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What drives 20th century polar motion?

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... More significantly, we shall argue that the coseismic contribution represents a certain portion of the total effect arising from the mass redistribution associated with the tectonic plate motions, rather than a transient effect to be annihilated by the interseismic motions in the long run as suggested by Cambiotti et al. (2016). Cambiotti et al. (2016) in addition modeled and estimated the total contribution of the tectonic plate motions to SPD and obtained a tectonic SPD in Direction A, whereas Adhikari et al. (2018), doing the same, obtained a tectonic SPD in the opposite Direction B instead. We shall raise the caveats of adopting such modeling in budgeting and interpreting the SPD, in particular when both seismic and aseismic contributions are involved. ...
... On the other hand, the recent work by Adhikari et al. (2018) yielded the conclusion opposite to the above. They surmised that the tectonic contribution (mantle convection) is rather important; in fact it, now pointing toward Direction B in their version is potentially able to account for the significant shortage of the purported (or calculated and statistically preferred) GIA and environmental contribution (toward 15°E, orthogonal to Directions A and B) from the observed SPD (toward Direction B) during the twentieth century. ...
... In the process they had placed the (relatively small) seismic contribution in the general Direction B as well on account of the coseismic-interseismic annihilation model with full seismic coupling of Cambiotti et al. (2016), whether justifiably or not (as discussed above). More specifically, the tectonic contribution preferred by Adhikari et al. (2018), to explain the aforementioned polar motion excitation shortage, was based on statistical fluctuations in the parameter space of tectonic models. The majority of the statistical outcomes were in Direction B and not quite sufficient in magnitude to fully account for the said shortage, but some of them that were dismissed turned out to be in Direction A, actually consistent with that modeled by Cambiotti et al. (2016). ...
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Earthquake‐induced mass redistribution in the Earth excites the polar motion; its cumulative coseismic effect has been found to cause a secular polar drift (SPD) toward ~140°E longitude with strong statistical tendency. Here we find numerically the cumulatively coseismic effect in SPD since 1952 to be at the rate of ~0.75 mas/year (or ~2.3 cm/year), amounting to nearly 20% of the observed SPD that points to the opposite geographical direction and hence is significant in the pursuit of understanding the source budget of SPD. We further argue on theoretical and observational ground that such behavior reflects that of the overall plate tectonic motion and in fact accounts for a fraction of the latter over long term. The exact amount of the fraction is indeterminate until mass transport models of plate tectonics prove adequate. This viewpoint is in contrast to that of Cambiotti et al. (2016, https://doi.org/10.1093/gji/ggw077) which required the coseismic effect to get annihilated completely by the interseismic effect under their earthquake cycle decomposition of the velocity field at the faulting system.
... As shown in Figure 3, the non-seasonal oscillations in GAO and HAM were characterized by both long-term and short-term oscillations. The main contributors to long-term non-seasonal variations in HAM are groundwater changes [57] and mass loss of ice sheets and glaciers caused mainly by the warming climate [57][58][59]. Other contributors include core-mantle coupling [60] and the flattening of the inner core and its tilt angle with respect to the outer core and mantle [61,62]. ...
... Keeping this in mind, we now decompose GAO and HAM series into As shown in Figure 3, the non-seasonal oscillations in GAO and HAM were characterized by both long-term and short-term oscillations. The main contributors to long-term non-seasonal variations in HAM are groundwater changes [57] and mass loss of ice sheets and glaciers caused mainly by the warming climate [57][58][59]. Other contributors include core-mantle coupling [60] and the flattening of the inner core and its tilt angle with respect to the outer core and mantle [61,62]. ...
... Keeping this in mind, we now decompose GAO and HAM series into As shown in Figure 3, the non-seasonal oscillations in GAO and HAM were characterized by both long-term and short-term oscillations. The main contributors to long-term non-seasonal variations in HAM are groundwater changes [57] and mass loss of ice sheets and glaciers caused mainly by the Remote Sens. 2020, 12, 138 9 of 29 warming climate [57][58][59]. Other contributors include core-mantle coupling [60] and the flattening of the inner core and its tilt angle with respect to the outer core and mantle [61,62]. ...
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From 2002 to 2017, the Gravity Recovery and Climate Experiment (GRACE) mission’s twin satellites measured variations in the mass redistribution of Earth’s superficial fluids, which disturb polar motion (PM). In this study, the PM excitation estimates were computed from two recent releases of GRACE monthly gravity field models, RL05 and RL06, and converted into prograde and retrograde circular terms by applying the complex Fourier transform. This is the first such analysis of circular parts in GRACE-based excitations. The obtained series were validated by comparison with the residuals of observed polar motion excitation (geodetic angular momentum (GAM)–atmospheric angular momentum (AAM)–oceanic angular momentum (OAM) (GAO)) determined from precise geodetic measurements of the pole coordinates. We examined temporal variations of hydrological excitation function series (or hydrological angular momentum, HAM) in four spectral bands: seasonal, non-seasonal, non-seasonal short-term, and non-seasonal long-term. The general conclusions arising from the conducted analyses of prograde and retrograde terms were consistent with the findings from the equatorial components of PM excitation studies drawn in previous research. In particular, we showed that the new GRACE RL06 data increased the consistency between different solutions and improved the agreement between GRACE-based excitation series and reference data. The level of agreement between HAM and GAO was dependent on the oscillation considered and was higher for long-term than short-term variations. For most of the oscillations considered, the highest agreement with GAO was obtained for CSR RL06 and ITSG-Grace2018 solutions. This study revealed that both prograde and retrograde circular terms of PM excitation can be determined by GRACE with similar levels of accuracy. The findings from this study may help in choosing the most appropriate GRACE solution for PM investigations and can be useful in future improvements to GRACE data processing.
... realistic estimates of ice mass loss and terrestrial water storage on sea-level changes in the South Atlantic. For this estimate, we use two datasets: the estimate compiled by Adhikari et al. (2018) and the estimate from Frederikse et al. (2020). Both sets contain realistic estimates of mass changes related to glaciers, ice sheets, terrestrial water storage related to the impoundment of water behind dams, and groundwater depletion, as well as an estimate of the uncertainties. ...
... The deviation due to GRD effects from present-day mass redistribution is confirmed by the estimates from Adhikari et al. (2018) and Frederikse et al. (2020), which are depicted in Figure 6. Over the 20th-century, both these estimates show that sea-level rise in the South Atlantic due to barystatic processes has been above the global mean: Adhikari et al. (2018) shows a difference of 0.15 mm year −1 , while Frederikse et al. (2020) report 0.19 mm year −1 . ...
... The deviation due to GRD effects from present-day mass redistribution is confirmed by the estimates from Adhikari et al. (2018) and Frederikse et al. (2020), which are depicted in Figure 6. Over the 20th-century, both these estimates show that sea-level rise in the South Atlantic due to barystatic processes has been above the global mean: Adhikari et al. (2018) shows a difference of 0.15 mm year −1 , while Frederikse et al. (2020) report 0.19 mm year −1 . ...
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Sea level in the South Atlantic Ocean has only been measured at a small number of tide-gauge locations, which causes considerable uncertainty in 20th-century sea-level trend estimates in this basin. To obtain a better-constrained sea-level trend in the South Atlantic Ocean, this study aims to answer two questions. The first question is: can we combine new observations, vertical land motion estimates, and information on spatial sampling biases to obtain a likely range of 20th-century sea-level rise in the South Atlantic? We combine existing observations with recovered observations from Dakar and a high-resolution sea-level reconstruction based on salt-marsh sediments from the Falkland Islands and find that the rate of sea-level rise in the South Atlantic has likely been between 1.1 and 2.2 mm year−1 (5%–95% confidence intervals), with a central estimate of 1.6 mm year−1. This rate is on the high side, but not statistically different compared to global-mean trends from recent reconstructions. The second question is: are there any physical processes that could explain a large deviation from the global-mean sea-level trend in the South Atlantic? Sterodynamic (changes in ocean dynamics and steric effects) and gravitation, rotation, and deformation effects related to ice mass loss and land water storage have probably led to a 20th-century sea-level trend in the South Atlantic above the global mean. Both observations and physical processes thus suggest that 20th-century sea-level rise in the South Atlantic has been about 0.3 mm year−1 above the rate of global-mean sea-level rise, although even with the additional observations, the uncertainties are still too large to distinguish a statistically significant difference.
... To obtain estimates of changes in global ocean mass (barystatic changes), we combine estimates of mass change for glaciers 16,21 , ice sheets 14,[22][23][24][25] and terrestrial water storage (TWS). For the TWS estimate, we consider the effects of natural TWS variability 17 , water impoundment in artificial reservoirs 26 and groundwater depletion 27,28 . ...
... For Antarctica, no mass-balance reconstruction exists before the satellite era, although observational evidence suggests twentieth-century mass loss, especially from West Antarctica 49,50 . Therefore, we assume a small Antarctic Ice Sheet contribution before 1993 of 0.05 ± 0.04 mm yr −1 , based on an existing compilation 22 . For 1993-2003, we use the multi-method assessments 23,24 to derive the mass changes. ...
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The rate of global-mean sea-level rise since 1900 has varied over time, but the contributing factors are still poorly understood¹. Previous assessments found that the summed contributions of ice-mass loss, terrestrial water storage and thermal expansion of the ocean could not be reconciled with observed changes in global-mean sea level, implying that changes in sea level or some contributions to those changes were poorly constrained2,3. Recent improvements to observational data, our understanding of the main contributing processes to sea-level change and methods for estimating the individual contributions, mean another attempt at reconciliation is warranted. Here we present a probabilistic framework to reconstruct sea level since 1900 using independent observations and their inherent uncertainties. The sum of the contributions to sea-level change from thermal expansion of the ocean, ice-mass loss and changes in terrestrial water storage is consistent with the trends and multidecadal variability in observed sea level on both global and basin scales, which we reconstruct from tide-gauge records. Ice-mass loss—predominantly from glaciers—has caused twice as much sea-level rise since 1900 as has thermal expansion. Mass loss from glaciers and the Greenland Ice Sheet explains the high rates of global sea-level rise during the 1940s, while a sharp increase in water impoundment by artificial reservoirs is the main cause of the lower-than-average rates during the 1970s. The acceleration in sea-level rise since the 1970s is caused by the combination of thermal expansion of the ocean and increased ice-mass loss from Greenland. Our results reconcile the magnitude of observed global-mean sea-level rise since 1900 with estimates based on the underlying processes, implying that no additional processes are required to explain the observed changes in sea level since 1900.
