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Open Journal of Geology, 2011, 1, 1-9
http://dx.doi.org/10.4236/ojg.2011.11001 Published Online April 2011 (http://www.SciRP.org/journal/ojg)
Copyright © 2011 SciRes. OJG
Distributions of Radionuclides (U & Th) and
Pedogenic Characteristics as Indicators of Wet
and Warm Climate during the Holocene in the
Western Part of the Upper Gangetic
Ahmedabad, Indian Institute of Technology Roorkee
Received January 3, 2011; revised February 7, 2011; accepted March 9, 2011
Distribution of radionuclides in the soil samples, Infra-red stimulated luminescence dating techniques, elec-
trical conductivity, pH measurements and grain size analysis of soils of the region between the Ganga and
Yamuna Rivers (in the Upper Gangetic plain) have been studied. Soil characteristics are highly sensitive to
climate changes and the degree of soil development indicated by higher thicknesses of A-Horizons, solum
and clay accumulation in b-horizon are higher during the periods 1.7 - 3.6 ka and 6.5 - 9.6 ka, marked by wet
and warm climates (inferred from earlier studies), the former period being marked by higher degree of soil
development than the later. Radionuclides are significantly in higher amounts in soils developed during the
period 1.7 - 3.6 ka, thus indicating that this was the wettest and warmest period, so these radionuclides could
be released by weathering of primary rocks and be preserved in sedimentary rocks deposited during that pe-
Keywords: Climate, Radionuclides, Gangetic Plain, Pedogenic Processes and India
Reconstruction of paleoclimates in the Gangetic Plains
has been attempted by several researchers using various
techniques like study of clay minerals (Srivastava et al.,
2003; Kumar et al., 1996; Pal et al., 1989; Srivastava et al.,
1998), calcretes (Singh, 2001; Andrews et al., 1998),
oxygen isotopes and organic remains (Sharma et al.,
2004; Singh et al., 1999, Singh et al., 1974), lacustrine
deposits (Sinha et al., 2006) and studies of the vertical
sections exposed along the rivers (Sinha et al., 2005).
This is the first attempt to reconstruct the past climates
by the use of elemental distribution of radionuclides
(Uranium and Thorium) and soil morphological aspects
like solum thickness and texture of soils like relative clay
accumulation in B-horizon, electrical conductivities and
pH values of soils of different ages. The study area cho-
sen is the Ganga-Yamuna Interfluve (locally called as
Doab) in the Upper Gangetic plains (Figure 1).
Indian Remote Sensing Satellite (IRS-1D): Wide Field
Sensors (WiFS, resolution: 188 m) and Linear Image
Scanning Sensor (LISS-III, resolution 23.5 m) digital
data were used to identify different soil-geomorphic units
in the laboratory using the elements of image interpreta-
tion as described by Gupta (2003). 33 soil-geomorphic
units were identified and mapped.
For ground truthing the laboratory generated data and
to collect samples for various laboratory analyses de-
tailed fieldwork was carried out. At least two soil pro-
files from each soil-geomorphic unit were studied in de-
tail (Figure 2). Samples from C- or BC-horizons of soil
profiles were taken for dating. The reasons for such type
of sampling are 1) C-horizons are close representatives
of the parent materials, 2) C-horizons are least affected
by the pedogenic and organic processes and 3) contami-
nation through clay translocation and leaching is minimal
Copyright © 2011 SciRes. OJG
Figure 1. Study area map.
in this horizon. Also, samples from different horizons of
one profile in a unit were collected for finding variation
of radionuclides in an entire soil profile. In the laboratory
samples from C-horizons were dated using Optically
Stimulated Luminescence Technique (OSL). Concentra-
tion of Uranium, Thorium and Potassium in C-horizons,
and electrical conductivity (EC) and Hydrogen ion con-
centration (pH) of samples from different horizons of
soil profiles were determined and grain size analysis
were carried out.
