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We present a new reference model for the evolution of oceanic lithosphere, which incorporates the effects of incomplete viscous relaxation, radiogenic heating, and secular cooling. The new reference model is based solely on thermal conduction, that is, without involving the occurrence of small‐scale convection, and unlike the plate model, it does not contain unphysical boundary conditions. Yet, our model can explain both bathymetry and the heat flow data on the normal seafloor. The success of the new model owes to the use of realistic material properties in conduction modeling as well as the consideration of all of major processes that take place ubiquitously beneath seafloor. The effect of secular cooling on the bathymetry of old seafloor is particularly notable. Whereas secular cooling brings only weak temperature variations with an amplitude of ∼20 K, it can nonetheless affect global bathymetry substantially owing to the deep sensitivity of long‐wavelength topography kernels. We suggest that the well‐known fact that Earth has been cooling, which was not considered in any of previous reference models, may be the key to the long‐standing puzzle of seafloor flattening. The new reference model is expected to be useful to better quantify the impact of the emplacement of hotspot islands and oceanic plateaus, the effect of small‐scale convection, and the regional history of secular cooling in the convecting mantle. Plain Language Summary: Seafloor deepens as it moves away from mid‐ocean ridges, and this subsidence reflects how the suboceanic mantle loses its heat by conduction. When seafloor becomes older than 70 million years old, however, it starts to deviate upward from what is predicted by the simple law of thermal conduction. A common approach to model such a deviation is to adopt the so‐called plate model, which can suppress seafloor subsidence with a constant temperature boundary condition at a shallow depth (120–140 km), although the real Earth does not contain such a boundary. Here we show that, by taking into account all of major processes intrinsic to the suboceanic mantle, from the cold shallow part to the hotter deep part, it is possible to explain the evolution of seafloor topography as well as heat loss, without invoking an unphysical boundary condition. In particular, this study illustrates that the fact that Earth is cooling, which is long known in Earth sciences, can have considerable effects on the large‐scale behavior of ocean basins. Key Points: A new reference model for the evolution of normal oceanic lithosphere is proposedThe model is based solely on thermal conduction and yet free of unphysical boundary conditionsThe model incorporates the effects of incomplete viscous relaxation, radiogenic heating, and secular cooling [ABSTRACT FROM AUTHOR]

Vigorous convection in Earth's core powers our global magnetic field, which has survived for over three billion years. In this study, we calculate the rate of entropy production available to drive the dynamo throughout geologic time using one-dimensional parameterizations of the evolution of Earth's core and mantle. To prevent a thermal catastrophe in models with realistic Urey ratios, we avoid the conventional scaling for plate tectonics in favor of one featuring reduced convective vigor for hotter mantle. We present multiple simulations that capture the effects of uncertainties in key parameters like the rheology of the lower mantle and the overall thermal budget. Simple scaling laws imply that the heat flow across the core/mantle boundary was elevated by less than a factor of two in the past relative to the present. Another process like the precipitation of magnesium-bearing minerals is therefore required to sustain convection prior to the nucleation of the inner core roughly one billion years ago, especially given the recent, upward revision to the thermal conductivity of the core. Simulations that include precipitation lack a dramatic increase in entropy production associated with the formation of the inner core, complicating attempts to determine its age using paleomagnetic measurements of field intensity. Because mantle dynamics impose strict limits on the amount of heat extracted from the core, we find that the addition of radioactive isotopes like potassium-40 implies less entropy production today and in the past. On terrestrial planets like Venus with more sluggish mantle convection, even precipitation of elements like magnesium may not sustain a dynamo if cooling rates are too slow. [ABSTRACT FROM AUTHOR]

The inception of plate tectonics on Earth and its subsequent evolution are discussed on the basis of theoretical considerations and observational constraints. The likelihood of plate tectonics in the past depends on what mechanism is responsible for the relatively constant surface heat flux that is indicated by the likely thermal history of Earth. The continuous operation of plate tectonics throughout Earth's history is possible if, for example, the strength of convective stress in the mande is affected by the gradual subduction of surface water. Various geological indicators for the emergence of plate tectonics are evaluated from a geodynamical perspective, and they invariably involve certain implicit assumptions about mantle dynamics, which are either demonstrably wrong or yet to be explored. The history of plate tectonics is suggested to be intrinsically connected to the secular evolution of the atmosphere, through sea-level changes caused by ocean-mande interaction. [ABSTRACT FROM AUTHOR]

