Deep Earth Dynamics Research Articles

Revising palaeoelevation estimates for the Tibetan Plateau by integrating latitude, paleogeographic and paleotemperature data

Quantifying the palaeoelevation of the Tibetan Plateau (TP) is important for investigations of deep earth dynamics, surface uplift, and climate evolution. In particular, high precision palaeogeographic data can provide a more comprehensive understanding of the processes and mechanisms of TP uplift and can help refine palaeoclimate simulations. As a whole, there are complex physical and chemical interactions between the six concentric layers of the Earth. In this study, we used published palaeogeographic and palaeotemperature data to revise the evolution of the TP elevation for six time intervals of the Cenozoic according to the time nodes of key changes in each layer. We then revised the palaeoelevations using palaeotemperature data and linear and polynomial regression models. Our revised palaeoelevation results solve the problems of the low resolution of palaeotemperature data and the low accuracy of palaeogeographic data, and they enable us to fine tune the palaeoelevation data for the studied intervals. Our findings contribute to reconstructions of the palaeogeomorphology and palaeoelevation of the TP and provide supporting data for future studies of the TP system on long timescales.

Misinterpreted Seismic Evidence for Localized Rapid Changes of the Inner Core Boundary Surface

AbstractThe nature of the inner core (IC) temporal changes is of great importance in understanding the deep earth dynamics. The comment by Tian and Wen (2023, https://doi.org/10.1029/2023gl103173) on our previous paper (Yang & Song, 2022, https://doi.org/10.1029/2022gl098393) provided a new observation as evidence against the IC rotation and proposed that our observations are instead from localized rapid changes at the IC surface. Here we argue the opposite to its conclusions. The comment misinterpreted our logic and many of our observations and arguments. Its one new waveform does not contradict with the “rotation” model. The original evidence for the “surface” model is demonstrated to be an artifact from station clock errors and instrument changes. Additionally, the surface model lacks a solid physical and quantitative basis to explain existing seismic evidence. We conclude that the rotation model is currently the best interpretation and the surface model is not a viable alternative.

Detecting dipolarity of the geomagnetic field in the paleomagnetic record

Recovering the geomagnetic field strength in the past is key to understanding deep Earth dynamics and detecting potential geodynamo regimes throughout the history of Earth. To better constrain the predictive power of the paleomagnetic record, we propose an approach based on the analysis of the dependency between geomagnetic field strength and inclination (angle made by the horizontal with the field lines). Based on the outcomes of statistical field models, we show that these two quantities should correlate for a wide range of Earth-like magnetic fields, even with enhanced secular variation, persistent nonzonal components, and severe noise contamination. Focusing on the paleomagnetic record, we show that the correlation is not significant for the Brunhes polarity chron, what we ascribe to inadequate spatiotemporal sampling. In contrast, the correlation is significant for the 1 to 130Ma interval, whereas it only marginally succeeds prior to 130Ma when strict filters on both paleointensities and paleodirections are applied. As we cannot detect significant variations in the strength of the correlation over the 1 to 130Ma interval, we conclude that the Cretaceous Normal Superchron may not be associated with enhanced dipolarity of the geodynamo. The strong correlation obtained prior to 130Ma when strict filters are applied indicates that the ancient field may not be on average so different from the present-day field. If long-term fluctuations nevertheless existed, detecting potential geodynamo regimes during the Precambrian is currently impeded by the sparsity of high-quality data passing strict filters in both paleointensities and paleodirections.

