Palaeogeography, Palaeoclimatology, Palaeoecology 215 (2004) 37 – 57
www.elsevier.com/locate/palaeo
A magnetic mineral record of Late Quaternary tropical climate
variability from Lake Bosumtwi, Ghana
John A. Pecka,*, Ryan R. Greena, Tim Shanahanb, John W. Kingc,
Jonathan T. Overpeckd, Christopher A. Scholze
a
Office for Terrestrial Records of Environmental Change, Department of Geology, The University of Akron, Akron, OH 44325, USA
b
Department of Geological Sciences, University of Arizona, Tucson, AZ 85721, USA
c
Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882-1197, USA
d
Department of Geological Sciences and Institute for the Study of Planet Earth, University of Arizona, Tucson, AZ 85721, USA
e
Department of Earth Sciences, Syracuse University, Syracuse, NY 13244-1090, USA
Received 3 October 2003; received in revised form 17 May 2004; accepted 20 August 2004
Abstract
We report magnetic hysteresis results from sediment cores obtained from Lake Bosumtwi, Ghana. As a hydrologically
closed basin, the water budget of Lake Bosumtwi is extremely sensitive to changes in the precipitation/evapotranspiration
balance. Lake Bosumtwi lies in the path of the seasonal migration of the intertropical convergence zone (ITCZ); hence, the
lake is ideally situated to study monsoon variability in West Africa. Five distinctive magnetic mineral zones (A–E) were
identified in the 11-m-long sediment cores that span the last 26,000 calendar years. Prior to 12 calendar (cal) ka, low
concentrations of multidomain, high-coercivity magnetic minerals are present. Three prominent shifts towards very high
concentrations of high-coercivity iron sulfide (greigite) magnetic minerals are centered at 12,470, 17,290, and 22,600
calendar years during the last glacial period (magnetic zones D1–3). Between 12 and 3.2 cal ka, there is an abrupt shift to
moderately high concentrations of mixed multidomain and single-domain, low-coercivity minerals and an organic-rich
sapropel lithology. Since 3.2 cal ka, the magnetic mineral parameters reveal a shift to increased amounts of high-coercivity
magnetic minerals.
These magnetic mineral zones document tropical climate variability on a variety of temporal scales. Glacial age sediments
have a high-coercivity magnetic mineralogy due to increased aeolian dust transport from the Sahel to Lake Bosumtwi as well as
postdepositional reductive diagenesis. During the last glacial period, the increased strength of Harmattan and North African
continental trade winds, the southward depression of the ITCZ, and weakened summer monsoon strength resulted in increased
regional aridity and greater dust flux out of Sahel source regions. The greigite-bearing D magnetic zones correspond to brief
lowstands in the level of Lake Bosumtwi and likely represent periods of intensified aridity in West Africa. The D magnetic
zones closely resemble the timing and duration of Heinrich events and suggest a hemispheric-scale climatic coupling between
the tropics and poles. The well-documented African humid period (AHP) is characterized by abrupt shifts in magnetic
parameters between 12 and 3.2 cal ka. Dust flux to Lake Bosumtwi is inferred to be very low during this humid interval due to
* Corresponding author. Fax: +1 330 972 7611.
E-mail address: [email protected] (J.A. Peck).
0031-0182/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.palaeo.2004.08.003
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J.A. Peck et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 215 (2004) 37–57
the strengthening of the summer monsoon. Since 3.2 cal ka, the magnetic mineral parameters suggest increased aridity as
compared to the AHP. This work demonstrates that the magnetic properties of Lake Bosumtwi sediment are a sensitive recorder
of abrupt climate change of global significance.
D 2004 Elsevier B.V. All rights reserved.
Keywords: West Africa; Rock magnetism; Younger Dryas; Heinrich events; Paleolimnology; Climatic change
1. Introduction
Low-latitude monsoon circulation, including the
North African monsoon, is a major component of the
Earth’s atmospheric system and plays an important
role in the global hydrologic cycle and atmospheric
transport of energy (Nicholson, 2000). Recent studies
indicate an important role for the tropical ocean and
atmosphere in producing globally distributed climatic
change including widespread, mid-latitude drought
(Hoerling and Kumar, 2003). Additional studies have
shown a temporal correlation between tropical and
high-latitude climatic change, particularly North
Atlantic thermohaline circulation, sea surface temperature, and trade wind strength (Stager and Mayewski,
1997; Gupta et al., 2003; Lea et al., 2003). However,
the underlying causal mechanism of these changes is
not completely understood (Broecker, 2003). In order
to gain a greater insight into the role of the tropics in
triggering, intensifying, and propagating climate
changes, more records of past climate changes in the
low latitudes are needed.
The magnetic properties of sediment have been
successfully used to provide insight on past climatic
conditions. As environmental conditions change (e.g.,
lake productivity, lake level, reductive diagenesis, and
weathering/pedogenic phases), the magnetic character
of the accumulating sediment varies. For example, in
North Atlantic sediment cores, magnetic parameters
have been used to identify ice rafted detritus (IRD)
layers associated with Heinrich events (Stoner et al.,
1996; Thouveny et al., 2000). In maar lake sediments
of the Massif Central, France, increases in magnetic
concentration reflect regional environmental changes
in weathering, sediment influx, and postdepositional
diagenesis, and are coeval with Heinrich events
(Thouveny et al., 1994; Vlag et al., 1997). From both
continental and marine sediments, magnetic parameters have revealed changes in the strength of monsoon
systems in terms of both wind strength and precip-
itation (Chen et al., 1997; Moreno et al., 2002; Chen
et al., 2003; Dinares-Turell et al., 2003).
This study provides new insight into West African
climate variability over the past 26 ka through an
examination of magnetic hysteresis parameters of
sediment cores from Lake Bosumtwi, Ghana. Previous work has demonstrated that lake level in this
hydrologically closed basin is principally controlled
by the precipitation/evapotranspiration balance,
inducing a strong hydrologic and sedimentological
link with regional climate (Talbot and Delibrias, 1980;
Talbot and Johannessen, 1992; Turner et al., 1996a).
