Coral reefs are too old to be young!

Corals are among the most valuable indicators of past climate conditions, due to their sensitivity to water depth, temperature, and acidity. We can infer from the age and depth of ancient reefs, for example, how global sea level has varied in response to past intervals of warming and cooling. Since reefs are formed by organisms secreting calcite, moreover, their isotopic and elemental chemistry is directly related to the temperature and salinity of the water in which they lived. But best of all, reefs form in the tropics (30°S – 30°N), which means that the researchers studying them do so while scuba diving in warm seas along pristine beaches. Perhaps it’s time for a career change?

Despite that corals can provide a wealth of information to geologists and climatologists studying the past, these data are useful only if we know the precise age of the sample! How then do we date a coral? A first order estimate can be obtained by simply measuring the size of a given reef and its average growth rate. Reefs such as the Great Barrier Reef (up to 120 meters thick, or more) could not have taken less than 12,000 years to form, assuming the most favorable conditions for reef growth, while atolls exceeding 1400 meters thickness must have taken far longer. Brian Thomas at Institute for Creation Research elaborated on why this may or may not be problem for YEC’s:

Some biologists like to say that massive coral reefs represent more than 100,000 years of growth, supposedly nullifying the Bible’s account of a world that is only thousands of years old. However, many known factors can affect coral reef growth rates. Now, a 50-year study of Caribbean coral reefs confirms the unpredictability of using such growth as a “clock.”

The study to which Mr. Thomas referred was reported by the USGS, and it involved photographic documentation of local reef growth within the Florida Keys from ~1960–2010. These corals grew at highly variable rate during each decade, due primarily to the prevalence or absence of disease, so it would be difficult to estimate their age by extrapolating a short-term growth rate over the whole thickness of the coral.

It is no surprise, however, that corals—like any other organism—can be affected by disease. Neither is it shocking to note that “available nutrition, physical weathering, water temperature, light penetration…and other factors” can also affect coral growth rates. So why should this hinder us from labeling corals among things that are older than Ken Ham thinks the Earth is? Brian Thomas, in line with nearly every proponent of young-Earth creationism, propagates a fatal error:

Researchers in the past have assumed that by measuring the rate of growth of a coral reef, as well as the total size of the reef, they can estimate how long it took corals to build it. One big problem with this “natural clock” system is that the growth rate of corals is inconsistent and relies on a host of changing variables.

Mr. Thomas’s concern would be valid if we had no other method to estimate the age of corals than to assume a constant growth rate over the lifespan of the reef. However, scientists have known for decades that coral growth can be highly variable, which is why nobody extrapolates growth rates blindly into the past to determine the age of coral reefs. This point is worth repeating, so I will bold and center the text:

Geologists do not determine the age of anything by extrapolating modern rates blindly into the past!

Beyond a century ago, it was still common to derive age estimates by this technique (e.g. salt accumulation in the sea, heat loss from a molten Earth, etc.), because we had no alternative. But today, any effort within the research community to extrapolate modern rates—especially those known to be variable—blindly into the past is shunned with ridicule. These studies must provide corroboratory evidence that the rate did not change in the past, such as with plate tectonic movements.

Coral reefs can be dated directly, without assuming anything about growth rate, by at least three techniques. The first involves counting of annual bands, which form in response to seasonal variations in nutrient/sunlight availability. The banding is especially visible under an x-ray scan (far right):

Example of annual coral banding in a slabbed core sample, reproduced from the post “How to lose your eyesight”.

Further analyses are available, if necessary, to confirm that annual banding was truly annual. These utilize chemical variations between bands, microscopic identification of growth hiatuses, and more. As with tree rings, multiple coral samples can be stacked together to produce a longer time series of growth and/or coral chemistry, some of which span more than 8,000 years. Continuous coral chronologies are more sparse further back in time, but the longest are sufficient to dismiss claims of a young Earth out of hand.

Alternatively, corals contain abundant carbon within their calcite structure, along with trace amounts of uranium. Radiocarbon dating of corals is complicated by the fact that approximately half of the carbon is derived from “¹⁴C-dead” inorganic carbonate dissolved in the ocean, rather than modern, atmospheric CO₂. Nonetheless, we can make a simple correction to account for this ‘reservoir effect’, which makes corals appear ~1 half-life of ¹⁴C (5,730 years) older than they really are.

Annual banding in a coral from the Gulf of Aqaba, from Al-Rousan (2012).

By far the most important dating technique applied to corals utilizes a rare isotope of uranium (²³⁴U), which decays to ²³⁰Th with a half-life of 245,000 years. If you are unfamiliar with uranium disequilibrium dating, I recommend this introduction to the method as it applies to speleothems. Corals also form in well oxygenated water, which means they absorb dissolved uranium into their mineral structure, but not thorium (which is insoluble in oxygenated water, so it is absent). Any thorium present must have been produced, therefore, by radioactive decay of uranium.

The last point is particularly difficult for the young-Earth creationist, who must consider modern coral reefs to be ‘post-Flood’ features, along with speleothems. While YEC’s try to dismiss the results of radiometric dating arbitrarily by invoking “accelerated nuclear decay” prior to and/or during the Flood, their ad hoc proposal is unavailable for geological formations that formed long after the Flood and/or the ‘biblical’ Ice Age. Either YEC’s must accept that some coral reefs are up to tens and hundreds of thousands of years old, or they must deny the abundance of thorium and lack of radiocarbon within the mineral structure of ancient reefs.

Are coral ‘clocks’ unreliable?

Within his article, Brian Thomas of ICR communicated a frightening level ignorance with respect to how corals are utilized and dated by geologists to study the past. If Mr. Thomas did not hold the title of ‘Science Writer’ at a popular media outlet for young-Earth creationism, then his error would be understandable. But thousands of enthusiastic readers rely on Mr. Thomas as a scientific authority to help bolster their own faith and share it with others.

If Mr. Thomas wants to persuade us that coral reefs could be younger than ~4,500 years, then he needs to address the actual methods by which geologists estimate their age. Additionally, he needs to address why multiple dating techniques corroborate each other when applied to corals, if he believes any or all of them to be flawed. Unfortunately, Mr. Thomas was rather eager to echo the egregious error of John Whitcomb and Henry Morris when he quoted them in his conclusion:

Drs. John Whitcomb and Henry Morris [cited] a study that found 20 centimeters of coral reef growth in five years. They wrote, “This rate of growth could certainly account for most of the coral reef depths around the world even during the few thousand years since the Deluge.” —The Genesis Flood, 1961

In other words, if a growth rate exists in nature, which—if applied to all coral reefs ubiquitously over the last 4,200 years—could explain the current size of coral reefs, then it must have been so! But scientific investigation does not progress by cherry picking, and we can not choose arbitrarily the data that support our hypotheses or speculations. To estimate coral growth over long periods of time, we rely on multiple chronometers to analyze directly the very reefs in question. Only then can we interpolate growth rates between known ages, rather than extrapolate using blind assumptions.

Mr. Thomas is attempting to do his science in reverse, as usual. But such is the anti-scientific nature of young-Earth creationism.

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For those interested, publicly available raw data from corals around the world can be accessed, plotted, and analyzed via the NOAA Paleoclimatology database. I recommend that you begin with the link to view a List of Coral Datasets by Location Name.

Featured image: Coral reef in Papua New Guinea