The Cenozoic inception and development of the Asian
monsoons remain unclear and have generated much debate, as several
hypotheses regarding circulation patterns at work in Asia during the Eocene
have ...been proposed in the few last decades. These include (a) the existence of
modern-like monsoons since the early Eocene; (b) that of a weak South Asian
monsoon (SAM) and little to no East Asian monsoon (EAM); or (c) a prevalence
of the Intertropical Convergence Zone (ITCZ) migrations, also referred to
as Indonesian–Australian monsoon (I-AM). As SAM and EAM are supposed to have
been triggered or enhanced primarily by Asian palaeogeographic changes, their
possible inception in the very dynamic Eocene palaeogeographic context
remains an open question, both in the modelling and field-based communities.
We investigate here Eocene Asian climate conditions using the IPSL-CM5A2 (Sepulchre et al., 2019)
earth system model and revised palaeogeographies. Our Eocene climate
simulation yields atmospheric circulation patterns in Asia substantially
different from modern conditions. A large high-pressure area is simulated over the
Tethys ocean, which generates intense low tropospheric winds blowing
southward along the western flank of the proto-Himalayan–Tibetan plateau
(HTP) system. This low-level wind system blocks, to latitudes lower than
10∘ N, the migration of humid and warm air masses coming from the
Indian Ocean. This strongly contrasts with the modern SAM, during which
equatorial air masses reach a latitude of 20–25∘ N over India and
southeastern China. Another specific feature of our Eocene simulation is the
widespread subsidence taking place over northern India in the midtroposphere (around 5000 m), preventing deep convective updraught that would
transport water vapour up to the condensation level. Both processes lead to
the onset of a broad arid region located over northern India and over the
HTP. More humid regions of high seasonality in precipitation encircle this
arid area, due to the prevalence of the Intertropical Convergence Zone
(ITCZ) migrations (or Indonesian–Australian monsoon, I-AM) rather than
monsoons. Although the existence of this central arid region may partly
result from the specifics of our simulation (model dependence and palaeogeographic uncertainties) and has yet to be confirmed by proxy records,
most of the observational evidence for Eocene monsoons are located in the
highly seasonal transition zone between the arid area and the more humid
surroundings. We thus suggest that a zonal arid climate prevailed over Asia
before the initiation of monsoons that most likely occurred following Eocene
palaeogeographic changes. Our results also show that precipitation
seasonality should be used with caution to infer the presence of a monsoonal
circulation and that the collection of new data in this arid area is of
paramount importance to allow the debate to move forward.
•Petrographical and geochemical (REE and other trace elements) analysis of sandstones from the Roraima group in the Guiana Shield (Brazil, 1.9 Ga) indicate that their source rocks are mainly ...tonalite, felsic volcanic rock, tholeiitic basalt and granite rocks.•The Chemical Index of Alteration (CIA) after correction of the metasomatism is ∼ 89.8 +/- 3.9 which is in line with the thermochemical calculations (PHREEQC) performed to constraint the main weathering pathways.•Global climatic simulations (FOAM) were performed to evaluate the respective contributions of pCO2), pluviometry and temperature and topographic parameters upon the weathering intensity, which seems to be two orders of magnitude greater than tropical present-day.
The weathering conditions prevailing during Proterozoic times differ likely from Present-day ones mainly because of the climate conditions driven by high CO2 atmospheric levels. The question of weathering intensity onto one of the first continent surface, so-called “Columbia”, is here addressed using the combination of a thermochemical and climatic model. Based on the dissolution of a mix of silicates representative of the presumed fresh rocks and the precipitation of secondary phases, the plausible reaction pathways of weathering seem to indicate that approximately 10% of initial silicates minerals should be dissolved to explain the measured high Chemical Index of Alteration (CIA), classically measured in the rocks belonging to this geological period. In addition, global climatic simulations were performed to evaluate the respective contributions of pCO2, pluviometry, temperature and topographic parameters upon the weathering intensity. This finding could suggest the weathering intensity have been, at the very most, two orders of magnitude greater than tropical present-day one over the Paleoproterozoic era: the runoff seemed to be similar whereas the dissolution reaction extent was enhanced. These calculations are confronted with the geological record of weathering conditions, namely well-preserved sandstone deposits belonging to the 1.9 Ga Roraima Supergroup in the Guiana Shield, Brazil. The field data indicating very high Chemical Index of Alteration are in line with the model calculations.
