Polar temperature seasonality from the Early Jurassic

The phenomenon of polar amplification causes high-latitude warming to exceed the global mean and enhances seasonal variations in temperature and precipitation. As such, it complicates the understanding of both modern and past global warming impacts in polar regions. Characterizing the climate change in polar regions is further hampered by limited data coverage and persistent challenges in interpreting local paleoenvironmental archives.

To help address these knowledge gaps, we present bulk and seasonally resolved stable oxygen (δ¹⁸O) and clumped isotope (Δ₄₇) data from exceptionally well-preserved bivalves from the Early Jurassic North Pole (eastern Siberia), spanning the late Pliensbachian icehouse-Toarcian hothouse transition. Upper Pliensbachian Harpax specimens show pronounced seasonal δ¹⁸O variability of 1.5-6 ‰, corresponding to apparent temperature ranges of 6-26 °C assuming invariant seawater δ¹⁸O. In contrast, seasonally resolved clumped isotope data do not yield a statistically significant seasonal temperature difference (1.4 ± 1 °C). Together with a low mean Δ₄₇ temperature of ~3.5 ± 1 °C, this discrepancy implies that the large δ¹⁸O amplitude reflects strong seasonal variability in seawater δ¹⁸O, likely driven by enhanced precipitation, and/or meltwater input in a relatively proximal sedimentary setting with near-freezing mean annual temperatures.

By contrast, Toarcian Dacryomya jacutica specimens show reduced δ¹⁸O variability of 1-2 ‰ (~4-8 °C) and a larger, statistically significant seasonal Δ₄₇ temperature difference of 5.9 ± 1.3 °C, with a mean Δ₄₇ temperature of ~10.3 °C. The agreement between δ¹⁸O and Δ₄₇-derived seasonality indicates a limited contribution of seawater δ¹⁸O variability during the Toarcian and points to a shift toward more distal sedimentary conditions. Collectively, these results provide one of the first quantitative constraints on Arctic temperature seasonality under greenhouse climate conditions in deep time.