Palaeogeography, Palaeoclimatology, Palaeoecology 356–357 (2012) 27–37
Contents lists available at ScienceDirect
Palaeogeography, Palaeoclimatology, Palaeoecology
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o
Clay minerals record from Late Quaternary drill cores of the Ganga Plains and their
implications for provenance and climate change in the Himalayan foreland
D.K. Pal a,⁎, T. Bhattacharyya a, R. Sinha b, P. Srivastava c, A.S. Dasgupta b, P. Chandran a, S.K. Ray a, A. Nimje a
a
b
c
Division of Soil Resource Studies, National Bureau of Soil Survey and Land Use Planning, Amravati Road, Nagpur 440 010, India
Geosciences Laboratory, Civil Engineering Department, Indian Institute of Technology, Kanpur 208 016, India
Department of Geology, University of Delhi, Delhi 110 007, India
a r t i c l e
i n f o
Article history:
Received 14 June 2010
Received in revised form 4 May 2011
Accepted 6 May 2011
Available online 17 May 2011
Keywords:
Ganga Plains
Interfluve
Clay minerals
Climate change
Provenance
Monsoonal forcing
a b s t r a c t
This study documents the coupling of provenance and climate change over the last 100 ka manifested in clay
mineralogy of sediments from two cores (~50 m deep) in the Ganga–Yamuna interfluve in the Himalayan
Foreland Basin, India. Depth distribution of the texture and clay mineral assemblage in the two cores show
notable differences on account of pedogenesis and sediment supply over the last 100 ka. Core sediments from
the northern part of the interfluve (IITK core) are micaceous and dominated by hydroxyl-interlayered
dioctahedral low-charge smectitea (LCS) in fine clay fraction but by trioctahedral high-charge smectite (HCS)
in silt and coarse clay fractions. In contrast, core sediments from the southern part of the interfluve
(Bhognipur core) are poor in mica and both LCS and HCS are recorded in the upper 28 m of the core while the
lower part is dominantly LCS in all size fractions. The paleosols in the two cores formed in the sub-humid to
semi-arid climatic conditions resulting in clay minerals such as 1.0–1.4 nm minerals, vermiculite, HCS and
also preserved the LCS, hydroxyl-interlayered vermiculite (HIV) and pseudo-chlorite (PCh), and kaolin that
formed earlier in a humid climate. The preservation of LCS, HIV, kaolin and PCh is a clear indicator of climate
shift from humid to semi-arid in the Ganga Plains as their formation does not represent contemporary
pedogenesis in the alkaline chemical environment induced by the semi-arid climate. As the simultaneous
formation of both HCS and LCS is not possible at the expense of mica, the abundance of LCS sediments from
both the cores suggests the role of plagioclase weathering in the formation of LCS. In the upper 28 m of the
Bhognipur core, the presence of both HCS and LCS in the fine clays suggests a change in sediment provenance
from cratonic to a dominantly Himalayan source during Holocene. The climatic records inferred from the
typical clay mineral assemblages of the two interfluve cores are consistent with the Marine Isotope Stages
(MIS). The humid interglacial stages (MIS 5, 3, and 1) are marked by dominance of HIV, PCh, and LCS whereas
the dominance of HCS together with pedogenic carbonate (PC) is noted in semi-arid stages (MIS 4 and 2).
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
The Ganga Plains of the northern India constitute one of the
world's most extensive alluvial tracts traversed by large rivers such as
the Ganga and the Yamuna that are sourced in the Himalayan orogen,
as well as rivers such as the Betwa, the Chambal, the Ken and the Son
that are sourced in the central Indian Craton and many smaller rivers
sourced within the plains. These rivers are distinctive in terms of their
source area which in turn translates into distinctive hydrological and
sediment transport characteristics (Sinha and Friend, 1994). The
Ganga Plains are of great significance as they hold important clues
regarding the tectonic and climatic factors that governed the
interaction between the Himalayan orogen and the foreland. It is
important to track changes in the alluvial landscape on different time
⁎ Corresponding author. Tel.: +91 712 2500545, + 91 712 2500075.
E-mail address: paldilip2001@yahoo.com (D.K. Pal).
0031-0182/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.palaeo.2011.05.009
scales as well as their spatial variability which is a function of rainfall
variability and hinterland characteristics including tectonic regime
(Sinha et al., 2005a). For a comprehensive understanding of the plains,
multiple approaches need to be adopted that combine the studies of
modern process as well as paleo-landscape development and
sedimentation history as recorded in the alluvial stratigraphy.
Evolutionary history of most landforms (mega- and meso-scale) in
the Ganga Plains remains inadequately understood because of the
methodological difficulties associated with the study of subsurface
deposits. Such studies require a multi-disciplinary approach including
geological, geophysical, geochemical, hydrological, atmospheric, soil
and agricultural, ecological and microbiological database and
knowledge.
The chemical equilibria of clay minerals are only apparent and in
any case ephemeral in nature. It is often difficult to determine as to
which minerals are diagnostic of different climatic zones. However,
those clay minerals which occur most frequently can be considered to
have climatic significance (Tardy et al., 1973). For example, minerals
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D.K. Pal et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 356–357 (2012) 27–37
such as kaolinite often remain unaltered through subsequent changes
in climate, and therefore, may preserve a paleoclimatic record. Singer
(1980) indicated that other layered silicates at a less advanced stage
of weathering may adjust to subsequent environmental changes and
thus may lose their interpretative value for paleoclimatic signatures.
There are various processes that are capable of producing distinctive
types of clay minerals (Birkland, 1984). Several workers have also
considered minerals of intermediate weathering stage as potential
indicators of paleoclimatic changes in parts of central India and
Gangetic Plains (Pal et al., 1989; Srivastava et al., 1998; Pal et al.,
2009).