2.1. Optically Stimulated Dating of Soil Samples
OSL dating of the samples were carried out to know the
ages of the last exposure of the sediments to day light
before present. Soil samples were collected in 5 cm (di-
ameter) X 15 cm (length) metal cylinders.
All laboratory procedures were carried out under sub-
dued red light conditions. 1 inch of the exposed sample
from both ends of the metal cylinder was removed. De-
tails of the procedures of the dating are given by Aitken
(1985, 1998). In the present study, optically stimulated
luminescence (infrared stimulated luminescence, IRSL)
was used, as it uses charges, which are removed rapidly
on exposure to sun (Godfrey-Smith et al., 1988, Hutt et al.,
1988). The sediments being dated are fluvial in nature
that could have been exposed to the Sunlight for short
periods during their transport. Feasibility study of OSL
dating of Gangetic alluvium was carried out by Rao et al.
Samples were treated with 1 N hydrochloric acid, hy-
drogen peroxide (6% - 30%) and 0.01 N sodium oxalate,
in order to remove carbonates, organic matter and to dis-
perse individual grains, respectively. The fine-grain frac-
tion (4-11 m) was collected using Stokes’ law in ace-
tone medium and it was deposited on 40 aluminum discs.
Equivalent dose was determined by additive dose method
(Aitken, 1998). Measurements were carried out on Day-
break version 1100 TL/OSL reader with infra-red (880
80 nm) light stimulation and luminescence detection was
through 7 mm Schott BG-39 and Corning 7-59 filters.
The aliquots were short-shined to determine the nor-
malization values. All the aliquots are divided in five
groups with four aliquots in each group. From one group,
the natural OSL signal is measured. The other groups are
given known amount of increased radiation doses on top
of their natural dose, using a calibrated source in the
laboratory. The aliquots were pre-heated to 220˚C for 0.5
seconds to remove unstable signals and given a range of
beta doses. Main signals were read after shining source
on a particular aliquot for 99 seconds to reduce the signal
to its residual value. Signals read were normalized with
short-shine values to remove disc-to-disc scatter. Growth
curves were plotted by fitting linear line to luminescence
vs. applied beta doses. Extrapolation of these curves to
Figure 2. Map showing sample locations.
Copyright © 2011 SciRes. OJG
residual value gave the equivalent dose. A plot of
equivalent dose vs. shine-down time representing expo-
sure energy was derived from ‘shine plateau‘. This rep-
resentative plateau value is used for calculating the pa-
leodoses. Rough estimates of water content were made
by taking weight a sample with moisture and without
moisture. The relative alpha efficiency for IRSL is taken
The concentrations of uranium (238U) and thorium
(232Th) were measured using thick source alpha counters.
The sample was crushed to about 10 µm grain size and
then evenly spread in a thick layer on a ZnS screen
placed at the base of a circular perpex container. Equal
counts were assumed for 238U - 232Th series and the series
were also assumed to be in radioactive equilibrium. All
the measurements were done using Daybreak 583 alpha
counters. Background was measured before measuring
each sample and background values were then subtracted
from the signal from the sample (Table 1).
For estimating potassium (40K) 0.5 mg of sample was
taken, powdered and digested using the standard proce-
dures of soil sample digestion. 40K concentration was
determined by using Atomic Absorption Spectrometry
2.2. Neutron Activation Analysis (NAA)
A general procedure for elemental analysis of soil sam-
ples involves several distinct stages, viz. sampling, sam-
ple preparation and analytical procedure. We have
adapted two methodologies NAA and AAS (especially to
determine the percentage of potassium in the samples).
In case of NAA, it involves packing, irradiation, activity
measurements, and calculations of results and data inter-
pretation. On the other hand AAS involves sample dis-
solution, instrumental measurements followed by data
Samples in NAA or AAS cannot be analyzed on site
as in case of multi channel portable Gamma Spectrome-
try. Hence their storage is also essential though it may be
for a short period. In general, sample must not be con-
taminated by any chemical treatment or destabilized
Table 1. Concentration of uranium and thorium in the soil samples.
