Earth-Science Reviews最新文献

South America figures as one of the most fruitful continents for paleontological research on the Ediacaran-Cambrian transition, with almost 100 years of studies on organisms preserved in carbonates and siliciclastic successions deposited during the birth of the Gondwana supercontinent. However, this scientific record is often scattered among local publications which is part of the reason for the unfamiliarity of geoscientists with the Ediacaran paleontology of this continent. To address this issue, this paper provides a comprehensive overview of Ediacaran paleontology in South America. It achieves this by conducting a thorough assessment of existing research alongside presenting ample new data concerning fossil discoveries. Following current efforts to add new pieces to the complex puzzle on metazoan evolution, this contribution resumes our understanding of the variety and diversity of Ediacaran assemblages in this part of the planet. Positioning South American successions in space and time and comparing them with occurrences worldwide helps us understand the different pulses of extinctions, and their forcings and consequences for life diversification during the Ediacaran. Lastly, by definitively adding the paleontological record of SW Gondwana to the global picture, we seek to contribute to current discussions on the subdivision of the Ediacaran, perhaps the most emblematic period in the geological record.

Submarine debris flows occur under the cloak of the sea and are giants among other types of landslides on planet Earth. They pose a significant threat to sustainable offshore development and marine ecosystems. Existing research on these flows mainly rely on back-analyzing field events and conducting miniaturized experiments. However, it is unclear whether the dynamics of miniaturized flows are similar to field ones. In this review, dimensional analysis is used to evaluate laboratory and field data collated from the literature to compare the dynamics of submarine debris flows at different scales. Miniaturized flows are demonstrated to have disproportionately low yield stress and viscosity compared to field flows. The low yield stress is caused by the need to reduce the clay content of a model debris mixture so that it can flow under substantially reduced gravitational driving stresses in laboratory conditions. Consequently, some proposed scaling relationships in the literature derived from laboratory experiments need to be used with caution. Specifically, both the Reynolds and Bingham numbers cannot independently provide a scale-invariant criterion for distinguishing between laminar and turbulent flows. Instead, the Hampton number, with a threshold >0.001, is proposed for the design of the yield stress and clay contents of laboratory flows. Moreover, reduced model viscous stress drastically reduces erosion potential, which limits the existing understanding of the excess fluid pressures generated at the flow-bed interface, and thus flow mobility. The mobility of field flows is generally attributed to hydroplaning. However, this conjecture mainly stems from experiments with impervious boundaries. Such an idealization exaggerates the effects of excess fluid pressures that develop during hydroplaning. An enhanced understanding of the differences in dynamics between field and modeled flows can improve the design of future experiments to model submarine debris flows.

Methane (CH₄) is a potent greenhouse gas that has a major impact on Earth's climate. CH₄ is accommodated in discrete bubbles in aquatic muds, whose sizes greatly exceed the pore size of the hosting sediment. This critical review examines the mechanics of CH₄ gas in consolidated aquatic muds at the scale of a single bubble and at a macroscale of gassy sediments, obtained from lab experiments, field observations, and numerical and analytical modeling. Linear elastic fracture mechanics (LEFM) theory is shown to control the single bubble shape, size, morphology, and inner pressure evolution over its entire life cycle. Reviewed implications focus on the effects of the inner bubble pressure on its solute exchange with ambient pore waters; on the dynamic water load effect (e.g., waves, tides) on the bubble growth rate and its release from sediment into the water column; and on competitive bubble pair growth in the aquatic muds, the process that presumably shapes the bubble size distribution pattern in muds. Alternatively, gassy sediment effective mechanical and physical characteristics and effective gassy media theories are examined at the macroscale, which makes them suitable for remote sensing acoustic applications. This review indicates, however, that most of the developed macroscale effective medium theories rely on the cumulative sediment gas content. Moreover, no theory for proper upscaling of the entire set of the microscale single bubble descriptors addressed in this review – bubble size distribution, their orientations and spatial locations, and inner bubble pressures – to the effective medium mechanical properties of gassy muds, exists. This review will serve, therefore, as a basis for the improved upscaling, while preserving the basic microscale bubble descriptors, their growth physics, and controls. Laying this foundation will enhance the accuracy of the acoustic applications. Improved assessment of sediment gas retention based on this upscaling will contribute to geohazard prediction and should reduce a long-persisting uncertainty related to CH₄ fluxes from the aquatic sediments.

