Pierre Sepulchre

Consequences of shoaling of the Central American Seaway determined from modeling Nd isotopes

The Central American Seaway played a pivotal role in shaping global climate throughout the late Cenozoic. Recent geological surveys have provided new constraints on timing of the seaway shoaling, while neodymium isotopic (e Nd) data measured on fossil teeth, debris, and ferromanganese crusts have helped define the history of water masses in the region. Here we provide the first 3-D simulations of e Nd responses to the shoaling seaway. Our model suggests that a narrow and shallow seaway is sufficient to affect interoceanic circulation, that inflow/ outflow balance between the Caribbean and the Antilles responds nonlinearly to sill depth, and that a seaway narrower than 400 km is consistent with an active Atlantic meridional overturning circulation during the late Miocene. Simulated e Nd values in the Caribbean confirm that inputs from radiogenic Pacific waters in the Caribbean decrease as the seaway shoals. Despite model limitations, a comparison between our results and e Nd values recorded in the Caribbean helps constrain the depth of the Central American Seaway through time, and we infer that a depth between 50 and 200 m could have been reached 10 Ma ago.

Tectonic-driven climate change and the diversification of angiosperms

Significance Angiosperm range expansion and diversification have been major biotic upheavals in the Earth history. Mechanisms involved in their successful diversification have mainly called upon intrinsic processes at the plant level, leaving the influence of the global tectonics poorly explored. We investigate evolution of paleogeography and climate and correlate it with the diversification of angiosperms by using a general circulation model. We show that Pangea breakup induced an important expansion of temperate zones during the late Cretaceous which was concomitant to the rise of angiosperms. We suggest that the breakup of Pangea led to the onset of new humid bioclimatic continents, which in turn may have provided new external conditions for ecological expansion of the angiosperms and their diversification. In 1879, Charles Darwin characterized the sudden and unexplained rise of angiosperms during the Cretaceous as an “abominable mystery.” The diversification of this clade marked the beginning of a rapid transition among Mesozoic ecosystems and floras formerly dominated by ferns, conifers, and cycads. Although the role of environmental factors has been suggested [Coiffard C, Gómez B (2012) Geol Acta 10(2):181–188], Cretaceous global climate change has barely been considered as a contributor to angiosperm radiation, and focus was put on biotic factors to explain this transition. Here we use a fully coupled climate model driven by Mesozoic paleogeographic maps to quantify and discuss the impact of continental drift on angiosperm expansion and diversification. We show that the decrease of desertic belts between the Triassic and the Cretaceous and the subsequent onset of long-lasting humid conditions during the Late Cretaceous were driven by the breakup of Pangea and were contemporaneous with the first rise of angiosperm diversification. Positioning angiosperm-bearing fossil sites on our paleobioclimatic maps shows a strong match between the location of fossil-rich outcrops and temperate humid zones, indicating that climate change from arid to temperate dominance may have set the stage for the ecological expansion of flowering plants.

Growth of subtropical forests in Miocene Europe: The roles of carbon dioxide and Antarctic ice volume

The middle Miocene is a crucial period for the evolution of apes, and it corresponds to their appearance in Europe. The dispersion of apes was made possible by tectonic changes and the expansion of their habitat, (sub-) tropical forest, in Europe. The context in which the middle Miocene climatic optimum occurred still lacks constraints in terms of atmospheric p CO 2 and ice-sheet volume and extent. Using a coupled atmosphere-ocean general circulation model (GCM) and dynamic vegetation model, we investigated the sensitivity of Miocene climate and vegetation to p CO 2 levels and Antarctic ice-sheet configurations. Our results indicate that higher than present p CO 2 is necessary to simulate subtropical forest in Western and Central Europe during the middle Miocene, but that a threshold at high p CO 2 makes subtropical forest partly collapse. Moreover, removing ice over Antarctica modifies oceanic circulation and induces warmer and slightly wetter conditions in Europe, which are consistent with the expansion of subtropical forest. These results suggest that a small East Antarctic Ice Sheet (25% of present-day ice volume) together with higher than present p CO 2 values are in better agreement with available European middle Miocene data.

