S U M M A R YExperiments simulating flow in the Earth's liquid core induced by luni-solar precession of the solid mantle indicate, to a first approximation, that the core behaves like a rigidized fluid sphere spinning slower than the mantle and with its spin axis lagging the mantle spin axis in precession. Secondary flow patterns are always present. At low precession rates the fluid sphere is subdivided into a set of cylinders coaxial with the fluid spin axis, the cylinders rotating alternately at slightly faster and slower rates relative to the net retrograde motion of the fluid as a whole.Slow non-axisymmetric columnar wave patterns develop between the differentially rotating cylinders. Axial flows between the spheroidal cavity boundary and the interior are observed. Fluid motion becomes turbulent only at precession rates large enough to cause the fluid spin axis to align nearly with the precession axis. There is no evidence that the Earth's liquid spin axis direction departs more than a fraction of a degree from geographic north. Our observations suggest precession induces a complex variety of laminar flows, including slowly varying and/or periodic patterns, in the Earth's liquid core.

Controlled spin and precession of liquid filled oblate spheroidal cavities in laboratory experiments that simulate luni-solar forced precession of the mantle and a simplified analytical liquid flow model, indicate Earth core flows approximated as spin about a core axis that lags behind the precessing spin axis of the mantle by a small angle (-0.01"). Core spin is slower than mantle spin by an amount consistent with westward drift of the magnetic field ( -O.T/yr). Theoretical and experimental evidence argue against inertia waves being effective in preventing such motion. This precessional flow model is consistent with geodynamo requirements indicating a kinematic over a magnetohydrodynamic dynamo in that viscous core-mantle coupling may dominate magnetic coupling, with only negligible topographic coupling. While the model appears to provide geodynamo requirements, it does not preclude simultaneous density gradient convective mechanisms.

Experiments using a precessing liquid‐filled oblate spheroid with ellipticity (a − b)/a =1/400 extend and clarify earlier research. They yield flow data useful for estimating flows in the Earth’s liquid core. Observed flows illustrate and confirm a nearly rigid liquid sphere with retrograde drift and lagging a cavity (mantle) axis in precession. The similarities of the observed lag angle with that computed for a rigid sphere, and earlier energy dissipation research both support the use of a rigid sphere analytical model to predict energy dissipation and first‐order flow within the core–mantle boundary (CMB). Second‐order boundary layer and interior cylindrical flow structures also are photographed and measured. Interior flows are never turbulent or unstable at near‐Earth parameters, although complex and transient flow patterns are observed within the boundary layer. Other mechanisms proposed to explain net heat loss from the Earth and maintenance of the geodynamo typically require acceptance of some critical but unproven premise. Precession and CMB configuration are known with certainty and precision. Analytical difficulties have been the obstacle. Experiments illustrate the consequences of precession and ellipticity, provide criteria for validating analytical and numerical models, and may yield direct knowledge of the Earth’s deep interior with careful scaling.

Modern microbial mats and stromatolites exhibiting a preferred orientation toward specular sunlight were found at two sites. In Hamelin Pool of Shark Bay, Western Australia, subtidal decimeter-sized columns and intertidal centimeter-sized tufts were found pointing north. In thermal spring effluents and pools of Yellowstone National Park, mats were found with columnar and conical centimeter-sized structures inclined to the south. These examples of heliotropism in modern stromatolites are each built by a different community of photosynthetic microbes under markedly different environmental conditions. These new observations support the proposal that stromatolites can orient themselves toward the sun.

Vanyo, J.P. and Awramik, S.M., 1985. Stromatolites and Earth--Sun--Moon dynamics.Precambrian Res., 29 : 121--142.Inclination of stromatolite columns, caused by nonvertical direction of averaged incident solar radiation, provides a signal for deducing astronomical and geophysical data at time of stromatolite formation. A sample of Anabaria juvensis (Bitter Springs Formation, central Australia) shows a sinusoidal growth pattern which, interpreted as an annual signal (obliquity of the ecliptic) with daily production of laminae, indicates 435 days per year, a result consistent with other available estimates. A sample of Inzeria from the same formation also shows a sinusoidal pattern and a similar estimate for days per year. Both samples indicate growth rates of centimeters per year. Several laboratory measurement techniques offer systematic procedures for extracting data.