Geological storage of CO₂ is a promising technique to mitigate anthropogenic CO₂ emissions. The effectiveness of CO₂ storage in the subsurface formations relies on various trapping mechanisms that immobilize the injected CO₂. Among these mechanisms, residual trapping has been identified as a critical factor, closely associated with residual CO₂ saturation. The extent of residual CO₂ saturation is strongly influenced by the petrophysical physicochemical and hydrodynamic properties of CO₂/fluid/rock systems and operational conditions, thereby governing the overall residual trapping efficiency.

This article reviews the published experimental datasets on the initial and residual CO₂ saturation and analyzes the corresponding trapping efficiency for a range of in-situ CO₂/fluid/rock systems. We explore the factors that influence trapping efficiency, including wettability, rock type, rock properties, and flow rate. The gas saturations and trapping efficiencies of different gas types (i.e., CO₂, N₂, and H₂) are also discussed. Finally, we present the knowledge gaps and outline prospects for future research. This review establishes a state-of-art data repository of gas saturations in different conditions, enhancing our understanding of residual trapping in subsurface gas storage.

The Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE 2, ca. 94 Ma) is characterized by a marked positive carbon isotope excursion (CIE) recorded in global marine basins. This CIE results from a global-scale increase in organic matter burial, facilitated by high productivity and seawater deoxygenation. To date, however, the precise pattern of changes in the burial rate of organic matter through the event has not been well constrained. In this work, we present a compilation of data from 42 globally distributed OAE 2 sites, as well as organic carbon isotope ($\delta$¹³C_org), total organic carbon (TOC), and trace element concentration data from a new OAE 2 interval in southern Tibet, China. In southern Tibet, the absence of redox-sensitive trace element enrichment through OAE 2 indicates prevailing oxic conditions. Organic carbon (OC) mass accumulation rate (MAR) at this site decreased from the lower part of the CIE to the upper part, in contrast to an approximate doubling of organic carbon MAR in the upper part observed globally. This result, coupled with detailed analysis of the compilation, shows that redox was a key factor controlling organic burial rates during OAE 2, with OC MAR scaling positively with increasing deoxygenation. Leveraging a biogeochemical model to simulate these data suggets that 5–20% of the seafloor became anoxic during OAE 2, and that this deoxygenation was accompanied by 100% to 200% increase in global seawater P concentration. Our findings indicate that during OAE 2, elevated nutrient levels may have resulted from enhanced recycling from sediments under reducing conditions, sustaining intensified primary production and subsequent organic carbon export and burial.

The East Africa - Arabia topographic swell is an anomalously high-elevation region of ∼4000 km long (from southern Ethiopia to Jordan) and ∼ 1500 km wide (from Egypt to Saudi Arabia) extent. The swell is dissected by the Main Ethiopian, Red Sea, and Gulf of Aden rifts, and characterized by widespread basaltic volcanic deposits emplaced from the Eocene to the present. Geochemical and geophysical data confirm the involvement of mantle processes in swell formation; however, they have not been able to fully resolve some issues, e.g., regarding the number and location of plumes and uplift patterns. This study addresses these questions and provides a general evolutionary model of the region by focusing on the present topographic configuration through a quantitative analysis and correlating long and intermediate wavelength features with mantle and rifting processes. Moreover, the isostatic and dynamic components of topography have been evaluated considering a range of seismic tomographic models for the latter. When interpreted jointly with geological data including volcanic deposits, the constraints do imply causation by a single process which shaped the past and present topography of the study area: the upwelling of the Afar superplume. Once hot mantle material reached the base of the lithosphere below the Horn of Africa during the Late Eocene, the plume flowed laterally toward the Levant area guided by pre-existing discontinuities in the Early Miocene. Plume material reached the Anatolian Plateau in the Late Miocene after slab break-off and the consequent formation of a slab window. During plume material advance, buoyancy forces led to the formation of the topographic swell and tilting of the Arabia Peninsula. The persistence of mantle support beneath the study area for tens of million years also affected the formation and evolution of the Nile and Euphrates-Tigris fluvial networks. Subsequently, surface processes, tectonics, and volcanism partly modified the initial topography and shaped the present-day landscape.