Coupled Rock-Eval pyrolysis and spectrophotometry for
lacustrine sedimentary dynamics: application for West Central
African rainforests (Kamalété and Nguène, Gabon)
David SEBAG1,2, Maxime DEBRET1, MAKAYA MVOUBOU3, Rolf MABICKA OBAME1,3, Alfred
NGOMANDA4, Richard OSLISLY5, Ilham BENTALEB6, Jean-Robert DISNAR7, Pierre GIRESSE8
1 Université de Rouen, Laboratoire M2C, CNRS, Mont-Saint-Aignan, France.
2 IRD, Laboratoire HydroSciences Montpellier, Université de Ngaoundéré, Cameroun.
3 Université des Sciences et Techniques de Masuku, URESTE, Franceville, Gabon.
4 Institut de Recherches en Ecologie Tropicale, CENAREST, Libreville, Gabon.
5 IRD, Laboratoire PALOC, Agence Nationale des Parc Nationaux, Libreville, Gabon.
6 Université de Montpellier II, Laboratoire ISEM, CNRS, Montpellier, France.
7 CNRS, Institut des Sciences de la Terre d’Orléans, Université d’Orléans, Orléans, France.
8 Université de Perpignan, Laboratoire CEFREM, Perpignan, France.
Corresponding author: firstname.lastname@example.org
Published online before print April 24, 2013
In recent years, Nguène Lake and Kamalété Lake (Gabon, West Central Africa) have been studied
repeatedly, providing comprehensive reconstructions of environmental changes over the last millennia. Both
lakes are in different geomorphological and environmental settings. They are therefore excellent sites to test
new methodological approaches. Indeed, the sedimentary cores provide various facies, and the previous
studies provide references for calibrating the results of new methods. In this methodological issue, the
present study aims to evaluate the potential of spectrophotometric and Rock-Eval coupled analysis to
describe the Holocene lake and marsh deposits from tropical moist forests. This assessment is carried out on
samples taken from two well-documented reference cores. The spectrophotometric analysis provides
reproducible colour measurements, which inform about the nature of the main colour-bearing constituents.
Coupled with Rock-Eval pyrolysis, this technique can be used to describe lithological changes and identify
the probable source of sedimentary organic matter. In the studied cases, this approach identified the facies
dominated by detrital terrigenous inputs (“iron bearing” signature and high OI values) and those associated
with a more abundant primary production (“chlorophyll” signature, low OI and high HI), providing a
distinction between palustrine and lacustrine dynamics. However, although the facies are comparable,
sedimentary dynamics and sediment sources may vary depending on geomorphological and climatic
Keywords: sedimentology, tropical lake, diffuse reflectance, organic matter, Africa
Numerous studies in West Central Africa help to
draw a clear picture of the evolution of tropical
rainforests during the Quaternary and Holocene
(e.g., Marchant & Hooghiemstra, 2004; Bonnefille,
2011), especially from marine records (Dupont et
al., 2000; Lézine et al., 2005; Weldeab et al., 2007;
Kim et al., 2010). However, continental reference
sites and continuous series from Central Africa
(Maley & Brenac, 1998; Marret et al., 2006) are too
scattered to assess the palaeohydrological models
recently developed for higher-resolution studies
(e.g., Ward et al., 2007). Therefore, recent works
have focused on several complementary approaches
to study some lacustrine key sites of West Central
Africa, such as Ossa (Nguetsop et al., 2010;
Kossoni & Giresse, 2010), Nyabessan (Ngomanda
et al., 2009; Sangen, 2010), and Kamalété and
Nguène (Ngomanda et al., 2005; Giresse &
Makaya-Mvoubou, 2010). In these studies, the
analytic methodologies are based on a combination
of sedimentological (e.g., grain size, X-ray
diffraction), palaeoecological (e.g., pollens,
diatoms), and geochemical techniques (e.g., C/N,
δ13C, δ15N) to reconstruct palaeoenvironments and
palaeoclimates. However, the routine application of
this multidisciplinary approach is limited by the
time-consuming procedures of pretreatments and
non-continuous measurements. On the other hand,
non-destructive methods exist such as X-ray
fluorescence core scanner or Scopix techniques.
SEBAG et al.
Such approaches have been proved useful for
assessing potential palaeoclimatic records by
continuous measurements from sedimentary
sequences. Yet, they often require large and
Spectrophotometric analysis offers probably a
good compromise: portable and easy to use, quick
high-resolution data acquisition from chromatic
properties of sediments. Mainly used for marine
and lacustrine investigations (e.g., Mix et al., 1995;
Debret et al., 2010, 2011), this method has recently
been used to study changes in the composition of
Quaternary sediments off the Gabon coasts in order
to reconstruct millennial-scale precipitation changes
(Itambi et al., 2010).
The present study aims to evaluate the potential of
sediment colour to describe the Late Holocene lake
and marsh deposits from West Central Africa moist
forests. Combined with Rock-Eval analysis of
sedimentary organic matter (OM), this assessment
is carried out on samples taken from two well-
documented reference cores (KAM1 and NGUE1;
e.g., Ngomanda et al., 2007; Giresse et al., 2009).
