Oxygen Isotope Seasonality in the Upper Mississippian Recorded by Well Preserved Brachiopod Shells at Yashui Section, Guizhou, South China

High‑resolution sampling across growth lines of brachiopod shells for oxygen isotope analysis can reconstruct sea‑surface seasonal temperature variations. The amplitude of intra‑annual δ¹⁸O oscillations (Δδ¹⁸O) is a proxy for seasonality intensity, and its increase is associated with climatic cooling and ice‑sheet expansion. Seasonality – the amplitude of seasonal sea‑surface temperature (SST) variation – is a fundamental regulator of marine physical, chemical and biological processes. Changes in seasonal extremes, rather than shifts in mean annual temperature, profoundly impact ecosystems and are critical for predicting climate‑change vulnerability. Yet deep‑time palaeoseasonality data remain scarce, especially for the Palaeozoic. This is a major gap because the Late Palaeozoic Ice Age (LPIA) was the longest and most intense icehouse of the Phanerozoic, with its main glaciation phase (middle‑upper Visean to Serpukhovian) coinciding with low atmospheric CO₂. Understanding seasonal SST dynamics during this interval is essential for constraining climate models and identifying ecosystem responses to past and future warming. Here we apply this brachiopod‑based approach to a stratigraphically‑ordered collection from the Yashui Section (Guizhou, South China), spanning the upper Visean to upper Serpukhovian. The brachiopod fauna (Shangsi and Baizuo formations) comprises 16 species and 14 genera of Productida, Orthida, Athyridida and Spiriferida, dominated by large linoproductids (Gigantoproductus, Datangia, Latiproductus, Striatifera) of the Latiproductus edelburgensis–Gondolina Association. Cathodoluminescence, SEM microstructural screening and elemental analyses reveal that Gigantoproductusspecies possess a well‑developed, pristine tertiary prismatic layer suitable for isotope analysis, whereas Datangia species show only limited posterior tertiary layer development, with the rest being a thickened, mostly poorly preserved laminar layer. Oxygen isotopes were sampled across growth lines in the tertiary layer of six Gigantoproductus specimens spanning the entire section (sampling step ~0.5 mm). Mean δ¹⁸O values vary from –3.16‰ to –4.02‰; intra‑specimen Δδ¹⁸O ranges from 2.05‰ to 3.65‰. Both parameters peak around 80 m, corresponding to the Visean–Serpukhovian boundary. More positive δ¹⁸O indicates cooler temperatures, while larger Δδ¹⁸O reflects stronger seasonality. The coincidence of both peaks might indicate a distinct glacial pulse in the late Visean, at the onset of sustained icehouse conditions These data from South China provide rare, high‑resolution evidence of sustained SST seasonality during the LPIA’s main phase in the eastern Paleo-Tethys, addressing a critical knowledge gap in Palaeozoic palaeoseasonality. They further demonstrate the value of pristine brachiopod shells as a palaeoclimatic archive capable of resolving seasonal‑scale climate dynamics under ancient icehouse conditions.