Drawing circles that don’t connect: Jake Hebert of AiG critiques ‘orbital tuning’

From confirmed hypothesis to logical fallacy?

I began last post with an overview of Milankovitch Theory and its application in sediment cores, because the most recent issue of the Answers Research Journal has featured a critique of dating methods used to correlate proxies for climate change over the last few millions of years. The lengthy article by Jake Hebert, entitled Circular Reasoning in the Dating of Deep Seafloor Sediments and Ice Cores: The Orbital Tuning Method, makes a bold claim: dating techniques are not truly independent, but lean on each other for support, which ultimately rests on a uniformitarian philosophy regarding the age of the Earth. Specifically, he states that the dating of ocean-floor sediment and ice cores, especially the use of orbital tuning, is “a gigantic exercise in circular reasoning.”

The argument goes like this: cores from ocean sediments and glacial ice are used to confirm the validity of Milankovitch Theory, but these cores are themselves dated (using orbital tuning) by assuming the theory is valid. Furthermore, independent dating methods are calibrated to orbitally tuned records to enhance their precision. Hence, geologists employ circular reasoning to establish long-term chronologies.

Before diving into some of the details, we might begin with a fundamental question: so what? If Mr. Hebert is correct, and the orbital tuning of sediment and glacial records is logically fallacious, where does that actually leave us? For the vast majority of time series, whether fossil trends in ocean sediments or stable-isotopes in ice cores, rectifying this error would shift the estimated age of layers only by a few percent in either direction. In many cases, the time series would be unaffected altogether, because so many independent methods exist to date these layers of rock and ice. Orbital tuning just happens to be the most precise clock that exists for dating events over the past few million years, because the laws that govern our planets movement are so rigid. Until Milankovitch theory is completely overturned, geologists are wise not to abandon its fruitful application.

Keep in mind also that Mr. Hebert’s alternative explanation is that all of the ocean’s sediments accumulated in a few hundreds of years following a global flood. In that scenario, the required sedimentation rates are orders of magnitude greater than the modern observed rates. Therefore, the burden of proof lies with Mr. Hebert, who needs to provide some evidence that ocean sedimentation or ice accumulation has varied so drastically in the recent past. Beyond this, he must explain why radiometric dating methods (radiocarbon, Ar-Ar, U-Th, and 10-beryllium) consistently yield ages for ocean sediments up to 10 million years and beyond. Even if we grant his assertion that cross-calibration of methods employs circular reasoning, it would not lead us to discard these methods altogether or to compress their model ages down by a factor of ~10,000.

Do the circles even connect?

Interestingly, Mr. Hebert’s broad accusation of circular reasoning does not originate with him, but has been a concern since the 1970’s. Neither has it gone unnoticed by paleoceanographers today. Dr. Lisiecki of UCSB (mentioned above) summarizes thusly:

The circular reasoning argument is the most fundamental challenge to the orbital tuning technique. However, excluding orbital tuning from all studies of orbital responses is unnecessary. Instead, conclusions based on orbitally tuned age models should be analyzed based on sensitivity to uncertainty in the data, as with any other scientific investigation. This presents the question of how the accuracy of orbitally tuned age models can be tested. The most convincing argument in favor of these age models is their close agreement with many CSR age models… which are based on entirely independent assumptions. If the same result is obtained from both the CSR and tuned age models, one can be fairly confident of its conclusion, assuming that at least two independent age estimates are used in the construction of the CSR model. If a study’s conclusion requires a tuned age model which produces some deviation from the CSR model, one must evaluate the probability that those deviations in sedimentation rate occurred. (emphasis mine)

So Mr. Hebert’s accusation is not entirely unfounded, but his argument only works if no truly independent method exists to date the cores, which we’ve already seen is not the case. Either way, the supposed logical circle doesn’t actually connect, because the geological records used to establish Milankovitch Theory are not the same records being dated by it (e.g. Edwards et al., 1987). Besides, the fine tuning of sediment and ice cores does not alter their chronologies significantly from other dating methods. As we saw in the data from Lisiecki and Raymo (2005), one could assume a constant sedimentation rate over the past 5 million years and arrive at approximately the same timeline. But Mr. Hebert writes in his introduction:

Since secular scientists adhere to a uniformitarian philosophy, they assume that sedimentation rates have been slow and gradual throughout earth history, and that millions of years were required for the deposition of these relatively thick layers of seafloor sediments.