Indeed, the studied sites (Kamalété and Nguène
Lakes, Gabon) present sufficiently contrasting
characteristics to provide reference terms for future
studies using the same methodology. In this
methodological issue, the results will be compared
to conclusions from previous studies, which they
may supplement about sources of sedimentary
2. Geographical settings and studied cores
2.1 Kamalété Lake
Kamalété Lake is a closed and small marshy basin
(50 to 100 m wide, 500 to 700 m long, ~2 m depth).
The basin covers ~0.1 km2 with a water depth that
falls to 1 m during the dry season. It located
southeast of Lopé National Park (Ogooué-Ivindo,
Gabon; 0°43′S, 11°46′E; 350 m asl; Figure 1) in a
narrow structural depression in the Early
Proterozoic formations (Francevillian) composed of
mudstones, micaceous and clayey sandstones,
feldspathic sandstones, cherts, dolomites and shales
(Weber, 1968). The yellow ferrallitic soils have a
clay texture to sandy clay and are composed of
kaolinite, illite, quartz, goethite, gibbsite, and
feldspars (Collinet and Forget, 1976; Chatelin,
1966). Coarse particles (i.e. pebbles) and traces of
gullies show that the catchment suffered an intense
erosion by runoff. Either side of the lake, steep
slopes are marked by landslides of tens of cubic
meter and by outcrops of ferruginous crusts. Some
hydromorphic soils (gley or pseudo-gley) are
visible on the marshy banks of the lake. In the
surrounding area, vegetation is characterized by a
colonizing forest/savanna mosaic. The lake is
surrounded by a heterogeneous vegetation
(savannah and colonizing forest) that combines
grassy species (Pobeguinea arrecta) and pioneer
species (Uapacca guineensis, Aucoumea
klaineana). Pioneer species reflect the long-term
transition from a savannah toward a forest
environment. On the banks, a ring of ferns
(Gleichenia) and sedges grow randomly in shallow
waters. Annual precipitation oscillates are around
1,500 mm, a low value for Gabon due to the rain
shadow effect of the Cristal Mounts and Massif of
Chaillu on the western side. But, present day
meteorological values show great inter-annual
variability related to variable timing and duration of
the dry summer season: the minimum rainfall are
recorded from mid-June to mid-September and
from mid-December to mid-January, while the
highest are in the other months of the year. Average
annual temperatures are between 20.6 and 30.8 ° C.
They are highest in the rainy season (February-
April) and minimum in the dry season (June-
August) because of the cloud cover reduces
The 375-cm-long core KAM1 (Figure 2) was
taken in the central part of Kamalété Lake (e.g.,
Ngomanda et al., 2007; Giresse et al., 2009). From
the base to 149 cm, sedimentation began with beige
mud. Abundant siderite aggregates and organic and
silty laminae several millimetres thick were noted
in this lithological unit. The upper 149-cm-thick
interval is a grey to grey-green mud with lower
abundance of sand and a darker colour. Lamination
frequency changes in different sections of the core:
numerous laminations were observed in the beige
mud, while only few laminations were observed in
the grey-green mud.
Figure 1: Map of Gabon showing dominant vegetation, isohyets, and site locations.
Figure 2: Down core variations of reflectance and classical PyRE parameters showing main lithological units and
some compositional changes in the KAM1 core. Coloured bands correspond to spectral signatures (A, B, and C)
defined from FDS (see text and Fig. 5).
2.2 Nguène Lake
Nguène Lake is located on the southern slopes of
the Cristal Mounts (Moyen-Ogooué, Gabon;
0°12′S, 10°28′E; 20 m asl; Figure 1). It was
primarily a fluvial depression, today partly isolated
from the Abanga River. The shallow lake (~3 to 5
m depth) covers ca. 3 km2 during the dry season (<
2 m depth). The swampy shore is largely flooded
during the rainy season. In the southeastern part, a
communication with the river plays seasonally as a
water supply or drain.
The basin is located in the clayey Permian-
Carboniferous formations between the Lambaréné
horst and the Precambrian basement. Most rocks
that outcrop along the Abanga River are part of
sandy-clayey Permo-Carboniferous series of
N'khom and Agula (Bassot, 1988). Common soils
are ferrallitic soils developed on sandstones,
providing a sandy clayey texture and composed of
kaolinite, illite (scarce), quartz, goethite and
feldspars (Collinet and Martin, 1973). In the area, is
covered by a dense evergreen rainforest
characteristic of the regional climate. The
catchment area (~6 km2) is colonized by a swamp
forest dominated by Anthostema aubryanum
(Euphorbiaceae), Uapacca spp. (with highly
developed aerial roots) and Alstonia congensis
(Apocynaceae). Near the banks (especially in the
northwest part), the abundance of Cyperus papyrus
(Cyperaceae) show the recent evolution of this
swampy area. Average rainfall is about 2150 mm
(Collinet and Martin, 1973) with a maximum
between March and May (long rainy season) and
minimum between June and August (long dry
season). Temperatures (around 26°C) are highest
during rainy season and lowest during dry season.