The study has attempted to reconstruct the Late Quaternary climatic
changes and source area variability in terms of sediment supply in the
Ganga–Yamuna interfluve using high resolution data on clay mineralogy of sediment samples from shallow cores (~50 m deep) covering a
time span of ~100 ka.
1.1. Study area description and methods
The study area constitutes a part of the Ganga–Yamuna interfluve
(GYI) in the Ganga Plains (Fig. 1) formed by continuous filling of the
Himalayan Foreland Basin through most of the Late Quaternary. The
Himalayan foreland is the largest foreland basin in the world consisting
of folded sedimentary sequences of the Siwaliks in the north and the
vast Ganga Plains in the south (Raiverman et al., 1983; Burbank et al.,
1993). The Ganga Basin has been filled with sediments derived from
both Himalayan as well as cratonic sources forming several kilometres
thick alluvial strata. While a predominance of the Himalayan source has
been widely suggested through the Quaternary (Burbank, 1992), Sinha
et al. (2009) showed that the contribution from the cratonic source
has also been significant during the Late Quaternary. This paper
examines the clay mineralogy of core sediments in the northern and
southern interfluves in various size fractions and attempts to establish
the pathways of the formation of these clay minerals as a function of
climatic and source area variability.
The stratigraphic framework of the study window is primarily
based on the cliff sections exposed along the river banks of the major
rivers (Sinha et al., 2002; Gibling et al., 2005; Sinha et al., 2005b)
which was later complemented by drill core studies (Sinha et al.,
2005c, 2007a, 2009). This study has utilized two available cores from
the G–Y interfluve, ~ 75 km wide as the crow flies. The core from
the northern part of the interfluve was located in the campus of the
Indian Institute of Technology Kanpur (IITK) (Fig. 1). Another core at
Bhognipur (BHOG) is located in the southern part of the interfluve
(Fig. 1). Most parts of the interfluves are monotonously flat as
reflected from the relief of only a few metres between the IITK and
Fig. 1. Study area in the Ganga–Yamuna interfluve (GYI) showing drill core at Indian Institute of Technology, Kanpur (IIT, K) and Bhognipur, Kalpi (BHOG, K).
D.K. Pal et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 356–357 (2012) 27–37
BHOG separated by ~55 km. Modern climatic condition in the study
area is marked by monsoonal regime; average annual rainfall is 550–
750 mm in IITK region and 450–700 mm in Bhognipur region. Most of
the rainfall is received in summer monsoon during June to September
(IMD, 2009).
A total of 92 sediment samples from IITK core and 62 samples from
the Bhognipur core were collected from different depths and analysed
for their texture, pH, electrical conductivity (EC), calcium carbonate
(CaCO3), organic carbon (OC), and the mineralogy of the silt, and clay
29
fractions. Detailed procedures for these analyses are given in the
supplementary material.
2. Core lithostratigraphy
The IITK core can be divided into four lithostratigraphic units
(Fig. 2a). The lowermost Unit 1 is primarily made up of deposits of
floodplain facies with occasional sand and silt beds/patches. This
unit represents a distal floodplain environment in the lower parts
Fig. 2. (a) Stratigraphy and paleosol distribution of the IITK drill core. A total of 4 major stratigraphic units and 13 paleosols were identified in this core covering a time span of
~ 100 ka. The entire core is dominated by muddy sediments with thin silt layers at regular intervals. (b) Stratigraphy and paleosol distribution of the Bhognipur drill core. A total of 6
major stratigraphic units and 10 paleosols were identified in this core. This entire core is distinctly coarser in the lower parts with N 10 m sand body representing a major channel.
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D.K. Pal et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 356–357 (2012) 27–37
(Unit 1A) which grades to a relatively less distal environment in the
upper half (Unit 1B). Unit-2 starts with a ~ 2 m thick bed of yellowish
brown micaceous fine sand with some intervening thin beds of
floodplain facies. Unit-3 is made up of repeated alternation of thin
laminae of brownish fine sand (fluvial) and yellow silt (eolian) within
floodplain facies. This unit is interpreted as a channel margin/levee
indicating further proximity to the channel than the underlying units.
The uppermost Unit-4 starts with a ~ 70 cm thick silty clay bed
containing distinct convolute laminations made of yellow silt and very
fine sand. The rest of the unit is made up of floodplain facies along
with some intervening silt beds of eolian facies. The unit has a
significantly high content of medium to large (1–3.5 cm) kankars
throughout the unit, especially near the lower boundary with Unit-3.
The IITK core represents a floodplain sequence typical of an interfluve
setting, with indications of a gradual change from distal to proximal
condition. The distal floodplain condition in the lower half of the core
led to intermittent exposure and pedogenesis of the sediments. In the
upper part of the core, proximal floodplain conditions prevailed, as
characterized by the presence of intermittent sand beds, including a
2 m thick crevasse splay deposit.
Four luminescence dates for the IITK core were reported by Sinha
et al. (2007a) and are shown in Fig. 2a. Two dates of 86 ± 7.39 ka
and ~ 63 ± 4.0 from Unit 1 correspond to MIS-5 and 4 respectively.
Two samples from the upper part of Unit 2 yielded ages of 38.7 ±
3.7 ka and 30.3 ± 3.4 ka both spanning MIS-3.
Based on varying depth functions of macro and micromorphic
features and maturity of the pedofeatures, Srivastava et al. (2010)
recognised 13 paleosols in the IITK core (Fig. 2a). Except for one Alfisol
(It12), most of the paleosols (It1–It11 and It13) are Inceptisol marked
by poorly developed pedogenic features and one by strongly
developed pedofeatures. Key micromorphic features of these paleosols are marked by weak pedality, undifferentiated to stipple speckled
b-fabric, few thin patchy clay pedofeatures, pedogenic carbonates,
and rhizocretes (Srivastava et al., 2010).