Furthermore, Lake Bosumtwi lies in the path of the
dust-laden NE harmattan winds, and may provide a
record of regional changes of dust transport from
Northwest Africa.
2. Study site
Lake Bosumtwi occupies a 1.07F0.05-millionyear-old meteorite impact crater (Koeberl et al.,
1997) located at approximately 6830VN, 1825VW in
the tropical forest lowlands of Ghana (Fig. 1). The
current lake is 99 m above mean sea level, has a
diameter of ca. 8 km, a surface area of 52 km2, and a
maximum depth of 78 m (Fig. 2) (Talbot and Kelts,
1986; Turner et al., 1996a). The surrounding watershed rises steeply to an elevation of 210 m above the
present lake, where the crater is ca. 11 km in diameter.
Because of the restricted size of its watershed, ca. 80%
of the annual water input to the lake is from rainfall
directly on the lake surface, which makes the lake
water budget extremely sensitive to the precipitation/
evapotranspiration balance (Turner et al., 1996a,b).
The lake waters are highly stratified with a well-mixed
surface epilimnion and anoxic hypolimnion below 15–
18 m depth (Turner et al., 1996a). Within the anoxic
zone, bioturbation is absent and thinly laminated
sediment varves are preserved, allowing the potential
J.A. Peck et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 215 (2004) 37–57
39
generally in phase with hydrologic fluctuations
recorded elsewhere (Gasse, 2000).
Lake Bosumtwi lies in the path of the seasonal
migration of the intertropical convergence zone
(ITCZ), the atmospheric boundary between NE
continental trade winds (Harmattan trades) and SW
onshore winds (Fig. 1) (Nicholson, 2000; Gasse,
2000; Ruddiman, 2001). During summer months,
increased continental insolation in the northern hemisphere causes low atmospheric pressure to develop
over North Africa, the ITCZ migrates to the north of
Lake Bosumtwi, and moisture-laden winds bring
heavy, monsoonal precipitation to western Africa.
The northward progression of the ITCZ also shifts the
zone of maximum African dust export, via the zonal
Saharan Air Layer (SAL), northward to between 158
and 258 N (Goudie and Middleton, 2001) (Fig. 1B).
The reverse occurs during winter months, as the ITCZ
is displaced southward to Lake Bosumtwi and dry,
aerosol-rich NE continental trade winds (Harmattan)
dominate over southern Ghana (Fig. 1A). At this time,
the zone of maximum dust export shifts southward to
between 58 and 108 N (Goudie and Middleton, 2001)
and substantial quantities of aerosols are deposited
across western Africa.
Fig. 1. Seasonal trends in atmospheric pressure centers and
atmospheric circulation over Africa (from Gasse, 2000). Dotted
lines represent the ITCZ; dashed lines represent the Congo Air
Boundary. The star marks the location of Lake Bosumtwi in West
Africa. (A) During winter months, the ITCZ is depressed south to
the Lake Bosumtwi region, thereby extending the southward
penetration of NE continental trade winds and increasing aerosol
dust supply to Lake Bosumtwi. (B) During summer months, the
ITCZ is well north of Lake Bosumtwi and humid onshore winds
cause the summer monsoon rainy season.
for high-resolution paleoclimatic reconstructions.
Bosumtwi lake level variability is correlative with
temporal reorganizations in regional moisture balance
(Talbot and Delibrias, 1977, 1980; Turner et al., 1996a;
Gasse, 2000). Several terraces representing previous
lake level high stands have been 14C-dated to establish
a history of lake level variability for the last 13,000
years (Talbot and Delibrias, 1977; 1980), which is
Fig. 2. Lake Bosumtwi bathymetry map contoured at a 10-m
interval showing core locations used in this study (Brooks et al., in
preparation). Cores 5N, 9P, 12P, and 15P are from the deepest
portion of the basin. Core 16P lies at a shallower depth and
preserves a lake level lowstand erosional contact dated to 16,330 cal
years ago.
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J.A. Peck et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 215 (2004) 37–57
3. Materials and methods
3.1. Sediment coring and logging
In June, 2000, 19 piston cores were extracted from
Lake Bosumtwi (Fig. 2). Eight short piston cores
(1N–8N), ranging from 108 to 134 cm in length, were
used to capture the sediment–water interface. These
cores were supplemented by two freeze cores obtained
from the deepest portion of the lake. The remaining 11
longer cores, ranging from 5.5 to 10.9 m in length, did
not preserve the sediment–water interface, but were
correlated with the shorter cores using well-defined
marker bands. Physical property logging of whole
core volumetric magnetic susceptibility and gammaray attenuation (an estimation of wet bulk density)
was conducted on the piston cores using an automated
Geotek multisensor core logger.
3.2. Magnetic hysteresis measurement
Sampling for magnetic hysteresis measurements
focused on freeze cores BOS-99-11 and BOS-99-12
and piston cores 5N, 9P, 12P, and 15P because they
are from the deepest portion of the lake and have
minimal coring disturbance (Fig. 2). Sampling intervals varied from 1 cm between 0 and 54 cm depth, to
2 cm between 54 and 110 cm, and to 8 cm below 110
cm. The resulting temporal resolution varied from
approximately 15 years at the core top to about 170
years for most of the core. Additional measurements
were made on a sample of modern soil from the
Bosumtwi crater collected in June 2000, and a sample
of North African dust collected in Barbados in April
1995 by R. Arimoto of the New Mexico State
University. Samples were dried at room temperature,
weighed to F0.001 mg on a microbalance, and
mounted on 3-mm2 glass cover slips with a small
amount of silicone grease. The effect of these
diamagnetic materials on the sediment magnetic
measurements is minimal and near constant across
all samples and measurements in this study.