The 415-cm-long core NGUE1 (Figure 3) was
taken from the southern part, 200 m from the
western bank (e.g., Ngomanda et al., 2007; Giresse
et al., 2009). From the base to 152 cm, sediments
began with light to dark grey hydromorphic
deposits, including some sandy layers and
bioturbation structures. This grey mud is similar to
gley forming on the current banks. The upper 152
cm show a dark clayey mud with conspicuous dark
and light grey alternations. Crystals of siderite and
vivianite accompany the clayey and organic
accumulations. However, these iron concretions are
more abundant in the lowermost deposits.
SEBAG et al.
Figure 3: Down core variations of reflectance and classical PyRE parameters showing main lithological units and
some compositional changes in the NGUE1 core. Coloured bands correspond to spectral signatures (A, B, C and
D) defined from FDS (see text and Fig. 6).
The present work aims to highlight the
complementarity of two techniques often used
separately: the spectrophotometry, which provides
continuous measurements of the sediment colour,
and the Rock-Eval pyrolysis, which provides
qualitative and quantitative data on the sedimentary
organic matter. Both techniques are detailed below.
In the discussion section, we show the interest of
these two methods by comparing the results
obtained here with previously published results
including mineralogical and palynological data. For
details of these conventional methods, lithological
description and chronological discussion, we refer
to these previous articles (Ngomanda et al., 2005;
Ngomanda et al., 2007; Giresse et al., 2009; Giresse
& Makaya-Mvoubou, 2010).
3.1 Spectrophometry (visible reflectance)
Analyses were carried out using a Minolta CM
2600d, commonly used for quantitative
measurements of soil and sediment colour (Croft &
Pye, 2004). This instrument provides reflectance
measurements in the visible wavelengths (400-700
nm). We used specular component excluded (SCE)
reflectance to eliminate any bias induced by
specular reflection. The illuminant was D65,
corresponding to the average daylight at a
temperature of 6504 K. Measurements were taken
with a 8 mm aperture.
For binary mixtures of contrasted components
(i.e., dark/light), CIELab standard parameters (L*,
a*, and b*) can eventually be used for
palaeoenvironmental or palaeoclimatic implications
(Mix et al., 1992; Debret et al., 2006). For more
complex mixtures, however, other parameters have
to be used and various approaches have been
proposed (e.g., first derivative methods, frequencies
gathering, factor analyses) to characterize the
sedimentary facies (Balsam & Beeson, 2003 ; Ji et
al., 2005) or quantify constituents such as
carbonates, goethite, hematite, or chlorite (e.g.,
Damuth & Balsam, 2003; Zhang et al., 2007; Ortiz
et al., 2009 ; see Debret et al, 2011 for a review).
In this work, we used two complementary tools (i)
L* as a measure of lightness ranging from 0 (black)
to 100 (white) to establish a lithological division,
and (ii) and the first derivative spectra (FDS) to
analyse the composition of sediments, and to
describe the changes in sedimentary dynamics.
3.2 Bulk geochemical analyses: Rock Eval
The Rock-Eval pyrolysis technique was designed
to screen automatically, without any preliminary
treatment, large sets of rock and sediment samples
(Espitalié et al., 1985). Initially designed for
petroleum applications, this routine method is now
used for a large variety of materials, e.g., soils and
recent sediments (Sifeddine et al., 1995; Patience et
al., 1996; Meyers & Lallier-Vergès, 1999; Disnard
et al., 2003; Marchand et al., 2003; Hetényi et al.,
2005; Sanei et al., 2005; Sebag et al., 2006).
The sampling interval was approximately 10 cm
for lithologic logging for the KAM1 and NGUE1
cores, but the intervals were shortened depending
on facies thickness. Analysis was carried out with
100 mg of powder sample using a ‘‘Turbo’’ Rock-
Eval 6 pyrolyzer manufactured by Vinci
Technologies. Standard parameters, namely, total
organic carbon (TOC), hydrogen index (HI), and
oxygen index (OI), were calculated by integrating
the amounts of hydrocarbon compounds (HC), CO,
and CO2 produced during thermal cracking of the
OM, between well-defined temperature limits
(Espitalié et al., 1985; Lafargue et al., 1998; Behar
et al., 2001). TOC (in % wt) is the sum of all the
organic carbon moieties (HC, CO, and CO2). TpS2
(in °C) is the corrected temperature of the oven,
which corresponds to the optimum HC release. HI
(in mg HC g-1 TOC) corresponds to the quantity of
HC released relative to TOC, and is correlated to
the H/C ratio. OI (in mg O2 g-1 TOC) corresponds
to the quantity of oxygen released as CO and CO2,
relative to TOC, and is correlated to the O/C ratio.
These parameters were defined to study mature OM
from sedimentary rocks (e.g., Disnar, 1994), but
previous works have shown that they could be used
to characterize immature OM (Disnar et al., 2003).
