Sand residence times of one million years in the Namib Sand Sea from cosmogenic nuclides

P. Vermeesch1, C.R. Fenton2, F. Kober3, G.F.S. Wiggs4, C.S. Bristow1 & S. Xu2
1 Birkbeck College, University of London, London WC1E 7HX, UK
2 NERC Cosmogenic Isotope Analysis Facility, East Kilbride G75 0QF, UK
3 Geological Institute and Institute of Geochemistry and Petrology, ETH Zürich, 8092 Zürich, Switzerland
4 Oxford University Centre for the Environment, Oxford OX1 3QY, UK

The Namib Sand Sea is one of the worlds oldest and largest sand deserts5, yet little is known about the source of the sand in this, or other large deserts1. In particular, it is unclear whether the sand is derived from local sediment or comes from remote sources. The relatively uniform appearance of dune sands and low compositional variability within dune fields2 make it difficult to address this question. Here we combine cosmogenic-nuclide measurements and geochronological techniques to assess the provenance and migration history of sand grains in the Namib Sand Sea. We use UPb geochronology of detrital zircons to show that the primary source of sand is the Orange River at the southern edge of the Namib desert. Our burial ages obtained from measurements of the cosmogenic nuclides 10Be, 26Al and 21Ne suggest that the residence time of sand within the sand sea is at least one million years. We therefore conclude that, despite large climatic changes in the Namib region associated with Quaternary glacialinterglacial cycles3, 4, the area currently occupied by the Namib Sand Sea has never been entirely devoid of sand during the past million years.

The Namib Sand Sea covers an area of 34,000 km2 along the Namibian coast6, 7. Previous cosmogenic nuclide studies north of the sand sea have indicated that aridity in this region dates back to at least the Miocene8, 9, although it has also been proposed that the linear dunes of the Namib only formed during the last glacial maximum3. Our study is the first to apply cosmogenic nuclide techniques to the dune sand itself, in order to directly determine the antiquity of the aeolian activity and investigate the response of a sandy desert to multiple cycles of Quaternary climate change4, beyond the timespan that can be determined through optical dating of sand10 or radiocarbon dating of organics11.

Our method is based on the widely applied technique for erosion-burial dating12, 13 using in situ produced cosmogenic 26Al and 10Be. The principle is as follows. In quartz-bearing rocks exposed to cosmic radiation (i.e. within the upper ~2 metres of the Earth’s surface), 26Al and 10Be are produced at a ratio of ~6. Namib desert sand predominantly originates from the Orange River in South Africa and quartz, therefore, enters the desert with an inherited cosmogenic nuclide signal, which is a function of the average erosion rate of the catchment area14. Within the desert the sand is wind-blown in a northerly direction by strong trade winds7, 15 and is then buried inside large (~100m high7) linear dunes. Upon burial and cessation of production, the 26Al and 10Be concentrations start decreasing due to radioactive decay. 26Al has a shorter half-life than 10Be and so the concentration of 26Al will decrease faster, changing the 26Al/10Be ratio. For material that has experienced a single erosion-burial event, the 26Al/10Be ratio can be used to calculate the burial age. Grains of sand moving through a dune field will be exposed and buried multiple times and so the ‘classic’ form of the 26Al/10Be erosion-burial method cannot be applied to these sands13, 16. To be able to do burial dating on the sand grains in the Namib Sand Sea, we extend the method by adding a third nuclide, 21Ne. Cosmogenic 21Ne, also produced in quartz, is stable and so its concentration in grains of sand moving through a dune field (and being continuously re-exposed and re-buried) can either (1) increase with time, if the episodes of exposure are substantial, or (2) remain constant, if the grains spend most of their time buried. Thus by combining 21Ne with 26Al and 10Be, one can ‘correct’ for any additional re-exposure events and calculate a mean burial age for the sand grains. To test these predictions, twelve samples of dune sand were collected from the edges of the Namib Sand Sea, plus one alluvial sample from the mouth of the Orange River (Figure 1).

In addition to alluvial sediments from the Orange River, which are similar in mineralogy to the Namib sands17, other possible sediment sources include the Great Escarpment and the Miocene Tsondab Sandstone Formation18. Before embarking on the cosmogenic nuclide work we need to first confirm that sand is indeed transported from south to north. To this end, we employed a zircon U-Pb provenance study, dating ~100 zircon grains in each of our samples. The resulting U-Pb age spectra look remarkably uniform, with the coastal samples (1, 2, 11, 12) being virtually indistinguishable from the Orange River sample (13). Arranging the thirteen U-Pb age distributions geographically reveals an ‘anisotropic’ pattern (Figure 1) in which samples that are close together geographically do not necessarily have similar age patterns. For example, samples 2 and 3 were collected only 40 km apart, but their age spectra look quite different, whereas samples 1 and 12 are separated by nearly 400 km, but their age spectra are almost indistinguishable. This confirms the validity of the assumption of predominant longitudinal sediment transport from south to north, parallel to the Benguela Current and the resultant wind direction15.


