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The Late Cenomanian Plenus Cold Event is one of the most enigmatic paleoclimate episodes in Earth history with potential to inform understanding of global climate system variability under greenhouse warming conditions, as well as internal feedback pathways that modulate such variability. Following an interpreted massive addition of volcanic CO 2 to the atmosphere and warming that led to a major ocean anoxic event (OAE2), there was a brief interval of cooling recorded in oxygen isotopes and biogeographic data. Here we present evidence that cooling was absent or muted within the Western Interior Seaway (WIS). Clumped isotope data from the basin suggest persistent extreme warmth during the Late Cenomanian, macroinvertebrate fossil assemblages do not record a decrease in temperature, and changes in other paleoceanographic proxies do not correlate temporally with Plenus interval signals from other locales. Using select proxy data to guide construction of GCM model simulations, we explore possible hypotheses to explain these observations. Our results suggest that the paleogeographic configuration of the basin and its gateways to adjoining oceans, which evolved in association with changing pCO 2 and sea level, influenced winter sea ice formation at the northern aperture of the seaway, water mass circulation, salinity, temperature, and water column stratification. We propose that northward advection of warm Tethyan water muted expression of Plenus cooling in the seaway. Understanding the unique character of the Western Interior paleoceanographic record provides critical input for the development of robust models of ancient Earth System dynamics and should aid predictions of future climate system dynamics. • The Plenus Cold Event, one of the most enigmatic paleoclimate episodes in Earth history, was initially documented in Europe. • The mechanistic cause for cooling may have been alkalinization of the global surface ocean related to ocean acidification. • Faunal and geochemical data collectively show that PCE cooling was absent or muted within the central Western Interior basin. • GCM modeling was used to test the impact of pCO 2 decrease on physical oceanographic parameters. • Modeling suggests pCO2 decrease drives northern sea ice formation, affecting water mass circulation and salinity in the basin. [ABSTRACT FROM AUTHOR]

The Arctic region is thought to play a key role in unraveling Mesozoic climate evolution. However, Late Cretaceous climate reconstructions in the high latitudes suffer from contradicting paleoclimatic interpretations. Toward the end of the Cretaceous hot-house, atmospheric CO 2 concentration declined potentially enabling the formation of sea-ice in the Arctic Ocean. We use a coupled atmosphere-ocean climate model to investigate possible effects of different atmospheric CO 2 levels and gateway configurations between the North proto-Atlantic Basin and the Arctic Ocean on the formation of Arctic sea-ice in the latest Cretaceous. Sensitivity tests were run with two atmospheric CO 2 levels (840 and 1120 ppm, representing 3× and 4× pre-industrial concentrations, respectively) with six paleogeographic configurations. In the experiment with 840 ppm CO 2 , seasonal Arctic sea-ice is observed in each gateway configuration in December–June, while for 1120 ppm sea-ice in the central Arctic is either limited or absent, depending on gateway configuration. This suggests the existence of a CO 2 threshold, estimated between 3× and 4× pre-industrial (PI) CO 2 levels. For higher atmospheric CO 2 levels sea-ice formation can only occur by the combined effect of cold winds blowing over the Arctic from continental North America during boreal winter and seawater freshening. The latter can be caused by either very limited or an absence of gateway connections between the Arctic and the open ocean. Such a configuration likely developed in the latest Cretaceous, i.e. close to the Cretaceous/Paleogene boundary interval. • Sea-ice formation in the Arctic is mainly controlled by atmospheric CO 2 concentration. • Simulations predict seasonal Arctic sea-ice formation in the latest Cretaceous. • Onset of winter/spring Arctic sea-ice ranges between 840 and 1120 ppm CO 2. • Enclosed gateway configuration favors seasonal Arctic sea-ice formation. [ABSTRACT FROM AUTHOR]

The early Eocene greenhouse climate maintained by high atmospheric CO 2 concentrations serves as a testbed for future climate changes dominated by increasing CO 2 forcing. In particular, the early Eocene Arctic region is important in the context of future CO 2 driven climate warming in the northern polar region and associated shrinking Arctic sea ice. Here, we present early Eocene Arctic sea ice simulations carried out by six coupled climate models within the framework of the Deep-Time Model Intercomparison Project (DeepMIP). We find differences in sea ice responses to CO 2 changes across the ensemble and compare the results with available proxy-based sea ice reconstructions from the Arctic Ocean. Most of the models simulate seasonal sea ice presence at high CO 2 levels (≥ 840 ppmv = 3× pre-industrial (PI) level of 280 ppmv). However, the threshold when sea ice permanently disappears from the ocean varies considerably between the models (from <840 ppmv to >1680 ppmv). Based on a one-dimensional energy balance model analysis we find that the greenhouse effect likely caused by increased atmospheric water vapor concentration plays an important role in the inter-model spread in Arctic winter surface temperature changes in response to a CO 2 rise from 1× to 3× the PI level. Furthermore, differences in simulated surface salinity in the Arctic Ocean play an important role in the control of local sea ice formation. These differences result from different implementations of river run-off between the models, but also from differences in the exchange of waters between a brackish Arctic and a more saline North Atlantic Ocean that are controlled by the width of the gateway between both basins. As there is no geological evidence for Arctic sea ice in the early Eocene, its presence in most of the simulations with 3× PI CO 2 level indicates either a higher CO 2 level and/or an overly weak polar sensitivity in these models. • Models provide diverging CO2 thresholds for Arctic sea ice presence in early Eocene. • Width of the gateway controls water exchange between Arctic and North Atlantic Oceans. • Higher CO2 levels or changes in Arctic gateway width might reduce model-data mismatch. [ABSTRACT FROM AUTHOR]

In greenhouse world conditions, low seasonality in the polar region can have significant effects on paleobotanical proxy data interpretation of environmental conditions in the Arctic region. Here we present two simulations with a Maastrichtian (~70 Ma) set-up that differ only by atmospheric CO2 levels, applying the Earth system model COSMOS in a coupled atmosphere-ocean configuration. In the first simulation with a CO2 level of 280 ppm (C-280) we observe a strong surface temperature contrast of ~20-25 °C between the summer and winter seasons over the Arctic Ocean. In the second experiment with a CO2 level of 1120 ppm (C-1120) the contrast is highly reduced to ~3 °C. Most of this seasonal temperature contrast reduction stems from relatively warm and sea ice free Arctic winter conditions in C-1120. The key to these winter conditions is the summer warming of the Arctic basin in C-1120 that effectively stores energy due to sea ice free conditions compared to ice covered conditions during summer in C-280. During the winter months, this heat reservoir and associated surface heat fluxes are sufficient to sustain relatively mild Arctic conditions and prevent sea ice formation during polar night. In this context, extremely low seasonality associated with relatively warm winters could have created unusual climatic conditions in the greenhouse Arctic. Therefore, Late Cretaceous sub/arctic plants could have been well adapted to such mild non-actualistic climate environments. [ABSTRACT FROM AUTHOR]