The previous works (Giresse et al., 2009; Giresse
& Makaya-Mvoubou, 2010) mention the presence
of siderite in studied cores. The analyse of deposits
rich in siderite by Rock Eval analyses has some
problems principally in the evaluation of the
Oxygen Index because this mineral starts to
decompose during the pyrolysis, when the
temperature approaches 485-520°C (Lafargue et al.,
1998). For this reason, we checked the possible
influence of this mineral through (i) a study of S3
signals that measure the fluxes of CO2 and CO
during pyrolysis and (ii) a comparison between the
total C contents and the TOC. In addition, the
figure 4 presents a comparison of S2 (i.e.
hydrocarbon compounds produced by thermal
cracking of OM) and S3CO2 (i.e. CO2 produced by
thermal cracking of OM and siderite) for Kamalete
samples. The correlation between the two
parameters underlines the dominance of OM-
derived compared to mineral-derived signal. In
addition, the similarities with free-siderite soil
samples collected in the studied region shows the
very weak possible influence of this mineral on the
parameters and S3CO S3CO2 measured (and hence
the IO). We conclude that major changes in the IO
are primarily related to sedimentary organic matter
and not to the mineralogical composition (i.e.
relative abundance of siderite).
Figure 4: Correlation between S2 (i.e. hydrocarbon
compounds released during thermal cracking of OM)
and S3CO2 (i.e. CO2 released during thermal
cracking of OM and mineral constituents as siderite)
measurements from KAM1 core and soil samples.
4.1 Reflectance (L*) and First Derivated
Coupling L* measurements and FDS analyses can
be used to draw quantitative limits within the
various lithological units, which were determined
visually at the core opening.
KAM1 core. The L* variations are used to define
three units (Figure 2), which correspond to the main
lithologic units described by Giresse et al. (2009).
Unit 1 (from the base of clayey beige deposits to
350 cm) presents the highest L* values (>50%),
followed by a decrease around 335 cm (<35%)
reflecting a darker colour in this organic rich
lithological boundary. Unit 2 (upper part of clayey
beige deposits from 335 to 150 cm) is characterized
by very stable L* values (around 48%). Only the
dark layer located at 270 cm is highlighted by very
low values (<35%). Unit 3 is characterised by lower
and more variable values (from 44 to 40%),
following a general decreasing trend reflecting a
gradual darkening of the upper silty grey deposits
(from 150 to the top).
As shown by Balsam et al. (1991, 1996, and
2003), FDS analysis can distinguish characteristic
spectral signatures. This approach identifies three
distinct signatures in core KAM1 (Figure 5). Two
very sharp peaks, centred respectively at 435 and
545 nm, characterize the first spectral signature A
(Figure 5A). The second signature B mainly differs
from the previous one by (i) a close amplitude of
the two peaks at 435 and 545 nm and (ii) the
presence of an additional broad peak centred
around 675 nm (Figure 5B).
SEBAG et al.
Figure 5: In-depth variations of first derivated spectra showing some compositional changes by
spectrocolorimetric analysis of the KAM1 core. A, B, C: characteristic FDS of each spectral signature (see text).
Figure 6: In-depth variations of first derivated spectra showing some compositional changes by
spectrocolorimetric analysis of the NGUE1 core. A to D: characteristic FDS of each spectral signature (see text).
Peaks at 435 and 545 nm are also present on the
third spectral signature C, which differs from the
previous signatures by an ascending trend in seesaw
pattern above 600 nm (Figure 5C). Finally, the in-
depth representation of FDS defines the lithological
units previously established by Makaya Mvoubou
(2005). The lower clayed beige deposits (units 1
and 2) have a dominant signature A, except the
darkest layers around 335 and 270 cm, which
respectively have the signatures B and C; the upper
silty grey deposits (unit 3) present a spectral
signature B (Figure 5).
NGUE1 core. Here also L* variations (Figure 3)
delineate the lithological limits described by
Giresse et al. (2009). Unit 1 (from the bottom of the
lower gley to 370 cm) is marked by a gradual
decline of L* values (from 50 to 46%) until the
darker layer highlighted by very low L* values (35
to 23%) between 370 and 340 cm. Unit 2 (upper
part of dark gley from 330 to 140 cm) is
characterized by a variable lightness ranging from
30 to 52%, which identifies four sub-units: 330 to
290 cm (40 to 43%), 280 to 230 cm (31 to 40%),
230 to 190 cm (40 to 45%), and 190 to 155 cm
(>50%). The bioturbated layer (150-140 cm) ending
unit 2 is marked by an L* decrease (~40%). Unit 3,
which corresponds to the upper dark deposits (from
135 cm to the top of the core), is characterized by
low L* values between 33 and 39%.
The FDS analysis identifies four distinct spectral
signatures in core NGUE1 (Figure 6). Two very
sharp peaks, respectively centred at 425 and 535
nm, characterize the spectral signature A (Figure
36A). The second signature B is mainly
characterized by a sharp peak around 675 nm (Fig.
6B). In some spectra, this peak is combined with
two other weak peaks more or less visible to 435
and 545 nm. The third signature C is characterized
by a flat spectrum, marked by a strong increase
after 600 nm (Fig. 6C). The fourth signature D (Fig.
6D) differs from signature A by (i) the presence of
twin peaks at 435 and 535 nm but with rather close
height and (ii) a declining trend from 360 nm to 700
nm. Finally, each lithological unit is dominated by
one of these four signatures (Figure 6) and is not a
result of a complex mixture of the four end-
members: D is dominant in the bottom of the core
(unit 1), C in the boundary dark layer, A and B in
the intermediate light gley (unit 2), and B in the
upper dark deposits (unit 3).