The Bhognipur core is located approx. 6 km north of the present
day Yamuna river, and is divided into six lithostratigraphic units
based on their facies association (Fig. 2b). The lowermost Unit 1 is
essentially composed of fine intercalations of floodplain and lacustrine facies with abundant kankars all through. Unit-2 is dominated by
sandy facies, and a kankar zone occurs near the bottom of this unit,
probably representing basal carbonate gravel lag. The sandy facies in
the lower part is made up of medium to coarse-grained, pink sand
devoid of any mica and resembles the modern day channel deposit of
the cratonic river Betwa, which presently flows ~50 km south of this
site. The upper part is micaceous fine sand devoid of any pink grains
(feldspar), and is similar to the present day channel deposit of
Yamuna and/or Chambal rivers. In-between the sandy beds, thin
layers of lacustrine (backswamp) facies are also present suggesting
brief interruptions in channel deposition. Unit-3 is made up of
intercalations of floodplain, lacustrine and channel facies. Unit-4 is
made up mainly of reddish brown silty clay and grades to Unit 5 with
a gradual change of colour from reddish brown below to yellowish
brown above. Unit-5 is made up of thin laminae of silty clay intervened by clayey beds. The uppermost Unit-6 is made up of a
combination of floodplain, eolian and lacustrine facies. The Bhognipur
core thus represents an interesting depositional history; this site was
initially an active channel belt of a major river which subsequently
migrated away from the site leading to deposition of proximal and
distal floodplain sediments in the upper parts. The river sand at the
bottom of the core has a strong imprint of a cratonic source (pink
coloured coarse ‘Betwa sand’), whereas the upper part has micaceous
sands typical of Himalayan-derived sediments.
Paleopedological investigations of the Bhognipur core have shown
10 paleosols in the ~ 50 m sequence of the core (Fig. 2b) and they
show different pedosedimentary characters in comparison to the
paleosols in the IITK core (Srivastava et al., 2010). The transition from
the lower feldspar rich sand to quartz and mica dominated sediments
is marked by a prominent erosional surface with extensive pedorelicts
and papules and very weak pedogenesis at 25–28 m depth. It is
overlain by several thick-cumulic paleosols with weakly developed
pedofeatures and one mature paleosol with strongly developed
pedofeatures in the upper half of the core (Srivastava et al., 2010).
3. Physical and chemical characteristics of the core sediments
The IITK core sediments along with 13 paleosols are generally finegrained, slightly to moderately alkaline, calcareous (2–24%) and nonsaline and also impoverished in organic carbon (b0.5%) (Fig. 3a, b,
and c). Texturally, core sediments are predominantly silty clay. An
increase in the clay fraction was observed at certain depth intervals
(e.g. 0.4–3.4 m, 12.0–15.0 m, 17.5–20.8 m, 29–30.5 m, 40.0–46.2 m)
that correspond to It1, It2, It6, It7, It10 and It12 paleosols with weakly
to strongly developed pedofeatures. However, an increase in clay
fraction at a couple of depth intervals (e.g. 20–25 m, 35–40 m) does
not seem to be related to pedogenesis.
The depth distribution of physical and chemical characteristics of
the Bhognipur core along with a log of paleosol intervals is presented in
Fig. 4 (a, b and c). Core samples down to the depth of about 1.5 m are
neutral to slightly alkaline in reaction representing Bh1 paleosol with
Inceptisol like characters. However, the remaining part of the core with
9 paleosols (Bh2–Bh9) having Inceptisol, Vertisol and Alfisol like
characters (Soil Survey Staff, 2003) is moderately to highly alkaline
with a very low concentration of soluble salts. The entire core with 10
paleosols is calcareous and the maximum concentration of CaCO3
(~20%) is noted in sediments between Bh1 and Bh2 paleosols at about
3.5 m, but organic carbon content is considerably low (b0.5%). The
texture of the core varies from sandy loam to silty clay with moderate to
high content of clay fraction in some of the paleosols. There is an
appreciable increase in clay fraction at depth intervals 1.55–1.86 m,
3.54–8.27 m, 8.37–10.90 m and 12.13–13.84 m that correspond to the
occurrences of Bh1, Bh2 and Bh3 paleosols (Srivastava et al., 2010). The
Bh3 paleosol occurring between 10 and 14 m depth is marked by
strongly developed argillic and vertic horizons (Soil Survey Staff, 2003)
whereas the others (Bh1 and Bh2) are marked by weakly developed
pedofeatures (Srivastava et al., 2010). The paleosols below 15 m depth
(Bh4–Bh10) are marked by a large amount (N30%) of clay that does not
appear to have any relationship with pedogenic development. These
paleosols are marked by weakly developed syn-depositional pedofeatures (Srivastava et al., 2010).
4. Mineralogical attributes of core sediments
Table 1 shows the major clay minerals identified in core sediments
and the diagnostic criteria used for their identification. The following
sections describe the results of semi-quantitative analysis of clay
mineralogy for both the cores highlighting the spatial as well as
temporal variability of clay mineralogy.
4.1. IITK core
The XRD analysis of core sediments from IITK core shows dominance of mica in the silt (50–2 μm) and coarse clay (2–0.2 μm) fractions
that also contain mixed-layer minerals, smectite, vermiculite, HIV, PCh,
kaolin, feldspar and quartz (see Figs. 1–3 in the supplementary
material). The fine clay fractions (b0.2 μm) are dominated by mica
and smectite along with vermiculite, HIV, PCh and kaolin. The smectite
is predominantly LCS and dioctahedral in nature in fine clay fraction
but silt and coarse clay fractions are dominantly HCS. The collapsing
characteristics of K-saturated fine clay on heating from 110 °C to 550 °C
indicate that most of the LCS has hydroxy-interlayering (Harward et al.,
1969).
