Episodic Convection Research Articles

Capturing a Mode of Intermediate Water Formation in the Red Sea

AbstractFormation of intermediate waters in the northern Red Sea was captured by Seagliders during the winter of 2016. The analysis is based on measurements collected by consequent deployments of Seaglider autonomous underwater vehicles along a 70 km transect, which was run repeatedly from the coast to midbasin during the period from October 2015 to May 2016. The observations revealed typical open ocean convection processes including a preconditioning phase followed by deep convection and spreading of the newly formed waters through lateral exchange. Winter conditions were characterized by surface cooling and enhanced evaporation, which were observed from late October until late February. During this period, the gradual surface cooling and salinity increase resulted in the weakening of the surface stratification. Recurrent cyclonic gyres forced by strong atmospheric events led to episodic convection and the formation of intermediate seawater typical of the upper overturning circulation cell of the Red Sea. Following deep mixing, a remarkable bloom of chlorophyll was detected at the top layer of the water column in response to the nutrients delivery from the deeper layers. The end of winter was marked by the intrusion of lower salinity water presumably advected from the south and the reestablishment of the general cyclonic circulation, typical for the northernmost part of the Red Sea. To the best of our knowledge, this is the first observation study of such convection events in the northern Red Sea.

Singularity analysis of global zircon U-Pb age series and implication of continental crust evolution

Recently compiled global databases of igneous and detrital zircon U-Pb ages have been integrated with other types of isotope data (e.g., neodymium, hafnium, and oxygen) to characterize the episodic growth of the continental crust and the development of supercontinents. However, to what extent do the age peaks reflect the rate of continent crust growth or the differential preservation due to supercontinent settings is a long-standing question. Instead of analyzing amplitudes and periodicity of the age series, here, the analysis focuses on shapes of the individual age peaks described by a power law model for measuring the scale invariant fractality and singularity of time-series records. The results indicate that zircon age distributions around peaks follow power law distribution, regardless of the bin-size used to measure the age distribution. Based on the commonly accepted mechanisms (phase transition, self-organized criticality and multiplicative cascade processes) for generation of power law distributions one can relate the nonlinearity of the age peaks to short spurts of accelerated magmatic activities due to “avalanches” (superplumes, slab breakoff etc.) occurred during episodic convection of the mantle. The exponents of the power law age distributions estimated from the age peaks can be used as an index to quantify the intensity of a singularity. The values of exponents calculated at all major age peaks from five global databases exhibit an episodic nature, with a mean duration of approximately 600–800Myr. Both intensity of the zircon episodes and their duration for the interval from 3Ga to 0.5Ga depict a descending trend, which may signify mantle cooling.

Observations of a Nondeveloping Tropical Disturbance in the Western North Pacific during TCS-08 (2008)

Abstract Large uncertainty still remains in determining whether a tropical cloud cluster will develop into a tropical cyclone. During The Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Campaign (T-PARC)/Tropical Cyclone Structure-2008 (TCS-08) field experiment, over 50 tropical cloud clusters were monitored for development, but only 4 developed into a tropical cyclone. One nondeveloping tropical disturbance (TCS025) was closely observed for potential formation during five aircraft research missions, which provided an unprecedented set of observations pertaining to the large-scale and convective environments of a nondeveloping system. The TCS025 disturbance was comprised of episodic convection that occurred in relation to the diurnal cycle along the eastern extent of a broad low-level trough. The upper-level environment was dominated by two cyclonic cells in the tropical upper-tropospheric trough (TUTT) north of the low-level trough in which the TCS025 circulation was embedded. An in-depth examination of in situ observations revealed that the nondeveloping circulation was asymmetric and vertically misaligned, which led to larger system-relative flow on the mesoscale. Persistent environmental vertical wind shear and horizontal shearing deformation near the circulation kept the system from becoming better organized and appears to have allowed low equivalent potential temperature () air originating from one of the TUTT cells to the north (upshear) to impact the thermodynamic environment of TCS025. This in turn weakened subsequent convection that might otherwise have improved alignment and contributed to the transition of TCS025 to a tropical cyclone.