I hope that from my last post, it is clear that no geologist simply assumes that sedimentation rates of any kind were slow and gradual. We observe that today, these rates are very slow. We observe no physical characteristics in deeper sediment layers that would indicate a drastic shift in sedimentation rate (such as coarser particles or color changes). We observe that short-lived, radioactive isotopes like 14C, 10Be, 234U, 231Pa, etc. decrease with depth in ocean sediments, at a rate consistent with slow sedimentation (~4 cm/1,000 years). Finally, we observe that certain geochemical signals exhibit cyclic fluctuations, as predicted by the basic laws of physics that describe our planet’s orbit around the sun. Geologists are not taking a giant leap of faith or resting on obscure philosophical claims when they conclude that ocean sediments took millions of years to accumulate.

Misinform and misrepresent, then attack when they’re not looking

The remainder of Mr. Herbert’s article makes it clear to me that while understands some of the fundamentals behind paleoclimatology and geochronology, he has little grasp on how they are applied. Below, I want to address a handful of erroneous claims among the scattered topics that fill his text. Since ARJ is called a ‘research journal’, however, Mr. Hebert should have been pressed by the reviewers to follow up on his problem statement. He has identified what he thinks is the problem (“orbital tuning employs circular reasoning”), so what is the solution? If the last interglacial didn’t really peak around ~125,000 years ago, then what is its true age and how do we know? If Mr. Hebert cannot suggest a better alternative for dating ocean sediments and ice cores, then he hasn’t really done any research, in the scientific sense, but only compiled snippets of relevant info from textbooks and journals.

Hebert continues his polemic with an attempt to undermine Milankovitch theory by posing questions he thinks can’t be answered:

Despite its current popularity, [Milankovitch] theory has serious problems, the most obvious of which is the fact that the changes in high latitude northern summer insolation that are thought to “pace” the ice ages are so small that they cannot, by themselves, account for ice ages. It is for this reason that many secular scientists are convinced that other factors such as greenhouse gases, the amount of sea ice, and ocean circulation also contribute to ice ages.

Strangely, Mr. Hebert suggests that Milankovitch theory is flawed because it requires positive feedbacks to explain how extensive continental ice sheets could wax and wane. But in the very next sentence, he names some of those feedbacks, which explain the so-called paradox. So where is the flaw? Presumably, Hebert believes that if a scientific theory is not perfect with its inception, it must be flawed forever. Despite this red herring, paleoclimatologists have little difficulty explaining how Milankovitch cycles cause ice ages to come and go (for more detail, see Davis and Brewer, 2009; Köhler et al., 2010).

Later, Mr. Hebert provides a brief explanation of δ18O in ocean sediments, which is affected by both local water temperature and global ice volume (total volume of ice on land). But apparently, he believes that paleoceanographers have no way of separating these variables:

…it seems reasonable that variations in global ice sheet volume could influence oceanic δ18O values, which is one of the two explicit variables affecting δ18O-calcite values. But since δ18O-calcite values also depend on temperature, and since larger ice sheets are also generally associated with lower temperatures, how does one de-convolve which part of the variation in δ18O-calcite is a result of changes in temperature per se, and how much is a result of changes in ice volume?

To determine the water temperature at which calcite was precipitated in the ocean, we don’t actually use δ18O, but the ratio Mg/Ca. The latter is unaffected by global ice volume, but varies predictably with temperature. Once the formation temperature is known, we can calculate the original δ18O value of the water. Two variables, two equations: this common algebraic trick allows us to calculate precisely how much calcite δ18O values have shifted in response to global ice volume. No mystery there!

The simplest possible “depth-age” model assumes that sediments at a given location have been deposited on the seafloor at exactly the same rate throughout earth history… However, even uniformitarian scientists do not believe that sedimentation rates have been that uniform… Moreover, if one were to assume perfectly constant sedimentation rates, the ages assigned to the sediments would not in general agree with expectations from the astronomical theory… Secular scientists are not bound by observations and feel free to select depth-age models that suit their purposes, and they use this fact in assigning dates to the seafloor sediments.

Mr. Hebert displays for us his unfamiliarity with how depth-age models are constructed. We don’t simply get to choose age models that suit our purposes, but must demonstrate rigorously why that model is the most plausible. To accomplish this, paleoceanographers regularly use independent markers, such as radiometric dates or magnetic reversals (Hebert arbitrarily claims these methods are not truly independent, but fails to explain how potassium-argon dating of ocean basalts and ash layers is affected by the principles of ocean stratigraphy). Perhaps the most convincing example of independent corroboration, however, comes from the use of speleothems to verify deep-ocean chronologies. A study by Caballero-Gill et al. (2012) illustrates how to turn high-precision, orbitally tuned sediment core data into ultra-high-precision, radiometrically tuned sediment core data:

Figure 2 from Caballero-Gill et al. (2012); cross-correlation of δ18O signals in multiple stalagmites (blue line) and planktonic foraminifera (red line). Note close overlap of the orbitally tuned series (red) with the radiometrically dated series (blue).