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D.K. Pal et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 356–357 (2012) 27–37
Paleosols
a)
b)
c)
Sand
Silt
Clay
Fig. 3. Depth distribution of pH, EC, CaCO3, OC, particle size distribution and textural class of the IITK core.
The depth distribution of these minerals (Fig. 5) allows us to
distinguish four zones in the IITK core. The lowermost Zone A (base to
40 m) is characterized by relatively low mica content in all size fractions
compared to the upper zones. Both silt and coarse clay fractions show
variable but significant amount of vermiculite and b10% kaolin whereas
the fine clay fraction is high in LCS. Srivastava et al. (2010) also reported
a mature paleosol with pedogenic carbonates from this zone. Zone B
(40–25 m) is marked by a gradual decrease in LCS in fine fraction
although HCS remains high particularly in the coarse clay fraction. Mica
is low in fine clay fraction, variable in coarse clay and high in silt fraction
in this zone. This zone also marks the highest but variable contents of
vermiculite in all size fractions but kaolin remains low.
Zone C (25–15 m) marks a sharp decrease in LCS in fine clay and
also in HCS in coarse clay and silt fractions. In contrast, mica is high in
Paleosols
a)
b)
Silt
Clay
Sand
Fig. 4. Depth distribution of pH, EC, CaCO3, OC, particle size distribution and textural class of the Bhognipur core.
c)
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D.K. Pal et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 356–357 (2012) 27–37
Table 1
Identification key for the clay minerals in IITK and Bhognipur cores, Ganga–Yamuna interfluve, India.
Mineral
Diagnostic criteria
Climatic and provenance implications
Mica (biotite/muscovite)
Characteristic peak at 1.0 nm, not affected by heating or
glycolation; both biotite and muscovite present if the ratio
of 001/002 reflection of 1.0 nm of mica is more than 1.
Characteristic peak at 1.40 nm on glycolation which
decreases on heating the K-saturated samples with
concurrent increase in 1.0 nm peak of mica
(Schultz et al., 1971).
Shows a peak at ~ 1.38–1.36 nm instead of 1.42 nm
(true chlorite) on heating the K-saturated samples
at 550 °C.
Characteristic peak at 0.7 nm, no shifting on glycolation
and heating up to 300 °C and but after heating the K saturated
samples to 550 °C, 1.0 nm peak of mica gets reinforced.
Himalayan sediments rich in biotite mica derived from granitic rocks
Vermiculite (Vm)
Pedogenic chlorite (PCh)
Kaolin (Sm/K and Vm/K)
Low charge smectite (LCS)
Mixed layer minerals
(1.0–1.4 nm)
Quartz
Characteristic peak at 1.7 nm on glycolation, shifts to
1.1–1.2 nm on K-saturation and heating to 110 °C; expansion
of 1.4 nm peak on glycolation of K-saturated and heated
(300 °C) samples; expansion of Ca-saturated samples to 1.8 nm
region with glycerol vapour (Harward et al., 1969).
Characteristic peak at 1.7 nm on glycolation, collapses readily
to 1.0 nm on K-saturation and heating to 110 °C and gets
destroyed in HCl treatment
Characteristic peak at 1.4 nm on glycolation and collapses not
readily to 1.0 nm peak on heating to 110 °C but shows tailings
on the low angle side of 1.0 nm peak after heating the K-saturated
samples at 550 °C.
Characteristic peak at 1.7 nm on glycolation but collapses with
tailings to 1.0 nm peak of mica on its low angle side on heating
the K-saturated samples at 550 °C.
Generally found in region between 1.0 and 1.4 nm peak areas
on glycolation and also on heating the K-saturated samples
Characteristic peaks at 0.334 and 0.426 nm
Feldspar
Characteristic peaks at 0.318, 0.403 and 0.32 nm
High charge smectite (HCS)
Hydroxyl-interlayered
vermiculite (HIV)
Hydroxyl-interlayered
smectite (HIS)
all the size fractions and particularly in the silt fraction. Vermiculite
content decreases significantly in coarse clay and silt fractions but
remains high in the fine clay fraction. Kaolin is present in this zone but
does not exceed 10% in any size fraction. The uppermost Zone D
(15 m–top) shows high mica content in all size fractions but very
significant drop in HCS in both silt and coarse clay fractions. The fine
clay fraction shows significant amount of LCS in this zone. This zone
also marks a distinct zone of high vermiculite in silt fraction but a
corresponding decrease in coarse clay fraction. The fine clay fraction
shows an extremely variable content of vermiculite throughout this
zone. This is also the zone in which kaolin content frequently exceeds
10% particularly in silt fraction in the upper parts of the core.
4.2. Bhognipur core
In the Bhognipur core, the mineral assemblage of the silt (50–
2 μm) fraction is marked by mica, mixed-layer minerals (1.0–1.4 nm),
vermiculite, PCh, HIV and smectite, kaolin, quartz and feldspars (see
Fig. 1 in the supplementary material). Semi-quantitative estimates of
these minerals indicate that none of the layer-silicate mineral was
dominant (N50%) in this fraction. The mica consists of both muscovite
and biotite as evident from the ratio of 001/002 reflection of 1.0 nm
peak, which is more than unity (Pal, 2003). In the X-ray diffractograms, both trioctahedral HCS and dioctahedral LCS are recorded (see
Table 1 for the criteria used for their identification). The LCS also
shows partial hydroxy interlayering because on heating to 550 °C the
K-saturated smectites collapsed to 1.0 nm peak but with a broadening
at the low angle side of 1.0 nm peak.