Figure 1: The detrital U-Pb data arranged geographically. The age spectra of the coastal samples (1, 2, 11, 12, and 13) look nearly identical, indicating that coastal sands of the Namib Sand Sea are exclusively derived from the Orange River. n is the number of concordant ages. Samples dated with cosmogenic nuclides are marked in red.


Having established the direction of sand transport, we set out to measure the time it takes for the sand to travel across the desert. Cosmogenic 10Be and 26Al measurements were performed on quartz sand collected along a longitudinal transect from the mouth of the Orange River at Alexander Bay (sample 13) to the southern margin of the Namib Sand Sea at Lüderitz (sample 12), and its northern margin just south of the Kuiseb River (samples 1 and 2). The cosmogenic 10Be and 26Al concentrations of sample 13 are compatible14 with steady state erosion at a rate of 4.04 ± 0.89 mm/kyr, followed by 280 ± 230 kyr of burial decay due to sediment storage in the Orange River catchment. Nearly identical cosmogenic radionuclide concentrations are found in sample 12 (Table 1), indicating that sand is quickly transported north along the coast by longshore drift and inland across the ‘Sperrgebiet’ deflation area (Figure 1) where the potential sand flow15 exceeds 1200 tonnes/m/yr. The 10Be concentrations in the southern samples (12 and 13) of the transect are a factor of two higher than those in the northern samples (1 and 2), while the 26Al concentrations differ by a factor of three (Table 1). Plotting the coastal samples on a 26Al/10Be two-nuclide diagram reveals a simple trend consistent with 0.82-1.08 million years of radioactive decay (Figure 2). The fact that all four samples plot on the same radioactive decay line, corresponding to the same palaeodenudation rate, is another strong argument in favour of the hypothesis that all samples are sourced from the same region, the Orange River catchment.


Figure 2: 26Al/10Be two-nuclide plot normalised to sea level and high latitude24, 25. The 26Al/10Be ratios in the southern (12 and 13) and northern (1 and 2) samples are 4.6-4.8 and 2.8-3.0, respectively, corresponding to an apparent burial age of 0.82 - 1.08 Myr. Their nearly exact alignment along a trend of simple radioactive decay indicates a simple burial history. Re-irradiation by cosmic rays is limited, as confirmed by the uniform 21Ne content of the same samples (Table 1). Error ellipses are 2σ.


The ~1 million year residence time is a minimum estimate for the following two reasons. First, it can be shown mathematically (see Methods section) that the apparent burial age calculated from the average nuclide concentration of multiple sand grains is less than or equal to the average burial age of those same grains. Second, every step of re-irradiation by cosmic rays would increase the 10Be and 26Al content of the quartz, and partially ‘erase’ the burial signal. The effect of this re-exposure should be relatively minor, again for two reasons. First, cosmogenic nuclide production rates along the Namibian coast are less than half those of the Orange River catchment, which has an average elevation of 1240m. Second, simple geometric considerations (see Methods section for details) show that the effective cosmogenic nuclide production rate in a well mixed, ~90m high linear dune is fifty times less than the surface production rate. By combining these two effects, even one million years of recycling inside the sand sea would produce only ~8×104 at/g 10Be and ~40×104 at/g 26Al.

In addition to these theoretical considerations, the importance of re-irradiation of the aeolian sand by cosmic rays on its way across the Namib Sand Sea can also be directly assessed by measuring its 21Ne content. Because 21Ne is a stable nuclide, it does not decay while buried inside sand dunes. Therefore, the 21Ne content of the quartz sand is not expected to decrease downwind, as is the case for 10Be and 26Al, but should remain constant or increase. The rate of increase provides a measure of the cumulative amount of re-exposure. Our measurements show nearly constant 21Ne concentrations in the coastal samples (1, 2, 12, and 13) (Table 1), confirming that re-exposure is minor (<75kyr). Finally, when jointly considered with the 10Be and 26Al data, the 21Ne content of the Orange River sand (sample 13) is not consistent with steady state erosion, indicating an inherited component. This is not surprising given that most of the Orange River catchment is underlain by sedimentary rocks of the Karoo Supergroup with a history of one or more erosion cycles prior to their deposition during the Mesozoic19.