The top plot in the figure above shows two time series that measured δ18O. The first (red line) comes from an ocean sediment core, recovered off the southeastern coast of China, whose age was determined by extrapolating recent sedimentation rates into the past and then ‘tuning’ the data to the known ages of Milankovitch cycles. On the other hand, the second series (blue line) was constructed from multiple stalagmites from caves in northeastern China. The δ18O value in these stalagmites reflects the strength of the Asian monsoon, which is impacted directly by long-term shifts in Earth’s orbit. Stalagmites are dated by the U-Th disequilibrium method, however, which operates independently of Milankovitch theory (i.e. no circles here!). If you’re unfamiliar with how the U-Th dating method works for cave samples, please check out this article for a brief explanation.

The bottom line is this: if orbital tuning is a valid method to refine ocean-sediment and ice-core chronologies, then these time series should overlap within uncertainty. As you can see in the figure above, the match is very strong, with only minor exceptions. Therefore, the researchers used the more precise stalagmite chronologies to calibrate the ocean sediment core:

The markers at the top of the graph indicate positions of 145 U-Th disequilibrium dates obtained from stalagmites, which were analyzed for δ18O (light purple line). Since the U-Th method is not calibrated to Milankovitch theory, and the assumptions behind the method are entirely independent from age models in ocean sediments, the overlap of these two records provides incredibly strong support to the use of orbital tuning in ocean sediment cores. Mr. Hebert’s criticism, therefore, does not hold. But what about other radiometric methods?

Creationists will likely not be surprised to learn that the astronomical theory has been used to “adjust” or “calibrate” radioisotope dates… A team of secular scientists…argued that the then-accepted age of 730,000 years for the Matuyama-Brunhes magnetic reversal (which was based upon K/Ar dating) should be revised upward to 780,000 years. Shackleton, Berger, and Peltier based their reasoning on the astronomical theory. Because the K/Ar dates were a little younger than those demanded by the astronomical theory, these K/Ar dates were revised upward. Likewise, the 40Ar/39Ar dates were deemed more accurate because they agreed with the astronomical theory.

The instance to which Mr. Hebert refers is misleading with regard to his case, because in no way were the K-Ar or Ar-Ar methods calibrated to Milankovitch theory. Moreover, the scientific reasoning for shifting this date was sound: an event is dated by three clocks; the two newer and more precise clocks both say it’s 5:33 PM, but the grandfather clock in the living room says it’s only 5:17 PM. Which do we trust? Yes, as technology advances, we all calibrate our clocks to the one that best tells time. All of you are connected to the internet, and so your clocks are being calibrated to a set of atomic clocks. Do we therefore insist that if my wind-up clock gives a slightly different time, we must throw them all away? The previously accepted date for the most recent reversal of Earth’s magnetic field was obtained through older equipment and a more problematic method. The fact that both ages are within 7% of each other actually speaks well to the accuracy of each method, especially since the age of the reversal is 130 times older than the age Mr. Hebert assigns to the Earth itself.

Conclusion

Additional topics and anecdotes are offered in support of Hebert’s bold claim, which deserve attention. The pattern of misrepresentation is the same, however, as he continues to draw circles that don’t really connect. Missing entirely from the discussion, moreover, are two necessary items: a sensitivity test, and a valid alternative. A sensitivity test would establish just how much geochronology is affected, if we were to accept Hebert’s criticism. In reality, the number is not large. Do you remember calling the local weather hotline so that you could recalibrate your wristwatch? Geology is not much different; we live in a room of imperfect clocks, but they’re never so far off that we lose track completely. Lastly, Mr. Hebert cannot provide for us a scientific method by which to date ocean sediments, ice cores, stalagmites, or any other paleoclimate archive. The date he assigns is arbitrarily chosen, resting on a philosophical commitment to one biblical chronology of many, despite the numerous assumptions behind it. Whatever the evidence may be, Flood geologists point right back to what they perceive as the superior chronology. As far as I can tell, it is the Flood geologist who must beware of logical circles.

 

 

Featured image credit: Heidi Roop