Role of the stratospheric chemistry-climate interactions in the hotclimate conditions of the Eocene

The stratospheric ozone layer plays a key role in atmospheric thermal structure and circulation. Although stratospheric ozone distribution is sensitive to changes in trace gases concentrations and climate, the modifications of stratospheric ozone are not usually considered in climate studies at geological timescales. Here, we evaluate the potential role of stratospheric ozone chemistry in the case of the Eocene hot conditions. Using a chemistry–climate model, we show that the structure of the ozone layer is significantly different under these conditions (4×CO2 climate and high concentrations of tropospheric N2O and CH4). The total column ozone (TCO) remains more or less unchanged in the tropics whereas it is found to be enhanced at midand high latitudes. These ozone changes are related to the stratospheric cooling and an acceleration of stratospheric Brewer–Dobson circulation simulated under Eocene climate. As a consequence, the meridional distribution of the TCO appears to be modified, showing particularly pronounced midlatitude maxima and a steeper negative poleward gradient from these maxima. These anomalies are consistent with changes in the seasonal evolution of the polar vortex during winter, especially in the Northern Hemisphere, found to be mainly driven by seasonal changes in planetary wave activity and stratospheric wavedrag. Compared to a preindustrial atmospheric composition, the changes in local ozone concentration reach up to 40 % for zonal annual mean and affect temperature by a few kelvins in the middle stratosphere. As inter-model differences in simulating deep-past temperatures are quite high, the consideration of atmospheric chemistry, which is computationally demanding in Earth system models, may seem superfluous. However, our results suggest that using stratospheric ozone calculated by the model (and hence more physically consistent with Eocene conditions) instead of the commonly specified preindustrial ozone distribution could change the simulated global surface air temperature by as much as 14 %. This error is of the same order as the effect of non-CO2 boundary conditions (topography, bathymetry, solar constant and vegetation). Moreover, the results highlight the sensitivity of stratospheric ozone to hot climate conditions. Since the climate sensitivity to stratospheric ozone feedback largely differs between models, it must be better constrained not only for deep-past conditions but also for future climates.

The role of East-Tethys seaway closure in the middle Miocene climatic transition (ca. 14 Ma)

The Middle Miocene Climatic Transition (MMCT, approximately 14 Ma) is a key period in Cenozoic cooling and cryospheric expansion. Despite being well documented in isotopic record, the causes of the MMCT are still a matter of debate. Among various hypotheses, some authors suggested that it was due the final closure of the eastern Tethys seaway and subsequent oceanic circulation reorganisation. The aim of the present study is to quantify the impact of varying Tethys seaway depths on middle Miocene ocean and climate, in order to better understand its role in the MMCT. We present four sensitivity experiments with a fully coupled ocean-atmosphere general circulation model. Our results indicate the presence of a warm and salty water source in the northern Indian Ocean when the eastern Tethys is deep open (4000 or 1000 m), which corresponds to the Tethyan Indian Saline Water (TISW) described on the basis of isotopic studies. This water source is absent in the experiments with shallow (250 m) and closed Tethys seaway, inducing strong changes in the latitudinal density gradient and ultimately the reinforcement of the Antarctic Circumpolar Current (ACC). Moreover, when the Tethys seaway is shallow or closed, there is a westward water flow in the Gibraltar Strait that strengthens the Atlantic Meridional Overturning Circulation (AMOC) compared to the experiments with deep-open Tethys seaway. Our results therefore suggest that the shoaling and final closure of the eastern Tethys seaway played a major role in the oceanic circulation reorganisation during the middle Miocene. The results presented here provide new constraints on the timing of the Tethys seaway closure and particularly indicate that, prior to 14 Ma, a deep-open Tethys seaway should have allowed the formation of TISW. Moreover, whereas the final closure of this seaway likely played a major role in the reorganisation of oceanic circulation, we suggest that it was not the main driver of the global cooling and Antarctica ice-sheet expansion during the MMCT. Here we propose that the initiation of the MMCT was caused by an atmospheric pCO2 drawdown and that the oceanic changes due to the Tethys seaway closure amplified the response of global climate and East Antarctic Ice Sheet.