Wavefield distortion imaging of Earth's deep mantle

The seismic wavefield, as recorded at the surface, carries information about the seismic source and Earth's structure along the seismic path, essential for the understanding of the interior of our planet. For 40 years seismic tomography studies have resolved the 3D seismic velocity structure in growing detail using seismic traveltimes and waveforms. These studies have been driving our understanding of the dynamics and evolution of the planet, but are limited in their spatial resolution to imaging scales of a few 100 s to 1000 km due to the constraints of the tomographic inversion. Detailed studies of seismic waveforms can resolve finer scale structure but are often reliant on serendipitous source-receiver combinations and provide very uneven coverage of the planet. Therefore, we often lack an understanding of the fine scale structure of the Earth that is important to understand structures and processes such as mantle plumes or details of slab recycling. Here we show evidence that we can exploit slowness vector deviations of the seismic wavefield to extend our knowledge of Earth structure to smaller scales using large datasets. Analysing seismic array data, we show strong and measurable focussing and defocussing effects of the teleseismic P and Pdiff wavefield sampling the deep Earth. We compare the P-wave results to additional S and Sdiff data and find good agreement between both wavetypes. We can link the wavefield deviations to strong velocity variations assuming sharp boundaries are sampled along the path in the deep mantle. The dataset samples the Pacific and Gulf of Mexico well and shows strong horizontal incidence (backazimuth) deviations in the Pacific (up to 14° westwards) and beneath the Gulf of Mexico (up to 5 to 8° east- and west-ward). The backazimuth deviations are also reflected in slowness deviations in the range of ± 0.8 s/° relating to velocity variations in the range of ± 9 km/s. Using 3D raytracing we are able to forward model the detected backazimuth variations of the P and Pdiff dataset. The high frequencies of the P-waves, density of the ray-paths, and low computational cost of our forward calculation allow us to construct a higher resolution and more detailed model of velocity anomalies under Hawaii than was possible with previous methods. The best-fitting velocity model for the Pacific contains two low-velocity regions located at N25°/W155° and N25°/W165° beneath the tip of the Hawaii Emperor chain. The Pacific anomalies have diameters (D) of 6° and 2° with velocity reductions (dVP) of 8% and 4% with heights (H) above the CMB of 70 km and at least 200 km, respectively. We also detect a fast region of 3% velocity increase in the North Pacific rising at least 300 km above the CMB with a diameter of 12° at N60°/W175°. Beneath the Gulf of Mexico we find ambiguous results with either a slow region (N25°/W85°, H = 200 km, dVP=-3%, D = 2°) or a fast region (N15°/W75°, H = 200 km, dVP = 3%, D = 4°) able to explain the data. We thus show that the directivity information of the seismic wavefield - largely underexploited - can be used to resolve the fine scale velocity structure of the Earth's interior with great accuracy and can deliver additional insight into deep Earth dynamics.

Dense mantle flows periodically spaced below ocean basins

Understanding mantle flow is key to elucidate how deep Earth dynamics relate to tectonics at the global scale. The convective mass transport is reflected in lateral variations of the gravity field, seismic velocities, as well as deformations of the Earth's surface. Yet, upper to mid-mantle dynamics have been difficult to constrain at the medium scales of thousands of km. Here, we analyze the second-order horizontal derivatives of seafloor topography and of the gravity potential over the Pacific and Northern Indian ocean basins, and provide evidence for periodic undulations of 1600-2000 km wavelength in both signals, elongated along the direction of absolute plate motion. We investigate potential crustal and lithospheric sources and show that at least part of this signal must originate below the lithosphere, with alignments of sub-lithospheric upper mantle mass excess below seafloor lows. Furthermore, we find that these alignments coincide geographically over wide areas with similarly periodic slow seismic velocity fingers located at upper mantle depths. These two fields may thus record an intermediate scale of mantle convection below ocean basins, which cannot be explained by purely thermal convection and requires instead lateral variations in composition in the upper mantle. Elucidating the nature of the detected mass excess sources coincident with the slow seismic velocities calls for a joint dynamical modeling of all observations in a thermo-chemical context.

