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2 Expected Cosmogenic In Situ Radionuclide Concentrations in Solids, for Eroding Surfaces

2 Expected Cosmogenic In Situ Radionuclide Concentrations in Solids, for Eroding Surfaces

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24 Using Cosmogenic Radionuclides for the Determination of Effective Surface Exposure Age

1. Cosmic ray flux and nuclide production during

different geomagnetic fields

If the past geomagnetic field is known, one can

calculate the expected cosmogenic nuclide production

rate at different geomagnetic latitudes (Lal et al. 1985;

Nishiizumi et al. 1989), basing on the present day field

(cf. nuclide production rates for different nuclides

as given by Lal and Peters 1967). The vertical cut-off

rigidity in the cosmic ray spectrum at the top of the

atmosphere, Rc (¼pc/Ze), is given by the following


Rc ẳ 14:9M=M0 ị cos4 lị

where p is the particle momentum, c is the speed of

light, Z is the particle’s atomic number, e is the electronic charge; M and M0 are the dipole fields during an

epoch in the past and during the present time, respectively, for which cosmogenic production rates

in the atmosphere have been calculated (Lal and

Peters 1967), and l is the geomagnetic latitude. The

corresponding geomagnetic latitude for the past, lM,

when the dipole field was M, is given by the relation

(Lal et al. 1985; Nishiizumi et al. 1989):

cos lM ị ẳ




cos l0 ị


where l0 is the geomagnetic latitude for the present

field, M0.

2. Cosmic ray flux and nuclide production during

periods of different solar activity

The primary galactic cosmic ray (GCR) flux incident at the top of the Earth’s atmosphere is anticorrelated with solar activity (with some phase lag).

See Gleeson and Axford (1968) for a convectiondiffusion theoretical framework for the transport of

GCR particles through the heliosphere, and see

Castagnoli and Lal (1980) and Papini et al. (1996)

for the essential features of the experimental data on

solar modulation of primary cosmic ray flux. In terms

of the solar modulation formulism (Gleeson and

Axford 1968), the effective change in the energy of a

charged particle moving from a point P on the boundary along a dynamical trajectory to an interior point Q

is Zef, irrespective of its path to Q. If the measured

kinetic energy of a charged particle at Q is Ek/nucleon,


then it follows (Randall and Van Allen 1986) that its

kinetic energy/nucleon at P must have been (Ek ỵ

Zef). As shown by Randall and Van Allen (1986),

this leads to the following simple equation relating

differential fluxes at points P and Q:


jQ T


jP T ỵ




T (T ỵ 2m c2 ị








In (24.2), A is the mass number; T is defined as

the kinetic energy per nucleon, Ek; the total kinetic

energy ¼ A Ek. If the point P is taken at the heliosphere boundary, and the point Q at 1 AU then the

charged particle is modulated through a potential

difference of f MeV/nucleon.

For more than four decades, the solar modulation of

cosmic ray flux has been studied using satellites/

spacecraft in orbit around the Earth, and more recently

on deep space probes. The range of observed variations in the modulated near-Earth differential flux

of GCR protons until the late 1970s are shown in

Fig. 24.2 for protons as described by different curves

designated by the modulation parameter, f (Randall

and Van Allen 1986; Castagnoli and Lal 1980). The

hypothetical curve, f ¼ 0, corresponds to the predicted shape of the interstellar GCR proton flux, i.e.

the GCR flux outside the heliosphere. Information

about near Earth changes in flux have come largely

from the charged particle monitoring experiment

(CPME) aboard the IMP 8 satellite since 1973. For a

recent summary of these data reference is made to

Simunac and Armstrong (2004).

Our knowledge of modulation of GCR through the

outer heliosphere comes from deep space probes

(Lockwood and Webber 1995) which transmitted in

situ cosmic ray data up to distances of more than

70 AU (Pioneers 10 and 11, Voyagers 1 and 2). For

details see Lal (2007). Nuclide production rates for

different geomagnetic field intensities and modulation

parameters, f, have been calculated earlier (cf. Lal


3. Present day nuclide production rates at different

latitudes and at altitudes 10 km

If the production rate of a nuclide in a rock/soil is

known at some latitude and altitude ( 10 km), then


D. Lal

Fig. 24.2 Estimated proton

spectra for different solar

modulation intensities as

defined by the parameter, f.

