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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.

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.

In the present paper, we describe several coleoid jaws discovered in the lower Toarcian black shales, cropping out along the Vilyui River (Yakutia, Russia). This is the first record of a Lower Jurassic coleoid jaw outside Europe and the first report of such a finding from the Mesozoic of Siberia. The described coleoid jaws demonstrate the same mode of preservation and morphology as the coeval jaws previously reported from Europe. Their preservation in Siberia became possible due to the widespread occurrence of black shale facies associated with the early Toarcian oceanic anoxic event (TOAE).

Diverse microfossil assemblages characterise the Lower Jurassic deposits on Svalbard, but until recently nearly all Lower Jurassic macrofossils from this region were known only from re-deposited phosphorite pebbles in the Brentskardhaugen Bed. Here, we briefly describe a unique in situ molluscan assemblage from the lower Toarcian part of the Agardhbukta section. This assemblage is dominated by ammonites (Dactylioceras only), while belemnites and bivalves are less common. Ammonites are typical for the Commune zone of the Arctic and belong to the new species Dactylioceras (D.) kopiki Rogov, sp. nov. and D. (Microdactylites) sp., and their findings are characteristic for a new kopiki biohorizon located in the top of this zone. Bivalves are dominated by Meleagrinella (Clathrolima) substriata providing additional evidence for the age of this assemblage. The pattern of ammonite and bivalve stratigraphical and geographical distributions in the Arctic suggest bilateral molluscan immigrations via the Greenland-Norwegian seaway during the latest early Toarcian: dactylioceratid ammonites migrated northwards, while oxytomid bivalves migrated southwards.