Abstract: We present a coupled atmosphere–mantle evolution model to investigate a possible connection between the present-day atmospheric budget of radiogenic xenon and the evolution of subsolidus mantle convection since the Hadean. Two different types of heat-flow scaling for mantle convection are tested; whereas a conventional scaling predicts more vigorous convection in the hotter past, a recent one predicts more sluggish dynamics. Extensive degassing expected for a putative magma ocean in the very early Earth as well as considerable atmospheric loss caused by an early intensive solar wind are taken into account by using an effective closure time. The success of modeling results is measured by how closely they can reproduce the present-day abundances of 129Xe⁎ and 136Xe⁎ in the atmosphere. The conventional scaling demands the present-day mantle to be highly radioactive, which in turn indicates a high initial abundance of 244Pu, and because the mantle is very efficiently processed with this scaling, a large amount of plutogenic 136Xe⁎ is predicted to have been degassed into the atmosphere. Various parameter uncertainties are explored by Monte Carlo sampling, and our modeling results suggest that the use of the conventional scaling leads to overprediction of the atmospheric 136Xe⁎ budget, whereas we can satisfy the xenon budget more easily with the recent scaling. [Copyright &y& Elsevier]

Abstract: Small-scale convection in the suboceanic mantle, if present, is commonly thought to manifest in surface heat flux, and the steady-state scaling of sublithospheric convection has often been used to interpret heat flow data from old ocean basins. Relations among small-scale convection, surface heat flux, and the steady-state scaling, however, have been vague. A series of transient cooling modeling are conducted here to quantify such relations. Given the strong temperature-dependency of mantle viscosity, results suggest that small-scale convection could take place without noticeably disturbing surface heat flux, and that the use of steady-state scaling may not be warranted for the present-day suboceanic mantle. [Copyright &y& Elsevier]

Abstract: Seafloor topography has been a key observational constraint on the thermal evolution of oceanic lithosphere, which is the top boundary layer of convection in Earth''s mantle. At least for the first ~70 Myr, the age progression of seafloor depth is known to follow the prediction of half-space cooling, and the subsidence rate is commonly believed to be ~350 m Ma−1/2. Here we show that, based on a new statistical analysis of global bathymetry, the average subsidence rate of normal oceanic lithosphere is likely to be ~320 m Ma−1/2, i.e., ~10% lower than the conventional value. We define the ‘normal’ seafloor as regions uncorrelated with anomalous crust such as hotspots and oceanic plateaus, but the lower subsidence rate appears to be a stable estimate, not depending on how exactly we define the normal seafloor. This low subsidence rate can still be explained by half-space cooling with realistic mantle properties, if the effective thermal expansivity of a viscoelastic mantle is taken into account. In light of a revised model of half-space cooling, the normal seafloor unperturbed by the emplacement of anomalous crust exists for all ages, and the so-called seafloor flattening seems to be mostly caused by hotspots and oceanic plateaus. [Copyright &y& Elsevier]

Abstract: Heat flux from convection in Earth''s mantle has recently been suggested to vary substantially (20–30%) with the Wilson cycle of continental aggregation and dispersal, because of possible changes in the aspect ratio of convective cells, and the present-day heat flux may be at the maximum at such a temporal variation. This possibility of strong temporal fluctuations in heat flux has an important bearing on how we should model the thermal evolution of Earth in general. As most of convective heat flux appears as oceanic heat flux, and changes in oceanic heat flux can cause changes in the global sea-level, the likely amplitude of such a temporal variation can be quantified by long-term eustasy. Though this inference may be complicated by other processes that can affect the global sea level, most of them predict sea-level fall when Pangea was present, allowing to place a likely bound on the temporal variability of heat flux. Given the geologically plausible age–area distribution of seafloor, the present-day oceanic heat flux is likely at the minimum (not the maximum) of a possible temporal fluctuation, and the oceanic heat flux at ∼ 200 Ma cannot be lower than today by more than a few percent. I also suggest that mantle warming by supercontinental insulation is probably up to only ∼ 20 K, though it still has a nontrivial consequence for the global sea level. [Copyright &y& Elsevier]

The frequent formation of large igneous provinces during the opening of the Atlantic Ocean is a surface manifestation of the thermal and chemical state of convecting mantle beneath the supercontinent Pangea. Recent geochemical and geophysical findings from the North Atlantic igneous province all point to the significant role of incomplete mantle mixing in igneous petrogenesis. On the basis of a whole-mantle convection model with chemical tracers, I demonstrate that sublithospheric convection driven by surface cooling can bring up dense fertile mantle without a thermal anomaly. When small-scale convection in the upper mantle breaks down into the lower mantle, strong counter upwelling takes place, entraining a large amount of dense crustal fragments accumulated at the base of the mantle transition zone. This multi-scale mantle mixing could potentially explain a variety of hotspot phenomenology as well as the formation of both volcanic and non-volcanic rifted margins, with a spatially and temporally varying distribution of fertile mantle. [Copyright &y& Elsevier]