Construction of new archaeointensity reference curve for East Asia from 200 CE to 1100 CE

Archaeomagnetism is a research area that reconstructs ancient geomagnetic fields mainly using baked samples (especially baked artifacts) containing magnetic minerals such as magnetite and hematite. Archaeomagnetism can be applied to reveal temporal changes in the geomagnetic field related to deep-earth dynamics, and to estimate the age of archaeological remains and artifacts. Reconstruction methods for ancient geomagnetic field intensity (archaeointensity) continue to develop rapidly, with various experimental methods having been proposed since 2000. The Tsunakawa-Shaw method is one of the latest experimental methods. The Tsunakawa-Shaw method was applied to reconstruction of the archaeointensity for a large number of samples of baked clay belonging to different ages from archaeological relics in Osaka Prefecture, Japan, and a new archaeointensity reference curve was constructed. The archaeointensities from 98 specimens belonging to 39 kilns were estimated. These data were screened and compiled for each pottery sequence, and finally 6 mean intensity values for the pottery sequence level were obtained. All of the mean intensity values satisfied the selection criteria, which indicates that they have sufficient reliability. It is notable that these newly estimated archaeointensities are lower than those in previous studies in Japan. On the other hand, new archaeointensity values show a complementary trend in the archaeointensity dataset obtained by the IZZI-Thellier method reported in Korea recently. We combined the dataset for Japan obtained in this study and the dataset for Korea, and drew an interpolation curve to construct an archaeointensity reference curve for East Asia covering from ca. 200 CE to ca. 1100 CE. This reference curve includes events of archaeointensity decrease and increase that occurred around 610 CE and around 950 CE, which are a characteristic pattern of the intensity variation found in this study.

Adakitic rocks at convergent plate boundaries: Compositions and petrogenesis

Adakitic rocks are intermediate-acid magmatic rocks characterized by enrichment in light rare-earth elements, depletion in heavy rare-earth elements, positive to negligible Eu and Sr anomalies, and high La/Yb and Sr/Y ratios. Cenozoic adakitic rocks generated by partial melting of subducted oceanic crust (slab) under eclogite-facies conditions (i.e., the original definition of “adakite”) occur mainly in Pacific Rim volcanic arcs (intra-oceanic, continental, and continental-margin island arcs), whereas those generated by partial melting of thickened lower crust occur mainly in Tethyan Tibetan collisional orogens. In volcanic arcs, adakitic melts derived from the melting of subducted oceanic crust metasomatize the mantle wedge to form a unique rock suite comprising adakite-adakite-type high-Mg andesite-Piip-type high-Mg andesite-Nb-rich basalt-boninite. This suite differs from the basalt-andesite-dacite-rhyolite suite formed from mantle wedge metasomatized by fluids derived from subducted oceanic crust. Previously published data indicate that partial melting of mafic rocks can generate adakitic magmas under pressure, temperature, and hydrous conditions of 1.2–3.0 GPa, 800–1000°C, and 1.5–6.0 wt.% H2O, respectively, leaving residual minerals of garnet and rutile with little or no plagioclase. Cenozoic Au and Cu deposits occur proximally to adakitic rocks, with host rocks of some deposits actually being adakitic rocks. Adakitic rocks thus have important implications for both deep-Earth dynamics and Cu-Au mineralization/exploration. Although studies of Cenozoic adakitic rocks have made many important advances, there remain weaknesses in some important areas such as their tectonic settings, petrogenesis, magma sources, melt-mantle interactions of pre-Cenozoic adakitic rocks, and their relationship with the onset of plate tectonics and crustal growth. Future research directions are likely to involve (1) the generation of adakitic magmas by experimental simulations of partial melting of different types of rock (including intermediate-acid rocks) and magma fractional crystallization at different temperatures and pressures, (2) the relationship between magma reservoir evolution and the formation of adakitic rocks, (3) the tectonic setting and petrogenesis of pre-Cenozoic adakitic rocks and related geodynamic processes, (4) interactions between slab melts and the mantle wedge, (5) the formation of Archean adakitic tonalite-trondhjemite-granodiorite and its link to the onset of plate tectonics and crustal growth, and (6) the relationship between the formation of adakitic rocks and metal mineralization in different tectonic settings.

