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.
Highlights
Proxy data indicate that the Cretaceous–early Paleogene (∼ 140 to 34 Myr 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)
Current temperature estimates predating the Cretaceous period are mostly derived from the oxygen isotope composition of marine carbonate fossils (δ18Oc), with wellknown limitations related to uncertainties in the past δ18O signature of seawater (δ18Ow) (Epstein et al, 1953; Roche et al, 2006; Laugié et al, 2020)
Considering the scarcity of other Early Jurassic temperature proxy data, model-based sea surface temperature (SST), and δ18Ow estimates, we extend the comparison to SST and δ18Ow estimates based on various proxy data and published Earth system simulations for other Jurassic to Eocene intervals (Fig. 5; Supplement)
Summary
Proxy data indicate that the Cretaceous–early Paleogene (∼ 140 to 34 Myr 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; Laugié et al, 2020; Zhu et al, 2020). It remains unclear whether higher climate sensitivity is unique to the Cretaceous–early Paleogene world or is rather a hallmark of Earth’s climate under high atmospheric pCO2. Current temperature estimates predating the Cretaceous period are mostly derived from the oxygen isotope composition of marine carbonate fossils (δ18Oc), with wellknown limitations related to uncertainties in the past δ18O signature of seawater (δ18Ow) (Epstein et al, 1953; Roche et al, 2006; Laugié et al, 2020).
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