Sample No. b-Horizon
Thickness (cm) Clay Accumulation IndexPedogenic Clay
238U Concentration (ppm)
1 17 153.85 9.05 5.50 9.90
2 30 286.5 9.55 6.64 16.77
3 40 1063.68 26.59 2.42 16.91
4 54 1377.49 25.51 8.36 25.42
5 67 1459.39 21.78 14.08 30.85
6 87 2233.55 25.67 7.00 15.06
7 72 1750.61 24.31 6.70 9.75
8 55 604.28 10.99 4.72 14.49
9 40 158.31 3.96 7.61 8.77
10 52 219.49 4.22 6.58 18.70
11 30 408.57 13.62 3.42 7.59
12 56 2009.95 35.89 6.81 14.14
13 80 2812.32 35.15 7.45 19.87
14 30 663.72 22.12 4.16 18.10
15 40 1522.36 38.06 7.74 18.16
16 46 1043.14 22.68 7.74 11.65
17 58 1663.61 28.68 7.55 11.81
18 100 1152.10 11.52 5.37 15.91
19 38 262.38 6.90 6.25 11.50
20 31 187.44 6.05 2.62 14.03
21 41 212.29 5.18 8.06 16.40
22 23 322.23 14.01 4.08 11.50
Copyright © 2011 SciRes. OJG
during sample handling and storage. Soil samples were
thoroughly mixed and small clusters if present are broken
manually; roots, organic remains, calcretes and Fe-Mn
nodules, if present, are taken out completely. Samples
were then air dried or dried in oven and small part of it is
taken, finely crushed to less than 2 µm size and then the
sample is stored for further processing.
Perhaps the simplest quality assurance procedure in
NAA is to include two or more Standard/Certified Ref-
erence Material (SRM) in each batch of unknown sam-
ples for short or long irradiation. Das (1990) has recom-
mended the simultaneous analysis of two or more SRMs
preferably of similar matrix but from different agencies
and known analysis by different techniques. In all our
studies, we have used at least three different SRMs all
having similar matrix from different agencies (Table 2).
2.2.1. NAA Methodology
All the powdered samples and standards each of 50 mg
were accurately weighed and packed in alkathene (pure
form of polypropylene) for long-term irradiation of 6 hrs.
Long (6h) irradiation at thermal neutron flux of 1012 n
-1 in APSARA reactor at the Bhabha Atomic Re-
search Centre (BARC), Mumbai, India. Activities due to
153Sm, 121Eu, 82Br, 117Lu, 239U, 233Th, 140La, 24Na, 51Cr,
181Hf, 137Cs, 46Sc, 59Fe, 40K and 42K were measured using
an 80 cm3 coaxial High Precision Germanium (HPGe)
detector (EG & G ORTEC) and 8k MCA at the reactor
site and later at the Radiochemistry Laboratories of BARC.
All irradiations were carried out in APSARA/Dhruva
reactors at the BARC, Mumbai. The irradiated samples
were then brought to our laboratories in Roorkee and
-activity was measured using a NaI (Sodium Iodide)
detector having 2.2keV resolution at 1332keV of 60Co
and 8k MCA with GENIE-2000 software (Canberra,
Australia). Counting was followed after a delay of 10
days for 1, 2, 6 and 12 hours at different intervals up to 2
months. A typical -ray spectrum showing various pho-
topeaks is shown in Figure 3. Peak areas were noted and
background was subtracted to obtain net counts. Care
was taken to obtain concentrations of the maximum
number of elements from more than one counting and
thus reproducibility of data was checked. Elemental con-
tents were calculated by comparing with more than one
The spectra of irradiated samples of natural matrices
are usually complex with closely-lying multiple peaks.
The peaks show Gaussian distribution as the first contri-
bution to the photo peak in a semiconductor detector is
due to statistical fluctuations in sharing the absorbed
energy between ionization and heating the crystal net-
work. The continuum under the peak is due to Compton
effect from the -rays of higher energies and the back
Table 2. List of standard reference materials.