The curve labeled

f ¼ 0 corresponds to the unmodulated proton spectra

outside the heliosphere

Table 24.1 Nuclear disintegration rates (Lal and Peters 1967) in the atmosphere (gÀ1 yearÀ1)

Geomagnetic latitude

Polynomial coefficientsa





2.559 Â 102



3.307 Â 102


3.379 Â 102

2.521 Â 102




3.821 Â 10

2.721 Â 102



4.693 Â 102

3.946 Â 102



5.256 Â 102

5.054 Â 102



5.711 Â 102

5.881 Â 102



5.634 Â 102

6.218 Â 102







































The polynomials (24.3) are valid for 0–10 km altitudes

the production rate at other latitudes and altitudes can

be derived using the scaling factors given by Lal and

Peters (1967). The scaling factors are primarily based

on thermal neutron measurements and take into

account the thresholds for nuclide production from

the targets of interest (Lal 1958). Third degree polynomials are presented by Lal (1991) for nuclear disintegrations (gÀ1 air yearÀ1) in the atmosphere in

Table 24.1, based on Lal and Peters (1967), and for


Be and 26Al in quartz (gÀ1 yearÀ1), based on measurements in quartz from glacially polished rocks

(Nishiizumi et al. 1989). The nuclear disintegration

rates or the production rates of 10Be, and 26Al in quartz

are given by the corresponding polynomial coefficients, P1, P2, P3 and P4 for q (L, y):

q L; yị ẳ P1 Lị ỵ P2 Lị y ỵ P3 Lị y2 ỵ P4 Lị y3


where q(L,y) is the nuclear disintegration rate in the

atmosphere or 10Be or 26Al production rate in quartz

(gÀ1 SiO2 yearÀ1) at latitude L, and altitude y (km).

The polynomial coefficients are given separately for

latitudes: 0, 10, 20, 30, 40, 50, 60–90 , in Tables 24.1

and 24.2.

For completeness we give here the third degree

polynomial for converting altitude (y km) to pressure,

P (g cmÀ2), based on the ICAO standard atmosphere

(International Civil Aviation Organization 1964):

P ẳ p1 ỵ p2 : y ỵ p3 : y2 ỵ p4 : y3


24 Using Cosmogenic Radionuclides for the Determination of Effective Surface Exposure Age


Table 24.2 Production rates of 10Be and 26Al in quartz (gÀ1 SiO2 yearÀ1)

Geomagnetic latitude

Polynomial coefficientsa






















































































The polynomials (24.3) are valid for 0–10 km altitudes

where p1 ¼ 1032.92; p2 ¼ À121.95; p3 ¼ 5.657;

p4 ¼ À0.1095.

For scaling factors for 14C, reference is made to

Miller et al. (2006) and Lifton et al. (2001). Earlier

scaling factors for 3He and 20,21,22Ne are given in

Lal (1991) but they have since been considerably

improved as a result of efforts made by scientists

working jointly in the CRONUS-Earth Project (Cosmic ray produced systematics on Earth Project; Stuart

and Dunai 2009).

4. Expected nuclide concentrations in exposed rocks

for first order exposure models

As expected, the principal obstacle in determining

surface exposure ages or erosion rates is the lack of

knowledge regarding the rock’s surface exposure history. For example, the rock may have been covered

with sediment or have had undergone exfoliation during its prior exposure history. Further, the erosion rate

may have varied with climate in the past. Clearly, any

evidence from the geologic history of the rock is of

paramount importance.

Since the production of cosmogenic nuclides

within the rock is a sensitive function of the geometry

of the rock, use of two or more cosmogenic nuclides

can help constrain the exposure history of the rock. In

an earlier paper, Lal (1991) considered the effects of

changes in exposure geometry, and also considered

first order exposure models. In a subsequent paper,

Lal and Chen (2005) considered more complicated

exposure geometries and considered possibilities

of constraining exposure histories using cosmogenic

data alone.

In the following, we will discuss first order exposure models and develop simple equations to bring

out salient features of the cosmogenic method, for

instance, the constraints on exposure history that are

possible using two or more cosmogenic nuclides.

For simplicity, we will consider a horizontal rock

surface; complex rock geometries have been considered by Lal and Chen (2005). As the surface of a rock

horizon is exposed, nuclear interactions of cosmic rays

produce nuclides at the exposed surface and at depths.