Distributed sensing of microseisms and teleseisms with submarine dark fibers

Sparse seismic instrumentation in the oceans limits our understanding of deep Earth dynamics and submarine earthquakes. Distributed acoustic sensing (DAS), an emerging technology that converts optical fiber to seismic sensors, allows us to leverage pre-existing submarine telecommunication cables for seismic monitoring. Here we report observations of microseism, local surface gravity waves, and a teleseismic earthquake along a 4192-sensor ocean-bottom DAS array offshore Belgium. We observe in-situ how opposing groups of ocean surface gravity waves generate double-frequency seismic Scholte waves, as described by the Longuet-Higgins theory of microseism generation. We also extract P- and S-wave phases from the 2018-08-19 {M}_{w}8.2 Fiji deep earthquake in the 0.01-1 Hz frequency band, though waveform fidelity is low at high frequencies. These results suggest significant potential of DAS in next-generation submarine seismic networks.

Earth history: A journey in time and space from base to top

The invention of a robust and accurate sea-going chronometer transformed navigation in the mid-eighteenth century. The calibration of longitude against the prime meridian at Greenwich, in combination with latitude derived from the positions of celestial bodies gave mariners for the first time confidence that they could calculate their position on the Earth's surface. Until recently, Earth scientists have been in a comparable position of having no way of calculating the longitudes of continents before the Cretaceous. Here I discuss Phanerozoic polar wander and paleogeographies and describe ways of quantitatively establishing ancient longitudes which also establish how the Earth's interior can be linked to its surface in geological time. The first method makes use of the fact that longitudinal uncertainty of continents that were assembled in Pangea can, for subsequent times, be eliminated, if longitude motion is known for only one of these continents. The best assumption is zero-longitude motion for Africa and with this assumption we can show that large igneous provinces (LIPs) and kimberlites almost exclusively erupted above the margins of TUZO and JASON in the lower mantle. This remarkable observation, also considering the effect of true polar wander, has led to a second method the plume generation zone reconstruction method unlocking a way forward in modelling absolute plate motions before Pangea and exploring links between plate tectonics, intra-plate volcanism and Deep Earth dynamics. Conceptually, that link can be viewed as a simple mass-balance: subducted lithosphere slabs restore mass to the mantle and trigger the return flow toward the surface including mantle plumes rising from the margins of TUZO and JASON. The surface manifestations of plumes are hotspot lavas, kimberlites and LIPs.

Erosion-driven vertical motions of the circum Arctic: Comparative analysis of modern topography

Deep and complex geodynamic processes, including the effects of plumes, heat, plate tectonics, and local tectonics control the Earth’s surface. In the Arctic these deep processes are masked by extensive glaciations and associated or roughly synchronous erosion. In this study we aim to reveal these hidden geodynamic processes by modeling erosion backward in time by numerically restoring eroded material and calculating the flexural isostatic response repeatedly iteratively until eroded features are filled. This method estimates erosion recorded in the modern topography and models the influence of that erosion. Although the obvious topographic response to erosion is a lowering of the elevation, our coupled erosion-isostatic response method results in dramatic vertical motions leading to km-scale uplift in fjord carved areas of Scandinavia, Greenland, and Canadian Arctic Archipelago and supporting ancient orogenic belts of northern Siberia and northern Alaska to stay at high elevation. Sensitivity testing confirms the utility of our method over a range of effective elastic plate thicknesses as well as for laterally varying elastic thickness. Comparison of modelling results with observed gravity anomalies shows that our method is valid for both glacial and fluvial affected landscapes but more importantly links surface and deep Earth dynamics. Combined analysis of the gravity anomalies and model results also explains erosion as one of the main mechanisms responsible for gravity signal for tectonically inactive regions and illustrates the interaction of short wavelength erosional processes and large scale, regional processes like active orogenesis and dynamic topography.