The pseudo-chlorite (PCh) (Table 1) described here is not a true
chlorite showing a peak at ~ 1.38–1.36 nm instead of 1.42 nm, on
heating K-saturated samples at 550 °C. The mineral assemblage in the
Weathering product of biotite mica and their content is
more in granitic rocks
Continued weathering of vermiculite under humid
conditions (through formation of HIV)
Forms from weathering of vermiculite and smectite through
hydroxy-interlayering in acidic conditions induced by humid
climate. It remains as interstratified smectite/vermiculite–kaolinite
stage, mostly as remnant in arid and semi-arid environments.
First weathering product from plagioclase feldspar under humid
conditions; can be derived from both Himalayan and cratonic sources
but a larger amount may suggest a dominant cratonic source
Derived from weathering of biotite under arid and semi-arid climate;
indicates a dominantly Himalayan source
Generally occurs in acidic soil conditions induced by humid climate
–do–
Forms when micas start weathering in ambient conditions
Mainly in detrital fraction; Himalayan source rich in quartz
compared to cratonic source
Mainly in detrital fraction; cratonic source rich in plagioclase
feldspar compared to Himalayan source.
coarse clay (2–0.2 μm) fraction is similar to those in the silt fraction.
Both HCS and LCS are present along with variable contents of mica and
kaolin which is stratified with hydroxyl-interlayed minerals (Sm/K
and Vm/K, see Table 1). This situation is similar to that of kaolin in the
silt fraction, suggesting this to be a part of the sediment. The fine clay
fractions (b0.2 μm) contain mica, vermiculite, HIV, PCh, smectite and
kaolin.
The depth distribution of mica and smectites allows us to divide
the Bhognipur core into five distinct zones (Fig. 6). The lowermost
Zone A (base to 35 m) has rather limited data points but this is
generally low in mica and LCS constitutes the most dominant clay
mineral (N60%) in all the size fractions. Coarse clay fraction has some
HCS as well. Significant amount of vermiculite (up to 20%) is noted in
this zone but kaolin content is low (b10%) in all the size fractions. The
overlying Zone B (35–28 m) is marked by a sharp increase in LCS
content constituting N90% of the total clay mineral assemblage in the
fine clay fraction. Mica increases slightly in this zone in all the size
fractions. Only silt fraction has some vermiculite (b10%) in this zone
and kaolin remains low in all the size fractions.
Zone C (28–19 m) marks a significant change in the type of
smectite from LCS to a mixture of LCS and HCS in silt and coarse clay
fractions (N20% and N60% respectively) but the fine clay fraction is
still dominated by LCS (N90%) in this zone. Mica follows an increasing
trend particularly in silt fraction. Vermiculite content picks up again in
this zone particularly in the coarse clay fraction (N20%). Kaolin starts
increasing in the silt and the coarse clay fraction but the fine clay
fraction still has b5% kaolin. Zone D (19–2.8 m) is characterized by a
sharp decrease in smectite in all the size fractions. In silt fractions, LCS
is less than 10% and the coarse clay fraction shows a drop from 60% to
~30% at the top of the zone. The fine clay fraction is still dominated by
LCS but a drop from 90% to ~50% is noted towards the top. In contrast,
D.K. Pal et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 356–357 (2012) 27–37
HCS
LCS
33
Zone D
High Mica in all fractions, HCS decreases in silt, LCS
increases in FC; Vm significant in FC and silt; PCh
significant in FC
HCS
HCS
HCS
HCS
LCS
LCS
Zone C
Mica increases in all fractions; Sharp decrease in LCS in
FC and HCS in silt and CC; PChin silt and CC
Zone B
Silt and CC high in Mica and HCS; FC low in mica and
high in LCS; PChin CC; Vm significant in FC
HCS
LCS
HCS
Zone A
Low in Mica, Silt and CC high in HCS; FC high in LCS; Vm
significant in Silt and CC
HCS
Silt
Coarse clay (CC)
Fine clay (FC)
Fig. 5. Depth distribution of mica, vermiculite (Vm), pseudo-chlorite (PCh), smectite (LCS and HCS), and kaolin in the silt, coarse clay (CC) and fine clay (FC) fraction of the IITK core.
mica content remains unchanged from the lower zone in all size
fractions except a few isolated peaks. Vermiculite content is variable
in this zone in all size fractions. Kaolin rises gradually in silt and coarse
clay fractions (~15–20%). Srivastava et al. (2010) reported a mature
paleosol with pedogenic carbonates from this zone. The uppermost
Zone E (2.8 m–top) shows a sharp rise in mica content particularly in
the coarse clay fraction (up to 50%). Smectites in silt and coarse clay
fractions are of HCS type whereas those in the fine fractions are both
HCS and LCS. Vermiculite is present in significant amount in this zone
and kaolin shows a distinct enrichment in the fine clay fraction but
decreases in the coarse clay and silt fractions.
5. Discussion
5.1. Systematic changes in physical and chemical properties in the
two cores
The paleosols in both cores are slightly to moderately alkaline,
calcareous, non-saline and impoverished in organic carbon but their
textural classes are different. The Bhognipur core is primarily sandy in
lower parts and silty clay loam in upper parts whereas the IITK core is
dominantly silty loam. At certain depths, an increase in the clay
content is indicative of an argillic horizon formed by clay illuviation
(Soil Survey Staff, 2003), suggesting a phase of landscape stabilization
during which pedogenic processes operated. Some of these intervals
correspond to mature paleosol layers in the cores inferred from
the micromorphological investigations (Srivastava et al., 2010).
These mature paleosols of the cores are similar to modern Alfisols of
the Ganga Plains with a soil-forming interval of 8000–13,500 years
(Srivastava et al., 2010). The absence of argillic horizon along with
increase in clay content suggests the occurrence of detrital clays or
paleosols with low maturity developed during short pedogenic
intervals. These paleosols of low maturity are similar to the modern
Inceptisols of the Ganga Plains with soil forming intervals of 500–
2500 years (Srivastava et al., 2010).
