A Tropically Hot Late Jurassic North American Sundance Sea: Evidence for the Oxfordian Warm Interval

The Late Jurassic greenhouse climate was characterized by elevated atmospheric CO ₂ (pCO ₂) at 600–1,000 ppm and sea-surface temperatures substantially warmer than today. These conditions peaked during the Oxfordian, when extensive epicontinental seaways, including the North American Sundance Sea, dominated mid-latitude paleoceanography. We present clumped isotope (Δ ₄₇) and ⁸⁷Sr/ ⁸⁶Sr analyses of Oxfordian belemnite and Gryphaea fossils from the Sundance Sea to reconstruct paleotemperatures (tΔ ₄₇) and assess the balance between open ocean and terrestrial inputs. The data reveal (a) a mean tΔ ₄₇ of 30.6°C (±1.6 1SE), or 27.6°C (±1.6 1SE) if subjected to maximum bond reordering, reflecting unusually high mid-latitude sea-surface temperatures; (b) δ ¹⁸O seawater values ranging from −2.3‰ (±1.5 1SE) to −0.1‰ (±0.7 1SE) within the Sundance Sea; and, (c) evidence of a slight radiogenic enrichment in ⁸⁷Sr/ ⁸⁶Sr (0.706850, offset by 2.0 × 10 ⁻⁵ to 3.0 × 10 ⁻⁵) relative to the reconstructed global seawater curve for the early-mid Oxfordian, indicating a limited connectivity between the Sundance Sea and Panthalassic Ocean. When integrated with existing Jurassic Δ ₄₇ records, these findings provide evidence for an Oxfordian Warm Interval and locally elevated sea-surface temperatures in the Sundance Sea relative to the coeval open ocean, owing to its marginal or semi-enclosed hydrography. This furthermore demonstrates the capacity of the Δ ₄₇ proxy to resolve the upper thermal bounds of Jurassic greenhouse climates.