Stable isotope analysis of freshwater gastropod carbonates from Swanscombe: Insights into climate stability and seasonality during MIS 11c

ABSTRACT Stable isotope analyses of freshwater mollusc carbonate can record environmental conditions at the time of mineralisation, yet this technique remains underused in British Middle Pleistocene fluvial sequences, despite the abundance of fossil shell material in these deposits. We present new δ 18 O and δ 13 C measurements from calcitic Bithynia tentaculata opercula and aragonitic Valvata piscinalis shells from fluvial sediments at Swanscombe, in the Lower Thames Valley, correlated with the Hoxnian Interglacial (MIS 11c). We assess whether isotope values show sustained change through units correlated with the MIS 11c thermal maximum, and whether inter‐taxon differences provide evidence for seasonal differences in carbonate mineralisation. Across the dataset ( n = 173), δ 18 O and δ 13 C values fall within the range expected for a groundwater‐influenced temperate lowland river system. Sample medians show little systematic stratigraphic change, indicating limited variation in environmental conditions during the sampled intervals of fluvial deposition. After accounting for mineralogy, the inter‐taxon δ 18 O offset is consistent with differences in growth season, whereas the δ 13 C offset likely reflects species‐specific differences in microhabitat and carbon incorporation. The Swanscombe isotope record therefore supports fully temperate MIS 11c conditions, with little evidence for sustained long‐term thermal change during the sampled intervals. Instead, the paired‐taxon record suggests that seasonal signals may be preserved even where mean environmental conditions remained relatively stable. These results demonstrate the value of paired‐taxon stable isotope analysis in fluvial archives for reconstructing climate stability and identifying seasonal signals during Middle Pleistocene interglacials.