Further, the impoverishment of organic carbon, calcareousness of
the sediments, and alkaline pH suggest that both the cores experienced dry sub-humid climate during the post-depositional period
(Pal et al., 2009). Such climatic conditions induce the precipitation of
CaCO3 causing depletion of Ca + 2 ions in sediment solution and the rise
in pH (Pal et al., 2003a, 2009) facilitates the illuviation of clay, thus
creating the most conducive conditions for the formation of argillic
horizon (Eswaran and Sys, 1979; Pal et al., 2003b). Presence of
pedogenic CaCO3 (PC) along with rare occurrence of non-pedogenic
CaCO3 (NPC), which is generally considered a pedorelict feature
(Brewer, 1976), provides evidence of the prevalence of the dry subhumid climate during the post-depositional period (Pal et al., 2009).
The mature paleosols with argillic horizons at 10 to 14 m depth of the
Bhognipur core and also at 40 to 45 m depth of IITK core show higher
amount of CaCO3 than the overlying and the underlying layers (Fig. 2a
and b), suggesting a drier phase within the dry sub-humid climatic
conditions.
5.2. Clay minerals as evidence of provenance changes
Depth distribution of clay minerals can be used to infer the
sediment provenance and its variability through time. As discussed
earlier, the parts of the interfluve under investigations have been filled
34
D.K. Pal et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 356–357 (2012) 27–37
HCS
HCS
HCS
(+LCS)
Zone E: Mica rises; HCS dominant in silt and CC;
HCS + LCS in FC; Kaolin rises in FC
LCS
(+HCS)
LCS
(+HCS)
LCS
(+HCS)
LCS
(+HCS)
LCS
LCS
(+HCS)
LCS
LCS
LCS
(+HCS)
LCS
LCS
Zone D: LCS decreases in all fractions; HCS and
kaolin significant in silt and CC; PCh significant in
all fractions
Zone C: LCS dominant in all fractions; HCS
significant in silt and CC; Vm significant in CC;
Kaolin rises in silt and CC
Zone B: LCS very high in FC; Vmlow and present
in silt fraction only
Zone A: Low in Mica; LCS dominant in all size
fractions + some HCS in CC; Vm significant; Kaolin low
LCS
Silt
Coarse clay (CC) Fine clay (FC)
Fig. 6. Depth distribution of vermiculite (Vm), pseudo-chlorite (PCh), smectite (LCS and HCS), and kaolin in the silt, coarse clay (CC) and fine clay (FC) fraction of the Bhognipur core.
by sediments from both Himalayan as well as cratonic sources, which
are quite distinctive in terms of their clay mineralogy. While the
Himalayan source is dominantly micaceous and consists of metamorphic and granitic rocks, the cratonic source, represented by Deccan
Basalt and Vindhyans is characterized by dioctahedral smectites (LCS)
which form as the first weathering product of plagioclase (Pal and
Deshpande, 1987). In contrast, the smectites in the Himalayan source,
if present, are generally trioctahedral (HCS) formed from the weathering of biotites (Srivastava et al., 1998).
The IITK core is highly micaceous throughout in contrast to the
Bhognipur core, which is poor in mica particularly in the lower parts of
the core (N10 m depth). The LCS dominates in fine clay fraction of the
IITK core and HCS in silt and coarse clay fractions throughout the core.
In the Bhognipur core, the presence of both HCS and LCS in the upper
28 m depth and almost exclusive presence of LCS beyond this depth
suggests that the sediment supply at Bhognipur changed from a
dominantly cratonic source (characterized by low mica and high LCS)
in the lower part to a Himalayan source (characterized by higher mica
and HCS) in the upper part. This interpretation is based on the fact that
the formation of such a large amount of smectite at the expense of mica
in semi-arid climate is improbable (Pal et al., 1989; Bhattacharyya et al.,
1993; Ray et al., 2006; Pal et al., 2009).
Further, as the biotite mica cannot yield both trioctahedral HCS
and dioctahedral LCS simultaneously, the genesis of LCS and its
accumulation in these two cores needs to be viewed in terms of the
influence of weathering products of the plagioclase present in both
Deccan basalt of Central Indian Craton (Pal and Deshpande, 1987) and
micaceous Himalayan river sediment. This smectite must have been
carried through the cratonic (Betwa, Chambal, Ken and Son) as well as
the Himalayan (Ganga–Yamuna) rivers to Bhognipur and Kanpur
areas in the past.
Low amount of kaolin in the fine clay fraction in the Bhognipur core
suggests that it may not have formed during the post-depositional
period but may have resulted due to physical communition of larger
kaolin particles present in the coarse silt fractions derived from the
Deccan basalt of the Craton. Unlike large amount of interstratified
(Sm/K) minerals reported from the Holocene soils of the Ganga Plains
(Srivastava et al., 1998), the Bhognipur core sediments and the
paleosols show only a small amount of Sm/K. It is likely that the Sm/K
formed along with the LCS during the weathering of the Deccan basalt
under humid climatic condition (Pal and Deshpande, 1987) and was
transported to the Ganga Plains over a short period of time.
5.3. Pathways of clay mineral transformations and implications for
climate changes
Although the chronological data for these cores is very limited, an
attempt has been made to associate transitions in clay mineralogical
assemblages through the cores to major climatic events during the
last ~100 ka. The mineralogical composition of core sediments shows
significant variability in different size fractions and in depth
distribution. For example, the contents of mica and vermiculite are
generally higher in silt fraction than in fine clay fractions whereas that
of smectite is higher in fine clay fraction. Further, the Bhognipur core
shows the presence of trioctahedral HCS in the silt and coarse clay
fractions in the upper 28 m (Fig. 6) below which the dominance of LCS
is reported. On the other hand, the IITK core has predominantly
trioctahedral HCS in both silt and coarse clay fractions all through the
core (Fig. 5). The fine clay fractions of both the cores contain
predominantly LCS with low amount of HCS (Figs. 5 and 6).