... considered to be mostly related to the solid Earth, especially the glacial isostatic adjustment (GIA) process and mantle convection (Adhikari et al., 2018;R. S. Gross & Vondrák, 1999;McCarthy & Luzum, 1996;Nakada et al., 2015). ...
... The TWS-related polar drift, which moves toward 22°E, is also in good agreement with that of the residual (28°E) in direction (Figures 2c and 2d), and the movement rate of the TWS-related polar drift (2.58 mas/yr) accounts for 65% of that of the residual (3.95 mas/yr). According to the study of Adhikari et al. (2018), the discrepancy in direction (6°) and, in particular, rate (1.37 mas/yr) is reasonable when mantle convection is not considered in this study. ...
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Plain Language Summary The Earth's pole, the point where the Earth's rotational axis intersects its crust in the Northern Hemisphere, drifted in a new eastward direction in the 1990s, as observed by space geodetic observations. Generally, polar motion is caused by changes in the hydrosphere, atmosphere, oceans, or solid Earth. However, short‐term observational records of key information in the hydrosphere (i.e., changes in terrestrial water storage) limit a better understanding of new polar drift in the 1990s. This study introduces a novel approach to quantify the contribution from changes in terrestrial water storage by comparing its drift path under two different scenarios. One scenario assumes that the terrestrial water storage change throughout the entire study period (1981–2020) is similar to that observed recently (2002–2020). The second scenario assumes that it changed from observed glacier ice melting. Only the latter scenario, along with the atmosphere, oceans, and solid Earth, agrees with the polar motion during the period of 1981–2020. The accelerated terrestrial water storage decline resulting from glacial ice melting is thus the main driver of the rapid polar drift toward the east after the 1990s. This new finding indicates that a close relationship existed between polar motion and climate change in the past.
... As far as the Indus, Gange, Brahmaputra, and Mekong are concerned, the basins have become wetter due to more intense monsoonal precipitation, and the contributions of snow-and ice-melt have become less relevant [2]. An unexpected consequence to changes in the HKKH water supply has been highlighted in recent studies by [3,4], where it was proposed that changes in the Earth's axis are related to water mass loss away from the Indian subcontinent and the Caspian Sea. The complex and irregular orography of the HKKH region means that accurately modelling and taking sufficient observations to survey the region adequately is rather difficult from ground level. ...
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The climatology of major sources and pathways of moisture for three locales along the Hindu-Kush-Himalayan region are examined, by use of Lagrangian methods applied to the ERA-Interim dataset, over the period from 1980 to 2016 for both summer (JJA) and winter (NDJ) periods. We also investigate the major flooding events of 2010, 2013, and 2017 in Pakistan, Uttarakhand, and Kathmandu, respectively, and analyse a subset of the climatology associated with the 20 most significant rainfall events over each region of interest. A comparison is made between the climatology and extreme events, in the three regions of interest, during the summer monsoon period. For Northern Pakistan and Uttarakhand, the Indus basin plays the largest role in moisture uptake. Moisture is also gathered from Eastern Europe and Russia. Extreme events display an increased influence of sub-tropical weather systems, which manifest themselves through low-level moisture transport; predominantly from the Arabian sea and along the Gangetic plain. In the Kathmandu region, it is found that the major moisture sources come from the Gangetic plain, Arabian Sea, Red Sea, Bay of Bengal, and the Indus basin. In this case, extreme event pathways largely match those of the climatology, although an increased number of parcels originate from the western end of the Gangetic plain. These results provide insights into the rather significant influence of mid-latitudinal weather systems, even during the monsoon season, in defining the climatology of the Hindu-Kush-Himalaya region, as well as how extreme precipitation events in this region represent atypical moisture pathways. We propose a detailed investigation of how such water pathways are represented in climate models for the present climate conditions and in future climate scenarios, as this may be extremely relevant for understanding the impacts of climate change on the cryosphere and hydrosphere of the region.
... To suitably interpret the (l, m) = (2, ±1) symmetry, it is worth to note that according to our computations, the GIA-induced polar motion presently occurs at a rate of ∼1.4 deg/Myr (roughly corresponding to 15 cm/year on the Earth's surface) along the meridian ∼80 • W (roughly, towards the Hudson Bay). Such rate and direction of polar drift match well the astronomical observations in the course of last century (see e.g., Lambeck [4]) and with recent analyses about the causes of secular polar motion [66]. Performing a further run of SELEN 4 in which we have adopted the traditional rotation theory (see e.g., Spada et al. [46]), we have verified that the (l, m) = (2, ±1) pattern ofĠ(ω) would be indeed much stronger, with a more than two-fold rate of polar drift of ∼3.5 deg/Myr in the same direction. ...
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Along with density and mass variations of the oceans driven by global warming, Glacial Isostatic Adjustment (GIA) in response to the last deglaciation still contributes significantly to present-day sea-level change. Indeed, in order to reveal the impacts of climate change, long term observations at tide gauges and recent absolute altimetry data need to be decontaminated from the effects of GIA. This is now accomplished by means of global models constrained by the observed evolution of the paleo-shorelines since the Last Glacial Maximum, which account for the complex interactions between the solid Earth, the cryosphere and the oceans. In the recent literature, past and present-day effects of GIA have been often expressed in terms of fingerprints describing the spatial variations of several geodetic quantities like crustal deformation, the harmonic components of the Earth’s gravity field, relative and absolute sea level. However, since it is driven by the delayed readjustment occurring within the viscous mantle, GIA shall taint the pattern of sea-level variability also during the forthcoming centuries. The shapes of the GIA fingerprints reflect inextricable deformational, gravitational, and rotational interactions occurring within the Earth system. Using up-to-date numerical modeling tools, our purpose is to revisit and to explore some of the physical and geometrical features of the fingerprints, their symmetries and intercorrelations, also illustrating how they stem from the fundamental equation that governs GIA, i.e., the Sea Level Equation.
... The GIA is the result of ice sheet retreat since the Last Glacial Maximum (~20-30 kyr BP) (e.g., Peltier, 1998;Mitrovica et al., 2005;Lambeck et al., 2001), while RIM has been evidenced over Greenland, Alaska, and Antarctica during the last decades (e.g., Cazenave & Llovel, 2010;Shepherd et al., 2012). Both phenomena induce deformation of the solid Earth (e.g., Khan et al., 2010;Peltier, 1974), sea level variations (e.g., Lambeck & Chappell, 2001;Peltier, 1998), gravity time variations (e.g., Khan et al., 2010;Tamisiea et al., 2007), geocenter motions (Argus, 2007;Greff-Lefftz, 2000;Greff-Lefftz et al., 2010;Métivier et al., , 2011, and rotation variations (e.g., Adhikari et al., 2018;Chambers et al., 2010;Mitrovica et al., 2005Mitrovica et al., , 2015. However, while GIA deformation is today the result of viscous relaxations (e.g., Caron et al., 2017), RIM deformation is generally considered as purely elastic. ...
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Using a selection of Global Navigation Satellite System vertical velocities from the latest solution of the International Terrestrial Reference Frame (ITRF) ITRF2014, we calculate the degree‐1 and degree‐2 spherical harmonics coefficients (SHC) of the solid Earth figure changes at different dates, with realistic errors that take into account the inhomogeneity of the network. We find that the SHC are globally close to zero except the zonal coefficients, which show values notably larger than those derived from different glacial isostatic adjustment (GIA) models and which have tended to increase during the time span of observations. We show that these differences are most probably due to global recent ice melting (RIM). Assuming elastic RIM deformation, we then investigate the Earth's geocenter velocity and the geoid oblateness time evolution (J2‐rate) derived from our SHC estimations. The obtained geocenter velocity reaches 0.9 ± 0.5 mm/year in 2013 with a z‐component of 0.8 ± 0.4 mm/year, which is slightly larger than previous estimations. We compare our J2‐rate estimations with observations. Our estimations show a similar acceleration in J2 after 2000. However, our estimates are notably larger than the observations. This indicates either that the J2‐rate due to GIA processes is lower than expected (as proposed by Nakada et al., 2015, 2016) or that the deformation induced by RIM is not purely elastic, or both. Finally, we show that viscous relaxation or phase transitions in the mantle transition zone may only partly explain this discrepancy. This raises the question of the accuracy of current mass estimations of RIM and GIA models.
... Polar motion (PM) is affected by a wide range of processes with different temporal variability ranging from several days to many decades [1]. Such disturbances include the gravitational influence of other celestial bodies, continuously changing mass distribution in the Earth's surficial fluids (atmosphere, oceans and land hydrosphere), the effects of core-mantle coupling, and also groundwater depletion and ice mass loss resulting from recent climate changes [2][3][4]. ...
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Over the last 15 years, the Gravity Recovery and Climate Experiment (GRACE) mission has provided measurements of temporal changes in mass redistribution at and within the Earth that affect polar motion. The newest generation of GRACE temporal models, are evaluated by conversion into the equatorial components of hydrological polar motion excitation and compared with the residuals of observed polar motion excitation derived from geodetic measurements of the pole coordinates. We analyze temporal variations of hydrological excitation series and decompose them into linear trends and seasonal and non-seasonal changes, with a particular focus on the spectral bands with periods of 1000–3000, 450–1000, 100–450, and 60–100 days. Hydrological and reduced geodetic excitation series are also analyzed in four separated time periods which are characterized by different accuracy of GRACE measurements. The level of agreement between hydrological and reduced geodetic excitation depends on the frequency band considered and is highest for interannual changes with periods of 1000–3000 days. We find that the CSR RL06, ITSG 2018 and CNES RL04 GRACE solutions provide the best agreement with reduced geodetic excitation for most of the oscillations investigated.
... Besides the relatively high noises and the remaining contaminations from other sources, one possible major reason is that the cumulative coseismic effect may have been removed numerically at the outset by the quadratic polynomial fit to ΔJ 2 that we have performed. Furthermore, postseismic variations can be comparable to, if not larger than, and augment the coseismic effects, as suggested by, for example, Adhikari et al. (2018) and Xu and Chao (2019). Such fine and detailed signals await further studies. ...