Spatial and temporal variations in crustal production at the Mid‐Atlantic Ridge, 25°N–27°30′N and 0–27 Ma

AbstractWe use high‐resolution multibeam bathymetry, shipboard gravity, side‐scan sonar images, and magnetic anomaly data collected on conjugate flanks of the Mid‐Atlantic Ridge at 25°N–27°30′N and out to ~27 Ma crust to investigate the crustal evolution of the ridge. Substantial variations in crustal structure and thickness are observed both along and across isochrons. Along isochrons within spreading segments, there are distinct differences in seafloor morphology and gravity‐derived crustal thickness between inside and outside corners. Inside corners are associated with shallow depths, thin crust, and enhanced normal faulting while outside corners have greater depths, thicker crust, and more limited faulting. Across‐isochrons, systematic variations in crustal thickness are observed at two different timescales, one at ~2–3 Myr and another at >10 Myr, and these are attributed to temporal changes in melt supply at the ridge axis. The shorter‐term variations mostly are in‐phase between conjugate ridge flanks, although the actual crustal thickness can be significantly different on the two flanks at any given time. We observe no correlation between crustal thickness and spreading rate. Thus, during periods of low melt supply, tectonic extension must increase to accommodate the full plate separation rate. This extension commonly is concentrated in long‐lived faults on only one side of the axial valley, resulting in strong across‐axis asymmetries in crustal thickness and seafloor morphology. The thin‐crust flank has few volcanic features and exhibits elevated, blocky topography with large‐offset, often irregular faults, while the conjugate thicker‐crust flank shows shorter‐offset, regular faulting, and common volcanic features. The variations in melt supply at the ridge axis most likely are caused either by episodic convection in the subaxial mantle or by variable melting of chemically heterogeneous mantle.

Episodic Earth evolution

Abstract U–Pb ages of zircons from Precambrian granitoids and major rivers are grouped into a series of major peaks at about 2.7, 2.5, 2.1, 1.9 and 1.1 Ga. Recently these peaks have been interpreted as times of enhanced preservation of the continental crust associated with the assembly of supercontinents. An older interpretation, which we support, is that they correspond instead to periods of accelerated crustal growth related to episodic convection of the mantle. In this paper we use fluid mechanics experiments to develop a new model of mantle convection and crustal growth. A dense layer at the base of the mantle persists until 2.7 Ga when it destabilizes and generates large domes that rise into the upper mantle. There they cause a large increase in the rate of subduction which leads to enhanced granite magmatism at convergent margins and thus to a pulse of crustal growth. The domes heat the upper mantle which partially melts at mid-ocean ridges to produce thick oceanic crust that resists subduction. The subsequent period of subdued plate motion is broken by the next generation of mantle domes. Before and after the Archean–Proterozoic period of episodic crustal growth, plate tectonics operated quasi-continuously.

Tectonothermal evolution of solid bodies: terrestrial planets, exoplanets and moons

A framework for understanding the tectonothermal evolution of solid planetary bodies has historically been lacking owing to sparse observational constraints. Developments in simulating the physical interiors and tectonic behaviour of terrestrial planets have allowed insights into the relevant physics and important factors governing planetary behaviour. This contribution summarises the critical factors in determining a planet's tectonic regime, and the application of this framework to understanding terrestrial planet evolution. Advances in modelling have led to the identification of new, unmapped tectonic regimes, such as episodic convection, which has relevance to our understanding of the evolution of the early Earth, Venus and Saturn's moon Enceladus. Coupling of tectonic and atmospheric models for planetary evolution has contributed to our knowledge of Martian and Venusian degassing histories, and recent debate on the tectonic regime of exosolar planets informs outstanding questions on their habitability. Ultimately, a framework for terrestrial planet evolution will couple available cosmochemical, geochemical and astrophysical constraints into an emerging generation of simulation tools, facilitating the mapping of terrestrial planet behaviour over a wide parameter space.

