Slow erosion without creep in the McMurdo Dry Valleys, Antarctica

Dan Morgan
Vanderbilt University, Earth and Environmental Sciences

Author Profile

Shortcut URL: https://serc.carleton.edu/93205

Location

Continent: Antarctica
Country:
State/Province:
City/Town:
UTM coordinates and datum: none

Setting

Climate Setting: Arid
Tectonic setting:
Type: Process, Computation

Fig. 1. Map of the McMurdo Dry Valleys, Antarctica

Show caption

Fig. 2. The Arena Valley ash sample site

Show caption

Figure 2. The Arena Valley ash and the stratigraphy of the sample site. Widths of the stratigraphic columns represent relatively larger average sediment grain size of the unit. D.P. indicates a desert pavement. At this site, the Arena Valley ash is 15 cm thick, beginning 3 cm below the desert pavement that makes the current surface. The Arena Valley ash has an 40Ar/39Ar age of 4.33 Ma (Marchant et al. 1993) and rests on a desert pavement that caps the Monastery colluvium. Details

Show caption

Figure 3. A cartoon diagram of how an exposure model represents the change in depth through time for a sample collected for cosmogenic nuclide analysis. The black lines represent different paths that a sample in an unconsolidated deposit could have traveled relative to the surface. Because the production rate of cosmogenic nuclides depends on the depth of a sample, we can integrate these depth through time curves to make unique predictions about the concentration of cosmogenic nuclides in the samples that we measure. In this manner, we can determine the stability, erosion rate, and potential for mixing and burial at the sample site. Details

Fig. 4. Data for the Arena Valley ash site

Show caption

Figure 4. Measured 10Be (circles) and 26Al (squares) concentrations (atoms g-1quartz) at the Arena Valley ash site. The small vertical and horizontal lines indicate the range of depth of the sediment sampled and the error in the nuclide measurement, respectively. In some cases, these values are smaller than the markers used to indicate each datum. The white box shows the approximate location of the Arena Valley ash in the soil column. The solid lines show the predicted nuclide concentrations for the best-fit model results where the erosion rate after the emplacement of the ash was ~0.2 m Ma-1. 4.B) A Klein-Nishiizumi-two-isotope diagram, which plots the 26Al/10Be ratio against the 10Be concentration. Nuclide concentrations for each sample have been divided by the production rate of the nuclides at each sample depth. For each sample that we have concentrations of both 10Be and 26Al, the measurements are shown as black circles with ellipses around them representing the 68% confidence interval. The general interpretation of this diagram is that cosmogenic nuclide concentrations that reflect only surface exposure plot on the upper line, concentrations that reflect erosion plot on the lower line, and concentrations that reflect a more complicated exposure history, such as burial or inheritance, plot below these lines. A more detailed explanation of this diagram can be found in Lal (1991). The grey boxes and black circles show the results of using an exposure model to consider the nuclide concentrations as the sum of their pre- and post-ash components, respectively. Both pre-and post-ash components plot directly on the steady erosion line, indicating that the measured nuclide concentrations can be explained by erosion alone and do not indicate that the sample site has been buried. Details

Description

Quantifying erosion rates is central to understanding how landscapes evolve through time and for discerning the links between landforms and the processes that shape them. The alpine valleys of the McMurdo Dry Valleys in Antarctica (MDV - Figure 1) have a unique hyperarid, cold, polar desert climate. The MDV are incredibly dry, receiving less than ~10 cm of water equivalent in precipitation each year, and extremely cold, with mean annual temperatures of ~-22°C. Because it is so cold and dry, running melt water is rarely observed, and bacteria and nematodes (a worm from the phylum Nematoda) are typically the highest life forms found in the MDV. Under these conditions, erosion rates are thought to be among the lowest in the world. The preservation of ash deposits of up to 15 Myrs old at the present-day surface has been interpreted as indicating that these cold and dry climate conditions have persisted for millions of years in the MDV, implying that erosion rates have remained low for millions of years too (e.g. Denton et al., 1993). Landscapes such as the MDV make geomorphologists wonder: How low can erosion rates be? Is it possible to have zero erosion across a landscape? What are the geomorphic processes operating in a landscape with very low erosion rates? How do processes and rates in the MDV compare to planetary bodies such as the Moon and Mars? In this vignette, we will briefly describe the method of quantifying erosion rates with cosmogenic nuclides, and discuss an example from the MDV to address these questions.