The number of atoms of radio-nuclides at time t, N(x,t)

within the rock at any depth, x(t) are given by the

following differential equation:

d Nx; tị

ẳ Nx; tị þ Pðx; tÞ



where l is the disintegration constant of the nuclide

and P(x,t) is the nuclide production rate at depth x (cm),

and time t (s). If one considers nuclide production

by nucleons, then the nuclide production rate, P(x,t)

is fairly well described by the equation:

P x; tị ẳ P 0; tị em r xðtÞ


where m (cmÀ2 gÀ1) is the absorption coefficient

(inverse of m is defined as the absorption mean free

path (g cmÀ2) in the rock). The absorption mean free

path is altitude and latitude dependant (Table 24.3).

The value of x(t) in (24.2) is given by the integral:


x tị ẳ e tị dt



(a) Special case of fixed erosion rate, fixed cosmic ray

production rate and uniform erosion history of the


If one assumes a constant erosion rate, e, and constant cosmic ray nuclide production rate, P0 at the rock


D. Lal

Table 24.3 Absorption mean free paths in the atmosphere for

nuclear disintegrations of energy release >40 MeV

Altitude range (km)

Mean free path (g cmÀ2)












follows from (24.11) that the quantities below are

invariant for different radionuclides:

P 0ị

N 0; Tị

l1 ẳ


surface, then the solution of (24.1) nuclide concentration at time t, at depth x can also be written in the

integral form:


N ðx; Tị ẳ P0 em r xỵe tị : el Ttị dt



integrating backwards in time, setting the present

time at 0, and t ¼ T, when the rock was first exposed

to cosmic radiation, as T, the total duration of the


Solving (24.5) or (24.8) gives the nuclide concentration, N(x, T) in the rock at depth, x:

N x; Tị ẳ


em x 1 eT lỵm eị ị

l ỵ me


If however, the rock sample had some initial concentration of the radionuclide at the start of the irradiation, N(x,0), then an additional term, N(x,0) exp

(Àl T) should be added to (24.9).

Equation (24.9) states that if a rock surface

undergoes steady state erosion, the in situ radionuclides attain secular equilibrium concentration

corresponding to an effective disintegration constant,

l + me. The effective irradiation time, Teff for the top

surface of the rock is then given by:

Teff ¼ N 0; T )



=P0 ẳ


l ỵ me

l ỵ me

The validity of a steady state model can be tested by

studying two or more radionuclides of different halflives in the rock surface. The model steady state erosion at the surface is then given by, based on (24.5):


P 0ị




l ẳ



m N 0; Tị

m Teff

P 0ị

N 0; TÞ

À l2


À l3



Note that the validity of (24.8) does not necessarily imply that surface has been undergoing erosion

on a micro-scale. However it does imply that if the

rock has undergone macro-erosion, then the chipoff distances have always been much ( than the

distance 1/m.

The suitability of a radionuclide for determining the

erosion rate of a rock horizon is determined by the

condition that l and me should be of the same order;

in other words Teff and the mean half-life of the radionuclide, l, should be of the same order. If l ) me, the

radionuclide would be built to its secular equilibrium

value (Teff ¼ 1/l) and no information can be obtained

from its study about the erosion rate. Else if me ) 1,

the nuclide concentrations would build up to the same

concentration independent of the half-life of the radionuclide. In this case, the nuclide decay during the

build-up period is unimportant and the nuclide behaves

similar to a stable nuclide. Also, in this case (me ) 1),

the apparent surface exposure age, Teff ¼ 1/me, i.e. it is

the time during which erosion removes a rock depth

equivalent to one absorption mean free path for cosmic

rays, 1/m, ~50 cm in common rocks.

Under steady state, the nuclide build up is shown in

Fig. 24.3 for five radionuclides, 39Ar, 14C, 36Cl, 26Al

and 10Be, and a stable nuclide. These curves are

dependent on the values of e and l, and provide useful

insight if the exposure history of the rock deviates

from the assumed steady state erosion (24.10).

