June 4, 2007 | David F. Coppedge

How Old Are Sand Dunes?

The Namib Desert has some of the largest sand dunes in the world.  How old are they?  Three scientists from the University of London decided to find out.  They took cores out of some dunes in Namibia and analyzed the sand, using multiple high-tech methods.  Their conclusion, just published in Geology: the dunes are much younger than previously thought – only about 5,700 years old.1  That means this huge field of dunes, some over 380 meters high, could have formed from the base up within recorded history.
    Previously, geologists were convinced that these dunes were far older:

It has been argued that large linear dunes are relics of a cooler, drier, and windier climate during the Last Glacial Maximum, a hypothesis supported by luminescence dating of linear dunes in many areas (Lancaster, 2007) and by the considerable inertia of large dunes, which require thousands of years to respond to changes in wind regime (Warren and Allison, 1998).  In addition, the large number of endemic species within the Namib Sand Sea has been given as evidence for a long and continuous period of hyperarid conditions within Namibia (Ward et al., 1983) and therefore potentially very old dunes.  These ideas gave rise to the hypothesis that, although the linear dunes of the Namib Sand Sea are currently active, they should have some older, Pleistocene core.

The Pleistocene epoch dates at 1.8 million to 11,5000 years in the standard geologic column.  The Last Glacial Maximum is thought to be 20,000 years ago.
    The team used ground-penetrating radar on a study dune 70m high and 600m wide, and surveyed a 4km area.  They also used a drilling rig to dig out a core, and dated it with optically stimulated luminescence (this gives an indication of the last time sand grains were exposed to sunlight).  They identified three domains within the core, which they said indicates three phases of dune construction with pauses in between.  The first hiatus lasted 2,830 years, they said, due to a prolonged period of increased rainfall.  In their revised model, the last hiatus was very short, occurring between 100 and 50 years ago.  It was due to “reworking of the western flank of the dune by superimposed transverse dunes migrating north along the dune flank within the past 50 yr.”  What does all this mean?

    Our studies show that large, complex, linear dunes in the northern Namib Sand Sea are younger than expected and are Holocene in age.  The relative youth of the dune indicates complete turnover of sand during the Holocene, leaving no relic of older Pleistocene dunes, if indeed they existed in this area.  It is possible that there are older dune deposits preserved within the Namib Sand Sea farther to the south, but we have no evidence for this.
    The lack of a preserved late Pleistocene core to the dune shows that large linear dunes can be entirely reworked during the Holocene in hyper-arid environments such as the Namib Desert. 

(The Holocene is charted from 11,500 years ago to the present.)  In their introduction, they stated that “Linear dunes are the most widespread type of desert sand dune, but they are rarely recognized in the geologic record.”  This study of a classic sand dune in the present, therefore, has further implications for interpreting the past as seen in the rocks:

Our studies therefore provide firm evidence for lateral migration of linear dunes and indicate that the deposits of many dunes preserved in the rock record previously interpreted to be transverse to the mean transport direction may in fact be those of dunes of linear form that combine the deposits of flow-parallel and flow-transverse elements.  This has important implications for interpretation of ancient eolian sandstones, past wind regimes, and resulting paleoclimatic and paleogeographic reconstructions.

It’s timely this study of the Namib dunes came out just when planetary scientists are trying to figure out the dunes on Saturn’s moon Titan (cf. 05/04/2006).  A new set of scientific papers about Titan was just reported in a series of press releases from Jet Propulsion Lab (see also Science Daily).  The series of articles announces the latest results from ongoing analysis of data from the Cassini and Huygens spacecraft by the European Space Agency and NASA.  The article on Titan’s surface says, “Most of Titan’s dunes are giants, each one stretching for up to 100 kilometres in length across the dark plains and separated by 10 kilometres.”  Instead of silica sand, these dunes are made up of “sugar-size hydrocarbon grains between 100 and 300 microns in diameter.”  Still, the dynamics of dune formation should share some commonalities on the two worlds, once the factors of grain size, wind speed and gravity are taken into account.  Perhaps the vast dune fields on Titan were formed in short order as well compared to the billions of years the moon is assumed to have existed.

1Bristow, Duller and Lancaster, “Age and dynamics of linear dunes in the Namib Desert,” Geology, June 2007, pp. 555-558, DOI: 10.1130/G23369A.1.

Let’s add this interesting discovery to our growing list of geological processes being revised drastically downward in age (e.g., 05/07/2007, 01/12/2007).  The questions they are not asking are as interesting as the ones they are answering.  We can think of three: (1) Why are there so few linear sand dunes in the rock record?  (cf. 06/27/2003, 07/11/2001).  (2) Will geologists now be willing to drastically revise downward the formation times for alleged fossilized dunes, like the Coconino Sandstone?  (3) If a wetter period lasted almost 3000 years between earlier stages of Namib dune formation, long before civilization began using oil and gas, how can we be sure humans today are causing significant climate change?  Try your hand.  What other questions does this paper raise in your perceptive, logical mind?  Tip: you can include Titan.

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