Geological StandardIdentification Number Source
Estuarine sediment IAEA-405 IAEA, Vienna
Pond Sediment No.2 NIES, Japan
Soil Soil-5 IAEA, Vienna
Basalts W-1 and BCR-1 USGS, USA
Figure 3 Typical -ray spectrum showing various pho-
ground. Steps involved in the computerized analysis of a
peak are smoothening of experimental data, peak search-
ing, selection of fitting intervals, peak energy calculation
and peak area calculation.
2.3. EC and pH Measurements
EC measurements of soil samples were carried out by
preparing soil:water (1:2) mixture (Jackson, 1967), using
distilled water. EC of the mixture was determined by
digital conductivity meter (R-314, Raina Instrument,
To determine pH a mixture of soil and distilled water
in the ratio of 1:2 was prepared by shaking it intermit-
tently for an hour (Jackson, 1967). pH reading for the
mixture was taken by using Lutron PH-201 digital pH
2.4. Grain Size Analysis
40 gm of sample was taken air or oven dried at low tem-
peratures. Samples were prepared by following the stan-
dard method given by Galehouse (1971). Coarse (> 50 µ)
and fine (< 50 µ, silt and clay) were separated by wet
sieving through American Society for Testing and Mate-
Copyright © 2011 SciRes. OJG
rials (A.S.T.M) Sieve Number 230 sieve. Fine fraction
was further analyzed for silt and clay contents by using
standard procedure of pipette analysis. Sand, silt and clay
percentages were calculated according to the size classi-
fication used by U.S.D.A. Soil Survey Manual (1999).
Pedogenic clay content was determined by subtracting
the percentages of clay of A or C-horizons from that of
the B-horizons (Birkeland, 1984). Clay accumulation
index (sum of increase in clay content over that of A or
C-horizon multiplied by thickness of B-sub-horizon/s,
C.A.I. index) proposed by Levine and Ciolkosz (1983)
was also calculated for typical pedons. Where trenches
were excavated up to C-Horizons, the clay percentages
were calculated taking the clay abundances of C-horizon
into account as C-horizon is least affected by pedogenic
From ages of C-horizons, soils have been categorized
into five members of a morphostratigraphic sequence
with ages < 1.7 ka (QGMS-I), 1.7 - 3.6 ka (QGMS-II),
3.6 - 6.5 ka (QGMS-III), 6.5 - 9.6 ka (QGMS-IV) and >
9.6 ka (QGMS-V).
B-horizon thickness of different pedons are higher in
the QGMS-II and IV member soils as compared to soils
of other members, as shown in bar diagrams for various
pedons (Figure 4(a)). Pedogenic clay content in QGMS-II
and QGMS-IV is higher as compared to the other mem-
bers (Figure 4(b)).
The C.A.I. calculated for the soil samples of QGMS-II
and IV show high values ranging from 1377.3.86 and
2233.55 as compared to soils of other members, where
C.A.I. ranges from 153.85 to 475.95 (Figure 4(c)).
Bar diagrams (Figure 5(a), (b) and Table 1) showing
concentration of Uranium and Thorium in different sam-
ples from five morphostratigraphic sequence members
bring out that the soils of the QGMS-II members show
higher concentrations as compared to the soils of other
members. Also, U and Th values remain almost un-
changed from horizon to horizon within a soil profile
from QGMS-II member.
The monsoonal variability during the last 18 ka inferred
by Overpeck et al. (1996), Prell and Kutzbach (1987,
1992) and Goodbread (2003) during the last 150 ka show
that the period at the beginning of the Holocene (about
10 to 11 ka) was a wet period. The periods 1.7 to 3.6 ka
(QGMS-II) (Sharma et al., 2004; Andrews et al., 1998;
Singh et al., 1974; Srivastava et al., 2003) and 6.5 to 9.6
ka (QGMS-IV) (Sharma et al., 2004; Srivastava et al.,
2003) were marked by wet, warm climate.