Magnetic mineral data were generated using a
Princeton Measurements Micromag 2900-02 alternating gradient magnetometer (AGM). The AGM is used
to first demagnetize the sample followed by the
application of a magnetic field and measurement of
the magnetic moment of the sample. The slope of the
initial magnetizations (IS) measured in low magnetic
fields (0–25 mT) is calculated first. Next, the applied
field is varied between F500 mT and the full
magnetic hysteresis loop is measured, allowing the
determination of the saturation magnetization (M s),
saturation remanent magnetization (M rs), and coercivity (H c). By 500 mT, all sample hysteresis loops had
closed. The remanent magnetization at 500 mT is
expressed as M rs even though all high-coercivity
phases may not have been completely saturated.
Separate isothermal remanent magnetization (IRM)
acquisition curves were made on each sample. Direct
current (DC) stepwise demagnetization was preformed with the AGM to determine the reversed field
required to reduce M rs to zero. This value is called the
coercivity of remanence (H cr).
First-order reversal curves (FORCs) were generated following the methods of Roberts et al. (2000)
and Pike et al. (2001) by measuring a set of 111 partial
hysteresis curves for representative samples from each
magnetic mineral zone, catchment soil, and North
African dust. FORC diagrams were generated with a
smoothing function of five using FORCAM, a
MatLab program by Michael Winklhofer of the
University of Munich, adapted from a code written
by Christopher Pike (Pike et al., 2001). On a FORC
diagram, the vertical axis (H b) shows the magnetostatic interactions between magnetic grains. Contours
with very little vertical spread indicate noninteracting
magnetic grains, whereas contours with vertical
spread reveal magnetically interacting grains in the
sample (Roberts et al., 2000; Pike et al., 2001). On a
FORC diagram, the horizontal axis (H c) is a measure
of the distribution of median switching field; thus, it
reveals the coercivity distribution of the different
magnetic grain size and mineralogy components
within the bulk sample (Roberts et al., 2000; Pike et
al., 2001).
3.3. Magnetic hysteresis interpretation
There are a number of magnetic hysteresis parameters that are useful in characterizing magnetic
concentration, grain size, and mineralogy of sediment
samples, and thus lend themselves to paleoenvironmental studies. In this section, we provide a general
description of how the magnetic parameters are
interpreted. In-depth explanations of the magnetic
J.A. Peck et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 215 (2004) 37–57
parameters can be found in several works (Thompson
and Oldfield, 1986; Dunlop and Ozdemir, 1997;
Evans and Heller, 2003).
The parameters IS, M s, and M rs are primarily
dependent upon magnetic mineral concentration.
Because initial slope (IS) is an in-field measurement,
samples with low concentrations of remanencebearing minerals (e.g., hematite and magnetite) and
substantial diamagnetic material (e.g., organic matter,
quartz, and carbonate) have very low IS values
because of the large dilutional effect of the diamagnetic material on the overall measurement.
Because IS is measured between the poles of an
electromagnet, it is comparable only, in a very
general sense, to low-field magnetic susceptibility
(X) as measured by Bartington systems (Dunlop and
Ozdemir, 1997). However, both IS and X downcore
profiles display similar trends and, in this study, we
approximate X by IS. Saturation magnetization (M s)
is primarily a concentration measurement; however,
M s is also a function of mineralogy and is
independent of grain size. Saturation remanent
magnetization (M rs) is primarily a concentration
parameter, although it is secondarily influenced by
mineralogy and grain size.
Ratios between parameters such as M rs/X , M rs/M s,
and H cr/H c and the FORC diagram are primarily
indicators of magnetic grain size. The ratio M rs/v is
partially influenced by grain size, with higher values
indicating finer grain size and very large values
indicating the presence of the iron sulfide mineral
greigite (Snowball, 1991). The M rs/M s and H cr/H c
ratios are measures of the domain state, or the grain
size of magnetite (Day et al., 1977). However,
because mixtures of superparamagnetic (SPM) and
single-domain (SD) or both multidomian (MD) and
SD grains can produce M rs/M s and H cr/H c values
similar to pseudosingle (PSD) domain grains, interpretation of these parameters can be ambiguous. The
FORC diagram contours the coercivity distribution of
the individual components of a sample; hence, SPM,
SD, PSD, and MD grain sizes each produces different
FORC contours. Therefore, a FORC diagram may be
used to better resolve the mixed character of magnetic
mineral samples than Day plots alone (Roberts et al.,
2000; Pike et al., 2001).
H cr is primarily a measure of grain size (higher
values indicating SD grains) and mineralogy (Maher
41
and Thompson, 1999). Hematite, goethite, and greigite are high-coercivity minerals. Hematite and
goethite are an important mineralogic component of
North African Sahara and Sahel aerosols with H cr
values of 42 mT (Oldfield et al., 1985). The S-ratio is
an indicator of magnetic mineralogy (King and
Channell, 1991; Maher and Thompson, 1999) and
has been used as a measure of African dust flux to the
Eastern Atlantic Ocean (Bloemendal et al., 1988;
1989). This ratio was calculated by dividing M r at 104
mT by M rs at 500 mT. Ratios close to 1 indicate an allmagnetite–maghaemite composition, whereas ratios
nearing 0.85 indicate increasing proportions (N50%)
of hematite–goethite mineralogy (Maher and Thompson, 1999).
3.4. Radiocarbon dating
AMS radiocarbon dating was performed by Jim
Russell of the University of Minnesota and Tim
Shanahan of the University of Arizona and the results
were made available for this study. Radiocarbon dates
were converted to calibrated (calendar) ages with the
program CALIB 3.0 using the INTCAL98 calibrated
dataset (Stuiver and Reimer, 1993; Stuiver et al.,
1998). Some AMS 14C dates extended beyond the
INTCAL98 calibration range. The equation of a linear
regression line through the 14C ages younger than
19,000 years BP, which were converted to calendar
years, was used to estimate the calibrated age of 14C
samples older than 19,000 14C years BP.