If two radionuclides are used, e.g., 10Be and 26Al,

their steady state concentrations are well described for

all erosion rates by the shadowed curve in Fig. 24.4,

labelled “steady state erosion island”, The temporal

evolution of the curves is given by the following:

Ratio ð26 Al =10 Be ịt

If the rock had been undergoing steady state erosion for periods of the order of !(4–5) times Teff, it

P 0ị

N 0; Tị

t l

ỵm eị

P0ị26 Al l10 Be ỵ m e 1 e 26 Al

ị (24.13)






P0ị10 Be l26 Al ỵ m e 1 e 10 Be

24 Using Cosmogenic Radionuclides for the Determination of Effective Surface Exposure Age


Fig. 24.3 Effective steady state surface exposure ages, Teff (24.6), as a function of erosion rates for five radionuclides, 39Ar, 14C,


Cl, 26Al and 10Be, and a stable nuclide

with the ratio lying between two limits:

Ratio at


Ratio, 26AI/ 10Be



Steady State



Pð0Þ26 Al

Pð0Þ10 Be


Pð0Þ26 Al l10 Be


Pð0Þ10 Be l26 Al


corresponding to large and small erosion rates,


(b) Complex cosmic ray irradiation histories




Be = 1 atom g–1 yr–1

26AI = 1 atom g–1 yr–1






Be concentration (atoms /gm)

Fig. 24.4 Theoretically calculated steady state build up of


Al/10Be ratio and 10Be concentration for hypothetical production rates of 1 atom gÀ1 rock for both 10Be and 26Al. The steady

state erosion island includes the temporal build up of radionuclide concentrations as given by (24.9) for all erosion rates. In

steady state all regions above the shaded curve are forbidden

If the exposure history of the rock is simple as so far

considered: a flat rock surface, eroding at a constant

rate, curves like that shown in Fig. 24.4 for 26Al and


Be can be used to ascertain its veracity. One can list

a number of complex rock irradiation histories: for

instance (1) rock’s of different geometrical shapes,

(2) the top surface spalls off sometime in the past

durting irradiation, and (3) the top surface is buried

under a layer of sediment. The first case has been

considered in some detail by Lal and Chen (2005),

who have cosidered a sphroidal target, a rectangular

geometry with a flat top, a sloping surface and a beach

terrace with a flat top. The second case of a rock


D. Lal

Forbidden Zone


2 x 10 5



Ratio, 28AI/ 10Be

5 x 10 5


10 6


2 x 10 6




10 Be


concentration (atoms / gm)

Fig. 24.5 The upper bound of the curve corresponds to temporal evolution for the case of e ¼ 0, as also in Fig. 24.4. The

straight lines denote the path traced by the rock surface after the

surface spalling of has been considered in detail by Lal

(1991), who have considered two cases as examples:

the effective exposure age of the rock is (1) small

compared to the mean life of the radionuclide, and

(2) comparable to the mean life. The third case was

in fact discovered in a real case of a soil profile based

on studies of 14C and 10Be (Lal et al. 1996). Reference

is also made to Miller et al. (2006), who studied ice

sheet erosion and complex exposure histories in Bafin

Island, Arctic Canada, based on 14C, 26Al and 10Be.

We will not consider here the cases of complex

rock geometries since these have been dealt with in

detail by Lal and Chen (2005), and also the case (2) of

rock spall during cosmic ray exposure (Lal 1991). The

third case is an interesting one since in some cases the

rock surface may be shielded by a thick sediment or

rock layer. In this case, if the shielding of rock surface

is substantial, the shorter lived radioisotope would

decay much faster than the longer lived radionuclide

(Lal 1991), providing an information on the time for

which the rock surface was shielded. As a case in point

we consider the special case of a rock surface which

was exposed under zero erosion rate evolving finally

to a ratio, 26Al/10Be:



top surface was shielded by a thick sediment or rock fragments.

The production rates are fixed at 1 atom gÀ1 10Be and 26Al in the


Ratio ð26 Al =10 Be ịSteady State and eẳ0 ẳ

P0ị26 Al l10 Be


P0ị10 Be l26 Al


corresponding to the top curve in Fig. 24.5. If during

the evolution of this curve, the rock surface is overlain

by a thick deposit of sediment or rock fragments, the

ratio 26Al/10Be and the 10Be concentration will evolve

along the example straight lines due to faster decay

of 26Al compared to 10Be. These points would generally lie outside the “steady state erosion island” in

Fig. 24.4, and can be used to find out if the rock

surface was part of the time or continuously shielded

on the top surface by a thick sediment layer or rock


24.3 Future Directions

In this chapter, we have discussed that cosmic ray

labeling of erosion surfaces allows one to determine

both cosmic ray exposure ages and surface erosion

rates. Several radionuclides are produced in situ in

24 Using Cosmogenic Radionuclides for the Determination of Effective Surface Exposure Age

rocks/soils from major target elements, e.g., 39Ar, 14C,


Cl, 26Al and 10Be. This task is accomplished fairly

well using two or more radionuclides produced in situ

in the rock by cosmic rays.

