CO2 and temperature decoupling at the million-year scale during the Cretaceous Greenhouse.

CO2 is considered the main greenhouse gas involved in the current global warming and the primary driver of temperature throughout Earth's history. However, the soundness of this relationship across time scales and during different climate states of the Earth remains uncertain. Here we explore how CO2 and temperature are related in the framework of a Greenhouse climate state of the Earth. We reconstruct the long-term evolution of atmospheric CO2 concentration (pCO2) throughout the Cretaceous from the carbon isotope compositions of the fossil conifer Frenelopsis. We show that pCO2 was in the range of ca. 150-650 ppm during the Barremian-Santonian interval, far less than what is usually considered for the mid Cretaceous. Comparison with available temperature records suggest that although CO2 may have been a main driver of temperature and primary production at kyr or smaller scales, it was a long-term consequence of the climate-biological system, being decoupled or even showing inverse trends with temperature, at Myr scales. Our analysis indicates that the relationship between CO2 and temperature is time scale-dependent at least during Greenhouse climate states of the Earth and that primary productivity is a key factor to consider in both past and future analyses of the climate system.

Oxygen isotope fractionation between bird bone phosphate and drinking water.

Oxygen isotope compositions of bone phosphate (δ18Op) were measured in broiler chickens reared in 21 farms worldwide characterized by contrasted latitudes and local climates. These sedentary birds were raised during an approximately 3 to 4-month period, and local precipitation was the ultimate source of their drinking water. This sampling strategy allowed the relationship to be determined between the bone phosphate δ18Op values (from 9.8 to 22.5‰ V-SMOW) and the local rainfall δ18Ow values estimated from nearby IAEA/WMO stations (from -16.0 to -1.0‰ V-SMOW). Linear least square fitting of data provided the following isotopic fractionation equation: δ18Ow = 1.119 (±0.040) δ18Op - 24.222 (±0.644); R 2 = 0.98. The δ18Op-δ18Ow couples of five extant mallard ducks, a common buzzard, a European herring gull, a common ostrich, and a greater rhea fall within the predicted range of the equation, indicating that the relationship established for extant chickens can also be applied to birds of various ecologies and body masses. Applied to published oxygen isotope compositions of Miocene and Pliocene penguins from Peru, this new equation computes estimates of local seawater similar to those previously calculated. Applied to the basal bird Confuciusornis from the Early Cretaceous of Northeastern China, our equation gives a slightly higher δ18Ow value compared to the previously estimated one, possibly as a result of lower body temperature. These data indicate that caution should be exercised when the relationship estimated for modern birds is applied to their basal counterparts that likely had a metabolism intermediate between that of their theropod dinosaur ancestors and that of advanced ornithurines.