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The Earth's oblateness varies slightly due to a host of physical processes that involve large‐scale meridional N‐S mass redistributions in the Earth. We analyze these minute, broadband signals in the observed ΔJ2 data series for 1976–2019 (43 years). We first remove the near‐quadratic variation (due to the glacial isostatic adjustment plus the accelerating land ice melting) and the seasonal terms (due to seasonality of the surface geophysical fluids) by least squares regression. Then we examine via cross‐correlation function and cross‐coherence spectrum the relationships of the remaining interannual‐to‐decadal ΔJ2 with various climate oscillations in terms of their respective indices. We elucidate the contributions of the Antarctic Oscillation and Arctic Oscillation (for timescales shorter than 5 years), Pacific Decadal Oscillation (for timescales longer than 5 years), and the absence thereof in El Niño–Southern Oscillation and Atlantic Multidecadal Oscillation. Removal of their contributions from ΔJ2 reveals two remaining, nonclimatic long‐period signals: An 18.61‐year tidal signal calls for an augmentation in the in‐phase value and a reduction of the out‐of‐phase value in the theoretical models per International Earth rotation and Reference systems Service (IERS), while a more detailed quantitative study awaits future, longer data. An additional 10.5‐year signal is found to be correlated with the solar cycle, but the origin of this apparent correlation is uncertain presently.
... Although these factors appear to control the direction of polar motion, they do not appear sufficient to account for its magnitude. Adhikari et al. (2018) have come to the conclusion that mantle convection, which drives plate tectonics, also seems to be a significant factor affecting polar motion. ...
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In a previous study of over two hundred ancient sites, the alignments of almost half of the sites could not be explained. These sites are distributed throughout the world and include the majority of Mesoamerican pyramids and temples that are misaligned with respect to true north, megalithic structures at several sites in Peru’s Sacred Valley, some pyramids in Lower Egypt, and numerous temples in Upper Egypt. A new model is proposed to account for the alignment of certain unexplained sites based on an application of Charles Hapgood’s hypothesis that global patterns of climate change over the past 100,000 years could be the result of displacements of the Earth’s crust and corresponding shifts of the geographic poles. It is shown that over 80% of the unexplained sites reference four locations within 30° of the North Pole that are correlated with Hapgood’s hypothesized pole locations. The alignments of these sites are consistent with the hypothesis that if they were built in alignment with one of these former poles they would be misaligned to north as they are now as the result of subsequent pole shifts.
... Although these factors appear to control the direction of polar motion, they do not appear sufficient to account for its magnitude. Adhikari et al. (2018) have come to the conclusion that mantle convection, which drives plate tectonics, also seems to be a significant factor affecting polar motion. ...
... Although these factors appear to control the direction of polar motion, they do not appear sufficient to account for its magnitude. Adhikari et al. (2018) have come to the conclusion that mantle convection, which drives plate tectonics, also seems to be a significant factor affecting polar motion. ...
... In general, relative sea level histories and postglacial rebound data, specifically those from Fennoscandia or Antarctica (Mitrovica & Peltier, 1993), constrain the upper mantle's viscosity, while postglacial signals from Canada are used to constrain the upper part of the lower mantle (Peltier, 2004). Other geophysical observables, including the rate of change of J 2 and the polar wander are used to constrain the viscosity of the rest of the lower mantle (Lau et al., 2016), though these constraints may be challenged (Adhikari et al., 2018;Nakada et al., 2015). Other radial profiles were derived from joint inversion of data that include these GIA effects along with data related to mantle convection , 2004Moucha et al., 2008). ...
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Plain Language Summary The Earth is a deformable body subject to inner, surface, and outer forces acting together to adjust its mass distribution. This mass distribution is quantified by the dynamical ellipticity, and its evolution is largely unknown over geological timescales. As this parameter plays an important role in the evolution of the Earth's rotational motion, its uncertainty propagates to long term solutions of the Earth's orientation. To minimize this uncertainty, we present here a solution of the Earth's dynamical ellipticity over the past 50 Myr, pertaining to the surface loading contribution of the Cenozoic glacial cycles. We do so by combining oceanic proxies of glacial volume with the proper mathematical formalism to reconstruct a self‐consistent history of the glacial and oceanic loading. As the Earth's response to this loading is highly dependent on its viscosity, we perform a parametric study and constrain the possible scenarios of dynamical ellipticity evolution. Combined with other processes involving mass redistribution, such as mantle convection and the tidal response, our findings add a missing puzzle piece toward a complete history of the dynamical ellipticity, and consequently valid extended rotational solutions.
... Observations suggest some mass loss in the mid-to late 20th century from West Antarctica (Smith et al., 2017), with more rapid rates during 21st century (Galassi and Spada, 2017;Shen et al., 2018;Shepherd et al., 2018). Our budget follows Frederikse et al. (2020) in assuming a small and unchanging contribution from Antarctica of 0.05 ± 0.04 mm a −1 before 1993, based on the compilation of Adhikari et al. (2018). The rapid rates of sea-level rise that we have reconstructed from New Zealand are unlikely to reflect enhanced Antarctic ice melt. ...
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In this paper we present new proxy‐based sea‐level reconstructions for southern New Zealand spanning the last millennium. These palaeo sea‐level records usefully complement sparse Southern Hemisphere proxy and tide‐gauge sea‐level datasets and, in combination with instrumental observations, can test hypotheses about the drivers of 20th century global sea‐level change, including land‐based ice melt and regional sterodynamics. We develop sea‐level transfer functions from regional datasets of salt‐marsh foraminifera to establish a new proxy‐based sea‐level record at Mokomoko Inlet, at the southern tip of the South Island, and to improve the previously published sea‐level reconstruction at Pounawea, located about 110 km to the east. Chronologies are based on radiocarbon, radiocaesium, stable lead isotope and pollen analyses. Both records are in good agreement and show a rapid sea‐level rise in the first half of the 20th century that peaked in the 1940s. Previously reported discrepancies between proxy‐based sea‐level records and tide‐gauge records are partially reconciled by accounting for barystatic and sterodynamic components of regional sea‐level rise. We conclude that the rapid sea‐level rise during the mid‐20th century along the coast of southern New Zealand was primarily driven by regional thermal expansion and ocean dynamics.
... The authors of [85] showed that groundwater variations affect mainly interannual TWS variability, so it should indirectly influence HAM/CAM as well. Other authors [86,87] pointed out that the climate changes affect long-term groundwater loss, which can contribute to the change in polar motion. The few-year change in HAM/CAM might also be related to a variation in continental water distribution due to different precipitation patterns, as the trend in continental water distribution between 2007-2013 was different than in other years [46]. ...
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In this study, we calculate the hydrological plus cryospheric excitation of polar motion (hydrological plus cryospheric angular momentum, HAM/CAM) using mascon solutions based on observations from the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) missions. We compare and evaluate HAM/CAM computed from GRACE and GRACE-FO mascon data provided by the Jet Propulsion Laboratory (JPL), the Center for Space Research (CSR), and the Goddard Space Flight Center (GSFC). A comparison with HAM obtained from the Land Surface Discharge Model is also provided. An analysis of HAM/CAM and HAM is performed for overall variability, trends, and seasonal and non-seasonal variations. The HAM/CAM and HAM estimates are validated using the geodetic residual time series (GAO), which is an estimation of the hydrological plus cryospheric signal in geodetically observed polar motion excitation. In general, all mascon datasets are found to be equally suitable for the determination of overall, seasonal, and non-seasonal HAM/CAM oscillations, but some differences in trends remain. The use of an ellipsoidal correction, implemented in the newest solution from CSR, does not noticeably affect the consistency between HAM/CAM and GAO. Analysis of the data from the first two years of the GRACE-FO mission indicates that the current accuracy of HAM/CAM from GRACE-FO mascon data meets expectations, and the root mean square deviation of HAM/CAM components are between 5 and 6 milliarcseconds. The findings from this study can be helpful in assessing the role of satellite gravimetry in polar motion studies and may contribute towards future improvements to GRACE-FO data processing.
... Long-term variations, contributing to a secular trend over the few decades during which we have observations, are dominated by viscous mantle flows from convection and postglacial rebound (e.g., Tamisiea et al. 2007). The accelerating rate of melting of continental ice sheets and mountain glaciers induced by global warming over the past two decades, together with the elastic response of the solid Earth to this mass redistribution, is now imprinting a change in this secular trend, both in the degree 2, order 0 (elliptical) component of gravity (e.g., Nerem and Wahr 2011;Cheng and Ries 2018;) but also in the degree 2, order 1 components and thereby inducing a displacement in the Earth's rotation axis (or, for short, a polar motion) (e.g., Chen et al. 2013;Adhikari et al. 2018;Deng et al. 2021). ...
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Fluid motion within the Earth’s liquid outer core leads to internal mass redistribution. This occurs through the advection of density anomalies within the volume of the liquid core and by deformation of the solid boundaries of the mantle and inner core which feature density contrasts. It also occurs through torques acting on the inner core reorienting its non-spherical shape. These in situ mass changes lead to global gravity variations, and global deformations (inducing additional gravity variations) occur in order to maintain the mechanical equilibrium of the whole Earth. Changes in Earth’s rotation vector (and thus of the global centrifugal potential) induced by core flows are an additional source of global deformations and associated gravity changes originating from core dynamics. Here, we review how each of these different core processes operates, how gravity changes and ground deformations from each could be reconstructed, as well as ways to estimate their amplitudes. Based on our current understanding of core dynamics, we show that, at spherical harmonic degree 2, core processes contribute to gravity variations and ground deformations that are approximately a factor 10 smaller than those observed and caused by dynamical processes within the fluid layers at the Earth’s surface. The larger the harmonic degree, the smaller is the contribution from the core. Extracting a signal of core origin requires the accurate removal of all contributions from surface processes, which remains a challenge. Article Highlights Dynamical processes in Earth's fluid core lead to global gravity variations and surface ground deformations We review how these processes operate, how signals of core origin can be reconstructed and estimate their amplitudes Core signals are a factor 10 smaller than the observed signals; extracting a signal of core origin remains a challenge
... Recently, NASA scientists have defined three main processes responsible for the Earth's spin axis drift, among which the melting of the global cryosphere, in particular Greenland, over the course of the 20th century is one of the most important (8). The melting of the glaciers could be the consequence of climate changes. ...
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O negacionismo anti-ciência dos nossos dias é claramente praticante de um obscurantismo que não pode ser pensado como sendo o fenômeno do obscurantismo como conhecido na história social até aqui. Sendo da ordem de racionalizações deliberadas de irracionalismos e dedicado a uma pauta política influente, somente pode ser compreendido – e designado por seu justo título – como um obscurantismo cínico. O exemplo da afirmação burlesca que “a terra é plana” é quase uma caricatura desse cinismo que milita pela negação do conhecimento científico, mas igualmente serve de metáfora suprema da opção dos negacionistas por tentar desautorizar e invalidar o pensamento teórico-filosófico-científico por qualquer que seja a via. Mesmo quando certas vias sejam elas mesmas pseudociências, opinião ignorante, retórica ideológica anti-intelectualista e anti-ciência.