The single most salient feature of cosmic ray labeling of erosion surfaces is the high sensitivity of the rate

of in situ nuclear interactions on the geometry of the

irradiation. Note also that the mean absorption distance for cosmic radiation in typical rocks is ~50 cm

only. The text in this chapter along with ideas discussed by Lal (1991) and Lal and Chen (2005) cover

most of the ideas, which allow one to put severe

constraints on the “unknown” exposure history of the

rock horizon. As an example: Use of the shorter halflife radionuclides, 39Ar, 14C and 36Cl, in conjunction

with 10Be allow one to put severe constraints on the

“unknown” rock exposure histories. This task can be

put on much firmer ground if information on the exposure history can be constrained based on geological


24.4 Conclusion

The cosmogenic nuclear method described in this

chapter, bases itself on the fact that several radionuclides are produced in situ in rocks/soils from major

target elements. As stated earlier, the single most

salient feature of cosmic ray labeling of erosion surfaces is the high sensitivity of the rate of in situ nuclear

interactions on the geometry of the irradiation, coupled with the fact that the mean absorption distance for

cosmic radiation in typical rocks is ~50 cm.

In the absence of information on the exposure

histories of the rock surfaces studied, several ideas

to constrain exposure histories were discussed by

Lal (1991) and Lal and Chen (2005); these allow one

to put severe constraints on the “unknown” exposure

history of the rock horizon. As an example, use of

the shorter half-life radionuclides, 39Ar, 14C and


Cl, in conjunction with 10Be allow one to put severe

constraints on the “unknown” rock exposure histories.

Estimation of exposure ages and erosion requires

an accurate knowledge of nuclide production rates.

Realizing the importance of determining accurate

time scales in earth sciences, several scientists have

joined a project “Cosmic ray produced nuclide systematics on Earth project (CRONUS-Earth Project)”


for improving rates of production of cosmogenic

nuclides in targets exposed to cosmic radiation under

different conditions. These improvements combined

with the steady improvements in the AMS sensitivity

(cf. Galindo-Uribarri et al. 2007) should considerably

widen the scope of applications of cosmic ray produced nuclides in geomorphology.


Bennett CL, Beukens RP, Clover MR, Gove HE et al (1977)

Radiocarbon dating using electrostatic accelerators: negative

ions provide the key. Science 198:508–510

Castagnoli G, Lal D (1980) Solar modulation effects in terrestrial production of carbon 14. Radiocarbon 22:133–158

Galindo-Uribarri A et al (2007) Pushing the limits of accelerator

mass spectrometry. Nucl Instr Meth Phys Res B 299:123–130

Gleeson LJ, Axford WI (1968) Solar modulation of galactic

cosmic rays. Astrophys J 423:426–431

Gosse JC, Phillips FM (2001) Terrestrial in situ cosmogenic

nuclides: theory and application. Quat Sci Rev 20:1475–1560

International Civil Aviation Organization (1964) Manual of the

ICAO standard atmosphere, 2nd edn. International Civil

Aviatory Organization, Montreal

Lal D (1958) Investigations of nuclear interactions produced by

cosmic rays. Ph.D. Thesis, Bombay University

Lal D (1988) In-situ produced cosmogenic isotopes in terrestrial

rocks. Ann Rev Earth Planet Sci Lett 16:355–388

Lal D (1991) Cosmic ray tagging of erosion surfaces: in situ

production rates and erosion models. Earth Planet Sci Lett


Lal D (1992) Expected secular variations in the global terrestrial

production rate of radiocarbon. In: Bard E, Broecker WS (eds)

The last deglaciation: absolute and radiocarbon chronologies.

NATO ASI series, vol 12. Springer, Berlin, pp 113–126

Lal D (2007) Cosmic ray interactions in minerals. In: Elias SA

(ed) Encyclopedia of quaternary science. Elsevier, Oxford,

pp 419–436

Lal D, Baskaran M (2011) Applications of cosmogenic-isotopes

as atmospheric tracers. In: Baskaran M (ed) Handbook of

environmental isotope geochemistry. Springer, Heidelberg

Lal D, Chen J (2005) Cosmic ray labeling of erosion surfaces II:

special cases of exposure histories of boulders, soils and

beach terraces. Earth Planet Sci Lett 236(3–4):797–813

Lal D, Peters B (1967) Cosmic ray produced radioactivity on the

earth. In: Flugge S (ed) Handbook der Physik, vol 46/2.

