Yannick Donnadieu

The Sturtian 'snowball' glaciation: fire and ice

Abstract The Sturtian ‘snowball’ glaciation (730 Ma) is contemporary with the dislocation of the Rodinia supercontinent. This dislocation is heralded and accompanied by intense magmatic events, including the onset of large basaltic provinces between 825 and 755 Ma. Among these magmatic events, the most important one is the onset of a Laurentian magmatic province at 780 Ma around a latitude of 30°N. The presence of these fresh basaltic provinces increases the weatherability of the continental surfaces, resulting in an enhanced consumption of atmospheric CO 2 through weathering, inducing a global long-term climatic cooling. Based on recent weathering laws for basaltic lithology and on climatic model results, we show that the weathering of a 6×10 6 km 2 basaltic province located within the equatorial region (where weathering of the province and consumption of CO 2 are boosted by optimal climatic conditions) is sufficient to trigger a snowball glaciation, assuming a pre-perturbation PCO 2 value of 280 ppmv. We show that the Laurentian magmatic province might be the main culprit for the initiation of the Sturtian ‘snowball’ glaciation, since the Laurentian magmatic province had drifted within the equatorial region by the time of the glaciation.

Fish tooth δ18O revising Late Cretaceous meridional upper ocean water temperature gradients

The oxygen isotope composition of fossil fi sh teeth, a paleo– upper ocean temperature proxy exceptionally resistant to diagenetic alteration, provides new insight on the evolution of the low- to middlelatitude thermal gradient between the middle Cretaceous climatic optimum and the cooler latest Cretaceous period. The new middle Cretaceous low to middle latitude thermal gradient agrees with that previously inferred from planktonic foraminifera δ 18O recovered from Deep Sea Drilling Project and Ocean Drilling Program drilling sites, although the isotopic temperatures derived from δ 18O of fish teeth are uniformly higher by ~3–4 °C. In contrast, our new latest Cretaceous thermal gradient is markedly steeper than those previously published for this period. Fish tooth δ18O data demonstrate that low- to middle-latitude thermal gradients of the middle Cretaceous climatic optimum and of the cooler latest Cretaceous are similar to the modern one, despite a cooling of 7 °C between the two periods. Our new results imply that no drastic changes in meridional heat transport are required to explain the Late Cretaceous climate. Based on climate models, such a cooling without any change in the low to middle latitude thermal gradient supports an atmospheric CO2 decrease as the primary driver of the climatic evolution recorded during the Late 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.

Modeling the early Paleozoic long-term climatic trend.

The early Paleozoic climate has been described as warm and equable. However, recent data based on conodont oxygen isotopic composition reveal a large, long, cooling trend through the Ordovician, followed by an abrupt cooling during the Late Ordovician glaciation. This long-term climate change is associated with a major radiation in the Earth life history. Nonetheless, the driving mechanisms for this cooling trend remain unknown. Carbon dioxide consumption by the weathering of fresh rocks from volcanic arcs has recently been suggested as a possible driver for this climate change. However, the impact of the plate motion context has not been explored yet, although it might have a major impact on atmospheric CO 2 levels. Simulations with a climate model coupled to a biogeochemical model (GEOCLIM) show that the atmospheric CO 2 decreased from more than 20 PAL (∼5600 ppmv) in the Furongian down to approximately 10 PAL (∼2800 ppmv) in the Llandovery before rising again in the Early Devonian. We suggest that changes in geography and exposure of fresh volcanic rocks on continents are required to explain the large CO 2 drawdown that led to the onset of cooler to glacial conditions from the Middle Ordovician to the Llandovery. The weathering of fresh volcanic rocks is itself responsible for 33% of the Late Ordovician atmospheric CO 2 decrease; the rest being related to the continent motion through the intertropical convergence zone (ITCZ). Mean annual continental temperature falls by 3°C in the Early Ordovician, reaching 13.5°C during the glacial interval, and rises to 16°C in the Early Devonian.

Scenario for the evolution of atmospheric pCO2 during a snowball Earth

The snowball Earth theory, initially proposed by J.L. Kirschvink to explain the Neoproterozoic glacial episodes, suggests that the Earth was globally ice covered at 720 Ma (Sturtian episode) and 640 Ma (Marinoan episode). The reduction of the water cycle and the growth of large ice sheets led to a collapse of CO2 consumption through continental weathering and biological carbon pumping. As a consequence, atmospheric CO2 built up linearly to levels allowing escape from a snowball Earth. In this contribution, we question this assumed linear accumulation of CO2 into the atmosphere. Using a numerical model of the carbon-alkalinity cycles, we suggest that during global glaciations, even a limited area of open waters (103 km2) allows an efficient atmospheric CO2 diffusion into the ocean. This exchange implies that the CO2 consumption through the low-temperature alteration of the oceanic crust persists throughout the glaciation. Furthermore, our model shows that rising CO2 during the glaciation increases the efficiency of this sink through the seawater acidification. As a result, the atmospheric CO2 evolution is asymptotic, limiting the growth rate of the atmospheric carbon reservoir. Even after the maximum estimated duration of the glaciation (30 m.y.), the atmospheric CO2 is far from reaching the minimum deglaciation threshold (0.29 bar). Accounting for this previously neglected carbon sink, processes that decrease the CO2 deglaciation threshold must be further explored.