A common feature in both the IITK and Bhognipur cores is
therefore the increasing amount of HCS from the silt to the clay
D.K. Pal et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 356–357 (2012) 27–37
fractions at the expense of mica and vermiculite. This suggests early
stages of weathering of biotite to mixed-layered minerals containing
vermiculite layers. As more interlayer regions were affected by
weathering there was a progressive formation of vermiculite and HCS.
Previous studies (Pal, 2003) indicated that the HCS of trioctahedral
type is generally an alteration product of biotite mica even in presence
of muscovite and can be regarded as an alteration product in semiarid and arid climate. Micromorphological studies on pedality and
formation of argillans can complement this observation. In some core
samples where clay illuviation is observed, major alteration of biotite
to vermiculite and smectite presumably occurred during the post-depositional period.
In the Bhognipur core, the dominance of LCS along with some HCS
below 28 m depth (Zones A and B) indicates that the source of LCS
may not be the dioctahedral muscovite mica because the weathering
of muscovite is very sensitive to potassium in soil solution. Biotite
releases considerable amount of K in soil solution, and as a result,
weathering of muscovite is inhibited (Pal, 2003). Thus, the formation
of HCS and LCS simultaneously from mica is very unlikely (Pal et al.,
1989; Ray et al., 2006). Moreover, in dry environments of sub-humid
and semi-arid climate that facilitates the formation of CaCO3 from
plagioclase (Srivastava et al., 2002), micas may not yield so much LCS
as observed in both the cores (Pal et al., 2009). We therefore suggest
that the large amount of LCS formed in an earlier humid climate in the
source area as an alteration product of plagioclase (Pal et al., 1989).
Plagioclase feldspars are present in both the Deccan basalt of the
Central Indian Craton and also in the Himalayan hinterland, which
contain sufficient silica to form smectite in tropical humid climates
(Tardy et al., 1973; Pal et al., 1989; Srivastava et al., 1998).
Further, the LCS also dominates in the fine clay fraction in the IITK
core but its content (25–75%) is less than that in the Bhognipur core
(30–95%). This is more explicit when the LCS content in the fine clay
fraction is calculated on fine earth basis (b2 mm) as Bhognipur core
sediments contain more amount of clay than the IITK core. This is due
to the impoverishment of plagioclase in the Himalayan rocks as
compared to the Deccan Basalt. On the other hand, the formation of
smectite from biotite is quite unlikely in humid climate because in this
climate silica and cations are removed thereby inducing separation of
layers precluding the formation of clay minerals more siliceous than
biotite itself (Tardy et al., 1973). However, during this weathering
some amount of vermiculite could have transformed to HIV, which
may have further weathered to form PCh because hydroxy-interlayering
in vermiculite and smectite generally occurs in acidic soil conditions
(Jackson, 1964) under humid climate. Thus, the HCS existing in arid
climate is the alteration product of biotite, which survived earlier
weathering. The aridity appears to have reduced leaching and helped
concentrate silica in solution for the formation of HCS. However, the
formation of smectite did not continue in humid tropical climate as
evidenced from the presence of very small amount of kaolin (Sm/K) in
the fine clay fractions of Bhognipur core (Fig. 6), and also from the
absence of kaolin (Sm/K) in the fine clay fractions of IIT K core (Fig. 5).
In the event of prolonged weathering of smectite in humid climate, the
content of kaolin (Sm/K) should have been dominant (Bhattacharyya
et al., 1993).
The presence of HIS, HIV and PCh in the fine clay fractions, and HIV
and PCh in the silt and coarse clay fractions indicates that the
hydroxy-interlayering in the vermiculite and smectite interlayers did
occur when positively charged hydroxy-interlayer materials such as
[Fe3(OH)6] 3+, [Al6(OH)15] 3+, [Mg2Al(OH)6] +, [Al3(OH)4] 5+, etc.
(Barnhisel and Bertsch, 1989) entered into the inter-layer spaces at
pH much below 8.3 (Jackson, 1964). Moderately acidic conditions
are optimal for hydroxy-Al interlayering of vermiculite and smectite and
the optimum pH for interlayering in smectite and vermiculite is 5.0–6.0
and 4.5–5.0 respectively (Rich, 1968) as small hydroxyl ions are most
likely to be produced at low pH (Rich, 1960). The pH of the sediments
from both the cores is well above 7.6 throughout suggesting mildly to
35
moderately alkaline conditions under which 2:1 layer silicates suffer
congruent dissolution (Pal, 1985) discounting hydroxy-interlayering of
smectites during the post depositional period. The hydroxy-interlayers
in vermiculite and smectite and the subsequent transformation of
vermiculite to PCh therefore do not represent contemporary pedogenesis in the prevailing dry climatic conditions. However, the crystallinity
of LCS is being preserved in the non-leaching environment of the arid
climatic conditions (Pal et al., 2009). This suggests that the presence of
HIS and also HIV, and PCh in arid and semi-arid climatic environments
could be used as an indicator of climate change from humid to arid (Pal
et al., 2009). The alkaline chemical reaction, formation of CaCO3,
formation of trioctahedral HCS, preservation of HIS, HIV and PCh,
indicate the role of climate change from humid to arid during the
development of soils within the cores.