Springer, Berlin, pp 551–612

Lal D, Arnold JR, Nishiizumi K (1985) Geophysical records of a

tree: new application for studying geomagnetic field and

solar activity changes during the past 104 years. Meteoritics


Lal D, Pavich M, Gu ZY, Jull AJT et al (1996) Recent erosional

history of a soil profile based on cosmogenic in-situ radionuclides 14C and 10Be. In: Basu A, Hart S (eds) Earth

processes reading the isotopic code. Geophysics monograph

series, vol 95, pp 371–376


Lifton NA, Jull AJT, Quade J (2001) A new extraction technique

and production rate estimate for in situ cosmogenic 14C in

quartz. Geochim Cosmochim Acta 65:1953–1969

Lockwood JA, Webber WR (1995) An estimate of the location

of modulation boundary for E >70 MeV galactic cosmic

rays using voyager and pioneer spacecraft data. Astrophys J


Miller GH, Briner JP, Lifton NA, Finkel RC (2006) Limited icesheet erosion and complex exposure histories derived from

in situ cosmogenic 10Be, 26Al and 14C on Baffin Island,

Arctic Canada. Quat Geochronol 1:74–85

Nelson DE, Koertling RG, Stott WR (1977) Carbon-14: direct

detection at natural concentrations. Science 198:507–508

Nishiizumi K, Winterer EL, Kohl CP, Lal D et al (1989) Cosmic

ray production rates of 10Be and 26Al in quartz from glacially

polished rocks. J Geophys Res 94:17907–17916

D. Lal

Papini P, Grimani C, Stephens SA (1996) An estimate of the

secondary proton spectrum at small atmospheric depths.

Nuovo Cimento 19C(3):367–388

Randall BA, Van Allen JA (1986) Heliocentric radius of

the cosmic ray modulation boundary. Geophys Res Lett


Simunac KDC, Armstrong TP (2004) Solar cycle variations in

solar and interplanetary ions observed with interplanetary

monitoring platform. J Geophys Res 109:A10101

Stone JO (2000) Air pressure and cosmogenic isotope production. J Geophys Res 105(10):23753–23759

Stuart GM, Dunai TJ (2009) Advances in cosmogenic isotope research from CRONUS-EU. Quat Geochronol 4(6):


Tuniz C, Bird JR, Fink D, Herzog GF (1998) Accelerator mass

spectrometry. CRC Press, Washington, p 371

Chapter 25

Measuring Soil Erosion Rates Using Natural (7Be, 210Pb)

and Anthropogenic (137Cs, 239,240Pu) Radionuclides

Gerald Matisoff and Peter J. Whiting

Abstract This chapter examines the application

of natural (7Be and 210Pb) and anthropogenic fallout

radionuclides (134Cs, 137Cs, 239,240Pu) to determine

soil erosion rates. Particular attention is given to


Cs because it has been most widely used in geomorphic studies of wind and water erosion. The chapter is

organized to cover the formation and sources of these

radionuclides; how they are distributed in precipitation

and around the globe: their fate and transport in undisturbed and tilled soils; and their time scales of utility.

Also discussed are methods for soil collection, sample

preparation for 137Cs analysis by gamma spectroscopy, and the selection of standards and instrument

calibration. Details are presented on methods for calculating soil erosion, including empirical methods that

are related to the Universal Soil Loss Equation

(USLE), box models that compare 137Cs activities in

a study site to a reference site, and time dependent

methods that account for the temporal inputs of 137Cs

and precipitation induced erosion. Several examples

of recent applications, including the combination of

radionuclides with other techniques or measurements,

are presented. The chapter concludes with suggestions

for future work: the value of new methods and instrumentation to allow for greater spatial resolution of

rates and/or greater accuracy; the need to incorporate

migration of radionuclides in the time-dependent models; the opportunities to concurrently use the global

and Chernobyl signals to better understand temporal

variation soil erosion processes and rates; and the

importance of the use of these tracers to characterize

C storage and cycling.

