Clumped isotope evidence for Early Jurassic extreme polar warmth and high climate sensitivity

Abstract

Abstract. Periods of high atmospheric CO2 levels during the Cretaceous–early Paleogene (∼ 140 to 34 Myr ago) were marked by very high polar temperatures and reduced latitudinal gradients relative to the Holocene. These features represent a challenge for most climate models, implying either higher-than-predicted climate sensitivity to atmospheric CO2 or systematic biases or misinterpretations in proxy data. Here, we present a reconstruction of marine temperatures at polar (> 80∘) and middle (∼ 40∘) paleolatitudes during the Early Jurassic (∼ 180 Myr ago) based on the clumped isotope (Δ47) and oxygen isotope (δ18Oc) analyses of shallow buried pristine mollusc shells. Reconstructed calcification temperatures range from ∼ 8 to ∼ 18 ∘C in the Toarcian Arctic and from ∼ 24 to ∼ 28 ∘C in Pliensbachian mid-paleolatitudes. These polar temperatures were ∼ 10–20 ∘C higher than present along with reduced latitudinal gradients. Reconstructed seawater oxygen isotope values (δ18Ow) of −1.5 ‰ to 0.5 ‰ VSMOW and of −5 ‰ to −2.5 ‰ VSMOW at middle and polar paleolatitudes, respectively, point to a significant freshwater contribution in Arctic regions. These data highlight the risk of assuming the same δ18Osw value for δ18O-derived temperature from different oceanic regions. These findings provide critical new constraints for model simulations of Jurassic temperatures and δ18Osw values and suggest that high climate sensitivity has been a hallmark of greenhouse climates for at least 180 Myr.

Temperature Dependence of Clumped Isotopes (∆47) in Aragonite

“…These findings corroborate measurements in calcitic mollusks showing that clumped isotope values in mollusk carbonates adhere to the same temperature relationship as other carbonates precipitated in equilibrium (except for juvenile oyster shells; Huyghe et al., 2022 ). Clumped isotope analyses in (fossil) mollusk shells thus provide an independent temperature proxy, allowing paleoclimatologists to disentangle the effects of variability in temperature and the hydrological cycle (as measured in δ 18 O w ) throughout geological history down to the seasonal timescale (e.g., Caldarescu et al., 2021 ; de Winter et al., 2021 ; Letulle et al., 2022 ).…”