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We provide a new analysis of glacial isostatic adjustment (GIA) with the goal of assembling the model uncertainty statistics required for rigorously extracting trends in surface mass from the Gravity Recovery and Climate Experiment (GRACE) mission. Such statistics are essential for deciphering sea level, ocean mass, and hydrological changes because the latter signals can be relatively small (≤2 mm/yr water height equivalent) over very large regions, such as major ocean basins and watersheds. With abundant new >7 year continuous measurements of vertical land motion (VLM) reported by Global Positioning System stations on bedrock and new relative sea level records, our new statistical evaluation of GIA uncertainties incorporates Bayesian methodologies. A unique aspect of the method is that both the ice history and 1-D Earth structure vary through a total of 128,000 forward models. We find that best fit models poorly capture the statistical inferences needed to correctly invert for lower mantle viscosity and that GIA uncertainty exceeds the uncertainty ascribed to trends from 14 years of GRACE data in polar regions.
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This study explores an approach that simultaneously estimates Antarctic mass balance and glacial isostatic adjustment (GIA) through the combination of satellite gravity and altimetry data sets. The results improve upon previous efforts by incorporating reprocessed data sets over a longer period of time, and now include a firn densification model to account for firn compaction and surface processes. A range of different GRACE gravity models were evaluated, as well as a new ICESat surface height trend map computed using an overlapping footprint approach. When the GIA models created from the combination approach were compared to in-situ GPS ground station displacements, the vertical rates estimated showed consistently better agreement than existing GIA models. In addition, the new empirically derived GIA rates suggest the presence of strong uplift in the Amundsen Sea and Philippi/Denman sectors, as well as subsidence in large parts of East Antarctica. The total GIA mass change estimates for the entire Antarctic ice sheet ranged from 53 to 100 Gt yr−1, depending on the GRACE solution used, and with an estimated uncertainty of ±40 Gt yr−1. Over the time frame February 2003–October 2009, the corresponding ice mass change showed an average value of −100 ± 44 Gt yr−1 (EA: 5 ± 38, WA: −105 ± 22), consistent with other recent estimates in the literature, with the mass loss mostly concentrated in West Antarctica. The refined approach presented in this study shows the contribution that such data combinations can make towards improving estimates of present day GIA and ice mass change, particularly with respect to determining more reliable uncertainties.
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Recent estimates of the contribution of glaciers to sea-level rise during the 20th century are strongly divergent. Advances in data availability have allowed revisions of some of these published estimates. Here we show that outside of Antarctica, the global estimates of glacier mass change obtained from glacier-length-based reconstructions and from a glacier model driven by gridded climate observations are now consistent with each other, and also with an estimate for the years 2003–2009 that is mostly based on remotely sensed data. This consistency is found throughout the entire common periods of the respective data sets. Inconsistencies of reconstructions and observations persist in estimates on regional scales.
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A classical Green's function approach to computing gravitationally consistent sea level variations, following mass redistribution on the earth surface, employed in contemporary state-of-the-art sea-level models naturally suits the spectral methods for numerical evaluation. The capability of these methods to resolve high wave number features such as small glaciers is limited by the need for large numbers of pixels and high-degree (associated Legendre) series truncation. Incorporating a spectral model into (components of) earth system models that generally operate on an unstructured mesh system also requires cumbersome and repetitive forward and inverse transform of solutions. In order to overcome these limitations of contemporary models, we present a novel computational method that functions efficiently on a flexible mesh system, thus capturing the physics operating at kilometer-scale yet capable of simulating geophysical observables that are inherently of global scale with minimal computational cost. The model has numerous important geophysical applications. Coupling to a local mesh of 3-D ice-sheet model, for example, allows for a refined and realistic simulation of fast-flowing outlet glaciers, while simultaneously retaining its global predictive capability. As an example model application, we provide time-varying computations of global geodetic and sea level signatures associated with recent ice sheet changes that are derived from space gravimetry observations.
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The dynamics of ocean-terminating outlet glaciers are an important component of ice-sheet mass balance. Using satellite imagery for the past 40 years, we compile an approximately decadal record of outlet-glacier terminus position change around the entire East Antarctic Ice Sheet (EAIS) marine margin. We find that most outlet glaciers retreated during the period 1974-1990, before switching to advance in every drainage basin during the two most recent periods, 1990-2000 and 2000-2012. The only exception to this trend was in Wilkes Land, where the majority of glaciers (74%) retreated between 2000 and 2012. We hypothesize that this anomalous retreat is linked to a reduction in sea ice and associated impacts on ocean stratification, which increases the incursion of warm deep water toward glacier termini. Because Wilkes Land overlies a large marine basin, it raises the possibility of a future sea level contribution from this sector of East Antarctica.
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Advances in global seismic tomography have increasingly motivated identification of subducted lithosphere in Earth's deep mantle, creating novel opportunities to link plate tectonics and mantle evolution. Chief among those is the quest for a robust subduction reference frame, wherein the mantle assemblage of subducted lithosphere is used to reconstruct past surface tectonics in an absolute framework anchored in the deep Earth. However, the associations heretofore drawn between lower mantle structure and past subduction have been qualitative and conflicting, so the very assumption of a correlation has yet to be quantitatively corroborated. Here we show that a significant, time-depth progressive correlation can be drawn between reconstructed subduction zones of the last 130 Myr and positive S-wave velocity anomalies at 600-2300 km depth, but that further correlation between greater times and depths is not presently demonstrable. This correlation suggests that lower mantle slab sinking rates average between 1.1 and 1.9 cm/yr.
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Earth’s spin axis has been wandering along the Greenwich meridian since about 2000, representing a 75° eastward shift from its long-term drift direction. The past 115 years have seen unequivocal evidence for a quasi-decadal periodicity, and these motions persist throughout the recent record of pole position, in spite of the new drift direction. We analyze space geodetic and satellite gravimetric data for the period 2003–2015 to show that all of the main features of polar motion are explained by global-scale continent-ocean mass transport. The changes in terrestrial water storage (TWS) and global cryosphere together explain nearly the entire amplitude (83 ± 23%) and mean directional shift (within 5.9° ± 7.6°) of the observed motion. We also find that the TWS variability fully explains the decadal-like changes in polar motion observed during the study period, thus offering a clue to resolving the long-standing quest for determining the origins of decadal oscillations. This newly discovered link between polar motion and global-scale TWS variability has broad implications for the study of past and future climate.
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Automatic estimation of velocities from GPS coordinate time series is becoming required to cope with the exponentially increasing flood of available data, but problems detectable to the human eye are often overlooked. This motivates us to find an automatic and accurate estimator of trend that is resistant to common problems such as step discontinuities, outliers, seasonality, skewness, and heteroscedasticity. Developed here, MIDAS is a variant of the Theil-Sen median trend estimator, for which the ordinary version is the median of slopes vij=(xj–xi)/(tj–ti) computed between all data pairs i > j. For normally distributed data, Theil-Sen and least-squares trend estimates are statistically identical; but unlike least squares, Theil-Sen is resistant to undetected data problems. To mitigate both seasonality and step discontinuities, MIDAS selects data pairs separated by one year. This condition is relaxed for time series with gaps so that all data are used. Slopes from data pairs spanning a step function produce one-sided outliers that can bias the median. To reduce bias, MIDAS removes outliers and recomputes the median. MIDAS also computes a robust and realistic estimate of trend uncertainty. Statistical tests using GPS data in the rigid North American plate interior show ±0.23 mm/yr RMS accuracy in horizontal velocity. In blind tests using synthetic data, MIDAS velocities have an RMS accuracy of ±0.33 mm/yr horizontal, ±1.1 mm/yr up, with a 5th percentile range smaller than all 20 automatic estimators tested. Considering its general nature, MIDAS has the potential for broader application in the geosciences.
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In 2002, Munk defined an important enigma of 20th century global mean sea-level (GMSL) rise that has yet to be resolved. First, he listed three canonical observations related to Earth's rotation [(i) the slowing of Earth's rotation rate over the last three millennia inferred from ancient eclipse observations, and changes in the (ii) amplitude and (iii) orientation of Earth's rotation vector over the last century estimated from geodetic and astronomic measurements] and argued that they could all be fit by a model of ongoing glacial isostatic adjustment (GIA) associated with the last ice age. Second, he demonstrated that prevailing estimates of the 20th century GMSL rise (~1.5 to 2.0 mm/year), after correction for the maximum signal from ocean thermal expansion, implied mass flux from ice sheets and glaciers at a level that would grossly misfit the residual GIA-corrected observations of Earth's rotation. We demonstrate that the combination of lower estimates of the 20th century GMSL rise (up to 1990) improved modeling of the GIA process and that the correction of the eclipse record for a signal due to angular momentum exchange between the fluid outer core and the mantle reconciles all three Earth rotation observations. This resolution adds confidence to recent estimates of individual contributions to 20th century sea-level change and to projections of GMSL rise to the end of the 21st century based on them.
