Coral Banding

Massive reef corals lay down skeleton continuously, and the density of that skeleton varies with the seasons. Sawn open and X-rayed, a colony shows paired light and dark bands running from the base to the living surface, one pair per year. Counting them dates the skeleton directly, with no reference to radioactive decay.

Coral is unusual among the counted archives for a specific reason. Tree rings, varves, and ice layers are counted and then compared against samples dated elsewhere. A coral skeleton carries both clocks in the same piece of aragonite: the bands can be counted, and the same material can be dated by uranium-thorium, an unrelated decay system. The two are read off one specimen.

How annual bands form#

Massive corals — Porites in the Indo-Pacific, Orbicella and Siderastrea in the Caribbean — deposit an aragonite skeleton beneath the living tissue layer. The skeleton is never resorbed, so the whole growth history is retained in position.

The couplet is one year. Knutson, Buddemeier, and Smith (1972) established the annual periodicity by X-radiography of Pacific colonies, and the pairing has since been confirmed in colonies growing under monitored conditions.

Which half of the cycle produces the dense band differs between sites and species, and depends on local temperature, light, and nutrient regimes. The couplet structure is what carries the year, not the sign of the density change.

Signals recorded alongside the bands#

The skeleton records several seasonal cycles at once, so a year can be identified by more than one property.

The luminescent lines are useful because they mark events with dates in written records. Isdale (1984) showed that fluorescent bands in Great Barrier Reef corals correspond to flood years of the Queensland rivers, which are documented in gauge and rainfall records.

Verification against known years#

Coral banding is checkable against calendar dates in a way few archives are, because the twentieth century wrote several markers into the ocean.

In the fallout check, the dates of the weapons tests are known exactly, the radionuclides had no prior presence in the ocean, and the band that contains them is the one the count predicts.

The records#

Enewetak is treated in Time in Sedimentary Rock. At the maximum observed reef accumulation rate of roughly a centimetre per year, that thickness of in-place reef requires well over 100,000 years, and each generation of coral grew on the skeletons of the one beneath it.

Extending the radiocarbon curve#

Corals carried the radiocarbon calibration curve past the range of tree rings, and they could do so because a single sample can be dated twice by unrelated means.

Bard and colleagues (1990) measured uranium-thorium and radiocarbon ages on the same Barbados coral samples, extending calibration to about 30,000 years — well beyond what tree rings then covered. The same drill cores produced Fairbanks' sea level record.

Uranium-thorium and carbon-14 rest on different elements, different half-lives, and different chemistry. Neither is adjusted to match the other, and the offset between them is what the calibration curve records. Corals and speleothems appear in the calibration table in Radiocarbon Dating for this reason.

Days in the ancient year#

Corals deposit fine ridges daily as well as bands annually, which means a fossil colony records how many days its year contained.

Tidal friction transfers angular momentum from Earth's rotation to the Moon's orbit. The rate is measured directly by lunar laser ranging, and it means the day has been lengthening and the year has therefore contained more days the further back one looks. For the Middle Devonian, the calculation gives a day of roughly 22 hours and a year of about 400 days.

Wells (1963) counted the daily ridges between annual bands on Devonian rugose corals and obtained counts averaging close to 400. Scrutton subsequently reported lunar monthly groupings in the same material, consistent with a shorter synodic month.

The counting is genuinely difficult and the published values scatter, since ridge preservation in fossil skeletons is uneven and the increments are not always cleanly resolved. The tidal rhythmite record described in Time in Sedimentary Rock is the cleaner version of the same measurement. What the coral counts contribute is that a prediction from celestial mechanics, calculated with no reference to fossils, was tested against ridges on a fossil and came out in the right range.

Common objections#

"Bands can form more than once a year"

Correct, and it is the standard objection. Corals produce stress bands in response to bleaching, storms, freshwater plumes, and sedimentation, and these can be mistaken for annual density bands.

The phenomenon is real, which is why counts are not made from density alone.

Stress bands are distinguishable in structure — often abrupt, sometimes accompanied by partial mortality or a breakage surface — and they do not repeat with the regularity of the seasonal couplet. Where the density record is ambiguous, the strontium/calcium and oxygen-18 sawtooths give an independent seasonal count from a different physical property.

The check that settles it is the fallout markers. If a colony were systematically banding more than once a year, the radionuclides from a test of known date would sit in the wrong band. They do not.

"Corals grew far faster in the warm post-flood oceans"

Growth rate and band count are separate quantities, and this is the difficulty for the objection.

Faster growth produces thicker bands, not more of them. A colony growing at three times the modern rate lays down the same one couplet per year, spread over more skeleton. Compressing a 400-year colony into a few decades requires the banding not to be annual at all, which is what the fallout and flood-year markers rule out.

Modern growth rates are also measured rather than inferred, by staining, buoyant weighing, and repeat survey of tagged colonies, and they fall in the range the band thicknesses imply.

For reef accumulation as opposed to individual colony growth, the limiting figure is around a centimetre per year, and the Enewetak core is 1,400 metres of reef in growth position.

"Coral dates are calibrated against radiometric dating, so the agreement is circular"

Band counting takes no input from any decay method. A band is counted on an X-radiograph, in the same sense that a tree ring is counted.

Where uranium-thorium ages and band counts are compared, the count is established first and the decay measurement either matches or does not. That is a test rather than a calibration.

The direction of the calibration that does occur runs the other way, as it does for tree rings and varves. Radiocarbon was adjusted to match coral uranium-thorium ages; the corals were not adjusted to match radiocarbon.

"The Devonian day-length result assumes the rocks are 380 million years old"

It does, and the objection identifies the structure of the argument correctly.

The comparison is a consistency test rather than an independent dating of the coral. The radiometric age supplies the epoch, celestial mechanics supplies a predicted number of days in that epoch, and the fossil is counted to see whether it agrees.

What makes it worth something is that it could have failed. The prediction is specific — about 400 days, not 365 and not 500 — and it was made from orbital dynamics with no input from palaeontology. A coral only a few thousand years old would carry roughly 365 ridges per band, since the day length has not measurably changed over that span.

Nothing in the young-earth model predicts 400.

"Fossil corals were buried in the flood, not grown in place"

Some fossil coral material is transported and broken, and is identifiable as such.

Much of it is not. Fossil reefs preserve colonies upright and in growth position, with the framework organisms in their living orientation and successive generations built on the skeletons below. Enewetak is 1,400 metres of this, and the corals face upward throughout.

A colony deposited by moving water arrives in random orientation and in fragments. This distinction, and what the rock record shows generally, is examined in Flood Geology.

What the evidence shows#

Massive corals deposit one density couplet per year, and the couplets are counted from a physical structure rather than calculated from a decay rate.

The counting is verified against dates known exactly: nuclear test fallout, the bomb radiocarbon rise, and documented river floods each appear in the band the count predicts.

A coral skeleton carries a counted record and a decay-based record in the same sample, which is why corals were used to extend the radiocarbon calibration curve beyond the range of tree rings.

Daily ridges in Devonian corals return roughly 400 days to the year, which is the value celestial mechanics predicts for that period from the measured slowing of Earth's rotation.

The other counted archives are in Dendrochronology, Varves, and Ice Cores, and the agreement between independent methods is set out in Cross-Calibration.