Frédéric Fluteau

Tectonic Uplift and Eastern Africa Aridification

The history of Eastern African hominids has been linked to a progressive increase of open grassland during the past 8 million years. This trend was explained by global climatic processes, which do not account for the massive uplift of eastern African topography that occurred during this period. Atmosphere and biosphere simulations quantify the role played by these tectonic events. The reduced topographic barrier before 8 million years ago permitted a zonal circulation with associated moisture transport and strong precipitation. Our results suggest that the uplift itself led to a drastic reorganization of atmospheric circulation, engendering the strong aridification and paleoenvironmental changes suggested by the data.

Oxygen isotopes of East Asian dinosaurs reveal exceptionally cold Early Cretaceous climates

Early Cretaceous vertebrate assemblages from East Asia and particularly the Jehol Biota of northeastern China flourished during a period of highly debated climatic history. While the unique characters of these continental faunas have been the subject of various speculations about their biogeographic history, little attention has been paid to their possible climatic causes. Here we address this question using the oxygen isotope composition of apatite phosphate (δ) from various reptile remains recovered from China, Thailand, and Japan. δ values indicate that cold terrestrial climates prevailed at least in this part of Asia during the Barremian—early Albian interval. Estimated mean air temperatures of about 10 ± 4 °C at midlatitudes (∼42 °N) correspond to present day cool temperate climatic conditions. Such low temperatures are in agreement with previous reports of cold marine temperatures during this part of the Early Cretaceous, as well as with the widespread occurrence of the temperate fossil wood genus Xenoxylon and the absence of thermophilic reptiles such as crocodilians in northeastern China. The unique character of the Jehol Biota is thus not only the result of its evolutionary and biogeographical history but is also due to rather cold local climatic conditions linked to the paleolatitudinal position of northeastern China and global icehouse climates that prevailed during this part of the Early Cretaceous.

Is there a conflict between the Neoproterozoic glacial deposits and the snowball Earth interpretation: an improved understanding with numerical modeling

The behavior of the terrestrial glacial regime during the Neoproterozoic glaciations is still a matter of debate. Some papers claim that the glacial sequences cannot be explained with the snowball Earth scenario. Indeed, the near shutdown of the hydrological cycle simulated by climatic models, once the Earth is entirely glaciated, has been put in contrast with the need for active, wet-based continental ice sheets to produce the observed thick glacial deposits. A climate ice-sheet model is applied to the older extreme Neoproterozoic glaciation (around 750 Ma) with a realistic paleogeographic reconstruction of Rodinia. Our climate model shows that a small quantity of precipitation remains once the ocean is completely ice-covered, thanks to sublimation processes over the sea-ice at low latitudes acting as a water vapor source. After 10 ka of the ice-sheet model, the ice volume in the tropics is small and confined as separate ice caps on coastal areas where water vapor condenses. However, after 180 ka, large ice sheets can extend over most of the supercontinent Rodinia. Several areas of basal melting appear while ice sheets reach their ice-volume equilibrium state, at 400 ka, they are located either under the two single-domed ice sheets covering the Antarctica and the Laurentia cratons, or near the ice-sheet margins where fast flow occurs. Only the isolated and high-latitude cratons stay cold-based. Finally, among the simulated ice sheets, most have a dynamic behavior, in good agreement with the needs inferred by the preserved thick formations of diamictite, and share the features of the Antarctica present-day ice sheet. Therefore, our conclusion is that a global glaciation would not have hindered the formation of the typical glacial structures seen everywhere in the rock record of Neoproterozoic times. < 2003 Elsevier Science B.V. All rights reserved.

A GEOCLIM simulation of climatic and biogeochemical consequences of Pangea breakup

