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Antarctica sits above Earth's strongest 'gravity hole' – how it got that way
by bikenaga
Original article: "Cenozoic evolution of earth’s strongest geoid low illuminates mantle dynamics beneath Antarctica" - https://www.nature.com/articles/s41598-025-28606-1
Abstract: "Constraining the long-term evolution of geoid anomalies is essential for unraveling Earth’s internal dynamics. While most studies focus on present-day geoid snapshots, we reconstruct the time-dependent evolution of Earth’s strongest geoid depression, the Antarctic Geoid Low (AGL), over the Cenozoic. Unlike geodetic reference frames that place the deepest geoid low in the Indian Ocean, a geodynamic perspective – relative to a hydrostatic ellipsoid – reveals the strongest nonhydrostatic geoid depression resides over Antarctica. Using a back-and-forth nudging technique for time-reversed mantle convection modeling, we leverage 3-D mantle density structures derived from seismic tomography and geodynamic constraints. Our results show that the AGL has persisted for at least ~70 Myr, undergoing a major transition in amplitude and position between 50 and 30 Ma. This transition coincides with an abrupt lateral shift in Earth’s rotation axis at ~50 Ma, independently validated through paleomagnetic constraints on True Polar Wander. Initially, the AGL was supported by stable lower mantle density anomalies, but over the past ~40 Myr, an increasing contribution from upper-mantle buoyancy – particularly above ~1300 km depth – amplified the AGL magnitude. This shift reflects the interplay between long-term deep subduction beneath the Northwest Antarctic margin and a broad, thermally driven upwelling of buoyant material sourced from the lowermost mantle. These results contrast with earlier interpretations by demonstrating the crucial role of time-dependent coupling between both positive and negative mantle buoyancy in shaping global geoid anomalies. By integrating seismic, geodynamic, and mineral-physics data, our reconstructions provide a dynamically consistent view of mantle flow beneath Antarctica and offer new insights into the coupling between deep and shallow mantle processes that govern Earth’s long-wavelength geoid evolution."
Just a fun interjection from my side: The best way we have at the moment to systematically measure groundwater reserves (and depletion) is via proxy of gravity measurements.
The only thing on earth which is heavy and mobile on short time-scales is water. We can measure gravity so exactly that we can measure this movement of water by measuring gravity.
It’s also probably the best single method of inferring ice sheet mass balance over wide areas.
Well, here goes an obligatory mention of "At the Mountains of Madness" by HP Lovecraft. (BTW: my favourite story by him.)
Good ol wet crystals
trying to click on the link i got a "security verification" screen.
i aborted after 5 seconds of waiting.
> Caused by different densities of rock far beneath Earth's surface, these variations in gravity are small in absolute terms. But they can have particularly large effects on the oceans. Where gravity is weaker, the ocean surface can sit slightly lower relative to Earth's center because water flows away toward areas of stronger gravity. Due to its gravity hole, the sea-surface height around Antarctica is measurably lower than it would otherwise be.
Isn’t that just a result of earth rotation, and not some magical rocks far beneath earth’s surface!? Plus, that’s not how gravity works anyway, this sounds like some high school way to explain gravity.. in fact, no one knows what gravity is, even quantum physics can’t explain gravity.
> Plus, that’s not how gravity works anyway, this sounds like some high school way to explain gravity.. in fact, no one knows what gravity is, even quantum physics can’t explain gravity.
There's an impressive amount of irony in these juxtaposed statements.
There are a lot of rocks beneath earth's surface, and it shouldn't be surprising or magical that some of them are heavier than others. Which does affect local gravity, especially if we talk about the rocks close enough to the surface.
Take for example a look at https://www.esa.int/ESA_Multimedia/Images/2015/04/Bouguer_gr... which has better resolution than the map in the article and tries to correct for oceans and mountains. There are clear deviations from the norm, and they are not shaped in a way that can be explained by simple rotational effects (even though those also exist)
Geophysicist here. Look up potential fields methods in geophysics.
Such things have been measured and mapped for quite some time.
The grandfather of all modern gravimeters was invented in 1936 by LaCoste and Romberg.
> it is weakest beneath the frozen continent of Antarctica *after accounting for Earth's rotation*.
If you weren't accounting for earths rotation, the area with lowest gravity would be near the equator due to the larger distance to earths center.
> in fact, no one knows what gravity is
Albert Einstein has entered the chat
We know how it behaves, not what it is [1].
Crafted by Rajat
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