Cross-checking Dating Methods: Tree Rings, Varves, and Carbon-14

When some Christians first consider the possibility that Earth might have a much longer history than a few thousand years, they face a daunting challenge. Conventional scientists claim that dating methods are robust and reliable, but young-earth advocates insist that all are based on untestable assumptions and circular reasoning. Without the tools or expertise to independently evaluate the competing claims, many Christians default to the young-earth view, assuming there must be scientific justification for the young-earth claims.

For those of us who actually use these dating techniques, it is equally challenging to find ways to communicate the reliability of these methods in an understandable way. Fortunately, the availability of new experimental data is starting to make this task easier. We offer an example here of how independent dating methods can be combined to test assumptions and verify conclusions. Much more detail on this can be found in our recently published article in Perspectives on Science and Christian Faith.

Tree rings

We’ve all seen the cross-section of a tree trunk and its characteristics rings. The thin darker lines grow during winter or dry seasons, and the thicker, lighter rings during the summer or rainy seasons. So each pair typically represents one year. There can be conditions when a specific tree forms a double ring or no ring at all in a year, but this can be discovered by measuring multiple trees in an area.

Of course individual trees only record the years they’ve been alive, but we can count back further by lining up the records of many trees that overlapped in time. Rings are not all the same width due to environmental factors, so when the same unique pattern of wider and narrower rings is found in different trees, this allows matching years to be lined up (called cross-dating). We currently have a cross-dated tree record over 14,000 rings in length before encountering a gap. (There are trees a lot older than this, but we don’t yet have identifiable overlaps with the continuous record from the present.)

Varves

In some lakes, different sizes or types of particles settle to the bottom in regular patterns throughout a year, resulting in pairs of layers that are very similar to tree rings – each pair, or couplet)  is called a varve. In Lake Suigetsu in Japan, spring algal blooms result in the growth of shelled microorganisms that settle out to create a lighter-colored layer on top of the normally darker sediments. Low oxygen levels at the bottom of the lake prevent burrowing organisms from stirring up layers, so the varves are preserved. Scientists have found tens of thousands of these yearly layers in Lake Suigetsu and other lakes.

Radiocarbon

Carbon-14 is a radioactive version of carbon that is continuously produced in the atmosphere, where it combines with oxygen to form CO2. This CO2 is taken in by plants during photosynthesis, and by animals that eat those plants. When those organisms die, the amount of carbon-14 in their bodies begins to diminish because of radioactive decay. If we know the amount of carbon-14 in the atmosphere when the organism perished, and if we know the half-life (decay rate), we can use the amount of carbon-14 we find in a dead organism today to estimate how long ago it died. With a half-life of 5730 years, carbon-14 is useful for dating back about 50,000 years (older samples don’t have enough carbon-14 left to reliably measure).

Objections and response

Young-earth advocates challenge all three of these methods, arguing that:

• conditions during and after Noah’s Flood could have caused trees to put on multiple rings each year
• cross-dated tree rings can be mismatched
• multiple sediment couplets can happen each year
• varve records are often discontinuous (intervals with no couplets)
• carbon-14 production and decay rates could have varied widely

We can put these competing claims to the test by cross-checking the three methods. We can simply address what our expectations should be if the sampled trees put on one ring per year, the sampled lake formed one sediment couplet per year, carbon-14 decay rates have been constant, and variation in atmospheric production of carbon-14 in the past is correctly understood. And we can compare with young-earth expectations.

It is true that for carbon-14 dating to be reliable, we need to know how much is produced in the atmosphere, and it is true that there is some variation in this. But we have independent ways of estimating atmospheric carbon-14 production over the past 50,000 years (e.g. using Be-10 concentrations in sediments). That allows us to predict how much carbon-14 should be left today in samples formed any time during the past 50,000 years.

The purple lines in the accompanying figure represent the upper and lower boundaries of the expected carbon-14 remaining today. We can then go to the record we have from tree rings and varves, count back a certain number of years, and see if the carbon-14 remaining there matches our prediction. You can see that the actual data from both the tree rings and the varves fits impressively into this expected range. If any of the young-earth claims about these measurement methods were correct, the observed data would not fall within the expected range.

These three different dating methods are like three different clocks. We might have a question about the accuracy of the clock on our stove, so we check it with the clock on the wall in the living room. Of course, it is possible that both are wrong in exactly the same way, but if we confirm these with the time on our cell phone (which is set independently), we can be highly confident we have the correct time. That is the situation we are in with carbon-14, tree rings, and varves.

It is awe inspiring to us that God created his natural world in a way that allows us to explore the unobserved past with such remarkable clarity. Nature reflects the orderliness and consistency of its Author. The young-earth view, in contrast, can explain such data only by calling on a God who manipulated multiple tree rings per year, multiple sediment layers per year, and varying carbon-14 production and decay rates, to make it precisely – and falsely – mimic the expectations of conventional science. To us, that describes the capricious gods of ancient Mesopotamia, not the God of Genesis.

Much more detail about these methods can be found in the full article: Davidson, G. and K. Wolgemuth (2018) Testing and verifying old age evidence: Lake Suigetsu varves, tree rings, and carbon-14. Perspectives on Science and Christian Faith, 70(2):75-89.