Large fluctuations in continental configuration occur throughout the Mesozoic. While it has long been recognized that paleogeography may potentially influence atmospheric CO2 via the continental silicate weathering feedback, no numerical simulations have been done, because of the lack of a spatially resolved climate-carbon model. GEOCLIM, a coupled numerical model of the climate and global biogeochemical cycles, is used to investigate the consequences of the Pangea breakup. The climate module of the GEOCLIM model is the FOAM atmospheric general circulation model, allowing the calculation of the consumption of atmospheric CO2 through continental silicate weathering with a spatial resolution of 7.5°long × 4.5°lat. Seven time slices have been simulated. We show that the breakup of the Pangea supercontinent triggers an increase in continental runoff, resulting in enhanced atmospheric CO2 consumption through silicate weathering. As a result, atmospheric CO2 falls from values above 3000 ppmv during the Triassic down to rather low levels during the Cretaceous (around 400 ppmv), resulting in a decrease in global mean annual continental temperatures from about 20°C to 10°C. Silicate weathering feedback and paleogeography both act to force the Earth system toward a dry and hot world reaching its optimum over the last 260 Myr during the Middle-Late Triassic. In the super continent case, given the persistent aridity, the model generates high CO2 values to produce very warm continental temperatures. Conversely, in the fragmented case, the runoff becomes the most important contributor to the silicate weathering rate, hence producing a CO2 drawdown and a fall in continental temperatures. Finally, another unexpected outcome is the pronounced fluctuation in carbonate accumulation simulated by the model in response to the Pangea breakup. These fluctuations are driven by changes in continental carbonate weathering flux. Accounting for the fluctuations in area available for carbonate platforms, the simulated ratio of carbonate deposition between neritic and deep sea environments is in better agreement with available data.

Extension of Cathaysian flora during the Permian: Climatic and paleogeographic constraints

Abstract Recent paleobotanic investigations have been described during the Late Permian intermingled Cathaysian (South China block), Gondwanian and Euramerian flora assemblage in Oman (Arabian plate). By analogy with present rainforest flora, Cathaysian floral elements in northeastern Gondwana require permanent precipitation, even if a short dry season may be endured. On the contrary, Eurasian flora is most probably adapted to dryness. We investigate the temporal and spatial extension of this surprising flora assemblage using numerical climatic simulations and paleomagnetic reconstructions. We suggest that the settlement of Cathaysian flora was possible as soon as this area reached about 20°S. According to paleomagnetic data, the southeastern Arabian plate reached this latitude during Late Permian, which exactly corresponds to the age of the oldest discovered Cathaysian flora elements in Oman. Moreover, the paleomagnetic configuration of plates allows for easy Chinese to Gondwana connections using migrating blocks in the Tethys or island arcs. However, the climatic simulations suggest the presence of a dry season lasting about 5 months in southeastern Arabia. Such precipitation conditions would probably prevent the settlement of Cathaysian plants. We thus propose that the intermingled assemblage reflected the paleoenvironmental conditions: Cathaysian plants grew in a flood plain with perennial water supply, whereas Euramerian plants lived out of the flood plain over adjacent hills where water was only supplied by seasonal precipitation. Migration paths of these different floras are also discussed.

The Syabru-Bensi hydrothermal system in central Nepal: 1. Characterization of carbon dioxide and radon fluxes

The Syabru-Bensi hydrothermal system (SBHS), located at the Main Central Thrust zone in central Nepal, is characterized by hot (30–62°C) water springs and cold (<35°C) carbon dioxide (CO2) degassing areas. From 2007 to 2011, five gas zones (GZ1–GZ5) were studied, with more than 1600 CO2 and 850 radon flux measurements, with complementary self-potential data, thermal infrared imaging, and effective radium concentration of soils. Measurement uncertainties were evaluated in the field. CO2 and radon fluxes vary over 5 to 6 orders of magnitude, reaching exceptional maximum values of 236 ± 50 kg m−2 d−1 and 38.5 ± 8.0 Bq m−2 s−1, with estimated integrated discharges over all gas zones of 5.9 ± 1.6 t  d−1 and 140 ± 30 MBq d−1, respectively. Soil-gas radon concentration is 40 × 103 Bq m−3 in GZ1–GZ2 and 70 × 103 Bq m−3 in GZ3–GZ4. Strong relationships between CO2 and radon fluxes in all gas zones (correlation coefficient R = 0.86 ± 0.02) indicate related gas transport mechanisms and demonstrate that radon can be considered as a relevant proxy for CO2. CO2 carbon isotopic ratios (δ13C from −1.7 ± 0.1 to −0.5 ± 0.1‰), with the absence of mantle signature (helium isotopic ratios R/RA < 0.05), suggest metamorphic decarbonation at depth. Thus, the SBHS emerges as a unique geosystem with significant deep origin CO2 discharge located in a seismically active region, where we can test methodological issues and our understanding of transport properties and fluid circulations in the subsurface.