4. Results
4.1. Core correlation
The variable core depth of a distinctive sapropel
lithology indicates that the piston corer plunged
beneath the sediment–water interface before coring
commenced (Fig. 3). Although core 15P failed to
recover approximately 2 m of surficial sediment core,
5N is missing only 8 cm of surface sediment based
upon correlation with freeze cores. In order to
construct a complete standardized sediment section,
lithology, Geotek whole core data, and AGM hysteresis data were used to correlate cores 12P, 15P, 9P,
and 5N, and the two freeze cores (Fig. 3). Original
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J.A. Peck et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 215 (2004) 37–57
Fig. 3. Whole core volumetric susceptibility (K) for cores 15P, 12P, 9P, and 5N on the original core depth scale. Tie lines join equivalent K
features between cores and the organic-rich sapropel unit is denoted by an bS.Q To the right are the portions of the piston cores used in this study,
plotted on the standardized depth scale.
core depths were converted to a standardized depth
scale through the use of AnalySeries (Paillard et al.,
1996).
4.2. Magnetic mineral zones
Magnetic hysteresis measurements of cores 15P,
12P, 9P, and 5N, and the two freeze cores have
produced several important results. First, downcore
magnetic measurements are highly reproducible
between the six cores measured (Figs. 3 and 4). This
magnetic mineral reproducibility is in agreement with
the high degree of lithologic correlation throughout
the lake. They suggest that the results are robust, and
reflect basinwide changes rather than effects caused
by localized sources (e.g., river input). Second,
magnetic hysteresis measurements display welldefined downcore shifts in magnetic mineral concentration, grain size, and mineralogy, presumably
reflecting past climatic or limnological conditions at
the lake. Based upon these results, five magnetic
mineral zones (A–E) have been identified (Table 1;
Fig. 4). Zones D and E are also readily distinguished
from zones A, B, and C in a plot of coercivity and
magnetization ratios (a Day plot; Fig. 5) (Day et al.,
1977). Because magnetic grain size mixtures often
plot in the PSD region, Fig. 5 serves largely to
identify distinct downcore magnetic zones rather than
to provide accurate domain state information. In
addition, below 3. 5 m standardized core depth, there
is a notable alternation between two dominant
magnetic mineral zones in all hysteresis parameters
(Fig. 4). The similarity of these recurring magnetic
mineral zones has prompted the labeling scheme of
D1, D2, D3 and E1, E2, E3 (Fig. 4). The remainder of
this section is comprised of a detailed description of
the magnetic properties that delineate each zone, the
atmospheric dust sample and the catchment soil.
4.2.1. Zone A
Magnetic zone A extends from 0 to 12.72 cm
standardized depth. Magnetic concentration measurements IS, M rs, and M s are high in surface sediments
and undergo a sharp decrease at approximately 12–13
cm depth at the transition into zone B (Fig. 4). Zone A
magnetic mineral grain size parameters M rs/IS and
J.A. Peck et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 215 (2004) 37–57
Fig. 4. Down core magnetic hysteresis parameters for cores BOS-99-11, BOS-99-12, 5N, 9P, 12P, and 15P, all plotted on the standardized depth scale. Horizontal lines delineate the
five magnetic mineral zones identified in the cores.
43
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J.A. Peck et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 215 (2004) 37–57
Table 1
Generalized characteristics of the interglacial and glacial magnetic mineral sediment zones from Lake Bosumtwi
Magnetic zone
Magnetic concentration
(IS, M rs, M s)
Magnetic grain size
(FORC, M rs/M s, H cr/H c)
Magnetic mineralogy
(M rs/IS, S-ratio, H cr)
Paleoclimatic interpretation
Interglacial
A
High
SD and SPM
B
Moderate to high
SD and MD
C
Moderate
SD and MD
High proportion of
low-coercivity minerals
High proportion of
low-coercivity minerals
Very high proportion of
low-coercivity minerals
Interglacial conditions of
reduced dust flux
Interglacial conditions of
reduced dust flux
Very humid conditions;
lake filled to overflowing
Glacial
D
Moderate to high
SD
E
Very low
MD
Very high proportion of
high-coercivity minerals
High proportion of
high-coercivity minerals
Correlative with Henrich events,
arid, very low lake level
Arid glacial period characterized
by reductive diagenesis
M rs/M s have moderate values (Fig. 4). On the Day
plot, zone A samples cluster tightly within the PSD
region (Fig. 5). The S-ratio of these sediments remains
at approximately 0.9 across the zone, indicating a
greater proportion of low-coercivity magnetic minerals such as magnetite and maghaemite as opposed to
high-coercivity magnetic minerals such as hematite
and goethite (Fig. 4). The presence of low-coercivity
Fig. 5. Day plot (after Day et al., 1977) of samples from the freeze cores and piston cores 5N, 9P, 12P, and 15P, plotted by magnetic mineral
zone. Also shown are the North African dust and Bosumtwi soil samples. The magnetic mineral zones clearly separate into different regions of
the Day plot.
J.A. Peck et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 215 (2004) 37–57
magnetic minerals is supported by the relatively low
H cr values of the surface sediments (Fig. 4). The zone
A FORC diagram shows a distribution, centered about
H b=0 and further out along the H c axis, that is almost
completely closed and the contours are near horizontal, which indicates the presence of an SD lowcoercivity component (Fig. 6A). The subparallel
contour lines close to the H b axis and a small peak
close to H c=0 likely indicate the presence of a very
fine-grained SPM component (Fig. 6A) (Roberts et
al., 2000). The mixture of SD and SPM grains
suggested by the FORC diagram accounts for the
zone A samples plotting in the PSD region of the Day
plot (Fig. 5).
4.2.2. Zone B
Zone B ranges from 12.72 to 183.7 cm. The base
of zone B occurs at the transition from finely
laminated clay and carbonate muds to the weakly
laminated black sapropel. The magnetic mineral
properties of zone B are similar to zone A except
that concentration parameters (IS, M rs, and M s) are
reduced throughout much of this section (Fig. 4).
However, at the base of zone B, the magnetic
concentration parameters increase to values similar
to, or greater than, zone A (Fig. 4). Zone B samples
cluster within the PSD region of the Day plot (Fig. 5).
Towards the bottom of zone B, the S-ratio mineralogy
parameter shows a slight and gradual trend towards
lower-coercivity mineralogies (Fig. 4). The zone B
FORC distribution is closed and centered further out
along the H c axis at about 35 mT, indicating the
presence of an SD low-coercivity (magnetite) component (Fig. 6B) The near-vertical open contours near
the origin suggest the presence of an MD component
as well (Roberts et al., 2000).
