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Abstract Mantle convection drives changes in Earth's ellipsoidal figure and corresponding moment of inertia, causing shifts in the planet's rotation axis known as true polar wander (TPW). Using seismic tomography and the Back-and-Forth Nudging (BFN) method, we developed a time-dependent convection model that reconstructs the evolution of mantle density distribution and Earth’s moment of inertia over the past 70 million years. This modelling approach provides a close match with independent paleomagnetic constraints on the Cenozoic shifts of Earth’s rotation pole, specifically resolving the previously unexplained U-turn in TPW trajectory at approximately 50 million years ago. Our findings reveal TPW shifts exceeding 5 degrees, despite the temporal stability imposed by high lower-mantle viscosity and the stabilizing effect of Earth's remnant rotational bulge. Verification of our predicted changes in Earth’s ellipsoidal figure through independent paleomagnetic data provides a robust calibration for new predictions of convection-induced dynamic flattening variations. Over the past 70 million years, we find convection-induced changes of flattening that shift from -0.2% to +0.1 % during the Paleogene. Our predictions of Earth's axial precession frequency in the Paleogene align with recent independent cyclostratigraphic studies, thus validating our model's accuracy and supporting the hypothesis of reduced luni-solar tidal dissipation during this period. 

Abstract We propose that the mantle lithospheric density and crustal thickness are correlated in such a way as to produce a flat Tibetan Plateau. We observe that the mantle lithosphere is relatively uniform beneath the Himalaya and southern and central Tibet, despite a near doubling of crustal thickness relative to India. Farther north, cratonic mantle lithosphere disappears over large regions of north-central Tibet, giving rise to large lateral variations in uppermost mantle Vs anomalies (>12%) that are uncorrelated with changes in surface elevation but are closely related to changes in crustal thickness. This decoupling of surface topography from spatial variations in upper mantle seismic velocity, and assumed buoyancy, implies that Tibetan topography is controlled by a crust-mantle interaction that is able to maintain its near constant elevation. This crust-mantle interaction is likely driven by gravitational potential energy with a very weak crust. Magmatism, with ages of ca. 20 Ma to Present, spatially correlated with this region with no sub-Moho mantle lithosphere implies destabilization of mantle lithosphere in northern Tibet. Cratonic Indian underthrusting for the past 25 m.y. has also not led to significant topography in the plateau through time. The magmatism may have helped weaken the crust, allowing it to respond to changes in uppermost mantle buoyancy, resulting in a flat plateau. 

Abstract Earth's topography arises from the linear superposition of isostatic and dynamic contributions. The isostatic contribution reflects the distribution of thickness and density of the crust overlying a static, non‐convecting mantle. We argue that isostatic topography should be limited to the crust, thereby delimiting all sources for dynamic topography below the Moho. Dynamic topography is the component of the topography produced by normal stresses acting on the Moho that deflect the isostatic topography away from crustal isostatic equilibrium largely as a consequence of mantle flow dynamics. These normal stresses arise from pressure variations and vertical gradients of the radial flow in the convecting mantle. The best estimate of dynamic topography is from the residual topography, which is the difference between observed topography and crustal isostatic topography. Dynamic and residual topography are the same. It is clear that thermal anomalies horizontally advected by plate motions would not exist if the mantle were not convecting, therefore their contribution to topography is inherently dynamic in origin. The global integral of dynamic topography that encompasses all non‐crustal buoyancy sources is demonstrated to be equal to zero. It follows that mantle convection cannot change the mean radius or mean elevation of the Earth. Since changes in ocean basin volume driven by changes in mean depth of the oceans are inherently part of dynamic topography, thereby requiring that continental elevations must also change, such that the global integral of these perturbations must also be equal to zero. This constraint has important implications for global long‐term sea level and the stratigraphic record, among other features of the Earth system impacted by changes in Earth's dynamic topography. 

As droughts become longer and more intense, impacts on terrestrial primary productivity are expected to increase progressively. Yet, some ecosystems appear to acclimate to multiyear drought, with constant or diminishing reductions in productivity as drought duration increases. We quantified the combined effects of drought duration and intensity on aboveground productivity in 74 grasslands and shrublands distributed globally. Ecosystem acclimation with multiyear drought was observed overall, except when droughts were extreme (i.e., ≤1-in-100-year likelihood of occurrence). Productivity losses after four consecutive years of extreme drought increased by ~2.5-fold compared with those of the first year. These results portend a foundational shift in ecosystem behavior if drought duration and intensity increase, from maintenance of reduced functioning over time to progressive and profound losses of productivity when droughts are extreme. 

The past elevation of the land surface provides a unique constraint on the underlying lithospheric structure during mountain and plateau formation. Development of new paleoaltimetry techniques that can be applied to a wide variety of sample types is therefore of continuing importance. This study evaluates organic substrates that preserve the δD ratio of surface waters as a new approach to reconstruct paleoaltimetry. We measured the hydrogen isotope composition of n-alkanes from epicuticular plant waxes preserved in lacustrine deposits to reconstruct the δD of precipitation in Cenozoic basins that have been elevated as part of the Tibetan Plateau. n-Alkane δD- and carbonate δ18O-inferred water compositions from the Eocene–Miocene Lunpola Basin and Miocene Hoh-Xil Basin plot near or at enriched values relative to the global meteoric water line, as expected for evaporative lakewater and leafwater systems that have the same precipitation source. n-Alkane δD-based water compositions are nearly identical to the minimum carbonate δ18O-based values, demonstrating that plant-wax δD is minimally affected by evaporation compared to lacustrine calcite δ18O. This agreement strongly supports the presence of similar precipitation isotopic compositions in both archives despite different isotope systems, source water reservoirs, archive materials, modes of incorporation, and diagenetic processes. Paleoelevations for each basin and time period were calculated from precipitation isotope ratios using the isotope–altitude relationship derived from both a simple thermodynamic model and modern precipitation sampling from the Plateau region. Our new results from the Hoh-Xil Basin suggest 1700 to 2600 m of uplift may have occurred some time between the late Eocene and early Miocene. The timing of this uplift is consistent with late-Oligocene compressional deformation of the Hoh-Xil Basin and northward growth of the Tibetan Plateau however, the calculated uplift is not a unique solution from the paleoisotope data because of uncertainties in Eocene and Miocene moisture sources and isotope gradients for the northern plateau. Our results demonstrate the utility of lipid-based estimates of paleoelevation and expand the types of deposits amenable to paleoaltimetry analysis.