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.

Error analysis of CO2 and O2 estimates from the long-term geochemical model GEOCARBSULF

Long-term carbon and sulfur cycle models have helped shape our understanding of the Phanerozoic history of atmospheric CO2 and O2, but error analyses have been largely limited to testing only a subset of input parameters singly. As a result, the full ranges of probable CO2 and O2 are not quantitatively known. Here we investigate how variation in all 68 input parameters of the GEOCARBSULF model, both singly and in combination, affect estimated CO2 and O2. We improve formulations for land area, runoff, and continental temperature, the latter of which now excludes land area not experiencing chemical weathering. We find our resampled model CO2 and O2 estimates are well bounded and provide high confidence for a “double-hump” in CO2 during the Phanerozoic, with high values during the early Paleozoic and Mesozoic, and low values during the late Paleozoic and late Mesozoic-to-Cenozoic. Our analyses also support a distinct atmospheric O2 peak during the late Paleozoic (>30%) followed by low values near the Triassic-Jurassic boundary (∼10%). Most of the spread in CO2 is contributed by three factors: climate sensitivity to CO2-doubling and the plant-assisted chemical weathering factors LIFE and GYM. CO2 estimates during the Paleozoic to early Mesozoic are highly concordant with independent records from proxies, but are offset to lower values during the globally warm late Mesozoic to early Cenozoic. The model-proxy mismatch for the late Mesozoic can be eliminated with a change in GYM within its plausible range, but no change within plausible ranges can resolve the early Cenozoic mismatch. Either the true value for one or more input parameters during this interval is outside our sampled range, or the model is missing one or more key processes.

A better-ventilated ocean triggered by Late Cretaceous changes in continental configuration

Oceanic anoxic events (OAEs) are large-scale events of oxygen depletion in the deep ocean that happened during pre-Cenozoic periods of extreme warmth. Here, to assess the role of major continental configuration changes occurring during the Late Cretaceous on oceanic circulation modes, which in turn influence the oxygenation level of the deep ocean, we use a coupled ocean atmosphere climate model. We simulate ocean dynamics during two different time slices and compare these with existing neodymium isotope data (ɛNd). Although deep-water production in the North Pacific is continuous, the simulations at 94 and 71 Ma show a shift in southern deep-water production sites from South Pacific to South Atlantic and Indian Ocean locations. Our modelling results support the hypothesis that an intensification of southern Atlantic deep-water production and a reversal of deep-water fluxes through the Caribbean Seaway were the main causes of the decrease in ɛNd values recorded in the Atlantic and Indian deep waters during the Late Cretaceous.

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.

Snowball Earth climate dynamics and Cryogenian geology-geobiology

We review recent observations and models concerning the dynamics of Cryogenian global glaciation and their biological consequences. Geological evidence indicates that grounded ice sheets reached sea level at all latitudes during two long-lived Cryogenian (58 and ≥5 My) glaciations. Combined uranium-lead and rhenium-osmium dating suggests that the older (Sturtian) glacial onset and both terminations were globally synchronous. Geochemical data imply that CO2 was 102 PAL (present atmospheric level) at the younger termination, consistent with a global ice cover. Sturtian glaciation followed breakup of a tropical supercontinent, and its onset coincided with the equatorial emplacement of a large igneous province. Modeling shows that the small thermal inertia of a globally frozen surface reverses the annual mean tropical atmospheric circulation, producing an equatorial desert and net snow and frost accumulation elsewhere. Oceanic ice thickens, forming a sea glacier that flows gravitationally toward the equator, sustained by the hydrologic cycle and by basal freezing and melting. Tropical ice sheets flow faster as CO2 rises but lose mass and become sensitive to orbital changes. Equatorial dust accumulation engenders supraglacial oligotrophic meltwater ecosystems, favorable for cyanobacteria and certain eukaryotes. Meltwater flushing through cracks enables organic burial and submarine deposition of airborne volcanic ash. The subglacial ocean is turbulent and well mixed, in response to geothermal heating and heat loss through the ice cover, increasing with latitude. Terminal carbonate deposits, unique to Cryogenian glaciations, are products of intense weathering and ocean stratification. Whole-ocean warming and collapsing peripheral bulges allow marine coastal flooding to continue long after ice-sheet disappearance. The evolutionary legacy of Snowball Earth is perceptible in fossils and living organisms.