4.2 Characterization of sedimentary OM
By combining the classical Rock-Eval parameters
(TOC, TpS2, HI, OI), it is possible to distinguish
OM geochemical signatures (Figure 7) related to
main lithological changes and to established
subdivisions (Figures 2 and 3).
Figure 7: Characterization of sedimentary OM from Rock Eval parameters using two diagrams (a and c:
TOCvsTpS2; b and d: HIvsOI) for comparisons between lithological units of the KAM1 and NGUE1 cores.
Colours of points correspond to spectral signatures defined from FDS.
SEBAG et al.
KAM1 core. As a whole TOC and Tmax, that
values range respectively from 0,3 to 6,7%, and
from 386 to 438°C, respectively (Figure 7a), allow
us distinguishing (i) mineral OM-poor (TOC <1%,
low or high Tmax), (ii) mineral OM-rich (2% <
TOC < 4%, with higher Tmax), and (iii) organic
facies (TOC >5%, high Tmax). This OM-base
typology corresponds to lithological subdivision:
e.g., lower clayey deposits (unit 2) present OM-
poor facies, whereas bottom layers (unit 1) and
upper silty deposits (unit 3) are dominated by OM-
rich facies. Organic facies are related to two layers
located around 330 and 30 cm (Figure 2).
Two sample groups can be distinguished through
HI and OI values (Figure 7b). The first group
corresponds to units 1 and 3, which presents a
homogeneous OM signature typified by low HI
values (from 133 to 245 HC/g Corg), and very low
OI values (from 128 to 230 mg O2/g Corg). The
second group, which corresponds to unit 2, presents
a distinct signature marked by very low HI values
(from 118 to 187 mg HC /g Corg), and variable OI
values (from 200 to 1500 mg O2/g Corg).
NGUE1 core. TOC and Tmax values, that range
from 0,6 to 6,4%, and from 420 to 435°C,
respectively (Figure 7c), distinguish OM-poor
(TOC <2,5%, constant Tmax up to 430°C), OM-
rich (2,5% < TOC < 6%, Tmax around 430 or
425°C), and organic facies (TOC >5%, Tmax
around 425°C). This typology corresponds to
lithological subdivisions (Figure 3).
Low HI and OI values (from 102 to 195 mg HC/g
Corg, and from 84 to 166 mg O2/g Corg,
respectively) define a homogenous signature in the
HI versus OI diagram (Figure 7d). Three samples
present different signatures: two of these ones (at
135 and 125 cm) have a higher HI values (ca. 240
mg HC/g Corg), and the third one (at 227 cm) a
higher OI value (311 mg O2/g Corg). In-depth HI
variations allow us the identification of significant
differences related to some lithological changes
(Figure 3). Briefly, HI values are (i) stable around
155 mg HC/g Corg belowr 300 cm; (ii) comprised
from 102 to 132 mg HC/g Corg between 300 and
230 cm; (iii) vary from 105 to 162 mg HC/g Corg,
between 230 and 150 cm; and finally (iv) remain
comprised between 149 and 248 mg HC/g Corg in
the unit 3 above 150 cm.
Previous studies are highlighted the possibility of
using spectrophotometric measurements to
determine qualitative changes in sediment
composition (e.g., Balsam & Deaton, 1996; Itambi
et al., 2010; Debret et al, 2011). In this way, the
FDS are often used to determine some main
sedimentary constituents (Barranco et al., 1989;
Balsam & Deaton, 1991; Deaton & Balsam, 1991).
Indeed, various studies have shown that some
sedimentary constituents have distinctive spectral
signatures identified by the position of first
derivative peaks, e.g., 445 and 525 nm for iron
oxyhydroxides such as goethite, and 555, 565, and
575 nm for iron oxides such as hematite (Barranco
et al., 1989), or from 605 to 695 nm for organic
compounds, and more precisely at 675 nm for a-
chlorophyll and some by-products (Wolfe et al.,
2006). Here, we used these characteristic FDS
peaks to determine the nature of main sedimentary
constituents, and thus to define distinct
spectrophotometric facies related to changes in
In the same way, numerous works use Rock Eval
pyrolysis to analyse the bulk composition of soil
and sedimentary OM (e.g., Meyers & Lallier-
Vergès, 1999; Sebag et al., 2006). Usually, the
standard parameters (TOC, HI, OI) can be used to
evaluate major stages of the OM transformations in
soils and sediments (mineralization, humification:
Disnar et al., 2003). In addition to distinguishing
terrestrial and aquatic OM, HI and OI can be used
to determine the characteristic signatures of plant
litter, humic horizons, and deepest organo-mineral
layers (Disnar et al., 2003). Here, we use these
previous studies to analyse the compositional
changes of sedimentary OM in each
5.1 Kamalété Lake
The results from KAM1 samples show that some
changes recorded by L* and FDS document
changes in sedimentary dynamics. Spectra analyses
(Figures 5 and 6) show that typical markers of iron
oxides (i.e., 435-545 twin peaks) are present on all
spectra. This feature reflects the ubiquity of iron-
bearing constituents in the KAM1 deposits, which
can be related to mineral terrigenous supplies from
the catchment area. This first point is consistent
with the steep slopes and colluvium mechanisms
that characterize the lake watershed. In addition,
our results are consistent with previous
palaeoenvironmental reconstructions proposed by
Ngomanda et al. (2005). For example, the base of
the sediment core corresponds a savannah-
dominated environment dated around 1300 yr BP.