The role of deep Earth dynamics in driving the flooding and emergence of New Guinea since the Jurassic

The paleogeography of New Guinea indicates fluctuating periods of flooding and emergence since the Jurassic, which are inconsistent with estimates of global sea level change since the Eocene. The role of deep Earth dynamics in explaining these discrepancies has not been explored, despite the strongly time-dependent geodynamic setting within which New Guinea has evolved. We aim to investigate the role of subduction-driven mantle flow in controlling long-wavelength dynamic topography and its manifestation in the regional sedimentary record, within a tectonically complex region leading to orogeny. We couple regionally refined global plate reconstructions with forward geodynamic models to compare trends of dynamic topography with estimates of eustasy and regional paleogeography. Qualitative corroboration of modelled mantle structure with equivalent tomographic profiles allows us to ground-truth the models. We show that predicted dynamic topography correlates with the paleogeographic record of New Guinea from the Jurassic to the present. We find that subduction at the East Gondwana margin locally enhanced the high eustatic sea levels from the Early Cretaceous (∼145 Ma) to generate long-term regional flooding. During the Miocene, however, dynamic subsidence associated with subduction of the Maramuni Arc played a fundamental role in causing long-term inundation of New Guinea during a period of global sea level fall.

Multispherical interactions and their effects on the Tibetan Plateau's earth system: a review of the recent researches

Abstract The Tibetan Plateau (TP) is a regional Earth system showing very strong interactions among its lithosphere, hydrosphere, cryosphere, biosphere, atmosphere, and anthrosphere. These interactions manifest TP's impact on surrounding regions and reflect TP's response to the global change. Quantifying the multispherical interactions is critically important to understand the TP environment. Our recent years researches including the ongoing program entitled ‘Tibetan Multi-Spheres Interactions and Their Resource-Environment Significance (TIMI)’, the completed program entitled. ‘Paleo-Altitudes of Tibetan Plateau and Environment (PATE)’, as well as the other relating projects have focused on multidisciplinary research approaches and emphasized on three major pathways: Eurasia-Indian plates collision on deep-Earth dynamics, uplift impact on Earth's mantle–crust dynamics, and contemporary interface on land surface and atmospheric dynamics. Our researches have taken in situ measurement as priority and developed several platforms of data acquisition and analysis, including the platforms of water-phase transformations, and ecosystem observations. Our field investigations have been conducted to obtain data about stratum, paleontology, paleoenvironment, genetic differentiation of animals and plants. We have developed conceptual and mathematical models for crust uplift formation, paleoclimate, glacial melt, water–air interface flux, vegetation climate, and soil erosion. We have also assessed the anthropogenic impacts on environment. Our researches have achieved new and reliable redating of the mantle–crust interaction and initial formation of the TP, found the interaction between tectonics and uplift of the TP and resultant paleoaltitude acting as a spreading source; discovered the interaction between the westerlies and Indian monsoon acting as a control chain that dominates the TP's contemporary environment. The scientific results can play fundamental roles in supporting the TP's resource exploration and societal sustainable development.

Probing connections between deep earth and surface processes in a land-locked ocean basin transformed into a giant saline basin: The Mediterranean GOLD project#

During the last decade, the interaction of deep processes in the lithosphere and mantle with surface processes (erosion, climate, sea-level, subsidence, glacio-isostatic readjustment) has been the subject of heated discussion. The use of a multidisciplinary approach linking geology, geophysics, geodesy, modelling, and geotechnology has led to the awareness of coupled deep and surface processes. Deep earth dynamics (topography, erosion, tectonics) are strongly connected to natural hazards such as earthquakes, landslides, and tsunamis; sedimentary mass transfers have important consequences on isostatic movements and on georesources, geothermal energy repartitions. The ability to read and understand the link between deep Earth dynamics and surface processes has therefore important societal impacts. Ground-truthing at carefully-selected sites of investigation is imperative to better understand these connections.Due to its youth (<30 Ma) and its subsidence history, the almost land-locked Gulf of Lion–Sardinia continental margins system provides a unique record of sedimentary deposition from the Miocene to present. Due to its high subsidence rate, palaeoclimatic variations, tectonic events and vertical evolution are all recorded here at very high resolution. The late Miocene isolation and desiccation of the Mediterranean, the youngest and most catastrophic event, the Messinian Salinity Crisis (MSC), induced drastic changes in marine environments: widespread deposition of evaporite (gypsum, anhydrite and halite) in the central basin, and intense subaerial erosion along its periphery. These extraordinary mass transfers from land to sea induced strong isostatic re-adjustments that are archived in the sedimentary record and represent a window to the lithospheric rheology and the deep processes.The GOLD (Gulf of Lion Drilling) project, proposes to explore this unique sedimentary record as well as the nature of the deep crustal structure, providing valuable information about the mechanisms underlying vertical motions in basins and their margins.

