Bierman Paul Robert

Bierman
Paul
Robert
Department of Geology, University of Vermont, Burlington, VT 05405 (802) 656-4411
(802) 656-0045 (fax)
6/13/96
Quantifying the rate of rock weathering in hyper-arid southern Africa
Geology and Geography
$16,630
1 year, field work summer 1997; lab work to follow
Namibia, southern Africa
Isolated, bare rock surfaces are a common, even dominant feature of deserts throughout the world. Over a
century of research has provided a plethora of hypotheses regarding the ages of and means by which these
rock outcrops, termed inselbergs, form (Twidale, 1982). However, until recently there were no means by
which to test these hypotheses nor to measure the rate at which inselbergs are eroding. Over the past
several years, we have been collecting samples of granitic rock and, in collaboration with physicists, have
used accelerator mass spectrometry to measure the abundance of very rare isotopes formed by the
bombardment of cosmic rays (Bierman and Turner, 1995; Bierman, 1994). These measurements, from
seven sites in Australia and North America, have allowed us to define an empirical relationship between
mean annual rainfall and the denudation rate of exposed rock (Bierman, 1995). This relationship suggests
that rock denudation in the absence of measurable precipitation should be 0.4 m/million years and should
increase at a rate of 2.6 m/My for each additional meter of precipitation. However, we have yet to sample
extremely arid sites. This proposal will fund the collection of samples from the hyperarid Namibian desert
in order to test the relationship we have established and to determine the rate of rock weathering and
landscape formation in what is thought to be one of the World’s driest and most tectonically stable areas.
Erosion, desert, granite, Namibia, cosmogenic isotope
This will be the first work to estimate the rate at which rock erodes in one of the most arid and tectonically
stable settings on Earth. Measuring such rates is fundamental to understanding the evolution of Earth’s
surface through time and space and is prerequisite to quantifying the rate at which topography develops and
sediment is generated.
Paul R. Bierman
Assistant Professor, University of Vermont, Department of Geology
Baltimore, MD 10/24/61
Ph.D., 1993, Geology, University of Washington, Seattle, WA with A. Gillespie
"Cosmogenic Isotopes and the Evolution of Granitic Landforms"
MS, 1990, Geology, University of Washington, Seattle, WA with A. Gillespie
"Accuracy and Precision of Rock Varnish Cation Ratio Dating"
BA, 1985, Geology and Environmental Studies, Williams College, Williamstown, MA
"Deglaciation of Northwestern Massachusetts," (cum laude and senior thesis)
Bierman has extensive experience collecting, processing, analyzing, and interpreting samples for cosmogenic
nuclide abundance. He developed and directs the extraction laboratory at the University of Vermont which has
prepared over 500 samples in the past three years. Bierman and his students are the authors of 11 papers
incorporating these nuclide measurements. In recognition of his research accomplishments, Bierman will be
awarded the Donath Young Scientist Medal from the Geologic Society of America during the 1996 annual meeting.
Bierman will oversee sampling and sample preparation. Isotopic analyses will be made in collaboration with Dr.
Marc Caffee of Lawrence Livermore National Laboratory. In southern Africa we will assisted by Dr. Tim Partridge
and at the Namibian field station by Mary Seely.
Paul R. Bierman
ARTICLES
Bierman, P., Gillespie, A. and Dunne, T. (in review) Basin-Scale Rates of Erosion Estimated Using 10Be And
26Al in Sediments. Geology.
Clapp, E., and P. Bierman (in press) COSMO-CALIBRATE A program for calibrating cosmogenic exposure
ages: Geophysical Research Letters.
Bierman, P. & Steig, E. (1996). Estimating rates of denudation and sediment transport using cosmogenic
isotope abundances in sediment. Earth Surface Processes and Landforms, 21, 125-139.
Gillespie, A. R. and Bierman, P. (1995). Precision of terrestrial exposure ages and erosion rates from analysis
of in-situ produced cosmogenic isotopes. Journal of Geophysical Research, 100, B12, 24637-24649.
