Abstract
Periods of high atmospheric CO2 levels during the Cretaceous-Early Paleogene (~140 to 33 My 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 mid (~40°) paleolatitudes during the Early Jurassic (~180 My ago) based on the clumped isotope (Δ47) and oxygen-isotope (δ18Oc) analyses of mildly 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 mid and polar paleolatitudes, respectively, point to a significant freshwater contribution in Arctic regions. This 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 is a hallmark of greenhouse climates since at least 180 My.
Highlights
30 Proxy data indicate that the Cretaceous-Early Paleogene (~140 to 33 My ago) was characterized by high atmospheric CO2 concentrations, extreme polar warmth and reduced latitudinal temperature gradients (Sluijs et al, 2006; Suan et al, 2017; Evans et al, 2018)
Reconstructed seawater oxygen isotope values (δ18Ow) of – 1.5 to 0.5‰ VSMOW and of – 5 to – 2.5‰ VSMOW at mid and polar paleolatitudes, respectively, point to a significant freshwater contribution in Arctic regions. This highlight the risk of assuming the same δ18Osw value for δ18O-derived 25 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 is a hallmark of greenhouse climates since at least 180 My
Mollusk 160 shells from Warcq showing an aragonite mineralogy revealed microstructures similar to those observed in Dacryomya jacutica, the main differences being that sheet nacreous structures of the studied ammonite shell (ARD-05) shows thinner tablets than those of bivalve shells (Fig. 3)
Summary
30 Proxy data indicate that the Cretaceous-Early Paleogene (~140 to 33 My ago) was characterized by high atmospheric CO2 concentrations, extreme polar warmth and reduced latitudinal temperature gradients (Sluijs et al, 2006; Suan et al, 2017; Evans et al, 2018). Most state-of-the-art climate models hardly reproduce such features, implying either a higher climate sensitivity under greenhouse conditions or systematic biases in proxy data interpretation (Huber and Caballero, 2011; Zhu et al, 2020; Laugié et al, 2020). It remains unclear whether 35 higher climate sensitivity is unique to the Cretaceous-Early Paleogene world or is rather a hallmark of Earth’s climate under high atmospheric pCO2. Modern local geothermal fluxes are lower than 50 mWm-2 (Kerimov et al, 2020), indicative of low geothermal gradient (
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