The first of Answers in Genesis’s ten best evidences for a young earth concerns ocean sediment. Every year, a certain amount of sediment is eroded from the land and washed into the sea by rivers. In theory, if we can compare this with the amount of sediment that we actually find on the ocean floors, we could calculate a rough, first-order estimate for the age of the earth. Adding up the rates and extrapolating backwards gives us a maximum age for the earth of 12 million years — far less than the 4.5 billion years of the scientific consensus.
Andrew Snelling, the author of the article, illustrates it with this drawing:
The first problem with this argument is that it is based on rates that are extremely difficult to measure, and that can not realistically be assumed to have been constant in the past. Sedimentation rates are strongly dependent on environmental and climatic conditions, and ice ages, deforestation, widespread farming, and more recently the building of dams and cities in the modern era, will all have affected them significantly.
The annual influx of 20 billion tons of sediment a year comes from Millman & Syvitski (1992). However, if you read their abstract, you will see that their figure of 20 billion tons a year is merely a finger-in-the-air estimate of historic sedimentation rates from the past 2,500 years or so up until the early twentieth century. It also tells us that prior to widespread farming and deforestation, rates were almost certainly much lower. Later in the paper, they tell us that ultimately the final figure is extremely difficult to estimate and that we simply don’t know what it is.
Since 1992, there has been a lot more research into ocean sedimentation rates by environmental scientists. For example, Willenbring et al. (2014) estimate that the long-term sedimentation rate is about 5.5 billion tons a year.
But then there is the question of what exactly is being measured? And where does the sedimentation actually end up?
The estimates for subduction rates and the total amount of sediment on the sea floor come from Hay et al (1998). However, it is quite clear from their abstract that these estimates only refer to the deep ocean floor, and not to the continental shelves and margins. Yet it is the continental shelves and the margin where the sediments are deposited — typically in river deltas and fjords.
We can illustrate the problem by annotating Snelling’s drawing accordingly:
In a nutshell, Snelling is citing the rate of accumulation of sediment on the continental shelf as evidence that there is not enough of it on the ocean floor. This is like trying to determine the amount of snowfall in London by taking measurements in the Cairngorms. It is patently absurd. More to the point, if this isn’t “having two differing measures in your house” which Deuteronomy 25:13-16 warns us against, then I don’t know what is.
Furthermore, in many places, the sediments are subsequently uplifted by tectonic plates colliding to form new mountain ranges. In fact, most if not all of the sedimentary rocks found inland are believed to have originated as sediment being washed into the oceans in this way. Yet Snelling does not include these in his calculations, despite the fact that the amount of sedimentary rock concerned is vast, in many places being several miles thick.
Snelling dismisses the possibility of sedimentation rates having been lower in the past as a “rescuing device.” He claims that the continental shelves and margins show evidence of having been deposited by catastrophic landslides and turbulence. Besides the fact that the continental shelves and margins aren’t what he’s counting, he does not cite a source for this claim. Nor does he give any examples, nor does he give any indication of how to differentiate between a single, global cataclysm and much more local events such as lahars, mud flows and landslides that we see today.
It simply is not scientific, nor even intellectually honest, to dismiss interpretations of the evidence that you don’t like as “meh, rescuing device.” You have to provide firm evidence that they are not consistent with the data, or at the very least that your own hypothesis fits the data better than they do. If you are not able to do so, then the evidence that you are providing is ambiguous, and does not confirm anything at all.
Nor is it scientific to blindly extrapolate rates of change back into the past as if they were constant. Either you have to establish firm theoretical and experimental reasons why this should be the case (as with radioactive decay rates or the speed of light, for example), or else you need to examine the historical record for evidence as to how those rates have varied in the past. Given that young-earthers are constantly berating “uniformitarians” for assuming rates were constant in situations where those assumptions are justified, it is quite hypocritical for them to make the same assumptions in situations where they are not.
Sedimentation rates are strongly dependent on climate changes. Temperature and precipitation affect rates of chemical/physical weathering. Glaciations significantly increase the sediment flux by grinding millions of tons of rock into dust and gravel, but widespread glaciation is far more common today than even 2 million years ago, and is absent from the vast majority of Earth history. Glacially and tectonically driven changes in sea level also affect sediment flux to the deep ocean, since higher sea levels reduce the area of exposed land that can be eroded into the oceans. On longer time scales, plate tectonics greatly affect the sediment flux by raising mountain ranges and deforming brittle rock formations. Today, massive mountain ranges span the whole western coasts of the Americas, and the Himalayas constitute the single largest source of sediment to the oceans. These mountain ranges are relatively young, however, so it’s reasonable to conclude that sediment flux is far greater today than for much of Earth history.
Ultimately, the amount of sediment in the oceans says nothing whatsoever about the age of the earth. Snelling’s calculations are invalid and do not reflect the reality of where the ocean sediment ends up. When the total worldwide amount of sedimentary rock is added up, the most up to date estimate of long-term sedimentation of about 5.5 billion tons a year is perfectly consistent with an age of the earth of 4.5 billion years.