The role of the palaeogeography on the geological evolution of the global carbon cycle has been suspected since the development of the first global geochemical models in the early 80s. The ...palaeogeography has been rapidly recognized as a key factor controlling the long-term evolution of the atmospheric CO2 through its capability of modulating the efficiency of the silicate weathering. First the role of the latitudinal position of the continents has been emphasized: an averaged low latitudinal position promotes the CO2 consumption by silicate weathering, and is theoretically associated to low CO2 periods. With the increase of model complexity and the explicit consideration of the hydrological cycle, the importance of the continentality factor has been recognized: periods of supercontinent assembly coincide with high pCO2 values due to the development of arid conditions which weaken the silicate weathering efficiency. These fundamental feedbacks between climate, carbon cycle and tectonic have been discovered by pioneer modelling studies and opened new views in the understanding of the history of Earth's climate. Today, some of the key features of the Phanerozoic climate can be explained by: (1) continental drift; (2) small continental blocks moving to tropical belts; and (3) modulation of the climate sensitivity to CO2 by palaeogeography changes. Those results emphasize the need for a careful process-based modelling of the water cycle and climate response to the continental drift.
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 CO2 (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 CO2 partial pressures of around 1 bar could have produced hot climates given a low land fraction and cloud feedback effects. However, such high CO2 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 CO2 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 CO2 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.
•Warm climates with oceans at 60 °C require 1 bar of CO2 at 3.8 Ga, but such CO2 is unstable to weathering.•The carbon cycle favors temperate climates on the early Earth.•Seafloor weathering was the main driver of the carbon cycle on the early Earth.•The weathering of ejecta cooled the Earth during the LHB.•Large impacts could have triggered glaciations.
The Cryogenian Period (717–635 Ma) experienced two low‐latitude “snowball Earth” glaciations, the Sturtian and the Marinoan of contrasting 57 and <16 Myr durations, respectively. A lack of reliable ...age controls on extensional tectonics and associated magmatic rocks during the Marinoan has hampered an understanding of the deglaciation. Furthermore, although deglaciation is generally assumed to have occurred once ongoing magmatism accumulated enough atmospheric CO2, as suggested by cap carbonates, specific geologic evidence linking volcanic events with deglaciation are lacking. Here, we present high‐precision zircon geochronology with chemical abrasion‐isotope‐dilution isotope ratio mass spectrometry that indicates an extensive and thick sequence of rift‐related magmatic rocks in South Qinling, Central China, erupted 2–6 Myr before the termination of the Marinoan. Climate modeling proposes a scenario explaining why the Marinoan was the shorter snowball and how volcanism may have driven the deglaciation.
Plain Language Summary
Volcanic CO2 and dust emissions have been regarded as the major driver for Marinoan deglaciation. Most of CO2 outgassing is associated with seafloor spreading and subduction (arc magmatism), which are ongoing processes on geological timescales. To drastically increase CO2 emissions, there must have been many active rift zones during Marinoan glaciation. We provide age constraints on a previously little‐known major Marinoan rift‐related volcanic suite in South Qinling, which reveals volcanic activity lasting 4 Myr and ending 2 Myr before the termination of the Marinoan snowball Earth. Climate modeling provides constraints for the ice‐age duration deduced from the geological setting.
Key Points
Volcanism is dated at 641–637 Ma using chemical abrasion‐isotope‐dilution isotope ratio mass spectrometry technique
Climate modeling on the volcanism‐related CO2 emission is conducted
Enhanced volcanic‐related dust and CO2 emission can shorten the Marinoan snowball Earth by 2–5 Myr
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 CO
was 10
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 CO
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.
The Ordovician glaciation represents the acme of one of only three major icehouse periods in Earth's Phanerozoic history and is notorious for setting the scene for one of the “big five” mass ...extinction events. Nevertheless, the mechanisms that drove ice sheet growth remain poorly understood and the final extent of the ice sheet crudely constrained. Here using an Earth system model with an innovative coupling method between ocean, atmosphere, and land ice accounting for climate and ice sheet feedback processes, we report simulations portraying for the first time the detailed evolution of the Ordovician ice sheet. We show that the emergence of the ice sheet happened in two discrete phases. In a counterintuitive sequence of events, the continental ice sheet appeared suddenly in a warm climate. Only during the second act, and set against a background of decreasing atmospheric CO2, followed steeply dropping temperatures and extending sea ice. The comparison with abundant sedimentological, geochemical, and micropaleontological data suggests that glacial onset may have occurred as early as the Middle Ordovician Darriwilian, in agreement with recent studies reporting third‐order glacioeustatic cycles during the same period. The second step in ice sheet growth, typified by a sudden drop in tropical sea surface temperatures by ∼8°C and the further extension of a single, continental‐scale ice sheet over Gondwana, marked the onset of the Hirnantian glacial maximum. By suggesting the presence of an ice sheet over Gondwana throughout most of the Middle and Late Ordovician, our models embrace the emerging paradigm of an “early Paleozoic Ice Age.”