4.2.3. Zone C
Zone C is coincident with the sapropel lithology
beginning at 183.7 cm and extends down to the first of
the D zones at 344.95 cm standardized depth (Fig. 4).
Zone C magnetic mineral concentrations are variable
but lower than those at the base of zone B (Fig. 4).
Magnetic mineral grain size remains rather uniform
across this zone and the parameters are just slightly
coarser than zone B values (Fig. 4). On the Day plot,
zone C samples plot within the PSD region just
slightly to the left of zone A and B samples (Fig. 5).
45
The low H cr and high S-ratio values of zone C
indicate greater proportions of magnetite–maghemite
mineralogy, which persist across the whole of zone C
(Fig. 4). In zone C, the FORC distribution is closed
and centered further out along the H c axis at about 35
mT, indicating the presence of an SD low-coercivity
(magnetite) component (Fig. 6C). The near-vertical
open contours near the origin suggest the presence of
an MD component as well (Roberts et al., 2000).
4.2.4. Zone D
Zone D is remarkable for its strong divergence in
magnetic mineral character from that of the other
zones (Figs. 4 and 5). Zone D magnetic mineral
concentration parameters are moderately high (Fig. 4).
Grain size parameters M rs/M s and H cr/H c display
much finer magnetic mineral grain size values and
range from PSD to SD on the Day plot (Figs. 4 and 5).
The magnetic mineralogy of zone D is also remarkable in the strong departure of the S-ratio and H cr
parameters towards greater proportions of highercoercivity mineralogies (Fig. 4). The zone D FORC
diagram has a wide, closed distribution centered
around 75 mT on the H c axis, indicating an SD
high-coercivity component (e.g., hematite, goethite,
greigite, and pyrrhotite) (Roberts et al., 2000; Weaver
et al., 2002). This mineral phase is not present in any
of the other zones (Fig. 6). The vertical width and shift
to negative H b values of this high-coercivity distribution indicate a positive mean-interacting field of the
mineral grains (Pike et al., 2001). The very high M rs/
IS ratio and low S-ratio are typical of greigite-bearing
sediment (Snowball and Thompson, 1990; Snowball,
1991; Reynolds et al., 1999). In addition, the high H cr
values and M rs/IS ratio (Fig. 4) are more suggestive of
the presence of greigite rather than pyrrhotite in the D
zones (Jelinowska et al., 1995; Peters and Thompson,
1998).
4.2.5. Zone E
Zone E is situated at 368.95–539.95, 671.15–
914.95, and 945.95–1127.0 cm standardized depth
(Fig. 4). Magnetic mineral concentration parameters
IS and M s remain very low across this zone, and
remanence-bearing material is extremely low as
shown by M rs (Fig. 4). Grain size parameters M rs/
M s and H cr/H c show a very distinct change to either
coarser MD grain sizes or to very fine SPM grain sizes
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J.A. Peck et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 215 (2004) 37–57
(Figs. 4 and 5), which do not as readily retain
remanent magnetizations. The zone E FORC diagram
does not contain an SD component and the contour
lines are open and parallel to the H b axis, indicating
that this zone contains predominantly MD grains (Fig.
6E) (Roberts et al., 2000). Magnetic mineralogy
parameters H cr and S-ratio also indicate a shift
towards moderately high-coercivity minerals between
the values of the D zones and those of the A, B, and C
zones (Fig. 4).
4.2.6. Barbados dust
The M rs/M s and H cr/H c ratios of the North African
dust sample place it in the PSD region of the Day plot
(Fig. 5). The S-ratio value of 0.68 and H cr value of 31
mT indicates a high proportion of high-coercivity
mineralogy (i.e., hematite and goethite). This low Sratio value is most similar to S-ratio values of zones D
and E (Fig. 4). A high-coercivity mineralogy is in
agreement with findings of other authors who describe
a prevalent reddish iron oxide staining on dust grains
emanating from North African Sahara and Sahel
source regions (Bloemendal et al., 1989; Balsam et
al., 1995; Pflaumann et al., 1998; Larrasoaña et al.,
2003a). Oldfield et al. (1985) also report the presence
of higher-coercivity (42F4.1 mT), reddish-brown
North African dust over Barbados. The FORC
diagram of North African dust displays the presence
of subparallel contour lines to the H b axis with a small
peak at H c=0 indicating the presence of an SPM
component as well as a possible PSD/MD component
(Fig. 6F).
4.2.7. Catchment soil samples
The catchment soil samples have an average Sratio of 0.84 and high magnetic concentration values
of IS (3410 8 m3 kg 1) and M s (17.5 mA m2 kg 1).
The soils plot within the PSD region of the Day plot,
near the E zone samples (Fig. 5). The FORC diagram
for one of the soil samples (Fig. 6G) shows the
characteristic SPM signature of subparallel contours
close to the H b axis with a small peak close to H c=0.
47
An SPM interpretation is supported by high percentages (between 9% and 11%) of the frequency
dependence of X.
4.3. Age model
An age model was produced from a suite of 30
AMS 14C dates from cores 12P, 15P, and 19P (Table
2). Despite efforts to constrain the 14C dating
procedure to more reliable materials such as wood
detritus, several age reversals appear in the dates
(Table 1), which exemplify some of the difficulty in
obtaining reliable 14C dates from many tropical
African lakes (Gasse, 2000). While a distinct downcore trend of older sediment ages exists in these data,
there is a significant amount of scatter below 700 cm
standardized depth (Fig. 7). The resulting age model
line was selected based upon clusters of more reliably
dated wood and sapropel organic material dates down
to a standardized depth of 705.25 cm (Fig. 7). It is
important to note that the resulting age model line
skirts the younger side of the main cluster of 14C dates
(Fig. 7). The tendency of bulk sediment to yield older
ages is likely a result of the frequent lowstands, which
increase the potential for reworking older material to
the lake basin. Below 705.25 cm standardized depth, a
linear regression was run through the remaining data
points, which have greater scatter. Data points
excluded from the regression calculation because they
yielded unrealistic ages are boxed (Fig. 7). This AMS
age model shows that the sediment cores span the last
26 cal ka (Fig. 7).