In this context, the terrigenous detrital contributions
are logically produced from the soil surficial layers,
rich in iron oxides (A signature) and containing a
moderately degraded OM (low OI). Between 370
and 335 cm, the pollen assemblages show an
expansion of forest environments, probably in
connection with wetter conditions (near the present)
between 1300 and 1200 yr BP (Ngomanda et al.,
2005). In this context, the first dark layer (around
335 cm) corresponds to a Cyperaceae peak
(Ngomanda et al., 2005), which can be related to a
greater contribution of autochthonous primary
production highlighted by the "chorophyll" specific
marker (i.e., 675 peak in signature B; Wolfe et al.,
2006; Debret et al., 2006; Michelluti, 2010). Above
this organic level, an abrupt increase in the
Pteridophytes percentage (c. 70%) suggests
significant lake-level decline (Ngomanda et al.,
2005). This change is marked at the base of unit 2
by an OM-poor facies dominated by "iron oxides"
markers (signature A). This back-to-a-terrigenous-
clastic sedimentation coincided with a period with
maximum opening of the forest canopy (Ngomanda
et al., 2005). The canopy opening combined with
greater seasonal contrasts could then explain more
intense erosion and transfer processes in the
watershed. Geochemical data will complete this
picture by providing information on the sources of
sedimentary OM (Figures 2, 3 and 7). Indeed, high
OI values (>300 mg O2/g Corg) indicate a strongly
altered OM (Copard et al., 2006), quite comparable
with samples from deep soil horizons (Disnar et al.,
2003; Mabicka Obame et al., 2009). Note that the
incoherent OI value (>1500 g O2/g Corg at 280 cm)
corresponds to a maximum contribution of sands
and a peak in charcoal abundance (Giresse et al.,
2009). It also coincides with a peak of Cyperaceae
pollens (Ngomanda et al., 2005). This level can be
related to a paroxysm of detritism, and may result
from a seasonal high contrast, favouring runoff and
mass wasting on the slopes covered with savanna.
The second dark layer (around 270 cm) presents a
combination between "iron oxides" markers and a
specific feature of altered OM (increasing trend in
signature C), which can be related to terrestrial
peaty deposits and some oxidized coal samples
probably linked to humic substance (Debret et al.,
2011). In addition, low HI and OI values confirm
the terrestrial OM origin. This peat layer could
result from the partial filling or from a temporary
drying up of the basin, around 1200 yr BP.
Ngomanda et al. (2005) note that the regression of
mature forest suggests longer and/or recurring
episodes of aridity from c. 1240 until 550 cal. BP.
They suggest that the dry season was more
prolonged and more severe than it is today. The top
of unit 2 corresponds to the installation of seasonal
contrasts in a drier general context. Ngomanda et al.
(2005) point to a high abundance of Pteridophyte
spores, which may indicate low lake-level
stabilization. In addition, Giresse et al. (2009) note
that the occurrence of siderite concretions in these
layers denotes anoxic conditions caused by the
reduction of the water input into the basin. These
climatic changes are documented across equatorial
Africa; they are characterized by a succession of
fluctuations in the regional rainfall regime, which
may be explained by shifts in the mean latitude of
the intertropical convergence zone, as proposed by
Nguetsop et al. (2004).
About 150 cm, the transition to unit 3 is marked
by a typical combination of "iron oxides" and
"chlorophyll" markers (signature B), with very low
OI values (<200 mg O2/g Corg), comparable with
those from surface samples of savanna soils, but
with some significantly higher HI (>200 mg HC/g
Corg), which are quite consistent with greater
autochthonous contributions (Figure 7). As revealed
by Ngomanda et al. (2005), the darker units 3 is an
OM-rich facies that may be related to a more humid
climate, entailing a lake-level rise and progressive
expansion of semi-evergreen rainforest after 550 yr
Thus, sedimentological and palaeoecological
proxies (Ngomanda et al., 2005; Giresse et al.,
2009) confirm our new results. Notably, Ngomanda
et al. (2007) observe that, when the lake level is
high, the δ13C signal records the local vegetation,
often dominated by C3 Cyperaceae, as well as the
C3 plants colonizing the lake basin. Conversely,
when the lake level shallows, the δ13C signal
records the isotopic composition of the local C4
plants (Pteridophytes and Poaceae).
5.2 Nguène Lake
It is important to note that the sedimentation in
Nguène Lake is very different from that in the
Kamalété basin. Indeed, the sedimentary dynamics
in Nguène Lake is directly controlled by
hydrodynamic changes of River Abanga (Giresse et
al., 2009), and is therefore only little influenced by
the vegetation surrounding the lake. In addition, the
morphology of the watershed is not conducive to
runoff and erosion as it is in Kamalété.