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We examine the geodetically derived rotational variations for the rate of change of degree-two harmonics of Earth's geopotential, $\skew5\dot J_2$ , and true polar wander, combining a recent melting model of glaciers and the Greenland and Antarctic ice sheets taken from the IPCC 2013 Report (AR5) with two representative GIA ice models describing the last deglaciation, ICE5G and the ANU model developed at the Australian National University. Geodetically derived observations of $\skew4\dot J_2$ are characterized by temporal changes of −(3.7 ± 0.1) × 10−11 yr−1 for the period 1976–1990 and −(0.3 ± 0.1) × 10−11 yr−1 after ∼2000. The AR5 results make it possible to evaluate the recent melting of the major ice sheets and glaciers for three periods, 1900–1990, 1991–2001 and after 2002. The observed $\skew4\dot J_2$ and the component of $\skew4\dot J_2$ due to recent melting for different periods indicate a long-term change in $\skew4\dot J_2$ —attributed to the Earth's response to the last glacial cycle—of −(6.0–6.5) × 10−11 yr−1, significantly different from the values adopted to infer the viscosity structure of the mantle in most previous studies. This is a main conclusion of this study. We next compare this estimate with the values of $\skew4\dot J_2$ predicted by GIA ice models to infer the viscosity structure of the mantle, and consequently obtain two permissible solutions for the lower mantle viscosity ( &eegr; lm ), ∼1022 and (5–10) × 1022 Pa s, for both adopted ice models. These two solutions are largely insensitive to the lithospheric thickness and upper mantle viscosity as indicated by previous studies and relatively insensitive to the viscosity structure of the D ″ layer. The ESL contributions from the Antarctic ice sheet since the last glacial maximum (LGM) for ICE5G and ANU are about 20 and 30 m, respectively, but glaciological reconstructions of the Antarctic LGM ice sheet have suggested that its ESL contribution may have been less than ∼10 m. The GIA-induced $\skew4\dot J_2$ for GIA ice models with an Antarctic ESL component of ∼10 m suggests two permissible lower mantle viscosity solutions of &eegr; lm ∼ 2 × 1022 and ∼5 × 1022 Pa s or one solution with (2–5) × 1022 Pa s. These results suggest that the effective lower mantle viscosity is larger than ∼1022 Pa s regardless of the uncertainties for an Antarctic ESL component. We also examine the polar wander due to recent melting and GIA processes, suggesting that the observed polar wander may be significantly attributed to convection motions in the mantle and/or another cause, particularly for permissible lower mantle viscosity solution of (5–10) × 1022 Pa s.
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Confidence in projections of global-mean sea level rise (GMSLR) depends on an ability to account for GMSLR during the twentieth century. There are contributions from ocean thermal expansion, mass loss from glaciers and ice sheets, groundwater extraction, and reservoir impoundment. Progress has been made toward solving the "enigma" of twentieth-century GMSLR, which is that the observed GMSLR has previously been found to exceed the sum of estimated contributions, especially for the earlier decades. The authors propose the following: thermal expansion simulated by climate models may previously have been underestimated because of their not including volcanic forcing in their control state; the rate of glacier mass loss was larger than previously estimated and was not smaller in the first half than in the second half of the century; the Greenland ice sheet could have made a positive contribution throughout the century; and groundwater depletion and reservoir impoundment, which are of opposite sign, may have been approximately equal in magnitude. It is possible to reconstruct the time series of GMSLR fromthe quantified contributions, apart from a constant residual term, which is small enough to be explained as a long-term contribution from the Antarctic ice sheet. The reconstructions account for the observation that the rate of GMSLR was not much larger during the last 50 years than during the twentieth century as a whole, despite the increasing anthropogenic forcing. Semiempirical methods for projecting GMSLR depend on the existence of a relationship between global climate change and the rate of GMSLR, but the implication of the authors' closure of the budget is that such a relationship is weak or absent during the twentieth century.
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The major cause of sea-level change during ice ages is the exchange of water between ice and ocean and the planet's dynamic response to the changing surface load. Inversion of ∼1,000 observations for the past 35,000 y from localities far from former ice margins has provided new constraints on the fluctuation of ice volume in this interval. Key results are: (i) a rapid final fall in global sea level of ∼40 m in <2,000 y at the onset of the glacial maximum ∼30,000 y before present (30 ka BP); (ii) a slow fall to -134 m from 29 to 21 ka BP with a maximum grounded ice volume of ∼52 × 10(6) km(3) greater than today; (iii) after an initial short duration rapid rise and a short interval of near-constant sea level, the main phase of deglaciation occurred from ∼16.5 ka BP to ∼8.2 ka BP at an average rate of rise of 12 m⋅ka(-1) punctuated by periods of greater, particularly at 14.5-14.0 ka BP at ≥40 mm⋅y(-1) (MWP-1A), and lesser, from 12.5 to 11.5 ka BP (Younger Dryas), rates; (iv) no evidence for a global MWP-1B event at ∼11.3 ka BP; and (v) a progressive decrease in the rate of rise from 8.2 ka to ∼2.5 ka BP, after which ocean volumes remained nearly constant until the renewed sea-level rise at 100-150 y ago, with no evidence of oscillations exceeding ∼15-20 cm in time intervals ≥200 y from 6 to 0.15 ka BP.
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Three marine sediment cores were collected along the length of the fjord axis of Barilari Bay, Graham Land, west Antarctic Peninsula (65 degrees 55'S, 64 degrees 43'W). Multi-proxy analytical results constrained by high-resolution geochronological methods (Pb-210, radiocarbon, Cs-137) in concert with historical observations capture a record of Holocene paleoenvironmental variability. Our results suggest early and middle Holocene (>7022-2815 cal. [calibrated] yr B.P.) retreated glacial positions and seasonally open marine conditions with increased primary productivity. Climatic cooling increased sea ice coverage and decreased primary productivity during the Neoglacial (2815 to cal. 730 cal. yr B.P.). This climatic cooling culminated with glacial advance to maximum Holocene positions and expansion of a fjord-wide ice shelf during the Little Ice Age (LIA) (ca. 730-82 cal. yr B.P.). Seasonally open marine conditions were achieved and remnant ice shelves decayed within the context of recent rapid regional warming (82 cal. yr B.P. to present). Our findings agree with previously observed late Holocene cooling and glacial advance across the Antarctic Peninsula, suggesting that the LIA was a regionally significant event with few disparities in timing and magnitude. Comparison of the LIA Antarctic Peninsula record to the rest of the Southern Hemisphere demonstrates close synchronicity in the southeast Pacific and southern most Atlantic region but less coherence for the southwest Pacific and Indian Oceans. Comparisons with the Northern Hemisphere demonstrate that the LIA Antarctic Peninsula record was contemporaneous with pre-LIA cooling and sea ice expansion in the North Atlantic-Arctic, suggesting a global reach for these events.
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We defined a new global moving hot spot reference frame (GMHRF), using a comprehensive set of radiometric dates from arguably the best-studied hot spot tracks, refined plate circuit reconstructions, a new plate polygon model, and an iterative approach for estimating hot spot motions from numerical models of whole mantle convection and advection of plume conduits in the mantle flow that ensures their consistency with surface plate motions. Our results show that with the appropriate choice of a chain of relative motion linking the Pacific plate to the plates of the Indo-Atlantic hemisphere, the observed geometries and ages of the Pacific and Indo-Atlantic hot spot tracks were accurately reproduced by a combination of absolute plate motion and hot spot drift back to the Late Cretaceous (˜80 Ma). Similarly good fits were observed for Indo-Atlantic tracks for earlier time (to ˜130 Ma). In contrast, attempts to define a fixed hot spot frame resulted in unacceptable misfits for the Late Cretaceous to Paleogene (80-50 Ma), highlighting the significance of relative motion between the Pacific and Indo-Atlantic hot spots during this period. A comparison of absolute reconstructions using the GMHRF and the most recent global paleomagnetic frame reveals substantial amounts of true polar wander at rates varying between ˜0.1°/Ma and 1°/Ma. Two intriguing, nearly equal and antipodal rotations of the Earth relative to its spin axis are suggested for the 90-60 Ma and 60-40 Ma intervals (˜9° at a 0.3-0.5°/Ma rate); these predictions have yet to be tested by geodynamic models.
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The viscosity of the mantle is indispensable for predicting Earth’s mechanical behavior at scales ranging from deep mantle material flow to local stress accumulation in earthquakes zones. But, mantle viscosity is not well determined. For the lower mantle, particularly, only few constraints result from elaborate high-pressure experiments (Karato, 2008) and a variety of viscosity depth profiles result from joint inversion of the geoid and postglacial rebound data (, and ). Here, we use inferred lower-mantle sinking speed of lithosphere subduction remnants as a unique internal constraint on modeling the viscosity profile. This entails a series of elaborate dynamic subduction calculations spanning a range of viscosity profiles from which we select profiles that predict the inferred sinking speed of 12 ± 3 mm/yr (van der Meer et al., 2010). Our modeling shows that sinking speed is very sensitive to lower mantle viscosity. Good predictions of sinking speed are obtained for nearly constant lower mantle viscosity of about 3–4 × 1022 Pa s. Viscosity profiles incorporating a viscosity maximum in the deep lower mantle, as proposed in numerous studies, only lead to a good prediction in combination with a weak postperovskite layer at the bottom of the lower mantle, and only for a depth average viscosity of 5 × 1022 Pa s.
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Land glacier extent and volume at the northern and southern margins of the Drake Passage have been in a state of dramatic demise since the early 1990s. Here time-varying space gravity observations from the Gravity Recovery and Climate Experiment (GRACE) are combined with Global Positioning System (GPS) bedrock uplift data to simultaneously solve for ice loss and for solid Earth glacial isostatic adjustment (GIA) to Little Ice Age (LIA) cryospheric loading. The present-day ice loss rates are determined to be −26 ± 6 Gt/yr and −41.5 ± 9 Gt/yr in the Southern and Northern Patagonia Ice Fields (NPI+SPI) and Antarctic Peninsula (AP), respectively. These are consistent with estimates based upon thickness and flux changes. Bounds are recovered for elastic lithosphere thicknesses of 35 ≤ h ≤ 70 km and 20 ≤ h ≤ 45 km and for upper mantle viscosities of 4–8 × 1018 Pa s and 3–10 × 1019 Pa s (using a half-space approximation) for NPI+SPI and AP, respectively, using an iterative forward model strategy. Antarctic Peninsula ice models with a prolonged LIA, extending to A.D. 1930, are favored in all χ2 fits to the GPS uplift data. This result is largely decoupled from Earth structure assumptions. The GIA corrections account for roughly 20–60% of the space-determined secular gravity change. Collectively, the on-land ice losses correspond to volume increases of the oceans equivalent to 0.19 ± 0.045 mm/yr of sea level rise for the last 15 years.
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Earth's orientation relative to its spin axis is determined by its nonhydrostatic inertia tensor. We show here that the present-day nonhydrostatic inertia tensor can be modeled by combining contributions due to large low shear velocity provinces (LLSVPs) in the lowermost mantle and due to subduction. With the first contribution only, the spin axis would be at ∼67°N, 96°E (north Siberia). The distribution of recent subduction, with largest amounts in the northwest Pacific (beneath East Asia) and the southeast Pacific (beneath South America), adds a secondary contribution which moves the spin axis toward the observed poles. We use plate reconstructions to infer subduction and inertia tensor through time, assuming that the LLSVP contribution has remained constant. Motion of the pole toward Greenland since ∼50 Ma is attributed to increased subduction beneath East Asia and South America and a decrease beneath North America since then. Motion of the pole toward Siberia before that is attributed to large amounts of subduction beneath North America between ∼120 and 50 Ma and decreasing amounts of subduction in East Asia after 60–70 Ma. Greater stability of the spin axis since ∼100 Ma can be attributed to a decrease in the amount of subduction in polar latitudes and an increase in equatorial latitudes.