5. Discussion
Climate-driven processes can control changes in
the magnetic mineral content of sedimentary material.
These processes have important implications for the
paleoclimatic interpretations made from sedimentary
magnetic data. First, because North African dust has a
distinctive high-coercivity character (Oldfield et al.,
Fig. 6. FORC diagrams of representative samples of (A) zone A, (B) zone B, (C) zone C, (D) zone D, (E) zone E, (F) North African dust, and
(G) Bosumtwi soil. The vertical axis (H b) shows the degree of magnetostatic interactions between grains. The horizontal axis (H c) displays the
coercivity distribution of the different magnetic grain size and mineralogy components within the sample. To the right of each FORC diagram is
the scale bar for the contouring of particle density on the FORC diagram. Warmer colors represent greater concentrations of a particular
coercivity distribution within the sample (see Section 4.2 text for an explanation of each sample).
48
J.A. Peck et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 215 (2004) 37–57
Table 2
Radiocarbon dates and calibrated ages for cores 19P, 16P, 15P, and 12P from Lake Bosumtwi
Laboratory Original Standardized Core 19P F(years) Core 16P F(years) Core 15P F(years) Core 12P F(years) Calibrated age
number
depth
depth
(14C years)
(14C years)
(14C years)
(14C years)
[maximum
(cm)
(cm)
(cal age)
minimum
(years)]
AA43708
90.0
182.2
3012
47
–
–
–
–
–
–
AA43982
92.0
184.7
2949
70
–
–
–
–
–
–
OS28023
407.0
–
–
–
13,600
90
–
–
–
–
OS28024
416.0
–
–
–
13,600
75
–
–
–
–
OS28025
445.0
–
–
–
14,800
120
–
–
–
–
AA47363
84.0
315.0
–
–
–
–
9586
82
–
–
AA46922
276.0
408.0
–
–
–
–
12,317
65
–
–
AA47364
304.0
432.0
–
–
–
–
13,460
100
–
–
AA47366
494.0
558.2
–
–
–
–
14,211
91
–
–
AA47365
748.9
803.2
–
–
–
–
18,200
170
–
–
AA46924
761.9
818.2
–
–
–
–
14,412
75
–
–
AA51287
AA51286
AA51288
OS27967
AA51285
OS27966
AA43709
1002.0
1006.0
1010.0
1029.0
1074.0
1083.5
75.0
1046.0
1050.0
1054.0
1073.0
1118.0
1127.5
157.0
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
19,860
20,310
20,900
23,600
21,480
22,900
–
890
420
690
160
850
180
–
–
–
–
–
–
–
3356
–
–
–
–
–
–
AA43981
93.0
181.2
–
–
–
–
–
–
3061
49
AA43710
95.0
183.7
–
–
–
–
–
–
3129
43
AA43980
115.0
204.0
–
–
–
–
–
–
4193
52
AA51280
225.4
314.4
–
–
–
–
–
–
9604
89
AA51276
AA51284
346.1
292.7
434.9
381.7
–
–
–
–
–
–
–
–
–
–
–
–
19,950
12,890
1135
240
AA51278
346.9
435.9
–
–
–
–
–
–
12,970
320
AA51277
412.6
501.6
–
–
–
–
–
–
13,750
100
AA51279
413.6
502.6
–
–
–
–
–
–
13,800
110
AA51275
506.5
595.5
–
–
–
–
–
–
15,540
410
46
3321 (3233)
3081
3239 (3128)
2971
16,581 (16,329)
16,088
16,574 (16,329)
16,095
18,009 (17,710)
17,427
11,159 (10,825)
10,701
15,301 (14,298)
14,135
16,422 (16,168)
15,923
17,300 (17,032)
16,778
22,014 (21,623)
21,243
17,530 (17,264)
17,012
a
23,865
a
24,416
a
25,139
a
28,445
a
25,849
a
27,588
3679 (3608)
3482
3354 (3302)
3174
3382 (3358)
3270
4832 (4747)
4630
11,165 (10,940)
10,705
a
23,975
15,859 (15,520)
14,475
16,032 (15,600)
14,955
16,761 (16,500)
16,252
16,825 (16,560)
16,303
19,120 (18,560)
18,021
J.A. Peck et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 215 (2004) 37–57
49
Table 2 (continued)
Laboratory Original Standardized Core 19P F(years) Core 16P F(years) Core 15P F(years) Core 12P F(years) Calibrated age
(14C years)
(14C years)
(14C years)
[maximum
number
depth
depth
(14C years)
(cal age)
(cm)
(cm)
minimum
(years)]
AA51274
563.5
652.5
–
–
–
–
–
–
17,460
630
AA51273
616.3
705.3
–
–
–
–
–
–
15,090
250
AA51272
689.8
778.8
–
–
–
–
–
–
17,060
520
AA51270
760.1
849.1
–
–
–
–
–
–
17,360
300
AA51271
844.1
933.1
–
–
–
–
–
–
20,260
800
21,568 (20,771)
119,981
18,444 (18,044)
17,660
20,990 (20,310)
19,638
21,130 (20,660)
20,189
a
24,355
a
Denotes estimated calendar ages (see section 3.4 for details).
1985; Bloemendal et al., 1988; Balsam et al., 1995;
Hunt, 1986; Maher, 1986), downcore shifts in
magnetic mineral content may reflect variations in
the input of dust to Lake Bosumtwi. Second, magnetic
changes may occur because of diagenetic alterations
of the sediments caused by changes in the physicochemical conditions of the lake water (Leslie et al.,
1990; Roberts et al., 1999; Larrasoaña et al., 2003b).