Cosmogenic nuclides, such as ¹⁰Be and ²⁶Al, accumulate in quartz in rocks and soil that are exposed to cosmic rays, which is generally the upper few meters of the surface. Because these nuclides accumulate only due to exposure to cosmic rays, we can use their concentration to address questions regarding the age and rate of processes like erosion on any geologic surface (e.g. Lal, 1991; Gosse and Phillips, 2001). We collected samples of surficial deposits in the MDV by digging a soil pit and taking bulk sediment samples of soil at various depths down to ~1 m. One of our sample sites includes an ash layer that is 4.34 Myrs old (Marchant et al., 1993) (Figure 2--The Arena Valley ash site). We collected samples from above and below this ash layer and measured the concentration of cosmogenic ¹⁰Be and ²⁶Al in the quartz from these sediments.

To analyze the measured concentrations of cosmogenic ¹⁰Be and ²⁶Al, we employ the concept of an exposure model, which describes how these samples have changed depth with time (Figure 3). Because the production rate of cosmogenic nuclides decreases exponentially with depth, the measured concentration depends on how these samples have changed depth with time. Exposure models based on erosion, mixing, and accumulation of sediment predict different nuclide concentrations with depth, and fitting these models to the measured concentrations can constrain the rate of these processes.

The measured concentrations of ¹⁰Be and ²⁶Al at the Arena Valley ash site fit a simple exponential curve and indicate that the site has been experiencing erosion at a rate of ~0.2 m Myrs^-1 for the past ~4 Myrs (Figure 4). Additionally, the concentrations of ¹⁰Be and ²⁶Al indicate that this site has not been shielded from cosmic rays, which indicates that glaciers have not expanded to cover this site, or experienced any vertical mixing due to cryoturbation or soil creep in the past ~4 Myrs. Various other surface deposits in the MDV have been sampled for similar cosmogenic nuclide analysis, and each have found that slow, steady erosion (0.2-4 m Myrs^-1) without creep has been going on in the MDV for the past ~1-4 Myrs (e.g. Putkonen et al., 2008; Schiller et al., 2009).

The slow, downslope movement of rocks, minerals, and soil due to gravity, regardless of the method or process responsible for the movement of the material, is called soil creep. Creep has the ability to move material downslope at rates on the order of cm yr^-1, and it is the dominant process responsible for transporting much of the soil from the landscape to river channels (Oehm and Hallet, 2005). Soil creep is thought to be a nearly universal process that occurs in all but the most hyperarid climates. Because all of the sites studied in this project (Morgan et al., 2010) have cosmogenic nuclide concentration profiles that are well-explained by an exposure model that predicts an exponential curve, vertical mixing of the regolith is not indicated as an active geomorphic process in the timeframe that the nuclide concentrations record (~1-4 Myrs for all sample sites). If vertical mixing was active during this time period, the nuclide concentration profiles would be expected to be well-mixed and would not fit an exponential profile. Thus, even though the surface is eroding and we observe wind-blown sediment fluxes (Lancaster, 2002), any downhill transport of material (e.g. Putkonen et al., 2007) must be limited to the upper few centimeters because creep is not indicated by the samples that lie just below the surface. Because soil creep rates affect the rates of landscape evolution (Oehm and Hallet, 2005), the lack of creep at the sample sites may contribute to the low erosion rates determined in this study.