G. Matisoff (*) and P.J. Whiting

Department of Geological Sciences, Case Western Reserve

University, Cleveland, OH 44106-7216, USA

e-mail: gerald.matisoff@case.edu; peter.whiting@case.edu

25.1 Introduction

25.1.1 Soil Erosion; Nature

of the Problem

Soil is among our most fundamental resources and soil

processes help regulate atmospheric composition and

climate. Soil anchors and sustains the vegetation that

provides sustenance for animals and humans and provides fibers and material used in everything from cotton for clothing to lumber for homes to biomass for

energy. The soil itself can be mined for key materials,

minerals and metals, and energy. The foundations of

most human structures – homes, buildings, and roads –

are built on soil. Soil and soil processes filter water,

reduce toxicity of airborne pollutants delivered to the

land surface, and store carbon and nutrients. The value

of soil in terms of ecosystem function and service has

been estimated in the hundreds of billions of dollars per

year (Pimental et al. 1995).

A comprehensive understanding of material fluxes

on the earth surface and its effects on geochemical

cycles (hydrologic, C, and N), atmospheric composition

and climate, and ocean chemistry depends upon an

understanding of soil and soil movement on the landscape including erosion, transport, and deposition. Soils

sequester C and N from the atmosphere and retain

certain metals during the weathering of rocks, but soil

erosion either moves those materials to places of longterm storage or exposes soils to greater reactivity. Soils

hold 2,300 Gt of carbon, about four times as much

carbon as is in the atmosphere (Lal 2003). It has been

suggested that if carbon on the landscape lost by erosion

is replaced by new vegetative growth, then intermediate

storage in fluvial systems of the eroded carbon represents a net removal of carbon from the atmosphere and

M. Baskaran (ed.), Handbook of Environmental Isotope Geochemistry, Advances in Isotope Geochemistry,

DOI 10.1007/978-3-642-10637-8_25, # Springer-Verlag Berlin Heidelberg 2011



may be the “missing” anthropogenic carbon (Harden

et al. 1992; Stallard 1998). Others note that oxidation of

a portion of the carbon in transport may produce

0.8–1.2 GtC per year. Thus anthropogenically enhanced

soil erosion may reinforce global warming.

Soil is moved by a variety of processes including

water (splash, sheetwash, rills), wind, ice (freezethaw, glaciers, periglacial), gravity (dry ravel, creep,

toppling, debris flows, earthflows), tillage, and bioturbation. Erosion is often accelerated by disturbance

(clearing, fire, plowing, overgrazing, compaction, or

desiccation) that disrupts soil structure and removes

vegetative covering. Oldeman (1994) estimated that

1,094 Mha (1 ha ¼ 104 m2) are affected by water

erosion and 549 Mha by wind erosion. These numbers

represent 12 and 6% of agricultural land areas, respectively. Total erosion of these areas is approximately

75 billion tons/year (Pimental et al. 1995).

The net loss of soil has both on-site and off-site

consequences as summarized by Pimental et al.

(1995). In croplands, the diminished fertility due to

topsoil erosion requires fertilization or results in

diminished yields, creates pressure to deforest new

areas as fertility of existing cropland decreases, and

results in the loss in water holding capacity of soils.

Fertilization, in turn, often has its own consequences.

Most fertilizers rely on fossil fuels to create, ship, and

apply the material and the applied fertilizer has the

potential for creating downstream water quality concerns. The additional water use required because

of diminished soil retention taxes another critical

resource. In forestlands, soil loss can change species

composition, diminish water-holding capacity, and

speed desertification. In suburban and urban areas,

soil loss can reduce the ability of soils to sustain

vegetative cover and trees helpful in addressing air,

water, heat, and sound pollution.

Fine sediments derived from erosion of soil are

disproportionately responsible for degradation of surface waters (Nelson and Logan 1983; Dong et al.

1984). Eroded soil impairs water quality (Sekely

et al. 2002; Sharpley et al. 1994; Pote et al. 1996) to

the point that drinking water supplies, aquatic environments, and opportunities for recreation are

threatened. Eroded soil often harms aquatic environments by inhibiting light penetration (Yamada and

Nakamura 2002), by siltation of rivers (Reiser 1998)

and reservoirs (Williams and Wolman 1984), by eutrophication of waterways, lakes, and seas (Rabalais et al.

G. Matisoff and P.J. Whiting

1999), and by contamination (Tarras-Wahlberg and

Lane 2003). In 2000, the US Environmental Protection

Agency reported that siltation debilitated 12% of the

stream reaches assessed by states and tribes and was

responsible for 33% of impairments to beneficial use

(USEPA 2000). In areas where wind is an important

process of erosion, the transported fine material can be

a health problem, foul equipment, and cause abrasion

requiring the repainting of structures (Lyles 1985).