Bierman, P., Gillespie, A., Caffee, M. (1995). Cosmogenic Ages for Earthquake Recurrence Intervals and
Debris-Flow Fan Deposition, Owens Valley, CA. Science, 270, 447-450.
Bierman, P. and Turner, J. (1995). 10 Be and 26Al evidence for exceptionally low rates of bedrock erosion and
the likely existence of pre-Pleistocene landforms. Quaternary Research, 44, 378-382
Clark, D. H. and Bierman, P. (1995). Improving cosmogenic chronometers. Quaternary Research, 44, 367377.
Bierman, P., Gillespie, A., Caffee, M. and Elmore, D. (1995). Estimating erosion rates and exposure ages with
36Cl produced by neutron activation. Geochimica et Cosmochimica Acta, 59, 3779-3798.
Bierman, P. (1994). Using in situ cosmogenic isotopes to estimate rates of landscape evolution: A review from
the geomorphic perspective. Journal of Geophysical Research (special issue on Tectonics and
Topography), 99, B-7, 13,885-13,896.
Bierman, P. and Gillespie, A. (1994). Short course notes for "Geomorphic application of cosmogenic isotopes".
Geological Society of America annual meeting, Seattle, 112 p.
Bierman, P. & Gillespie, A. (1991). Range fires: A significant factor in exposure-age determination and
geomorphic surface evolution. Geology, 19, 641-644 and subsequent comment and reply, Geology, 20,
283-285.
ABSTRACTS
Bierman, P. (1996) Cosmogenic clues to the tempo of environmental change, AMSIE '96, American
Association or the Advancement of Science, p. A-146
Bierman, P., Larsen, P., Clapp, E. and Clark, D. (1996). Refining estimates of 10-Be and 26-Al production
rates, Radiocarbon.
Bierman, P. (1995). How fast do rocks erode? New answers from atom counting. Geological Society of
America Abstracts with Programs. 27, A-44 (National)
Bierman, P. R., Gillespie, A. R. and Caffee, M. (1995). First 10Be , 26Al, and 36Cl age-estimates for earthquake
recurrence intervals and debris flow fan deposition, Owens Valley, California. Geological Society of
America Abstracts with Programs. (National)
Bierman, P. R. (1995) How quickly does granite erode -- evidence form analyses of in situ produced 10-Be, 26Al, and 36-Cl. Terra Nostra, INQUA, Berlin, 26. (International)
Bierman, P. R. (1995) A new method of estimating basin scale erosion rates -- measurement of in situ produced
10Be and 26Al in sediments: EOS, 76, S143. (National)
Bierman, P. and Caffee, M. (1994) Cosmogenic erosion rate estimates for granite landforms; Eyre Peninsula,
South Australia. Geological Society of America Abstracts with Programs, 26(7), A256. (National).
Bierman, P., Massey, C., Gillespie, A., Caffee, M. (1993) Cosmogenic isotope estimates for Lone Pine Creek
Alluvial fan ages, Geological Society of America Abstracts with Programs, 25(6), A461. (National).
Bierman, P., Gillespie, A., Caffee, M. and Elmore, D. (1993) Erosion rate and exposure age of granite
landforms estimated using 36Cl, Geological Society of America Abstracts with Programs, 25(6),
A141. (National).
Bierman, P. & Steig, E. (1992) Using cosmogenic isotopes to measure basin-scale rates of erosion. Geological
Society of America Abstracts with Programs, 24(7), A122. (National)
Bierman, P. & Gillespie, A. (1991) Lowering rates of granitic landforms determined by measurement of in situ
produced cosmogenic nuclides. EOS, 72(44), 575. (National)
Bierman, P. & Gillespie, A. (1991) The evolution of granitic landforms -- field observations and cosmogenic
insights. Geological Society of America Abstracts with Programs, 23(5), A89. (National)
Research results will be submitted to major journals most likely, Earth
Surface Processes and Landforms and the journal Science, an editor of which has requested Bierman prepare a
manuscript on world-wide rates of rock weathering.