Elastic ice shells of synchronous moons: Implications for cracks on Europa and non-synchronous rotation of Titan

A number of synchronous moons are thought to harbor water oceans beneath their outer ice shells. A subsurface ocean frictionally decouples the shell from the interior. This has led to proposals that a weak tidal or atmospheric torque might cause the shell to rotate differentially with respect to the synchronously rotating interior. Applications along these lines have been made to Europa and Titan. However, the shell is coupled to the ocean by an elastic torque. As a result of centrifugal and tidal forces, the ocean would assume an ellipsoidal shape with its long axis aligned toward the parent planet. Any displacement of the shell away from its equilibrium position would induce strains thereby increasing its elastic energy and giving rise to an elastic restoring torque. In the investigation reported on here, the elastic torque is compared with the tidal torque acting on Europa and the atmospheric torque acting on Titan. Regarding Europa, it is shown that the tidal torque is far too weak to produce stresses that could fracture the ice shell, thus refuting an idea that has been widely advocated. Instead, it is suggested that the cracks arise from time-dependent stresses due to non-hydrostatic gravity anomalies from tidally driven, episodic convection in the satellite’s interior. Two years of Cassini RADAR observations of Titan’s surface have been interpreted as implying an angular displacement of ∼0.24° relative to synchronous rotation. Compatibility of the amplitude and phase of the observed non-synchronous rotation with estimates of the atmospheric torque requires that Titan’s shell be decoupled from its interior. We find that the elastic torque balances the seasonal atmospheric torque at an angular displacement ≲0.05°, effectively coupling the shell to the interior. Moreover, if Titan’s surface were spinning faster than synchronous, the tidal torque tending to restore synchronous rotation would almost certainly be larger than the atmospheric torque. There must either be a problem with the interpretation of the radar observations, or with our basic understanding of Titan’s atmosphere and/or interior.

On the evolution of salt lakes: Episodic convection beneath an evaporating salt lake

The transient evolution of salt lake brines and the accompanying underlying groundwater salinity distribution with depth is discussed. A convective‐diffusive equation with a novel surface source term modeling the evaporative process is introduced, and an approximate analytical solution of this equation is obtained using a perturbation approach. A linear stability analysis of the resulting solution is carried out. For typical field‐scale parameters, the model predicts that instability may initially occur within timescales on the order of weeks or decades and that a near‐surface layer of salty fluid whose thickness is on the order of decimeters is responsible for the initial onset of convection. Furthermore, the analysis indicates that when convection starts, the decimeter layer mixes, and this results in the loss of the destabilizing force. In this period, the salty fluid diffuses downward. This results in a return to a hydrologic system, where the salt profile is then well mixed throughout, but now with a background concentration with a higher average salinity. The process can then repeat for an extensive but finite period as the surface layer and subsurface groundwater concentration eventually become hypersaline and where salt precipitation occurs. We have called this phenomenon episodic convection. Variable‐density groundwater flow and solute transport modeling experiments of a field‐based salt lake system confirm the existence of episodic convection and semiquantitatively validate the findings of the analytical approach.

Elasticity, Yielding and Episodicity in Simple Models of Mantle Convection

We explore the implications of refinements in the mechanical description of planetary constituents on the convection modes predicted by finite-element simulations. The refinements consist in the inclusion of incremental elasticity, plasticity (yielding) and multiple simultaneous creep mechanisms in addition to the usual visco-plastic models employed in the context of unified plate-mantle models. The main emphasis of this paper rests on the constitutive and computational formulation of the model. We apply a consistent incremental formulation of the non-linear governing equations avoiding the computationally expensive iterations that are otherwise necessary to handle the onset of plastic yield. In connection with episodic convection simulations, we point out the strong dependency of the results on the choice of the initial temperature distribution. Our results also indicate that the inclusion of elasticity in the constitutive relationships lowers the mechanical energy associated with subduction events.