Fig. 7. Age model showing AMS 14C ages from cores 12P, 15P, and 19P that have been converted to calendar ages using the methods described
in the text. An age of 154 years BP at 29.4 cm is derived from varve counting and 210Pb dating of the freeze cores (W. Wheeler, written
communication, 2002). Below 705.25 cm standardized depth, the age model was constructed using a linear regression, ignoring two ages
outlined with boxes.
50
J.A. Peck et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 215 (2004) 37–57
The combination of these two factors can be used to
identify several distinct climatic periods recorded in
the Bosumtwi sediments.
5.1. Glacial interval
Magnetic mineralogy, grain size, and sediment
organic content record a well-defined transition from
the last glacial period (zones D and E) to interglacial
(zones A–C) climate at ca. 12 cal ka (Table 1; Fig. 8).
Magnetic mineralogy parameters H cr and S-ratio
indicate a pronounced shift from high-coercivity
magnetic mineralogy (average S-ratio, 0.73) to lowcoercivity magnetic mineralogy (average S-ratio,
0.91) during the glacial to interglacial transition.
Studies of marine sediment cores surrounding Africa
have similarly documented greater levels of highcoercivity magnetic minerals in glacial-aged sedi-
ments (Bloemendal et al., 1988; 1989; Bloemendal
and deMenocal, 1989; Pflaumann et al., 1998;
Dinares-Turell et al., 2003). The magnetic grain size
parameters in Lake Bosumtwi also clearly show
glacial to interglacial differences ((Figs. 5, 6, and
8)). A low-coercivity SD component is present during
the interglacial, whereas the glacial is characterized by
a high-coercivity SD component in zone D and a
high-coercivity MD assemblage in zone E. The
origins of the distinctive glacial to interglacial shift
in the magnetic mineral signal are discussed in the
remainder of this section.
We interpret the increased proportion of highcoercivity magnetic mineral grains in zones D and E
to result in part from increased soil deflation and
aerosol production within the Sahel during the arid
glacial period (Fig. 8). Similar characteristics have
been documented during current arid conditions in the
Fig. 8. Lake Bosumtwi magnetic mineral (A) concentration (M s), (B) grain size (M rs/M s) and (C) mineralogy (S-ratio) parameters, and (D) losson-ignition organic content (Brooks et al., in preparation) on calibrated calendar age scale. The range of sapropel deposition during the AHP is
also shown. (E) Also shown is the June–August (dashed line) and June–October (solid line) summer insolation for 68 N (Paillard et al., 1996)
(see text for explanations).
J.A. Peck et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 215 (2004) 37–57
Sahel source region. Currently, aerosol concentration
varies inversely with precipitation in the Sahel source
region (Brooks and Legrand, 2000), and modern
North African dust is rich in high-coercivity iron
oxide minerals (Oldfield et al., 1985; Hunt, 1986;
Maher, 1986). Therefore, increased aridity in the
Sahel source region can result in elevated aerosol dust
flux of high-coercivity minerals into Lake Bosumtwi
and the adjacent ocean.
North African aridity is directly related to the
strength of the summer monsoon, with strong
summer monsoons bringing in moisture, and weak
summer monsoons creating arid conditions. On
glacial–interglacial time scales, the orbital precession
cycle modulates land surface insolation and thus
may strengthen or weaken the summer monsoon
(Pokras and Mix, 1987). Decreases in summer
season insolation weakens the summer monsoon
circulation, decreasing the onshore advection of
moisture laden ocean air. Thus, during summer
insolation minima, North African lakes experienced
negative moisture balance and greatly reduced lake
volume. Numerous North African lake records,
including Lake Bosumtwi, document arid conditions
throughout North Africa during the last glacial
period (Talbot and Delibrias, 1980; Kutzbach and
Street-Perrott, 1987; Talbot and Johannessen, 1992;
Kutzbach et al., 1996; Gasse, 2000). Owing to
increased aridity in the aerosol source areas, peaks of
terrestrial sediment supply have been shown to
match precessional minima (Bozzano et al., 2002;
Larrasoaña et al., 2003a). Atmospheric dust flux
emanating from Saharan and Sahel source regions
was increased by two to four times during the arid
and windy conditions of the last glacial period
relative to the Holocene (Goudie and Middleton,
2001; Swezey, 2001). Because North African aerosol
has a high-coercivity character, we interpret the
glacial period rock magnetic record from Lake
Bosumtwi to be partially due to the increased influx
of aerosol dust during that time.
In addition to the increased dust flux discussed
above, there is evidence that reductive diagenetic
processes related to physico-chemical lake characteristics also contribute to the high-coercivity character
of zones D and E. Microbially mediated reductive
diagenesis of iron oxide to iron sulfide minerals
such as greigite, pyrrhotite, and ultimately pyrite is
51
typical of anoxic bottom waters such as in Lake
Bosumtwi, (Canfield and Berner, 1987; Karlin,
1990; Leslie et al., 1990). The magnetic minerals
of zones D and E have the characteristics of
reductive diagenesis.
Zone E sediment contains little remanence-bearing
minerals as indicated by the exceedingly low values of
M rs (Fig. 4). Zone E samples plot near the MD region
of the Day plot (Fig. 5) and the FORC diagram (Fig.
6E) indicates the presence of MD grains. Reductive
diagenesis yields a higher-coercivity sediment assemblage because it preferentially affects fine-grained
followed by coarse-grained ferrimagnetic iron oxides
first, and has less of an effect on canted antiferromagnetic iron oxides (Karlin, 1990; Bloemendal et al.,
1993; Robinson et al., 2000). Reduction diagenesis is
likely to have contributed to the very low remanence
values and MD high-coercivity character of the E
zones.
The D zones display characteristics of iron
sulfide minerals produced by reduction diagenesis.