Key Points
Earth system model providing the first detailed simulation of Middle to Late Ordovician land ice growth
Model/data comparison suggests a Darriwilian age for glacial onset
A single ice sheet of large extent covered Gondwana during the Hirnantian glacial maximum
The rise of eukaryotes to ecological prominence represents one of the most dramatic shifts in the history of Earth's biosphere. However, there is an enigmatic temporal lag between the emergence of ...eukaryotic organisms in the fossil record and their much later ecological expansion. In parallel, there is evidence for a secular increase in the availability of the key macronutrient phosphorus (P) in Earth's oceans. Here, we use an Earth system model equipped with a size‐structured marine ecosystem to explore relationships between plankton size, trophic complexity, and the availability of marine nutrients. We find a strong dependence of planktonic ecosystem structure on ocean nutrient abundance, with a larger ocean nutrient inventory leading to greater overall biomass, broader size spectra, and increasing abundance of large Zooplankton. If existing estimates of Proterozoic marine nutrient levels are correct, our results suggest that increases in the ecological impact of eukaryotic algae and trophic complexity in eukaryotic ecosystems were directly linked to restructuring of the global P cycle associated with the protracted rise of surface oxygen levels. Our results thus suggest an indirect but potentially important mechanism by which ocean oxygenation may have acted to shape marine ecological function during late Proterozoic time.
The threat posed by powerful Plinian explosive eruptions, which inject large quantities of ash into the atmosphere and produce pyroclastic density currents (PDC) on ground, is mainly controlled by ...eruptive parameters and by the direction and strength of the wind field during the eruption. In most studies, mean wind profiles are used to investigate potential tephra deposit dispersion and to assess volcanic risk. Here we present a detailed reconstruction and reinterpretation of a poorly-understood eruption of Mount Pelée volcano (Martinique), and use it to demonstrate that exclusive use of the average trade wind profile can lead to a misrepresentation of the volcanic risk. The great interest of this eruption stems from its unusual southward dispersion, which encompasses areas that are considered to be safe in current hazard maps and that host major infrastructure. Our new field study and radiocarbon dating show that these deposits are not part of the 2010 BP P3 eruptive sequence as previously thought, but define a so far unknown eruptive event dating back to 13,516 cal BP, which we propose to name the Bellefontaine eruption. The Bellefontaine sequence consists of a basal grey lithic-rich layer resulting from an explosive opening phase that destroyed a pre-existing lava dome, immediately followed by a much thicker, slightly reverse-graded white pumice-fall layer. Their dispersal, thickness, and grain-size distribution are used together with physical models of a volcanic plume to reconstruct the time evolution of the eruption. We find that the mass eruption rate reached 5 × 107 kg s−1, producing a 20-km-high Plinian plume, and that the minimum volume of pyroclastic deposits was 0.18 km3 DRE. 2D simulations of tephra dispersion in the atmosphere performed with HAZMAP show that, unlike the recent eruptions at Mount Pelée volcano, mean seasonal wind profiles cannot explain the southward dispersal of the Bellefontaine deposits. To understand the origin of this unusual dispersion axis, we retrieve forty years of wind data over Martinique by using global atmospheric reanalyses from 1979 by the European Center for Medium-Range Weather Forecasts ERA Interim (hereafter ERA-Interim) and ERA5 (hereafter ERA5). We find that, contrary to previous assumptions, this eruption did not necessarily occur during extreme weather conditions associated with the passing of a hurricane. Looking in detail into the ERA Interim datasets, we observe that the wind direction variability over the past 40 years is very low during the dry season (from December to May), and much larger during the wet season (from June to November), even in the troposphere (≈0 to 18 km), which can occasionally result in northerly winds in the mid- to high troposphere over Martinique. As a similar eruption today would spread volcanic material as far as the prefecture of Fort-de-France and its international airport, a zone classified as safe in current hazard maps, this study highlights the importance of including daily variability of winds in hazard assessment models when considering Plinian eruptions.
•We identify a Plinian fall deposit heading south from Mount Pelée volcano.•We estimate eruptive parameters of the Bellefontaine event dated at 13.5 ka cal BP.•The southward dispersion axis of this event is not consistent with mean wind profiles.•ERA-Interim wind data from 1979 show that northerly winds are possible in Martinique.•Updated volcanic hazard maps in Martinique should consider wind direction variability.
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