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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.
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Jellyfish and Fish Solve the Challenges of Turning Dynamics Similarly to Achieve High Maneuverability
Turning maneuvers by aquatic animals are essential for fundamental life functions such as finding food or mates while avoiding predation. However, turning requires resolution of a fundamental dilemma based in rotational mechanics: the force powering a turn (torque) is favored by an expanded body configuration that maximizes lever arm length, yet minimizing the resistance to a turn (the moment of inertia) is favored by a contracted body configuration. How do animals balance these opposing demands? Here, we directly measure instantaneous forces along the bodies of two animal models—the radially symmetric Aurelia aurita jellyfish, and the bilaterally symmetric Danio rerio zebrafish—to evaluate their turning dynamics. Both began turns with a small, rapid shift in body kinematics that preceded major axial rotation. Although small in absolute magnitude, the high fluid accelerations achieved by these initial motions generated powerful pressure gradients that maximized torque at the start of a turn. This pattern allows these animals to initially maximize torque production before major body curvature changes. Both animals then subsequently minimized the moment of inertia, and hence resistance to axial rotation, by body bending. This sequential solution provides insight into the advantages of re-arranging mass by bending during routine swimming turns.
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- PAR ID:
- 10187121
- Date Published:
- Journal Name:
- Fluids
- Volume:
- 5
- Issue:
- 3
- ISSN:
- 2311-5521
- Page Range / eLocation ID:
- 106
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/fluids5030106