Paul R. Bierman
$16,630
$16,630
$4300
airfare, BTV-Johnassberg-Windhook: Bierman and assistant; economy coach; quote
6/20/96, UVM travel store
$1680
4 wheel drive rental vehicle, Kessler 4x4 hire, Windhook/Namibia
14 days at $120/day
$1400
$300
$300
$100
$1050
$7500
per diem, 2 people, 14 days $50/day including petrol and lodging at research station
sample equipment: sample bags, field note books, chisels
slide film and processing
Polaroid film for recording sampling sites
lab costs (acid, labware @$70/sample; 15 samples)
AMS analyses at Livermore Laboratory for 10-Be and 26-Al
University of Vermont, Office of Sponsored Programs, Waterman Building, Burlington, VT 05405
50% on 3/1/97
50% on 8/1/97
Paul R. Bierman
The University of Vermont will provide laboratory facilities for the preparation of samples and computer
facilities for data reduction. Total University cost-share is $9596 and consists of three weeks of Dr.
Bierman’s academic year effort to process samples plus associated fringe benefits and indirect costs.
No other grants have been submitted for this work.
No other grants have been received by Bierman from the Society
Dr. Raymond Burke, Dept. of Geology, Humboldt State University, Arcata, CA 95521, (707) 826-4292
Dr. David Dethier, Geology Department, Williams College, Williamstown, MA 01267 413 597 2078
Dr. Milan Pavich, USGS MS 298, 12201 Sunrise Valley Drive, Reston, VA 22092, (703) 648-6963
Dr. Les McFadden, Department of Geology, UNM, Albuquerque, NM 87131, (505) 277-4204
Dr. John Stone, Research School of Earth Sciences, ANU GPO Box 4 Canberra, ACT 2601, AUSTRALIA,
06 249 3406
3/1/98
12/30/98
Paul Bierman
Paul R. Bierman
Background -- Understanding rates of rock weathering over space and through time is a fundamental geologic
and geographic question. It is the breakdown of rock that creates sediment as spatial differences in weathering
rates generate spectacular landscapes and create topography.
Rates of rock weathering have been notoriously difficult to measure although a variety of techniques
have been employed (Saunders and Young, 1983). Until recently, there was no way to determine erosion rates
of rock directly over 1,000 to 1,000,000 year time scales; the meaning of measurements made over shorter
(human) time scales remains uncertain. The advancement of accelerator mass spectrometry for measuring
isotopes produced near Earth’s surface by cosmic rays (Elmore and Phillips, 1987) and the development of
models for interpreting these data (Lal, 1991) have revolutionized the study of bedrock landforms and at last
provide some constraint on the rate at which bare bedrock surfaces erode (Bierman, 1994). To the first
approximation, the abundance of cosmogenic nuclides such as 10Be and 26Al is related to the residence time of a
sample within a meter of Earth’s surface. Using models, isotope abundances can be interpreted as erosion rates
expressed in meters of lowering per million years (m/My; Lal, 1991; Bierman, 1994).
Crystalline rocks, such as granite and gneiss, underlie much of the continents and are the lithologies
upon which extensive geomorphic surfaces, described by some as peneplains, develop. Isolated rock
outcroppings or inselbergs project from relatively flat landscapes. Field observation and geologic reasoning
have been used to generate many hypotheses regarding the origin and age of inselbergs but little data exist to
test these hypotheses. For example, since weathering reactions are mediated by water, one might expect that
inselberg surfaces in humid regions would be shed sediment more rapidly than those in arid regions.
In order to determine the effect of water on the rate of rock weathering, we have sampled and made
cosmogenic isotopic analyses of granitic and gneissic rocks in seven different locations in Australia and North
America (Bierman, 1995). Mean annual precipitation (MAP) at these sample sites ranges from 150 mm/yr to
>1500 mm/yr. Comparing the lowest model erosion rate at each sample site, we find that rates of mass loss
from the inselberg surfaces are linearly relate to MAP (Figure 1). The lowest rates we have measured so far
(0.5 m/My; Bierman and Turner, 1995) are from the tectonically stable, semi-arid (340 mm/yr) Eyre Peninsula
of southern Australia. These rates are very low but are still an order of magnitude higher than rates measured
on sandstones in the Dry Valleys of Antarctica where liquid water is absent (Nishiizumi et al., 1991).
