July 8, 2016 | David F. Coppedge

Bad Assumptions Confuse Geological Ages and Processes

The best models in a scientific field can be overturned at any time when someone takes a critical look at the underlying assumptions.

Geologists have trouble understanding events that happen right before their eyes. Live Science shows them struggling to understand the effects of water on lava. Science Daily says that scientists don’t understand why lightning bolts tend to be more powerful over salt water. How much harder is it to explain processes taking thousands or millions of years? Geologists can build models, but their assumptions can make the best models subject to ruin.

California dreaming: A river in southern California was thought to show slow, periodic buildup of terraces along its banks over many years. A paper in the GSA Bulletin tells the tale:

In the North Fork of the San Gabriel River, an arid bedrock landscape in the San Gabriel Mountains, California, a series of prominent fill terraces was previously related to climate-change–induced pulses of hillslope sediment supply that temporarily and repeatedly overwhelmed river transport capacity during the Quaternary.

That was then. Geologists took a closer look and changed that tale completely.

Based on field observations, digital topographic analysis, and dating of Quaternary deposits, we suggest instead that valley aggradation was spatially confined to the North Fork San Gabriel Canyon and was a consequence of the sudden supply of unconsolidated material to upstream reaches by one of the largest known landslides in the San Gabriel Mountains.

It could have happened in one day or one hour. Does this have implications for the interpretation of other locales? You bet. “Our study highlights the potential for valley aggradation by debris flows in arid bedrock landscapes downstream of landslides that occupy headwater areas.”

Positive feedback: When you picture a mountain arising slowly, you might overlook an important fact. The strain on the rock makes it weaker. Other geologists writing in the GSA Bulletin started to take that into consideration, and found that the strain dramatically speeds up erosion. It speeds up detachment of blocks of material, making them easier for rivers to carry away. More strain produces more strain, and more erosion.

The subsequent rapid erosion of exposed shear zones reforms the topographic stress field in a way that encourages continued accommodation of strain, a positive feedback response that becomes more prominent with greater shear damage.

A cautionary tale: Seashells are a “mainstay for reconstructing ocean-climate change and carbon cycle dynamics,” three geologists explain in the GSA Bulletin. Noticing the assumption that the white, opaque shells are best for dating, they wondered if the effects of diagenesis (rock formation) had been taken into consideration. They hadn’t. The geologists decided to compare dates of opaque shells with translucent shells:

Results support a diagenetic mechanism as opaque shells yield 14C ages invariably older and trace element ratios consistently higher than those of translucent shells.

The radiocarbon dates of shells taken from the same horizon, in fact, differed by as much as 22,000 years. What will this do to climate change models?

These results demonstrate that the use of translucent foraminifera enhances reproducibility and accuracy of 14C ages by minimizing the deleterious effects of diagenesis. This study serves as a cautionary tale since white, opaque foraminifera are common in pelagic sediments, and 14C ages derived from their ostensibly well-preserved shells can lead to discrepancies in the timing of Quaternary climate events and ocean circulation reconstructions.

These geologists now think that the translucent shells give better dates, but why? Do the new assumptions give a better fit to preferred models? It seems so. They say that the translucent-shell dates are “congruent with the established age ranges for these climate events,” such as the accepted “Last Glacial Maximum” (LGM).

Not that it matters, but the acronym LGM appeared in another case of scientific reversal back in 1967, this time in astronomy. Antony Hewish and Jocelyn Bell discovered regular pulses coming from a star in space, later identified as a new class of star called a pulsar. They thought they had discovered “Little Green Men” and called the star LGM-1.

Geologists are too smart to be fooled by that kind of thing. Now they know all about diagenesis, orogeny and radiocarbon for good. It won’t happen again.

Scientists need to be constantly reminded of the vulnerability of their models to bad assumptions. It’s not as bad for repeatable observations like Faraday made in the physics lab as it is for unrepeatable, unobservable events from prehistory, where the best you can do is compare present processes with similar-looking effects in the field. In over 15 years of reporting, we have seen many, many assumptions overthrown, sometimes to very significant models. We remember one case where a rock that had been dated to the oldest end of the geologic column was reassigned to the youngest!

Even if these three cases are not that damaging – even if they allow geologists to maintain their evolutionary timeline with a few well-placed tweaks – they illustrate the problem that there are usually more unknowns than scientists like to acknowledge. “Now we know” are the famous last words of many a failed paradigm. You can rearrange the deck chairs for a better fit, but that doesn’t mean the deck of underlying assumptions is robust. Nor does it mean that the ship of your underlying worldview assumptions can withstand the blows of the next iceberg. Just remember that collapsing decks and sinking ships tend to carry a lot of other baggage down with them.




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