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Mitrovica et al. (2005), following calculations by Nakada (2002), demonstrated that the traditional approach for computing rotation perturbations driven by glacial isostatic adjustment significantly overestimates present-day true polar wander (TPW) speeds by underestimating the background oblateness on which the ice-age loading is superimposed. The underestimation has two contributions: the first originates from the treatment of the hydrostatic form and the second from the neglect of the Earth's excess ellipticity supported by mantle convection. In Mitrovica et al. (2005), the second of these two contributions was computed assuming a biaxial nonhydrostatic form (i.e., the principal equatorial moments of inertia were assumed to be equal to their mean value). In this article we outline an extended approach that accounts for a triaxial planetary form. We show that differences in the TPW speed predicted using the Mitrovica et al. (2005) approach and our triaxial theory are relatively minor (˜0.1°/Myr) and are limited to Earth models with lower mantle viscosity less than ˜5 × 1021 Pa s. However, for this same class of Earth models, the angle of TPW predicted for a triaxial Earth is rotated westward (toward the axis of maximum equatorial inertia) by as much as ˜20° relative to the biaxial case. We demonstrate that these effects are a consequence of the geometry of the ice-age forcing, which has a dominant equatorial direction that is intermediate to the axes defining the principal equatorial moments of inertia of the planet. We complete the study by computing updated Frechet kernels for the TPW speed datum, which provide a measure of the detailed depth-dependent sensitivity of the predictions to variations in mantle viscosity. We show, in contrast to earlier efforts to explore this sensitivity based on the traditional rotation theory, that the datum does not generally have a sensitivity to viscosity that peaks near the base of the mantle.
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The longstanding question of how fast episodes of true polar wander (TPW) can be excited is addressed here by analyzing the impact of the distribution and activity of subduction zones on polar motion. Nonlinear Liouville equations, which allow large excursions of the polar axis to be considered, are used to show that unrealistically fast TPW is excited by subduction episodes unless the lower mantle has a viscosity at least 10 times that of the upper mantle. This need for a viscosity increase with depth in the mantle reinforces the conclusions of previous studies on postglacial rebound and geoid anomalies, theoretical creep laws, and some preliminary results on TPW induced by density anomalies embedded in the mantle. The lower viscosity in the upper mantle means that upper-mantle density anomalies are most effective in exciting TPW. Changes in the pattern of subduction through time may be responsible for both episodes of fast TPW and times of quiescence in polar motion.
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Earth's spin axis follows the maximum moment of inertia axis of mantle convection, with some delay due to adjustment of the rotational bulge. Here we compute this axis for geodynamic models based on subduction history, assuming constant slab sinking speed, with another contribution due to thermochemical piles. For a wide range of parameters, a large shift of ≈90° is predicted around 80-90 Ma. It can be largely attributed to a change in circum-Pacific subduction from predominantly in the North and South toward East and West. Actual amounts of true polar wander are much smaller, pointing toward additional inertia tensor contributions, possibly due to slabs in the lowermost mantle below both polar regions. These slabs would have been subducted before ≈150 Ma, when plate motions in the Panthalassa basin are largely unknown. Matching predicted and observed true polar wander can serve at constraining such plate motions.
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We challenge the perspective that seismicity could contribute to polar motion by arguing quantitatively that, in first approximation and on the average, interseismic deformations can compensate for it. This point is important because what we must simulate and observe in Earth Orientation Parameter time-series over intermediate timescales of decades or centuries is the residual polar motion resulting from the two opposing processes of coseismic and interseismic deformations. In this framework, we first simulate the polar motion caused by only coseismic deformations during the longest period available of instrumental seismicity, from 1900 to present, using both the CMT and ISC-GEM catalogues. The instrumental seismicity covering a little longer than one century does not represent yet the average seismicity that we should expect on the long term. Indeed, although the simulation shows a tendency to move the Earth rotation pole towards 133°E at the average rate of 16.5mmyr-1, this trend is still sensitive to individual megathrust earthquakes, particularly to the 1960 Chile and 1964 Alaska earthquakes. In order to further investigate this issue, we develop a global seismicity model (GSM) that is independent from any earthquake catalogue and that describes the average seismicity along plate boundaries on the long term by combining information about presentday plate kinematics with the Anderson theory of faulting, the seismic moment conservation principle and a few other assumptions. Within this framework, we obtain a secular polar motion of 8mmyr-1 towards 112.5°E that is comparable with that estimated from 1900 to present using the earthquake catalogues, although smaller by a factor of 2 in amplitude and different by 20° in direction. Afterwards, in order to reconcile the idea of a secular polar motion caused by earthquakes with our simplest understanding of the seismic cycle, we adapt the GSM in order to account for interseismic deformations and we use it to quantify, for the first time ever, their contribution to polar motion. Taken together, coseismic and interseismic deformations make the rotation pole wander around the north pole with maximum polar excursions of about 1 m. In particular, the rotation pole moves towards about Newfoundland when the interseismic contribution dominates over the coseismic ones (i.e. during phases of low seismicity or, equivalently, when most of the fault system associated with plate boundaries is locked). When megathrust earthquakes occur, instead, the rotation pole is suddenly shifted in an almost opposite direction, towards about 133°E.
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Subducting slabs create strong short wavelength seismic anomalies in the upper mantle where much of Earth's seismicity is located. As such, they have the potential to bias longerwavelength seismic tomography models. To evaluate the effect of subducting slabs in global tomography, we performed a series of inversions using a global synthetic shear wave traveltime data set for a theoretical slab model based on predicted thermal anomalies within slabs. The spectral element method was applied to predict the traveltime anomalies produced by the 3-D slab model for paths corresponding to our current data used in actual tomography models. Inversion tests have been conducted first using the raw traveltime anomalies to check how well the slabs can be imaged in global tomography without the effect of earthquake mislocation. Our results indicate that most of the slabs can be identified in the inversion result but with smoothed and reduced amplitude. The recovery of the total mass anomaly in slab regions is about 88 per cent. We then performed another inversion test to investigate the effect of mislocation caused by subducting slabs. We found that source mislocation largely removes slab signal and significantly degrades the imaging of subducting slabs-potentially reducing the recovery of mass anomalies in slab regions to only 41 per cent. We tested two source relocation procedures-an iterative relocation inversion and joint relocation inversion. Both methods partially recover the true source locations and improve the inversion results, but the joint inversion method worked significantly better than the iterative method. In all of our inversion tests, the amplitudes of artefact structures in the lower mantle caused by the incorrect imaging of slabs (up to ~0.5 per cent S velocity anomalies) are comparable to some large-scale lower-mantle heterogeneities seen in global tomography studies. Based on our inversion tests, we suggest including a-priori subducting slabs in the starting models in global tomography studies and use joint relocation in the inversion.
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Climate-driven changes in land water storage and their contributions to sea level rise have been absent from Intergovernmental Panel on Climate Change sea level budgets owing to observational challenges. Recent advances in satellite measurement of time-variable gravity combined with reconciled global glacier loss estimates enable a disaggregation of continental land mass changes and a quantification of this term. We found that between 2002 and 2014, climate variability resulted in an additional 3200 ± 900 gigatons of water being stored on land. This gain partially offset water losses from ice sheets, glaciers, and groundwater pumping, slowing the rate of sea level rise by 0.71 ± 0.20 millimeters per year. These findings highlight the importance of climate-driven changes in hydrology when assigning attribution to decadal changes in sea level.
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Large-scale topography may be due to several causes, including (1) variations in crustal thickness and density structure, (2) oceanic lithosphere age differences, (3) subcrustal density variations in the continental lithosphere and (4) convective flow in the mantle beneath the lithosphere. The last contribution in particular may change with time and be responsible for continental inundations; distinguishing between these contributions is therefore important for linking Earth's history to its observed geological record. As a step towards this goal, this paper aims at such distinction for the present-day topography: the approach taken is deriving a 'model' topography due to contributions (3) and (4), along with a model geoid, using a geodynamic mantle flow model. Both lithosphere thickness and density anomalies beneath the lithosphere are inferred from seismic tomography. Density anomalies within the continental lithosphere are uncertain, because they are probably due to variations in composition and temperature, making a simple scaling from seismic to density anomalies inappropriate. Therefore, we test a number of different assumptions regarding these. As a reality check, model topography is compared, in terms of both correlation and amplitude ratio, to 'residual' topography, which follows from observed topography after subtracting contributions (1) and (2). The model geoid is compared to observations as well. Comparatively good agreement is found if there is either an excess density of ≈0.2 per cent in the lithosphere above ≈150 km depth, with anomalies below as inferred from tomography, or if the excess density is ≈0.4 per cent in the entire lithosphere. Further, a good fit is found for viscosity ≈1020 Pa s in the asthenosphere, increasing to≈1023 Pa s in the lower mantle aboveDLL. Results are quite dependent on which tomography models they are based on; for some recent ones, topography correlation is≈0.6, many smaller scale features are matched, topography amplitude is less than≈30 per cent too large, while geoid variance reduction exceeds 70 per cent-overall a considerable improvement compared to previous models. Correlation becomes less if smaller scale features (corresponding to spherical harmonic degrees 15 and higher), which are probably largely due to anomalies in the lithosphere, are also considered. Comparison of results with different viscosity structures, and a regional comparison of amplitude ratios, indicates that lateral viscosity variations can be quite strong, but only leading to moderate variations in model topography of a factor probably less than two.