In order to compliment our existing data set, we propose to collect samples from one of the driest
places on Earth, the inselbergs of the Namibian desert in southern Africa. Inselbergs are common Namibian
landscape elements and their location has been mapped by several workers (Ollier, 1978; Selby, 1977; Selby,
1982c). The Namibian inselbergs have been well studied (Selby, 1977; Selby, 1982c) and provide the
opportunity for us to examine the weathering rates of different lithologies including both Precambrian schist
and younger granites, which in some cases are exposed on the same inselberg (Selby, 1982a). The Namib
Desert receives extremely little precipitation (15-23 mm/yr, MAP). Prior research suggests that the area has
been arid for much, if not all, of the Quaternary (Ollier, 1978; Selby et al., 1979). Fog may add up to 30 mm/yr
of moisture in some regions of the Namib Desert (Goudie, 1972).
Samples we collect from the Namib Desert will
allow us to estimate isotopically the rate at which crystalline
rock erodes in the near absence of precipitation.
Furthermore, our data will be analyzed in the framework of
the Namib rock strength data gathered by Selby (Selby,
1982a; Selby, 1982b; Selby, 1982c) and will be compared to
the extensive isotopic data sets we have gathered from the
inselbergs of southern Australia, central Texas, southern
California, and the southeastern United States (Bierman,
1993; Bierman et al., 1995; Bierman and Turner, 1995).
Work Plan -- Bierman and a field companion, most likely
Caffee, will travel to Namibia in order to collect samples. In
the interest of safety, our field party will be a minimum of
two people. We will stay at the Gobabeb research station
(Desert Research Foundation of Namibia) and we have
already been advised as to permit, sampling and entry
requirements by Mary Seely who is associated with the
station. The station and its staff provide local expertise in
natural history and in
Paul R. Bierman
Namibian logistics; furthermore, the station is located near many of the well-studied Namibian inselbergs.
Working out of the research station will allow us to collate sample information in the evening and provide
a secure repository for our field equipment, supplies, and samples. We have also been in contact with Dr.
Timothy Partridge, Professor of Physical Geography, University of Witwatersrand, who has studied
extensively southern African geomorphic surfaces (Partridge and Maud, 1987). If possible, we plan to
spend time in the field with Dr. Partridge.
In the field, we plan to collect 3 to 5 samples from each of 8 to 10 inselbergs. These samples
will be collected from the tops of the inselbergs which have been shown to have the highest isotope
abundances and correspondingly lowest erosion rates (Bierman, 1993; Bierman and Turner, 1995) and
from the side slopes which typically have lower isotope abundances by a factor of >2 even after
geometrical corrections for cosmic ray dosing have been made. Sampling the tops of numerous
inselbergs will indicate whether the erosion rates we measure are spatially uniform such as those for the
Eyre Peninsula Australian inselbergs (Bierman and Turner, 1995) or whether there are significant
differences between inselbergs in terms of erosion rate and cover history.
Samples will be returned to the UVM where they will be processed using our standard protocols.
Rocks will be cleaned of lichen, jaw crushed, plate ground, and sieved before acid etching and mineral
separation to isolate a quartz-rich fraction. This fraction will be ultrasonically etched to provide 10-20 g of
nearly pure quartz which will be dissolved in the presence of Al and Be carrier. Be and Al will be separated
using ion exchange and AMS targets will be prepared at UVM and transported to Livermore National
Laboratory for isotopic analysis. Samples will be handled in batches of eight; each batch includes a process
blank. Although we are only requesting funding for processing and analysis of 15 samples, we plan to
oversample while in Namibia given the cost of travel and the difficulty of access. On the basis of the initial 15
samples, we will choose additional samples to run in the future as laboratory time and funds become available.