Sedimentary markers in the Provençal Basin (western Mediterranean): a window into deep geodynamic processes

AbstractDeep Earth dynamics impact so strongly on surface geological processes that we can use sediment palaeo‐markers as a window into the deeper Earth. Derived from climatic and tectonic erosive actions on the continents, and related to eustasy, subsidence and isostasy, the sediment in a deep basin is the main recorder of these processes. Nevertheless, defining and quantifying the relative roles of parameters that interact to give the final sedimentary architecture is not a simple task. Using a 3D‐grid of seismic and wide‐angle data, boreholes and numerical stratigraphic modelling, we propose here a quantification of post‐rift vertical movements in the Provençal Basin (West Mediterranean) involving three domains of subsidence: seaward tilting on the platform and the slope and purely vertical subsidence in the deep basin. These domains fit the deeper crustal domains highlighted by previous geophysical data. Post‐break‐up sedimentary markers may therefore be used to identify the initial hinge lines of the rifting phase and the subsidence laws.

Crustal structure of northeastern margin of the Tibetan Plateau by receiver function inversion

Using seismic data of about one year recorded by 18 broadband stations of ASCENT project, we obtained 2547 receiver functions in the northeastern Tibetan Plateau. The Moho depths under 14 stations were calculated by applying the H-κ domain search algorithm. The Moho depths under the stations with lower signal-noise ratio (SNR) were estimated by the time delay of the PS conversion. Results show that the Moho depth varies in a range of ∼40–60 km. The Moho near the Haiyuan fault is vague, and its depth is larger than those on its two sides. In the Qinling-Qilian Block, the Moho becomes shallower gradually from west to east. To the east of 105°E, the average depth of the Moho is 45 km, whereas the west is 50 km or even deeper. Combining our results with surface wave research, we suggest a boundary between the Qinling and the Qilian Mountains at around 105°E. S wave velocities beneath 15 stations have been obtained through a linear inversion by using Crust2.0 as an initial model, and the crustal thickness that was derived by H-κ domain search algorithm was also taken into account. The results are very similar to the results of previous active source studies. The resulting figure indicates that low velocity layers developed in the middle and lower crust beneath the transition zone of the Tibet Block and western Qinling, which may be related to regional faults and deep earth dynamics. The velocity of the middle and lower crust increases from the Songpan Block to the northeastern margin of Tibetan Plateau. Based on the velocity of the crust, the distribution of the low velocity zone and the composition of the curst (Poisson’s ratio), we infer that the crust thickening results from the crust shortening along the direction of compression.

Constraining shear wave velocity and density contrast at the inner core boundary with PKiKP/P amplitude ratio

Shear velocity and density contrast across the inner core boundary are essential for studying deep earth dynamics, geodynamo and geomagnetic evolution. In previous studies, amplitude ratio of PKiKP/PcP at short distances and PKiKP/P at larger distances are used to constrain the shear velocity and density contrast, and shear velocity in the top inner core is found to be substantially smaller than the PREM prediction. Here we present a large dataset of PKiKP/P amplitude ratio measured on 420 seismic records at ILAR array in Alaska for the distance range of 80°–90°, where the amplitude ratio is sensitive to shear velocity and density contrast. At high frequency (up to 6 Hz), mantle attenuation is found to have substantial effects on PKiKP/P. After the attenuation effects are taken into account, we find that the density contrast is about 0.2–1.0 g/cm3, and shear velocity of inner core is 3.2–4.0 km/s, close to the PREM (Preliminary Reference Earth Model) prediction (0.6 g/cm3 and 3.5 km/s, respectively). The relatively high shear velocity in inner core does not require large quantities of defects or melts as proposed in previous studies.