History shows that civilization can collapse as the

soil resource is depleted (Montgomery 2007; Diamond

2005; Hyams 1952). Plato ascribed the poor soils of

his native Attica to erosion after land clearing and his

view of the causative factors of poor soil was shared

by Aristotle (Montgomery 2007). Loss of production

associated with soil loss and degradation ultimately

affected the stability of the Greek civilization as it did

the Romans later. Lowdermilk (1953) describes a trail

of societies from Judea to Syria to China where poor

stewardship of the land and resulting erosion led indirectly to conquest or societal discord. More recent

examples of societal dislocations (famine and migration) associated with soil erosion and land degradation

include the Dustbowl of the 1930s, the Sahel of the

1970s, and Haiti.

25.1.2 Tools for Measuring Soil Erosion

Critical to the understanding and quantification of soil

erosion are tools for its measurement. Erosion pins,

sediment accumulated in reservoirs, measured sediment concentration in streamflow, photographic techniques, and soil tracers each have their usefulness and

limitations. Sediment budgets (Dietrich and Dunne

1978) are often a basis for quantifying the various

processes and paths that move soil on the landscape

and result in local loss (erosion) and local gain (deposition) of soil.

A particularly useful tool for measuring soil erosion

is a conservative tracer of the soil particles, especially

when the tracer is relatively easy to measure. Important considerations in the use of a tracer are that the

concentration of the tracer is relatively uniform;

adsorption of the tracer to soil is strong and quick;

variation in adsorption to various sizes or mineralogic/

organic constituents is minor or can be accounted for;

and methods exist to measure the tracer.

25 Measuring Soil Erosion Rates Using Natural (7Be, 210Pb) and Anthropogenic (137Cs, 239,240Pu) Radionuclides

The best known of the tracers for estimating soil

erosion are natural and anthropogenic radionuclides.

The anthropogenic radionuclides found on the landscape were produced largely by atmospheric nuclear

bomb testing and the fallout was distributed globally.

The list of fission products is extensive, although many

of these radionuclides are too short-lived to be useful

tracers of soil erosion. Of the longer-lived fission

products, the best known is 137Cs, but the list of

other useful tracers includes 134Cs, 238,239,240Pu, and


Am as minute solid particles or sorbed to soil

particles; and 3H and 90Sr as soluble tracers. The

naturally-occurring radionuclides are produced by

various nuclear reactions, or uranium or thorium

decay chains (Porcelli and Baskaran 2011) and include


Be, 210Pb and a few others. 137Cs, 7Be, and 210Pb are

each suitable as particle tracers because they have a

global distribution, adsorb efficiently to soil particles

and thus move with soil, and are relatively easily



Cs is the most widely used radionuclide tracer for

soil erosion (Ritchie and McHenry 1990). For years,

Ritchie and Ritchie (2008) maintained a bibliography

of publications that utilized 137Cs in the study of soils

and sedimentation. Figure 25.1, redrafted from Ritchie

and Ritchie (2008) and updated here to include papers

published after December 15, 2008, illustrates how

widespread the use of 137Cs as a tracer has been. There

are a total of about 4,500 Cs references with the vast


majority of the papers following the Chernobyl accident

in 1986. In comparison, there are about 2,700 references

to the use of 210Pb in studies of soils and sedimentation.

The use of 210Pb in such studies now exceeds the use of


Cs. The use of 7Be as a tracer in the context of soils

and sediment is relatively new and has resulted in about

90 papers to date. It should be noted that substantially

less than half of the total number of papers have used the

respective tracer to quantify soil erosion.

25.1.3 Summary of Contents of Book

Chapter and the Approach Used

This chapter focuses primarily upon the use of anthropogenic and naturally-occurring radionuclides to

study soil erosion processes and to quantify rates of

soil erosion. Many other processes affect soil and

sediment transport and deposition and other radionuclides used in those studies are detailed in other chapters of this collection. Here we devote much of our

attention to the anthropogenic radionuclide 137Cs but

we also look at other radionuclide tracers in part to

show which radionuclides may be the most suitable for

given applications. Specifically, we describe the

source of the radionuclides; their characteristics, deposition, sorption, and transport; the methods of measurement; the assumptions associated with their use as

Fig. 25.1 The annual number of papers utilizing 137Cs, 210Pb, and 7Be in chronologic, geomorphic and sedimentologic studies.

The total number of 137Cs papers is now about 4,500. Modified and updated from Ritchie and Ritchie (2008)

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