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The response of the Greenland Ice Sheet (GIS) to changes in temperature during the twentieth century remains contentious, largely owing to difficulties in estimating the spatial and temporal distribution of ice mass changes before 1992, when Greenland-wide observations first became available. The only previous estimates of change during the twentieth century are based on empirical modelling and energy balance modelling. Consequently, no observation-based estimates of the contribution from the GIS to the global-mean sea level budget before 1990 are included in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Here we calculate spatial ice mass loss around the entire GIS from 1900 to the present using aerial imagery from the 1980s. This allows accurate high-resolution mapping of geomorphic features related to the maximum extent of the GIS during the Little Ice Age at the end of the nineteenth century. We estimate the total ice mass loss and its spatial distribution for three periods: 1900-1983 (75.1±29.4 gigatonnes per year), 1983-2003 (73.8±40.5 gigatonnes per year), and 2003-2010 (186.4±18.9 gigatonnes per year). Furthermore, using two surface mass balance models we partition the mass balance into a term for surface mass balance (that is, total precipitation minus total sublimation minus runoff) and a dynamic term. We find that many areas currently undergoing change are identical to those that experienced considerable thinning throughout the twentieth century. We also reveal that the surface mass balance term shows a considerable decrease since 2003, whereas the dynamic term is constant over the past 110 years. Overall, our observation-based findings show that during the twentieth century the GIS contributed at least 25.0±9.4millimetres of global-mean sea level rise. Our result will help to close the twentieth-century sea level budget, which remains crucial for evaluating the reliability of models used to predict global sea level rise.
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The viscosity structure of Earth’s deep mantle affects the thermal evolution of Earth, the ascent of mantle plumes, settling of subducted oceanic lithosphere, and the mixing of compositional heterogeneities in the mantle. Based on a reanalysis of the long-wavelength nonhydrostatic geoid, we infer viscous layering of the mantle using a method that allows us to avoid a priori assumptions about its variation with depth. We detect an increase in viscosity at 800- to 1200-kilometers depth, far greater than the depth of the mineral phase transformations that define the mantle transition zone. The viscosity increase is coincident in depth with regions where seismic tomography has imaged slab stagnation, plume deflection, and changes in large-scale structure and offers a simple explanation of these phenomena.
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Plumes of hot upwelling rock rooted in the deep mantle have been proposed as a possible origin of hotspot volcanoes, but this idea is the subject of vigorous debate. On the basis of geodynamic computations, plumes of purely thermal origin should comprise thin tails, only several hundred kilometres wide, and be difficult to detect using standard seismic tomography techniques. Here we describe the use of a whole-mantle seismic imaging technique--combining accurate wavefield computations with information contained in whole seismic waveforms--that reveals the presence of broad (not thin), quasi-vertical conduits beneath many prominent hotspots. These conduits extend from the core-mantle boundary to about 1,000 kilometres below Earth's surface, where some are deflected horizontally, as though entrained into more vigorous upper-mantle circulation. At the base of the mantle, these conduits are rooted in patches of greatly reduced shear velocity that, in the case of Hawaii, Iceland and Samoa, correspond to the locations of known large ultralow-velocity zones. This correspondence clearly establishes a continuous connection between such zones and mantle plumes. We also show that the imaged conduits are robustly broader than classical thermal plume tails, suggesting that they are long-lived, and may have a thermochemical origin. Their vertical orientation suggests very sluggish background circulation below depths of 1,000 kilometres. Our results should provide constraints on studies of viscosity layering of Earth's mantle and guide further research into thermochemical convection.
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A three-dimensional, self-consistent numerical model of the geodynamo is described, that maintains a magnetic field for over 40,000 years. The model, which incorporates a finitely conducting inner core, undergoes several polarity excursions and then, near the end of the simulation, a successful reversal of the dipole moment. This simulated magnetic field reversal shares some features with real reversals of the geomagnetic field, and may provide insight into the geomagnetic reversal mechanism.
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The radially anisotropic shear velocity structure of the Earth's mantle provides a critical window on the interior dynamics of the planet, with isotropic variations that are interpreted in terms of thermal and compositional heterogeneity and anisotropy in terms of flow. While significant progress has been made in the more than 30 yr since the advent of global seismic tomography, many open questions remain regarding the dual roles of temperature and composition in shaping mantle convection, as well as interactions between different dominant scales of convective phenomena. We believe that advanced seismic imaging techniques, such as waveform inversion using accurate numerical simulations of the seismic wavefield, represent a clear path forwards towards addressing these open questions through application to whole-mantle imaging. To this end, we employ a `hybrid' waveform-inversion approach, which combines the accuracy and generality of the spectral finite element method (SEM) for forward modelling of the global wavefield, with non-linear asymptotic coupling theory for efficient inverse modelling. The resulting whole-mantle model (SEMUCB-WM1) builds on the earlier successful application of these techniques for global modelling at upper mantle and transition-zone depths (≤800 km) which delivered the models SEMum and SEMum2. Indeed, SEMUCB-WM1 is the first whole-mantle model derived from fully numerical SEM-based forward modelling. Here, we detail the technical aspects of the development of our whole-mantle model, as well as provide a broad discussion of isotropic and radially anisotropic model structure. We also include an extensive discussion of model uncertainties, specifically focused on assessing our results at transition-zone and lower-mantle depths.
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[1] The analytical formulation of the theories of nutation and wobble reveals the combinations of basic Earth parameters that govern the nutation-wobble response of the Earth to gravitational (tidal) forcing by heavenly bodies and makes it possible to estimate several of them through a least squares fit of the theoretical expressions to the high-precision data now available. This paper presents the essentials of the theoretical framework, the procedure that we used for least squares estimation of basic Earth parameters through a fit of theory to nutation-precession data derived from an up-to-date very long baseline interferometry data set, the results of the estimation and their geophysical interpretation, and the nutation series constructed using the estimated values of the parameters. The theoretical formulation used here differs from earlier ones in the incorporation of anelasticity and ocean tide effects into the basic structure of the dynamical equations of the theory and in the inclusion of electromagnetic couplings of the mantle and the solid inner core to the fluid outer core, though this generalization comes at the cost of making some of the system parameters complex and frequency dependent; it is also more complete, as it takes account of nonlinear terms in these equations, including effects of the time-dependent deformations produced by zonal and sectorial tides, which had been traditionally neglected in nonrigid Earth theories. Among the geophysical results obtained from our fit are estimates for the dynamic ellipticity e of the Earth (e = 0.0032845479 with an uncertainty of 12 in the last digit), for the dynamical ellipticity ef of the fluid core (3.8% higher than its hydrostatic equilibrium value, rather than ∼5% as hitherto), and for the two complex electromagnetic coupling constants. Our best estimates for the RMS radial magnetic fields at the core mantle boundary and at the inner core boundary, based on the estimates for these coupling constants, are ~6.9 and 72 gauss, respectively, when the magnetic field configurations are restricted to certain simple classes. The field strength needed at the inner core boundary could be lower if the density of the core fluid at this boundary or the ellipticity of the solid inner core were lower than that for the Preliminary Reference Earth Model. Our estimate for the resonance frequency of the prograde free core nutation mode, with an uncertainty of ∼10%, constitutes the first firm detection of the resonance associated with this mode; the period found is ∼1025 days, double that with electromagnetic couplings ignored. (Throughout this work, “days,” referring to periods, stands for “mean solar days.”) A new nutation series (MHB2000) is constructed by direct solution of the linearized dynamical equations (with our best fit values adopted for all the estimated Earth parameters) for each forcing frequency, and adding on the contributions from the nonlinear terms and other effects not included in the linearized equations. This series gives a considerably better fit to the nutation data than any of the earlier series based on geophysical theory. In particular, the residuals in the out of phase amplitudes of the retrograde 18.6 year and annual nutations, which had long remained at ∼0.5 milliseconds of arc (mas), are now reduced to the level of the uncertainties in the observational estimates, thanks mainly to the role played by the electromagnetic couplings. The largest remaining discrepancy is that in the out of phase prograde 18.6 year nutation, of ∼72 micorseconds of arc (μas). The frequency dependence of the nutation amplitudes cannot be exactly represented through a resonance formula, nor may the resonance frequencies themselves be interpreted as the eigenfrequencies of free modes because of the presence of complex and frequency-dependent system parameters. Nevertheless, we have constructed a new resonance formula which reproduces our nutation series accurately for almost all nutation frequencies; for the few remaining frequencies, a listing is given of the corrections to be applied in order to reproduce the exact results of the direct solution.
Article
[1] A new P wave tomographic model of the mantle was constructed using more than 10 million travel times. The finite-frequency effect of seismic rays was taken into account by calculating banana-donut kernels at 2 Hz for all first arrival time data, and at 0.1 Hz for broadband differential travel time data. Based on this model, a systematic survey for subducted slab images was developed for the circum-Pacific; including the Kurile, Honshu, Izu-Bonin, Mariana, Java, Tonga-Kermadec, southern and northern South America, and Central America, arcs. This survey revealed a progressive lateral variation of the configuration of slabs along arc(s), which we interpret as an indication for successive stages of slab subduction through the Bullen's transition region with the 660 km discontinuity at the middle. We identified the four distinct stages: I - slab stagnant above the 660 km discontinuity; II - slab penetrating the 660 km discontinuity; III - slab trapped in the uppermost lower mantle (at a depth of 660–1000 km); and IV - slab descending well into the deep lower mantle. The majority of slab images are found to be either at Stage I or III, suggesting that Stages I and III are relatively stable or neutral and II and IV are relatively unstable or transient. There is a remarkable distinction for the deepest hypocentral distribution between slabs at Stage I and slabs at Stages II or III.
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Space geodetic observations of polar motion show that around 2005, the average annual pole position began drifting toward the east, an abrupt departure from the drift direction seen over the past century. Satellite gravity measurements from the Gravity Recovery and Climate Experiment (GRACE) show that about 90% of this change is due to accelerated melting of polar ice sheets and mountain glaciers and related sea level rise. The close relationship between long-term polar motion and climate-related mass redistribution established using GRACE data indicates that accurately measured polar motion data offer an additional tool for monitoring global-scale ice melting and sea level rise and should be useful in bridging the anticipated gap between GRACE and follow-on satellite gravity missions.