Our data relating to soil morphology, texture and distri-
bution of U and Th for different soil-morphostratigraphic
sequence members can be explained in terms of climatic
changes during the Holocene Period.
Studies on relation between clay content accumulation
and climate suggests that both have a linear relationship
i.e., rates of clay production increases with the increasing
moisture content and temperature and the rate is highest
in wet and warm climates (Jenny, 1985, McFadden, 1982
and Dan et al., 1982). Thus in wet, warm periods we
expect highest pedogenic contents, high solum thickness
and A-horizon thickness (indicating higher degree of soil
development). Indeed that is the case in the present study,
pedogenic clay content in B-horizons, clay accumulation
index and solum thickness of soils are the highest during
the wet and warm periods of QGMS-II and IV as com-
pared to rest of the Holocene deposits in the study area.
The QGMS-II soils are relatively more developed as
compared to QGMS-IV soils. Thickness of A-horizons
of soils of member QGMS-II (12 to 52 cm) is higher than
soils of other members (0 to 45 cm). All these features
thus indicate that probably periods of formation of
QGMS-II soils showing higher degree of soil develop-
ment was warmer and wetter than that of QGM_IV soils.
The soils belonging to the QGMS-II and IV members are
acidic in nature. An electrical conductivity measurement
of these soils shows high conductivity values ranging
from 0.041 - 0.60 mmhos/cm, which are higher than the
soils of other ages (0.013 to 0.040 mmhos/cm). Electrical
conductivities are the measure of the total dissolved sol-
ids in any medium so high values means that the period
was marked by wetter and warmer climates, which are
necessary for high rates of chemical weathering of min-
erals, leading to release to large amounts of soluble
products. It will explain acidic nature and higher electri-
cal conductivity of soils of the QGMS-II and IV depos-
ited during warm and wet periods.
As Uranium and Thorium precipitation increases when
the climatic conditions are wet and warm (Ramli et al.,
2005; Vargas et al., 1997; Miah et al., 1998; Echevarria
et al., 2001; Martinez-Aguirre et al., 1995). Distribution
of Uranium is highly controlled by the insitu soil mois-
ture and pH values of the deposition medium (water in
this case) as envisaged by Langmuir (1997), which are
high in warm and wet periods. High values of U, Th and
K in parent materials for QGMS-II are as expected. Their
low values during the period 6.5 - 9.6 ka indicates that
though this period was marked by warm, wet climate, but
it was subdued as compared to 1.7 - 3.6 ka period, so that
smaller amounts of these elements were released by
weathering of primary rocks. It supports the conclusions
reached by degree of soil development, as discussed
Copyright © 2011 SciRes. OJG
Figure 4. Variation in (a) B-horizon thickness, (b) Percentages of pedogenic clays and (c) Clay accumulation index (CAI) in
soil profiles of different members.
Copyright © 2011 SciRes. OJG
Figure 5. Bar Diagram showing variations in concentration of (a) Uranium and (b) Thorium in different samples.
Minor variation of U and Th within a profile of
QGMS-II unit (warmest and wettest) suggests that pe-
dogenesis does not redistribute these elements signifi-
cantly and values from C-horizons are representative of
the parent material.
Following conclusions can be drawn from the studies:
1) Pedogenic processes are highly sensitive to climatic
changes and warmer and wetter periods of the Early
Holocene period (6.5 - 9.6 ka) and late Holocene period
(1.7 - 3.6 ka) are marked by higher thickness of A-horizons
and solum and higher clay accumulations in B-horizons
than the other periods.
2) Concentration of Uranium and Thorium show sig-
nificantly high values in sediments deposited during rela-
tively highly wet and warm period(1.7 - 3.6 ka), though
it is not possible to provide exact climatic conditions.
This is also supported by the degree of development of
Thanks are due to Drs. A.V.R. Reddy and A.G.C. Nair,
Radiochemistry Division of BARC, Mumbai, for pro-
viding necessary facilities during short irradiation work.
Fellowship from the Ministry of Human Resource and
Development, Government of India to B.B and R.P.C. is
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