FDS analysis shows facies changes that we can
try to relate with changes in sediment dynamics. It
should be noted that the “iron oxides” twin peaks
are present through the whole of the core, except in
the organic layer (about 360 cm, signature C;
The bottom deposits (unit 1; from 413 to 375 cm)
present FDS characterized by a combination of
"iron oxide" markers and a specific feature of
limestones (decreasing trend in signature D; Itambi
et al., 2010; Debret et al., 2011). These iron
deposits could be related to the presence of siderite
and vivianite crystals already described by
mineralogical analyses (Giresse et al., 2009). These
minerals result from anoxic conditions induced by a
gradual confinement of the basin around 4600 yr
BP. These pieces of information are consistent with
the reconstructions proposed by Ngomanda et al.
(2007) who noted that dense stands of mature
rainforest occupied the catchment area during the
mid-Holocene. The increase and progressive
closing of forest cover would have caused a drastic
SEBAG et al.
decrease in runoff, reducing water flows from the
lake watershed and thus, terrigenous proximal
supplies. Accordingly, Giresse et al. (2009) pointed
out that the detrital sedimentation was limited to
contributions of the river Abanga between 4600 and
2460 cal yr BP. This confinement trend is
highlighted by increasing TOC (Figure 3), which
prefigures the installation of a “peaty marsh”
(signature C of the organic layer around 360 cm).
This episode coincides with a regional
environmental change as revealed by Ngomanda et
al. (2007), who noted that from 4100 yr BP
surrounding evergreen rainforest was progressively
replaced by semi-deciduous rainforest.
The base of unit 2 (from 350 to 280 cm) presents
a spectral signature dominated by "iron oxides"
twin peaks (signature A) and OM-poor facies. This
new facies reflects major changes that occurred in
both the marsh and rainforest pollen signal. Indeed,
Ngomanda et al. (2007) noted that pioneering plants
progressively replaced mature rainforests between
3200 and 2400 yr BP. This trend indicates openings
in the closed canopy forest surrounding the lake
basin. That might explain an accentuation of runoff
and a more detrital sedimentation, and the higher
sand contents measured by Giresse et al. (2009;
Figure 3). Nevertheless, the absence of changes in
geochemical properties (i.e., HI and OI) shows that
the OM sources (surfical soil layers) have not
strongly varied during these times suggesting that
the river still the main contributor.
The overlying layers (from 280 to 230 cm) seem
to reflect a more radical change marked by the
presence of the "chlorophyll" fingerprint (peak
around 675 nm in signature B; Figure 6). This
spectrophotometric fingerprint shows an
autochthonous primary production, but this aquatic
contribution can hide an allochthonous altered OM
signature because of the low detection level of the
"chlorophyll" fingerprint by spectrophotometry
(<0.01%, Wolfe et al., 2006). Indeed, Rock Eval
analyses (i.e. significant low HI, <150 mg HC/g
Corg; Figure 3) shows the terrestrial origin of
dominant OM fraction. Ngomanda et al. (2007)
suggested that lake levels were lower after 2400 yr
BP, which would explain the higher contribution of
terrestrial plants, the decrease in terrigenous detrital
sedimentation, and the gradual increase of sand
contents (Giresse et al., 2009; Figure 3).
A change in spectrophometric facies (around 240
cm) is highlighted by the highest OI value (>300
mg O2/g Corg), which is consistent with more
intense weathering and erosion processes in the
catchment area. This layer marks the change toward
the upper unit 2 (from 230 to 150 cm), and presents
the same colour properties as the lower part (“iron
oxides” signature A). These detrital facies,
however, are distinguished by the lack of sand,
which may indicate that the sources have changed,
or that the lake basin is protected by coarser inputs.
Ngomanda et al. (2007) note that maximal
regression of the mature evergreen rainforest
occurred between 2000 and 1450 yr BP.
Unit 3 corresponds to the installation of lacustrine
conditions, with significant autochthonous primary
production (“chlorophyll” in signature B, high HI
values). This hypothesis is confirmed by pollen
analyses, which indicate that the basin became a
proper lake, but was marked by recurring seasonal
fluctuations. Thus, the pollen assemblages suggest
that an open forest occupied the catchment and the
mean water level of the lake was lower prior to
1450 yr BP (Ngomanda et al., 2007). The same
authors indicate that the marsh expanded in
response to the increasingly long duration of lower
lake levels from c. 1450 to 1250 cal yr BP.
Ngomanda et al. (2007) note the renewed spread of
dense closed canopy rainforest in the catchment and
an expansion of swamp forest and probably higher
water levels in the lake basin after 950 yr BP.
From 950 yr BP, lacustrine conditions result not
only from rainfall (with a consequential increase of
canopy), but also from changes in the Abanga
hydrodynamics, which then feeds the lake with a
suspended sedimentary load. Note here that Abanga
is very high compared to the lake. There is
therefore no possible drain for the lake into the
Abanga river (Giresse et al., 2009).
