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Thursday, May 29, 2014

"Best evidences for a young Earth": Snelling and the ocean-sediment flux

In September, 2012, Answers in Genesis featured a review article of what they deemed the 10 Best Evidences that Confirm a Young Earth. From this list, I have already addressed #2 (Bent Rock Layers), #6 (Helium in Radioactive Rocks), and #7 (Carbon-14 in Fossils, Coal, and Diamonds) in previous articles:

• "Rock layers folded, not fractured" — or are they?
• Clearest evidence that the Earth is 6,000 years old: helium diffusion in zircons
• Radiocarbon evidence for the antiquity of the Earth

So today, I want to discuss one of Andrew Snelling's arguments from the content of our oceans that the Earth cannot be hundreds of millions of years old. Snelling presents us with two claims on his list: #1, that too little sediment is accumulated on the ocean floor, and #9, that the oceans contain too little salt. Let's begin with #1.

Evidence #1: Very Little Sediment on the Seafloor

The argument here is rather simple. Snelling cites an article by Milliman and Syvitski (1992) to claim that every year, rivers discharge 20 billion tons of sediment into the world's oceans. He further cites Hay et al. (1988) to claim that only 1 billion tons of sediment are removed by subduction of oceanic plates (and with them, marine sediments). At this rate, Snelling claims, the whole of the marine sediments on the ocean floor would accumulate in only 12 million years.

Snelling's claims and calculations are riddled with errors and what I can only deem very dishonest research. First, Milliman and Syvitski (1992) note up front in their abstract that their estimate of sediment discharge applies only to the modern period prior to human damming of rivers. Dams tend to reduce the sediment output of rivers to the ocean by mitigating floods that carry substantially more sediment. Secondly, and most important, the authors suggest that "prior to widespread farming and deforestation... sediment discharge probably was less than half the present level." In their discussion, Milliman and Syvitski (1992) estimate the total sediment discharge to the oceans was "considerably less" than 10 billion tons per year.

So already, Snelling's calculation is off by a factor of 2 or more, due to his unwillingness to read the entire article that he cited (or even the entire abstract!). Correcting the misquotation already shifts the age estimate of ocean sediments to more than ~30 million years. Granted, this number is less than 3 billion years (the estimated mean age of our oceans), but if Snelling is willing to deceive his audience by misquoting a publicly available abstract, his methods are already suspect. One should also note that of the dozens of studies regarding sediment flux to the oceans, Snelling cites only one. Why? Since Snelling published his article in 2012, estimates of sediment flux to the ocean have multiplied and improved. One of the most recent publications places a long-term estimate at 5.5 billion tons per year (Willenbring et al., 2014), which is far less than the 20 Gt/yr used by Snelling.

Uniformitarian extrapolation

Anticipating criticism, Snelling writes that "those who advocate an old earth insist that the seafloor sediments must have accumulated at a much slower rate in the past." He provides no citation for this response, presumably because his readers would never ask for one. Thus he sets up a strawman to persuade his readers that us 'old-earthers' are desperate dogmatists. In truth, the main errors (below) in Snelling's article have less to do with the actual rate of sediment flux to the oceans. Nonetheless, we should consider the way that Snelling twists uniformitarianism in his approach to persuading readers of a young Earth.

There is nothing scientific about extrapolating rates of any natural process blindly into the past. Either one should establish good physical reason for these rates never to change (as with the speed of light or radioactive decay), or one should make predictions about past rates that can be tested by historical evidence (like the kind preserved in rocks). Thus Snelling's age estimate of the oceans, based on sediment flux, remains only conjecture and is as weak as his poorly derived assumptions.

First, why should we expect the rate of sediment delivery to the oceans to have remained constant over time? Apart from human influence, natural variations in climate affect sediment delivery, because the 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.

Ocean sediments in the dividend: Snelling's big error

To estimate a maximum age of 12 million years for the oceans, Snelling used his sediment flux of 19 billion tons per year to determine how long it would take to accumulate 1,300 ft (400 m) of sediment, which Hay et al. (1988) take to be the average thickness of ocean sediments. However, this average thickness includes only deep ocean pelagic sediments, of which ~74% is biogenic calcite that formed in surface waters (i.e. was not carried in by rivers). Since most riverine sediments accumulate on the continental shelves, deltas, and other nearshore basins, Snelling is comparing apples and oranges to make his case. In fact, he does not even include continental shelves, deltas, and other nearshore basins in the calculation, despite that he depicts the continental shelf in his Figure 1, titled "Where is all the sediment?".

Snelling also cites Hay et al. (1988) when he claims that only 1 billion tons of sediment are removed from the oceans via subduction, but this amount only accounts for deep ocean sediments removed via subduction, so it is incomplete. Total subduction is sufficient to recycle the entire crust in 1.8 billion years (Clift et al., 2009). Snelling's calculation thus falls to pieces with very little examination.

Most of the sediment carried to the ocean by rivers accumulates in deltas and along the continental shelves. In fact, the Ganges-Brahmaputra delta accumulated up to 70 m during the Holocene alone (i.e. in 11,700 years), according to Khan and Islam (2008). Prior to dam construction, this river system also carried nearly 1/10 of the global sediment flux. The sedimentary layers in delta sequences tend to be hundreds of meters to miles in thickness, so if we are to ask "Where is all the sediment?", the first answer should be: deltas. Fjords constitute another significant catchment, and provide ~25% of recent riverine sediments (Syvitski et al., 1987), due to the efficiency of glacial ice (particularly when it melts) in discharging sediment to the oceans.

Ocean sediments in the rocks: Snelling's really big error

A bulk of sedimentary rocks in the Earth's crust have been interpreted to be marine sediment sequences. If this interpretation is correct, then nearly all of the sediment load from rivers since the beginning of Earth history is bound up in the rocks beneath your feet. Strangely, Snelling omits this massive reservoir of riverine sand, silt, and clay while determining that our Earth's oceans cannot be older than 12 million years. The sheer volume of sedimentary rock is astounding, but is consistent with the estimate by Willenbring et al. (2014) that approximately 5.5 billion tons of sediment were discharged each year into the oceans.

We can visualize this process from a cross section of just one record of nearshore deposition, which begins with the Marcellus Shale (a major source of petroleum today). Notice that the sedimentary layers thicken to the east, where the sediment source (now the Appalachian Mountains) was once located. More than a mile of sediment accumulated in this region in a few millions of years:

Image from Wikipedia commons.
Yet Dr. Snelling omits this volume of sediment from his calculation, asking us rhetorically to account for its fate. How long will YEC's continue to trust Andrew Snelling to do their research?

1 comment:

  1. How long will YEC's continue to trust Andrew Snelling to do their research?

    For as long as he continues to reach conclusions they agree with I suspect