) derived from C. pumila shells were found to be in excellent agreement with chamber temperatures of 12, 18 and 24 °C, especially in the ATOMKI laboratory (mean T offset: 2.1 ± 2.2 °C, 1 SD), while the other two species exhibited larger discrepancies on the order of 5–8 °C (mean offset). Single shell-based mean T_Δ47 values of field-collected molluscs indicated growth temperatures spanning values from the frost-free period to summer season, or even above summer maxima. The sites of natural populations, characterized by different climate conditions, could be grouped using the aridity index (AI) with values of 0.96–1.06 representing mean temperature of the frost-free period (T_FFP), and AI ranging either from 0.85 to 0.91 or 1.14 to 1.51 reflecting mean summer season or July temperatures (T_JJA/T_J). Multi-shell T_Δ47, obtained by homogenizing the material of 3–5 shells of C. pumila and T. hispidus, reflect T_JJA in most cases. We observed a dependence of the shell oxygen isotope compositions of the three species on oceanity/continentality with more negative/positive values characteristic for more continental/oceanic sites, likely due to moisture source distance and depletion of rainfall in ¹⁸O towards the continental interior. In addition, deviation of body fluid δ¹⁸O values (derived from δ¹⁸O_shell and Δ_47shell) from seasonal precipitation δ¹⁸O is also controlled by an additional temperature effect, being independent of the climate zone of the collection site, but co-varying with the site-specific difference between single shell T_Δ47 and T_JJA.