6. FDS analysis, OM properties, and
In recent years, Nguène Lake and Kamalété Lake
have been studied repeatedly, providing
comprehensive reconstructions of environmental
changes over the last millennia (Ngomanda et al.,
2005; Ngomanda et al., 2007; Giresse et al., 2009;
Giresse & Makaya-Mvoubou, 2010). Thus, we do
not dwell on probable or even proven causes of
these changes, referring to the previous studies. In
this way, Table 1 synthesises additional information
gained for the particular study sites. However, this
work is the first combination of Rock Eval
pyrolysis and spectrophotometry applied to the
study the Holocene lake deposits from tropical
moist forests. In this methodological perspective,
the two lakes considered have sufficient
commonalities and differences to provide a
controlled assessment of the methods used and the
various reference terms for future studies.
In the studied cases, this double approach allowed
us identifing the facies dominated by detrital
terrigenous inputs (“iron oxides” signature and high
OI values) and those associated with a more
abundant primary OM production (“chlorophyll”
signature, low OI and high HI), providing a
distinction between palustrine and lacustrine
dynamics. However, although the facies are
comparable, sedimentary dynamics and sediment
sources may vary depending on geomorphological
and climatic contexts. Thus, the lower detrital
sequence of Kamalété Lake corresponds to the
intense processes of erosion and colluviation on the
slopes of the very narrow watershed (Giresse et al.,
2009). On the other hand, Nguène Lake is located
in a different context, surrounded by forested
lowlands but frequently supplied by Abanga River
waters. Here, the detrital sequence is interpreted as
resulting from fluvial inputs derived from the
upstream part of the Abanga River. During these
times, lake-level fluctuations could be related to
regional climate changes. Thus, the first OM
change (“chlorophyll” signature but low HI), dated
around 2400 cal. yr BP, could have been caused by
a diminution in the mean rainfall and, mainly in the
sedimentary inputs from river overflow, inducing a
relative increase of terrigenous supplies. Inversely,
the upper lacustrine sequence (“chlorophyll”
signature, high HI) would be related to increasing
rainfall and more frequent Abanga River floods
since 1900 cal. yr BP.
In addition, this comprehensive approach also
distinguishes between OM-rich facies from
terrestrial origin and those related to accumulation
of aquatic OM, providing general information about
the sources of sedimentary OM. Thus, a specific
signature of altered terrestrial OM (“oxidized OM”
in signature C, low HI and OI) should reflect the
installation of swamp and/or anoxic conditions
before 2700 cal. yr BP in the Nguène basin, and
around 1180 cal. yr BP in the Kamalété basin. On
the other hand, for more details about this
sedimentary OM, Rock Eval pyrolysis provides
bulk geochemical indices, which can be used to
track the soil horizon-sources of terrigenous OM in
detrital swamp (OI variations in KAM1 core), or to
draw water-level variations in a more lacustrine
system (HI variations in NGUE1 core).
Table 1: Contributions of “Spectrophotometry and Rock-Eval” coupled approach compared to more classical
techniques used in previous studies for the particular study sites.
Previous works have shown that FDS features
may be related to some sedimentary constituents.
Here, we show that these features may be used to
establish spectrophotometric facies reflecting major
lithological changes. Coupled with these
comprehensive measurements, the Rock Eval
parameters can be used to determine the sources of
sedimentary OM, to analyse the changes in
sedimentary dynamics, and to track the evolution of
In summary, the combination of spectro-
photometric and Rock Eval measurements of the
Kamalété core highlights a drastic change in
sedimentary dynamics around 500 yr BP. The first
part of the record (>150 cm) is dominated by
detrital terrigenous inputs (signature A, high OI) in
the swampy environment, probably consequential
to intense erosional processes in the watershed. The
second part (<150 cm) is marked by an apparent
increase of local primary production (signature B,
low OI), which can be induced by an attenuation of
detrital terrigenous fluxes in a lacustrine basin.
In Nguène core, the combination of FDS analysis
and Rock Eval pyrolysis allowed us to identify two
separate episodes: signature A (“iron oxides”) and
low HI values (<150 mg HC/g Corg) in basal
deposits (units 1 and 2), as a result of erosion of soil
surface layers; and signature B (“chlorophyll”) and
high HI values (>150 mg HC/g Corg) in the upper
deposits (unit 3), consistent with higher
autochthonous contribution. These drastic changes
are coherent with previous works concluding on the
replacement of swamp conditions, marked by
changes in water level and/or fluctuations in fluvial
flooding, by permanent lacustrine conditions. In
SEBAG et al.
addition, small variations in FDS and HI (and OI)
values could be related to minor changes in
sedimentary dynamics, OM sources, or terrigenous
input frequencies, as documented from
palynological and geochemical data.
From a methodological perspective, this work is
similar to a calibration on well-documented
reference cores, which validates matching
techniques, and provides elements of comparison
for future studies. The spectrophotometric analysis
can be performed quickly after the coring, and it
produces a high-resolution signal (up to 0.5 cm),
which assesses the quality of the sedimentary
records and guides the sampling strategy. Rock
Eval pyrolysis does not require pretreatments, and it
can provide comprehensive information on the
quality and nature of sedimentary OM. The
combined use of both techniques seems to be an
appropriate response to a preliminary analysis of
lake deposits of humid tropical forests.
This study was conducted under the OMARD
action (Organic Markers Dynamics in tropical
terrestrial environments) funded by the FED 4116
SCALE with support from the Institut de Recherche
pour le Développement (IRD), the University of
Masuku (USTM, Gabon), and the University of
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