Sample ID 26Al σ(26Al)10Be σ(10Be)21Neσ(21Ne)







1 163.0 7.1 58.6 2.0 1990 46
2 167.0 7.5 55.5 1.9 1716 40
12 479 15 105.0 3.7 1840 42
13 600 19 124.6 4.4 2005 38







Table 1: Cosmogenic nuclide data (×104 at/g).

Combined ground penetrating radar measurements and optical dates for a linear sand dune in the northern Namib Sand Sea indicate complete overturning in a time period of ~104 years10. In light of these findings, the >106 year residence time measured in our cosmogenic nuclide study can be interpreted in terms of two different end-member models. Either the Namib Sand Sea has been active throughout this entire period, and sand grains are, on average, buried, recycled and re-exposed at least 100 times on their northward journey across the sand sea. Or the sand sea has been immobilised for extensive lengths of time during relatively humid climate conditions20, alternating with dry conditions of increased dune mobility and sand mixing13. The above two models can be tested by analysing additional samples along a longitudinal transect through the central parts of the sand sea. If the sand sea is in a ‘steady state’, the 10Be and 26Al concentrations should decrease gradually from the South to the North. In the case of an episodic history, the 10Be and 26Al concentrations would be more uniform, with all but the southernmost dunes containing largely reset 10Be and 26Al concentrations.

Aridity in southwest Africa is at least 5 Myr old8, 9, predates the onset of the Benguela Current, and may have initiated shortly after the opening of the Atlantic Ocean5. The present study provides the first direct evidence that the occurrence of aeolian sand is an equally old and long-lived feature. A residence time of greater than one million years for the sand compares favourably with recent estimates for the age of the Namib Sand Sea based on the speciation of endemic beetles around 2.35 -2.6 Ma21 and indicates that while the individual dunes may only be a few thousand years old10, the area between the Sperrgebiet deflation area and the Kuiseb River Canyon has remained covered by sand through multiple cycles of Quaternary climate change3, 4.

Methods

Proof that residence time is a minimum estimate.

Consider a large number (n) of sand grains having common inherited cosmogenic radionuclide (26Al or 10Be) concentration N 0, but different residence times ti in the sand sea (1in). Assuming, for the sake of simplicity, that re-irradiation is negligible, then the nuclide concentration of the ith grain is N i = N0 e-λti, where λ is the radioactive decay constant. Define ˆtas the ‘apparent’ burial age, calculated from the average nuclide concentration N, i.e. ˆ
t≡-1
λln(N∕N0) with N 1
n i=1nN i. And define t as the true average burial age of all sand grains, i.e. t 1n i=1nt i. Then e-λˆ
t = N∕N 0 = n1 i=1nN i∕N0 = -1
n i=1ne-λti = Ce-λt, with C 1
n i=1neλ(t-ti). It can be shown that C 1. Therefore, e-λˆt e-λt, and hence ˆtt.

Effective production rate of a linear dune.

Cosmogenic nuclide production is restricted to a surface layer with an effective thickness22 of 160g/cm2. Using a dune density23 of 1.7g/cm3, this is equivalent to 90cm of sand. Approximating the linear dunes of the Namib by triangular prisms of 90m height, the volume of this ‘active layer’ is 1/50th of the total dune volume. Therefore, the average production rate for the entire dune is 1/50th of the surface production rate.

Supplementary Information 

Data tables and further details about the analytical methods and data reduction procedures are provided in the Supplementary Information item.

Correspondence 

Correspondence and requests for materials should be addressed to PV (email: p.vermeesch@ucl.ac.uk).

Acknowledgments 

This work was funded by a Marie Curie postdoctoral fellowship at ETH-Zürich in the framework of the CRONUS-EU network (RTN reference 511927), a faculty research grant at Birkbeck, University of London, and a NERC CIAF grant (allocation no. 9059.1008), all awarded to Pieter Vermeesch. We would like to thank Hartmut Kolb of the Gobabeb Desert Research Centre for his dune driving skills, Hans Schreiber and Piet Swiegers for granting access to their land for sampling, Allan Davidson, Heinrich Baur and Andy Carter for technical/lab assistance, and Rainer Wieler for feedback.

Author Contributions 

PV designed the study, collected the samples, performed the U-Pb analyses, and wrote the paper; CRF did the 10Be and 26Al measurements; FK performed the noble gas analyses; GFSW provided field assistance; CSB helped writing the paper; SX was in charge of the AMS measurements.

Competing Interests 

The authors declare that they have no competing financial interests.

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