Eruptive history of the Karoo lava flows and their impact on early Jurassic environmental change

This paper reports new paleomagnetic and geochronologic data from a ~1500 m-thick composite section belonging to the Drakensberg group, the thickest remnant of the Karoo lavas in Northern Lesotho. Flow-by-flow analysis of paleomagnetic directions reveals twenty-one magnetic directional groups (DG), corresponding to single eruptive events (SEE), and sixteen individual lava flows. The new age determinations of lava flows range from 180.1 ± 1.4 to 182.8 ± 2.6 Ma. These data, combined with previous results allow us to propose that the main part of the Drakensberg group and the Karoo intrusive complex dated around 181-183 Ma may have been erupted over a period as short as 250 kyr, and may have coincided with the two main phases of extinction in the Early Toarcian. This scenario agrees well with the discontinuous rhythm of environmental and biotic perturbations in the late Pliensbachian-Toarcian interval.

A warm or a cold early Earth? New insights from a 3-D climate-carbon model

Abstract Oxygen isotopes in marine cherts have been used to infer hot oceans during the Archean with temperatures between 60 °C (333 K) and 80 °C (353 K). Such climates are challenging for the early Earth warmed by the faint young Sun. The interpretation of the data has therefore been controversial. 1D climate modeling inferred that such hot climates would require very high levels of CO 2 (2–6 bars). Previous carbon cycle modeling concluded that such stable hot climates were impossible and that the carbon cycle should lead to cold climates during the Hadean and the Archean. Here, we revisit the climate and carbon cycle of the early Earth at 3.8 Ga using a 3D climate-carbon model. We find that CO 2 partial pressures of around 1 bar could have produced hot climates given a low land fraction and cloud feedback effects. However, such high CO 2 partial pressures should not have been stable because of the weathering of terrestrial and oceanic basalts, producing an efficient stabilizing feedback. Moreover, the weathering of impact ejecta during the Late Heavy Bombardment (LHB) would have strongly reduced the CO 2 partial pressure leading to cold climates and potentially snowball Earth events after large impacts. Our results therefore favor cold or temperate climates with global mean temperatures between around 8 °C (281 K) and 30 °C (303 K) and with 0.1–0.36 bar of CO 2 for the late Hadean and early Archean. Finally, our model suggests that the carbon cycle was efficient for preserving clement conditions on the early Earth without necessarily requiring any other greenhouse gas or warming process.

Oxygen isotopes suggest elevated thermometabolism within multiple permo-triassic therapsid clades

The only true living endothermic vertebrates are birds and mammals, which produce and regulate their internal temperature quite independently from their surroundings. For mammal ancestors, anatomical clues suggest that endothermy originated during the Permian or Triassic. Here we investigate the origin of mammalian thermoregulation by analysing apatite stable oxygen isotope compositions (δ18Op) of some of their Permo-Triassic therapsid relatives. Comparing of the δ18Op values of therapsid bone and tooth apatites to those of co-existing non-therapsid tetrapods, demonstrates different body temperatures and thermoregulatory strategies. It is proposed that cynodonts and dicynodonts independently acquired constant elevated thermometabolism, respectively within the Eucynodontia and Lystrosauridae + Kannemeyeriiformes clades. We conclude that mammalian endothermy originated in the Epicynodontia during the middle-late Permian. Major global climatic and environmental fluctuations were the most likely selective pressures on the success of such elevated thermometabolism. DOI: http://dx.doi.org/10.7554/eLife.28589.001

Emplacement of inflated Pāhoehoe flows in the Naude’s Nek Pass, Lesotho remnant, Karoo continental flood basalt province: use of flow-lobe tumuli in understanding flood basalt emplacement

Physical volcanological features are presented for a 710-m-thick section, of the Naude’s Nek Pass, within the lower part of the Lesotho remnant of the Karoo Large Igneous Province. The section consists of inflated pāhoehoe lava with thin, impersistent sedimentary interbeds towards the base. There are seven discreet packages of compound and hummocky pāhoehoe lobes containing flow-lobe tumuli, making up approximately 50% of the section. Approximately 45% of the sequence consists of 14 sheet lobes, between 10 and 52-m-thick. The majority of the sheet lobes are in two packages indicating prolonged periods of lava supply capable of producing thick sheet lobes. The other sheet lobes are as individual lobes or pairs, within compound flows, suggesting brief increases in lava supply rate. We suggest, contrary to current belief, that there is no evidence that compound flows are proximal to source and sheet lobes (simple flows) are distal to source and we propose that the presence of flow-lobe tumuli in compound flows could be an indicator that a flow is distal to source. We use detailed, previously published, studies of the Thakurvadi Formation (Deccan Traps) as an example. We show that the length of a lobe and therefore the sections that are ‘medial or distal to source’ are specific to each individual lobe and are dependent on the lava supply of each eruptive event, and as such flow lobe tumuli can be used as an indicator of relative distance from source.