Article
Modelling the geoid has been a widely used and successful approach in constraining flow and viscosity in the Earth's mantle. However, details of the viscosity structure cannot be tightly constrained with this approach. Here, radial viscosity variations in four to five mantle layers (lithosphere, upper mantle, one to two transition zone layers, lower mantle) are computed with the aid of independent mineral physics results. A density model is obtained by converting s-wave anomalies from seismic tomography to density anomalies. Assuming both are of thermal origin, conversion factors are computed based on mineral physics results. From the density and viscosity model, a model of mantle flow, and the resulting geoid and radial heat flux profile are computed. Absolute viscosity values in the mantle layers are treated as free parameters and determined by minimizing a misfit function, which considers fit to geoid, `Haskell average' determined from post-glacial rebound and the radial heat flux profile and penalizes if at some depth computed heat flux exceeds the estimated mantle heat flux 33 TW. Typically, optimized models do not exceed this value by more than about 20 per cent and fit the Haskell average well. Viscosity profiles obtained show a characteristic hump in the lower mantle, with maximum viscosities of about 1023 Pa s just above the D'' layer- several hundred to about 1000 times the lowest viscosities in the upper mantle. This viscosity contrast is several times higher than what is inferred when a constant lower mantle viscosity is assumed. The geoid variance reduction obtained is up to about 80 per cent-similar to previous results. However, because of the use of mineral physics constraints, a rather small number of free model parameters is required, and at the same time, a reasonable heat flux profile is obtained. Results are best when the lowest viscosities occur in the transition zone. When viscosity is lowest in the asthenosphere, variance reduction is about 70-75 per cent. Best results were obtained with a viscous lithosphere with a few times 1022 Pa s. The optimized models yield a core-mantle boundary excess ellipticity several times higher than observed, possibly indicating that radial stresses are partly compensated due to non-thermal lateral variations within the lowermost mantle.
Article
Predictions of glaciation-induced changes in the Earth's rotation vector exhibit sensitivities to Earth structure that are unique within the suite of long-wavelength observables associated with glacial isostatic adjustment (henceforth GIA), and, despite nearly a quarter of a century of research, these sensitivities remain enigmatic. Previous predictions of present-day true polar wander (TPW) speed driven by GIA have indicated, for example, a strong sensitivity to variations in the thickness of the elastic lithosphere and the treatment (phase or chemical?) of the density discontinuity at 670-km depth. Nakada recently presented results that suggest that the predictions are also sensitive to the adopted rheology of the lithosphere; however, his results have introduced an intriguing paradox. In particular, predictions generated using a model with an extremely high-viscosity lithospheric lid do not converge to results for a purely elastic lithosphere of the same thickness. Mitrovica (as cited by Nakada) has suggested that the paradox originates from an inaccuracy in the traditional rotation theory (e.g. Wu & Peltier) associated with the treatment of the background equilibrium rotating form upon which any load- and rotation-induced perturbations are superimposed. We revisit these issues using a new treatment of the linearized Euler equations governing load-induced rotation perturbations on viscoelastic earth models. We demonstrate that our revised theory, in which the background form of the planet combines a hydrostatic component and an observationally inferred excess ellipticity, resolves the apparent paradox. Calculations using the revised theory indicate that earlier predictions based on earth models with purely elastic lithospheric lids are subject to large errors; indeed, previously noted sensitivities of TPW speed predictions to the thickness and rheology (elastic versus viscous) of the lithosphere largely disappear in the application of the new theory. Significant errors are also incurred by neglecting the stabilizing influence of the Earth's excess ellipticity. Finally, we demonstrate that the contribution from rotational feedback on predictions of present-day rates of change of the geoid (sea surface) and crustal velocities are overestimated by the traditional rotation theory, and this has implications for analyses of ongoing satellite (e.g. GRACE) missions and geodetic GPS surveys.
Article
New finite-frequency tomographic images of S-wave velocity confirm the existence of deep mantle plumes below a large number of known hot spots. We compare S-anomaly images with an updated P-anomaly model. Deep mantle plumes are present beneath Ascension, Azores, Canary, Cape Verde, Cook Island, Crozet, Easter, Kerguelen, Hawaii, Samoa, and Tahiti. Afar, Atlantic Ridge, Bouvet(Shona), Cocos/Keeling, Louisville, and Reunion are shown to originate at least below the upper mantle if not much deeper. Plumes that reach only to midmantle are present beneath Bowie, Hainan, Eastern Australia, and Juan Fernandez; these plumes may have tails too thin to observe in the lowermost mantle, but the images are also consistent with an interpretation as ``dying plumes'' that have exhausted their source region. In the tomographic images, only the Eifel and Seychelles plumes are unambiguously confined to the upper mantle. Starting plumes are visible in the lowermost mantle beneath South of Java, East of Solomon, and in the Coral Sea. All imaged plumes are wide and fail to show plumeheads, suggesting a very weakly temperature-dependent viscosity for lower mantle minerals, and/or compositional variations. The S-wave velocity images show several minor differences with respect to the earlier P-wave results, including plume conduits that extend down to the core-mantle boundary beneath Cape Verde, Cook Island, and Kerguelen. A more substantial disagreement between P-wave and S-wave images reopens the question on the depth extent of the Iceland plume. We suggest that a pulsating behavior of the plume may explain the shape of the conduit beneath Iceland.
Article
We demonstrate that earth nutation measurements made with very long baseline interferometry are of sufficient accuracy to be sensitive to the properties of the core-mantle boundary. The retrograde nutation with annual frequency is particularly sensitive, since this frequency is closest to that of the free core nutation, a nutational normal mode which produces relative motion of the core and mantle. Our nutation measurements imply a deviation of the amplitude of the retrograde annual nutation from its value as calculated by Wahr and by Sasao et al. If this deviation is interpreted as the effect of a departure of the core-mantle boundary from its hydrostatic figure, then the observed amplitude is consistent with a core-mantle boundary that has a second zonal harmonic deviation from the hydrostatic equilibrium figure, with the peak-to-valley deviation being 490±110 m. The part of the retrograde annual nutation out of phase with the driving torques yields an upper limit on the kinematic viscosity at the surface of the fluid core of 0.54 m2/s (99.5% confidence limit).
Article
The linear stability problem for a number of models of the mantle of the earth is considered. For appropriate values of the physical parameters of the mantle it seems likely that the Rayleigh number for mantle-wide convection is far in excess of the value necessary for marginal instability. For very high Rayleigh numbers the velocities in the models can be derived from solutions for turbulent convection. But even for very high Rayleigh numbers the inhomogeneity in Bullen's region C is amply strong enough to prevent mantle-wide convection from occurring, whether the inhomogeneity involves a phase transition or represents a chemical inhomogeneity. Convection on a smaller scale is also considered. Convection in the upper mantle may occur. These events are not widespread and are of small scale, having dimensions of about 1200–1500 km in lateral extent and depths of the order of 400 km. Large Rayleigh numbers and associated turbulent convection are not ruled out for the lower mantle. The conclusions depend crucially on the assumptions of the values of the viscosity and of the strength of the mantle. The model of turbulent convection in the lower mantle is consistent with localizing a material of high strength and high viscosity in the upper mantle and with the observation that earthquakes are not observed to occur in the lower mantle.
Article
Polar wander, the secular motion of the Earth's rotation axis relative to its surface, has been studied for many years. Dynamical arguments1-3 show that polar wander can arise from the redistribution of mass in a plastic deformable Earth, the rate depending on both the rate of mass redistribution and the rate at which the Earth's rotational bulge can readjust to the changing rotation axis. Here we use a viscosity structure obtained through geoid modelling4, a mantle flow field consistent with tomographic anomalies5, and time-dependent lithospheric plate motions6 to calculate the advection of mantle density heterogeneities and corresponding changes in the degree-two geoid during the Cenozoic era. We show that the rotation axis will follow closely any imposed changes of the axis of maximum non-hydrostatic moment of inertia. The resulting path of the rotation axis agrees well with palaeomagnetic results7, with the model predicting a current rate of polar motion that explains 40% of that observed geodetically8.
Article
A formal inverse procedure is used to infer radial mantle viscosity profiles from several observations related to the glacial isostatic adjustment process. The data sets consist of Late Pleistocene and Holocene sea level data from Scandinavia, the Barents Sea, Central Europe, Canada, and the far field, as well as observations of changes in the Earth's rotation and gravitational field, and present-day uplift and gravity changes in Scandinavia. Inferences of mantle viscosity are robust against assumptions such as the a priori viscosity model and model discretization. However, the quality of ice sheet reconstruction remains crucial for the inverse inference. The importance to discuss regional mantle viscosity models in view of the lateral variability in mantle properties has been evident. Our inference suggests a two order of magnitude increase of mantle viscosity with depth, and volume-averaged upper and lower mantle viscosities around 7 × 1020 and 2 × 1022 Pa s, respectively. Mantle viscosity does not need to increase sharply across the 660-km seismic discontinuity. The viscosity profiles suggested are also able to reconcile the large-scale geoid anomaly related to mantle convection.
Article
We have constructed new apparent polar wander paths (APWPs) for major plates over the last 200 Myr. Updated kinematic models and selected paleomagnetic data allowed us to construct a master APWP. A persistent quadrupole moment on the order of 3% of the dipole over the last 200 Myr is suggested. Paleomagnetic and hot spot APW are compared, and a new determination of ``true polar wander'' (TPW) is derived. Under the hypothesis of fixed Atlantic and Indian hot spots, we confirm that TPW is episodic, with periods of (quasi) standstill alternating with periods of faster TPW (in the Cretaceous). The typical duration of these periods is on the order of a few tens of millions of years with wander rates during fast tracks on the order of 30 to 50 km/Myr. A total TPW of some 30° is suggested for the last 200 Myr. We find no convincing evidence for episodes of superfast TPW such as proposed recently by a number of authors. Comparison over the last 130 Myr of TPW deduced from hot spot tracks and paleomagnetic data in the Indo-Atlantic hemisphere with an independent determination for the Pacific plate supports the idea that, to first order, TPW is a truly global feature of Earth dynamics. Comparison with numerical modeling estimates of TPW shows that all current models still fail to some extent to account for the observed values of TPW velocity and for the succession of standstills and tracks which is observed.
Article
We obtain a three-dimensional (3D) model of shear wave velocity heterogeneity of the Earth's mantle by inverting a large set of seismic data consisting of 27,000 long-period seismograms and 14,000 travel time observations. About 60% of the data has been collected through the efforts of several research groups and used in earlier studies. The new data, which come from stations of different seismic networks including the Chinese Digital Seismographic Network (CDSN) and Geoscope, are extracted to provide sampling of mantle heterogeneity as uniform as possible. Because of the improved data coverage, we expand our model to degree 12 in spherical harmonics to describe horizontal variations, and to order of 13 Chebyshev polynomials to describe radial variations. The resulting model shows a clear pattern of slower-than-average shear velocities at shallow depths underlying the major segments of the world-wide ridge system. The model is dominated by a few megastructures of velocity heterogeneity below the depth of 2000 km, in agreement with previous studies. The model predicts well the large-scale pattern of observed S, SS absolute travel times, and SS-S, ScS-S differential travel times. It also predicts well the waveforms of mantle wave and body wave. We compare our model with several other recently published models. There is generally a good agreement in the long-wavelength pattern of the models.