The erosion rates of unconsolidated deposits in the MDV (0.2-4 m Myrs^-1) are so low that they are comparable to the lowest bedrock erosion rates ever reported, which also come from other arid regions (e.g. Belton et al., 2004; Dunai et al., 2005). The lack of creep and vertical mixing in these deposits is likely related to the lack of moisture in the regolith that could induce frost heave, and because the MDV lack higher lifeforms that would cause bioturbation. The lack of creep suggests that slopes should retreat in parallel, which may help to preserve the fresh appearance of surface deposits in the MDV for millions of years. Because unconsolidated deposits in the MDV can be viewed as an end-member in the spectrum of both soil moisture content and biotic activity, we can consider these erosion rates to be similar to what is experienced on the surface of Mars and the Moon today, and that the preservation of surface landforms on these planetary bodies may result from a lack of soil creep that would act to degrade these features.

Associated References

Belton, D. X., R. W. Brown, B. P. Kohn, D. Fink, and K. A. Farley (2004), Quantitative resolution of the debate over antiquity of the central Australian landscape: implications for the tectonic and geomorphic stability of cratonic interiors, Earth and Planetary Science Letters, 219(1-2), 21-34.
Denton, G., D. Sugden, D. Marchant, B. Hall, and T. Wilch (1993), East Antarctic Ice Sheet Sensitivity to Pliocene Climatic Change from a Dry Valleys Perspective, Geografiska Annaler. Series A, Physical Geography, 75(4, A Special Volume Arising from the Vega Symposium: The Case for a Stable East Antarctic Ice Sheet), 155-204.
Dunai, T. J., G. A. G. Lopez, and J. Juez-Larre (2005), Oligocene-Miocene age of aridity in the Atacama Desert revealed by exposure dating of erosion-sensitive landforms, Geology, 33(4), 321-324.
Gosse, J., and F. Phillips (2001), Terrestrial in situ cosmogenic nuclides: theory and application, Quaternary Science Reviews, 20(14), 1475-1560.
Lal, D. (1991), Cosmic ray labeling of erosion surfaces - In situ nuclide production rates and erosion models, Earth and Planetary Science Letters, 104, 424-439.
Lancaster, N. (2002), Flux of Eolian Sediment in the McMurdo Dry Valleys, Antarctica: A Preliminary Assessment, Arctic, Antarctic, and Alpine Research, 34(3), 318-323.
Marchant, D., G. Denton, and C. Swisher (1993), Miocene-Pliocene-Pleistocene Glacial History of Arena Valley, Quartermain Mountains, Antarctica, Geografiska Annaler. Series A, Physical Geography, 75(4, A Special Volume Arising from the Vega Symposium: The Case for a Stable East Antarctic Ice Sheet), 269-302.
Morgan, D., J., J. Putkonen, G. Balco, and J. Stone (2010), Quantifying regolith erosion rates with cosmogenic nuclides 10Be and 26Al in the McMurdo Dry Valleys, Antarctica, Journal of Geophysical Research, 115, F3, doi:10.1029/2009JF001443.
Oehm, B., and B. Hallet (2005), Rates of soil creep, worldwide: weak climatic controls and potential feedback, Zeitchrift fuer Geomorphologie, 49, 353–372.
Putkonen, J., G. Balco, and D. Morgan (2008), Slow regolith degradation without creep determined by cosmogenic nuclide measurements in Arena Valley, Antarctica, Quaternary Research, 69(2), 242-249, doi:10.1016/j.yqres.2007.12.004.
Putkonen, J., M. Rosales, N. Turpen, D. Morgan, G. Balco, and M. Donaldson (2007), Regolith transport in the Dry Valleys of Antarctica, Antarctica: A Keystone in a Changing World--Online Proceedings of the 10th ISAES X, edited by AK Cooper and CR Raymond et al., USGS Open-File Report.
Schiller, M., W. Dickinson, R. G. Ditchburn, I. J. Graham, and A. Zondervan (2009), Atmospheric ¹⁰Be in an Antarctic soil: Implications for climate change, Journal of Geophysical Research, 114(F1), doi:10.1029/2008JF001052.

« Complexity of suspended sediment transport in an agricultural drainage system in Imperial Valley, southern California The degradation of glacial deposits in the McMurdo Dry Valleys, Antarctica »