The deep carbon cycle and melting in Earth's interior

Carbon geochemistry of mantle-derived samples suggests that the fluxes and reservoir sizes associated with deep cycle are in the order of 10 12–13 g C/yr and 10 22–23 g C, respectively. This deep cycle is responsible for the billion year-scale evolution of the terrestrial carbon reservoirs. The petrology of deep storage modulates the long-term evolution and distribution of terrestrial carbon. Unlike water, which in most of the Earth's mantle is held in nominally anhydrous silicates, carbon is stored in accessory phases. The accessory phase of interest, with increasing depth, typically changes from fluids/melts → calcite/dolomite → magnesite → diamond/Fe-rich alloy/Fe-metal carbide, assuming that the mass balance and oxidation state are buffered solely by silicates. If, however, carbon is sufficiently abundant, it may reside as carbonate even in the deep mantle. If Earth's deep mantle is Fe-metal saturated, carbon storage in metal alloy and as metal carbide cannot be avoided for depleted and enriched domains, respectively. Carbon ingassing to the interior is aided by modern subduction of the carbonated oceanic lithosphere, whereas outgassing from the mantle is controlled by decompression melting of carbonated mantle. Carbonated melting at > 300 km depth or redox melting of diamond-bearing or metal-bearing mantle at somewhat shallower depth generates carbonatitic and carbonated silicate melts and are the chief agents for liberating carbon from the solid Earth to the exosphere. Petrology allows net ingassing of carbon into the mantle in the modern Earth, but in the hotter subduction zones that prevailed during the Hadean, Archean, and Paleoproterozoic, carbonate likely was released at shallow depths and may have returned to the exosphere. Inefficient ingassing, along with efficient outgassing, may have kept the ancient mantle carbon-poor. The influence of carbon on deep Earth dynamics is through inducing melting and mobilization of structurally bound mineral water. Extraction of carbonated melt on one hand can dehydrate the mantle and enhance viscosity; the presence of trace carbonated melt on other may generate seismic low-velocity zones and amplify attenuation.

Integrating deep Earth dynamics in paleogeographic reconstructions of Australia

It is well documented that the Cenozoic progressive flooding of Australia, contemporaneous with a eustatic sea level fall, requires a downward tilting of the Australian Plate towards the SE Asian subduction system. Previously, this large-scale, mantle-convection driven dynamic topography effect has been approximated by computing the time-dependent vertical shifts and tilts of a plane, but the observed subsidence and uplift anomalies indicate a more complex interplay between time-dependent mantle convection and plate motion. We combine plate kinematics with a global mantle backward-advection model based on shear-wave mantle tomography, paleogeographic data, eustatic sea level estimates and basin stratigraphy to reconstruct the Australian flooding history for the last 70 Myrs on a continental scale. We compute time-dependent dynamic surface topography and continental inundation of a digital elevation model adjusted for sediment accumulation. Our model reveals two evolving dynamic topography lows, over which the Australian plate has progressively moved. We interpret the southern low to be caused by sinking slab material with an origin along the eastern Gondwana subduction zone in the Cretaceous, whereas the northern low, which first straddles northern Australia in the Oligocene, is mainly attributable to material subducted north and northeast of Australia. Our model accounts for the Paleogene exposure of the Gulf of Carpentaria region at a time when sea level was much higher than today, and explains anomalous Late Tertiary subsidence on Australia's northern, western and southern margins. The resolution of our model, which excludes short-wavelength mantle density anomalies and is restricted to depths larger than 220 km, is not sufficient to model the two well recorded episodes of major transgressions in South Australia in the Eocene and Miocene. However, the overall, long-wavelength spatio-temporal pattern of Australia's inundation record is well captured by combining our modelled dynamic topography with a recent eustatic sea level curve. We suggest that the apparent Late Cenozoic northward tilting of Australia was a stepwise function of South Australia first moving away northwards from the Gondwana subduction-related dynamic topography low in the Oligocene, now found under the Australian–Antarctic Discordance, followed by a drawing down of northern Australia as it overrode a slab burial ground now underlying much of the northern half of Australia, starting in the Miocene. Our model suggests that today's geography of Australia is strongly dependent on mantle forces. Without mantle convection, which draws Australia down by up to 300 m, nearly all of Australia's continental shelves would be exposed. We conclude that dissecting the interplay between eustasy and mantle-driven dynamic topography is critical for understanding hinterland uplift, basin subsidence, the formation and destruction of shallow epeiric seas and their facies distribution, but also for the evolution of petroleum systems.