5.4. A model for clay mineral formation and transformation in interfluve
sequences of the Ganga Plains in response to climate change over the last
100 ka
In view of clay mineral distribution in the two interfluve cores that
is related to sedimentation and pedogenesis in Ganga Plains over the
last 100 ka, a summarized account of clay mineral formation and their
transformation in response to the climatic changes during this period
is proposed for three major climatic transitions namely, Marine
Isotope Stage (MIS) 5–4, 3–2 and 2–1 (Fig. 7).
(a) MIS 5–4 transition:
The available climatic reconstructions (Prell and Kutzbach,
1987; Clemens and Prell, 2003) and earlier studies in this
region (Gibling et al., 2005; Sinha et al., 2007b; Gibling et al.,
2008) suggest that the MIS 5–4 transition is marked by weakening of the SW monsoon after a prolonged humid phase
during MIS 5. The lower part of the IITK core is marked by the
transformation [Biotite → HIV → PCh] as the dominant process
and [Plagioclase → LCS] as the sub-dominant process in a
relatively humid climate and slightly acidic pedogenic environment (Fig. 7a). Later, the semi-arid climate in MIS 4 was
dominated by transformations [Biotite→ HCS] and [Plagioclase →
Pedogenic carbonate (PC)] in moderate to slightly alkaline
pedogenic environment (Pal et al., 1989; Srivastava et al.,
2002).
In the Bhognipur core, the formation of the mature paleosol
(Bh3 paleosol, Zone D) shows [Plagioclase → LCS] as the
dominant and [Biotite → HIV → PCh] as the subdominant clay
mineral transformations indicative of humid climate and
acidic pedogenic environment. This is attributed to the
dominance of plagioclase in the Bhognipur core in contrast
to that of mica in the IITK core. In the upper part of Zone D in
the Bhognipur core, the dominant clay mineral transformations are recorded as [Plagioclase → PC] and [Biotite → HCS]
which suggest semi-arid climate and alkaline pedogenic
environment (Fig. 7a).
It is important to note however that the formation of LCS, HIV,
and PCh is not a part of contemporary pedogenesis in the
prevailing semi-arid climatic conditions. Instead, they formed
in the source area when the climate was relatively more
humid and were transported and preserved as such in the
Ganga Plains when the climate changed to semi-arid.
(b) MIS 3–2 transition:
Further up in both the interfluve cores, a dominance of the
Himalayan source and frequent development of immature
paleosols are noted that are related to MIS 3–2 transition. The
clay mineral transformations during the initial phase of this
transition are characterized by [Biotite → HIV → PCh] and
[Plagioclase → LCS] in both the cores in a relatively humid
climate and slightly acidic pedogenic environment (Fig. 7b).
36
D.K. Pal et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 356–357 (2012) 27–37
Fig. 7. A schematic climate driven model for clay mineral alteration for the last 100 ka in northern and southern parts of the G–Y interfluve represented by IITK and Bhognipur cores.
The change to semi-arid climate during MIS 2 is marked by the
transformations of [Plagioclase → PC] and [Biotite → HCS] in a
moderately alkaline pedogenic environment in both the cores
(Fig. 7b).
(c) MIS 2–1 transition
In the uppermost parts of both the cores, the formation of
weakly developed cumulative paleosols suggests that sedimentation reached to a steady state. A climatic shift from semi-arid
to humid conditions related to MIS 2–1 transition is inferred on
the basis of the shift from the dominance of the [Biotite → HCS]
and [Plagioclase → PC] transformations to [Biotite → HIV → PCh]
and [Plagioclase → LCS] (Fig. 7c). While the former indicates
D.K. Pal et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 356–357 (2012) 27–37
slightly alkaline pedogenic environment during semi-arid
conditions of MIS 2, the latter suggests a relatively humid
climatic condition during Holocene time.
6. Conclusions
We conclude that the subtle spatial differences in clay mineralogy
and their depth distribution in cores from northern and southern parts
of the Ganga–Yamuna interfluve are manifestations of coupling of
sediment source and climatic transitions. The influence of micaceous
sediments of the Himalayan orogen in the upper part of both the cores
is reflected in dominance of HCS whereas the lower part of the core
from the southern part of the interfluve is characteristically LCS-rich.
Since the formation of large amounts of LCS at the expense of biotite
from either Himalayan or cratonic sources is not possible and also the
hydroxy-interlayering in smectite is improbable in the alkaline soil
environments of the present-day semi-arid climate, this reflects a
major change in sediment supply in the southern interfluve from
cratonic to Himalayan source.
Distinct clay mineral transformations are recorded in both the
cores under humid and arid climatic conditions during the last
~ 100 ka. Typical clay mineral assemblage in a humid climate (e.g. MIS
5, 3 and 1) is marked by HIV, LCS and PCh formed under acidic soil
conditions. In a drier climate (MIS 4 and 2), formation of trioctahedral
HCS from biotite weathering and precipitation of pedogenic CaCO3
were the dominant processes that created conducive conditions for
illuviation of clays forming argillic (Bt) horizon. Thus, the presence of
minerals such as LCS, HIV and PCh in soils of MIS 4 and 2 indicates that
these minerals were preserved in the subsequent drier climates.
Supplementary materials related to this article can be found online
at doi:10.1016/j.palaeo.2011.05.009.
Acknowledgements
The authors gratefully acknowledge the financial support of the
Department of Science and Technology, New Delhi, India under the
Science of Shallow Sub-surface (SSS) programme (SR/S4/ES-21/Ganga
Plain/P2). We thank the Director, NBSS&LUP, Nagpur, India and IIT
Kanpur for providing facilities for this work. Help received from other
colleagues in the Division of Soil Resource Studies at NBSS&LUP is also
thankfully acknowledged. This is a contribution to IGCP 582 on
Tropical Rivers: Hydro-Physical Processes, Impacts, Hazards and
Management.
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