Bierman, P., 1993, Cosmogenic isotopes and the evolution of granitic landforms [Doct. Thesis]: Univ. Wash..
Bierman, P., Gillespie, A., Caffee, M., and Elmore, D., 1995, Estimating erosion rates and exposure ages with
36Cl produced by neutron activation: Geochimica et Cosmochimica Acta, v. 59, no. 18, p. 3779-3798.
Bierman, P., and Turner, J., 1995, 10Be and 26Al evidence for exceptionally low rates of Australian bedrock
erosion and the likely existence of pre-Pleistocene landscapes: Quaternary Research, v. 44, p. 378-382.
Bierman, P. R., 1994, Using in situ cosmogenic isotopes to estimate rates of landscape evolution: A review
from the geomorphic perspective: Journal of Geophysical Research, v. 99, no. B-7, p. 13885-13896.
Bierman, P. R., 1995, How fast do rocks erode? New answers from atom counting: Geological Society of
America Abstracts with Programs, v. 27, no. 6, p. A-44.
Elmore, D., and Phillips, F., 1987, Accelerator mass spectrometry for measurement of long-lived radioisotopes:
Science, v. 236, p. 543-550.
Goudie, A., 1972, Climate, weathering, crust formation, dunes, and fluvial features of the Central Namib
Desert, near Gobabeb, south west Africa: Madoqua, Series II, v. 1, no. 54-62, p. 15-31.
Lal, D., 1991, Cosmic ray labeling of erosion surfaces: In situ production rates and erosion models: Earth and
Planetary Science Letters, v. 104, p. 424-439.
Nishiizumi, K., Kohl, C. P., Arnold, J. R., Klein, J., Fink, D., and Middleton, R., 1991, Cosmic ray produced
10Be and 26Al in Antarctic rocks: exposure and erosion history: Earth and Planetary Science Letters, v.
104, p. 440-454.
Ollier, 1978, Inselbergs of the Namib Desert, process and history: Zeitschrift fur Geomorphologie, supplement,
v. 31, p. 161-176.
Partridge, T. C., and Maud, R. R., 1987, Geomorphic evolution of southern Africa since the Mesozoic: Southern
Africa Journal of Geology, v. 90, no. 2, p. 179-208.
Saunders, I., and Young, A., 1983, Rates of surface processes on slopes, slope retreat, and denudation: Earth
Surface Processes and Landforms, v. 8, p. 473-501.
Selby, M. J., 1977, On the origin of sheeting and laminae in granitic rocks: evidence from Antarctica, the
Namib Desert and the central Sahara: Madoqua, v. 10, no. 3, p. 171-179.
Selby, M. J., 1982a, Rock mass strength and the form of some inselbergs in the central Namib Desert: Earth
Surface Processes and Landforms, v. 7, p. 489-497.
Selby, M. J., 1982b, Controls on the stability and inclinations of hillslopes formed on hard rock: Earth Surface
Processes and Landforms, v. 7, p. 449-467.
Selby, M. J., 1982c, Form and origin of some bornhardts of the Namib Desert: Zeitschrift fur Geomorphologie,
v. 26, no. 1, p. 1-15.
Selby, M. J., Hendy, C. H., and Seely, M. K., 1979, A late Quaternary lake in the central Namib Desert,
southern Africa, and some implications: Paleogeography, Paleoclimatology, Paleoecology, v. 26, p.
37-41.
Twidale, R., 1982, Granite Landforms: Amsterdam, Elsevier, 372 p.
Paul Bierman
Rates of rock weathering are poorly known but of great importance to
understanding how Earth’s surface and environment have changed over time.
New techniques now allow direct measurement of rock erosion rates and allow
estimation of how quickly topography is generated and landscapes change. This
study will measure rates of rock weathering in one of the driest regions of the
world, the Namibian Desert. Data collected in Namibia will complement data
gathered from other regions of the world and will address the fundamental
scientific question, “How quickly do the landscapes in which we live, change?”