Geomagnetic excursions: Knowns and unknowns

Geomagnetic excursions are short‐lived episodes when Earth’s magnetic field deviates into an intermediate polarity state. Understanding the origin, frequency, amplitude, duration, and field behavior associated with excursions is a forefront research area within solid earth geophysics. Recent advances in excursion research are summarized here, and key further research is suggested to resolve major unanswered questions. Improving the global distribution of excursion records, particularly from the southern hemisphere, obtaining high‐resolution sedimentary excursion records with good age control from sites with sedimentation rates &gt;10 cm/kyr, obtaining volcanic excursion records coupled with high‐precision geochronology, and estimating excursion duration with high chronological precision will all facilitate hypothesis testing concerning the deep earth dynamics that generate geomagnetic excursions.

Global tomographic images of mantle plumes and subducting slabs: insight into deep Earth dynamics

A new model of whole mantle P-wave tomography is determined with a novel approach. A grid parameterization instead of blocks and spherical harmonics is adopted to express the Earth structure. Depth variations of the Moho, 410 and 660 km discontinuities are taken into account in the inversion. Ray paths and travel times are computed with an efficient 3-D ray tracing scheme. This new approach was applied to a large data set of ISC (International Seismological Center) travel times (P, PP, PcP, pP, Pdiff) to determine a whole mantle P-wave tomography. For the shallow mantle, the new model contains the general features observed in the previous models: a low-velocity ring around the Pacific Ocean basins and high-velocity anomalies under the old and stable continents in the depth range of 0–300 km. One significant difference from the previous models is that stronger and wider high-velocity anomalies are visible in the transition zone depths under the subduction regions, which suggests that most of the slab materials are stagnant in the transition zone before finally collapsing down to the lower mantle as a result of very large gravitational instability from phase transitions. Very slow anomalies exist in the upper mantle right beneath the Wudalianchi and Changbai active volcanoes in Eastern China, right above the stagnant Pacific slab in the transition zone, suggesting that the origin of the intraplate volcanism in East Asia is closely related to the Pacific plate subduction process, such as deep slab dehydration and convective circulation in the mantle wedge. Plume-like slow anomalies are clearly visible under the major hotspot regions in most parts of the mantle, in particular, under Hawaii, Iceland, South Pacific and Africa. The slow anomalies under South Pacific and Africa have lateral extensions of over one thousand kilometers and exist in the entire mantle, representing two superplumes. The Pacific superplume has a larger spatial extent and stronger slow anomalies than that of the Africa superplume. The Hawaiian plume is not part of the Pacific superplume. The slow anomalies under hotspots usually do not show a straight pillar shape, but exhibit winding images, suggesting that plumes are not fixed in the mantle but can be deflected by the mantle flow. As a consequence, hotspots are not really fixed but can wander on the Earth’s surface, as evidenced by the recent paleomagnetic and numeric modeling studies. Wider and more prominent slow anomalies are visible at the core–mantle boundary (CMB) than most of the lower mantle, and there is a good correlation between the distribution of slow anomalies at the CMB and that of hotspots on the surface, suggesting that most of the strong mantle plumes under the hotspots originate from the CMB. However, there are some small-scaled, weak plumes originating from the transition zone or mid mantle depths.