Soluble iron, organic matter, and limited sulfate are
conducive to the production of authigenic greigite
(Fe3S4) under anoxic, sulfate-reducing lake conditions (Snowball, 1991; Maher and Thompson, 1999;
Reynolds et al., 1999; Roberts et al., 1999; Weaver
et al., 2002). The very large M rs/IS ratio, high H cr,
and low S-ratio values of the D zones are
indicative of the iron sulfide, greigite (Fig. 9)
(Snowball, 1991; Peters and Thompson, 1998; Hu
et al., 2001). Although greigite and hematite have
similar high-coercivity properties, greigite has a
large magnetization as do the D zones (Fig. 4)
(Thompson and Oldfield, 1986). The high-coercivity SD phase present on the FORC diagram closely
resembles that of greigite (Roberts et al., 2000) and
pyrrhotite (Weaver et al., 2002) (Fig. 6D). Lastly,
zone D samples extend into the SD region of the
Day plot up to M rs/M s values of 0.6, a feature
frequently reported for iron sulfide minerals (Snowball, 1991; Snowball, 1997; Jelinowska et al., 1999;
Weaver et al., 2002). Future study will examine the
zone D iron sulfide phase in greater detail; however,
from the data presented here, it is clear that significant
paleolimnologic changes occurred during the D zones
of the glacial period reflecting periods of intense
drought. A mechanism for the extreme aridity during
the D zones is proposed in Section 5.2.
52
J.A. Peck et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 215 (2004) 37–57
Fig. 9. Lake Bosumtwi magnetic mineral (A) concentration (M s), (B) mineralogy (S-ratio), (C) mineralogy (M rs/IS), and (D) grain size (M rs/M s)
parameters on calibrated calendar age scale. (E) North Atlantic record of ice-rafted debris (Bond et al., 1997). Large increases in lithic grains per
gram (plotted toward left) represent the YD and Heinrich ice rafting events (H1 and H2). DC denotes layers rich in detrital carbonate. The YD
event and H1 correlate well in time and duration with magnetic mineral zones D1 and D2, respectively. The age offset between zone D3 and
Heinrich layer 2 is within the uncertainty of the Lake Bosumtwi age model below 700 cm core depth (Fig. 7).
5.2. Heinrich events
The D zones described above are present in three
discrete intervals, each displaying strikingly similar
magnetic concentration, grain size, and mineralogy
parameters (zones D1–3) (Figs. 4, 8, and 9)). Zone D1
is situated between 12.0 and 12.9 cal ka, approximately centered upon and equal in duration to the
Younger Dryas (YD or H0) event (Fig. 9). Zone D2 is
situated between 16.8 and 17.8 cal ka and is also
correlative with the temporal onset of Heinrich event
1 (H1) (Fig. 9). Zone D3 is situated between 22.3 and
23.0 cal ka, occurring approximately 1500 years after
the Heinrich 2 event (H2) (Fig. 9). However, this age
offset is easily within the uncertainty of the Lake
Bosumtwi age model (Fig. 7). The magnetic data
presented in Section 5.1 suggest that the three D zones
are greigite-bearing intervals. Below, we discuss how
these D zones were likely diagenetically produced
under extreme arid conditions and lake lowstands.
Reynolds et al. (1999) document lacustrine greigite
formation during periods of drought in a detailed
study of 20th century reservoir sedimentation in
Texas, USA. During drought years, reduced lake
level leads to increases in pore water sulfate concentrations, thus favoring the formation of greigite via
microbially mediated sulfate reduction (Reynolds et
al., 1999). We hypothesize a similar mechanism
operating in Lake Bosumtwi, where during intense
periods of evaporation, enhanced reductive diagenesis
produces the greigite in the D zones. In addition to the
presence of greigite, there are other sedimentary lines
of evidence to support the interpretation of the D
zones as representing intervals of intense drought in
West Africa. First, a compact dense mud with rootlets
is abruptly overlain by low-density, unconsolidated
mud with a high water content at 4.1 m in core 16P.
The lower compact lithology is interpreted as desiccated lacustrine mud that represents a brief lake level
lowstand of about 60 m below present lake level.
Radiocarbon dates on either side of this contact place
the low stand at about 16,330 cal years BP, close to
the timing of zone D2 (Peck et al., 2000; Brooks et al.,
in preparation) (Table 2). Such an extreme lake level
lowstand could have produced the conditions for
greigite formation as described by Reynolds et al.
J.A. Peck et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 215 (2004) 37–57
(1999). Second, other paleoclimate records from the
region also strongly suggest intense aridity at H1 time
(Maley and Brenac, 1998; Gasse, 2000; Stager et al.,
2002).
Elsewhere in Africa and the adjacent oceans,
paleoclimate records of the YD and H1 events have
also been documented (Williamson et al., 1993;
Sirocko et al., 1996; Gasse, 2000; DeMenocal et al.,
2000a; Abell and Plug, 2000; Gasse, 2002; Stager et
al., 2002). Vegetation records from Lake Barombi
Mbo, Cameroon, show an abrupt reduction in
precipitation during the YD (Maley and Brenac,
1998). Talbot and Johannessen (1992) characterize
Lake Bosumtwi as being bgenerally low with intervals
of relatively intense evaporationQ prior to about 10 14C
ka. Their isotopic study of sedimentary organic matter
identified the YD as a dry phase in the lake’s history
as well as additional dry periods at about 17.5, 21.9,
and 25.9 cal ka (Talbot and Johannessen, 1992).
Talbot and Kelts (1986) found that low lake levels in
Bosumtwi corresponded to saltier lake conditions and
more intense diagenetic alteration of primary carbonate precipitates to dolomite.
The reasonable temporal correlation of the D zones
with North Atlantic Heinrich events (Bond and Lotti,
1995; Bond et al., 1997) suggests that these high- and
low-latitude climatic events are linked through ocean–
atmosphere processes and driven by the same forcing
mechanism. Heinrich event-induced changes to North
Atlantic surface currents, SST, and the thermohaline
circulation are thought to have far-reaching teleconnections to lower-latitude sites (Bond et al.,
1997; Sirocko et al., 1996; Moreno et al., 2002;
Broecker, 2003). Changing ocean conditions near
West Africa may influence the strength of the summer
monsoon and position of the ITCZ (Chang et al.,
1997; Nicholson, 2000; DeMenocal et al., 2000b;
Stager et al., 2002; Bard, 2002). Such teleconnections
may result in the abrupt and intensely arid D zones
corresponding in time to the Heinrich events. A
similar coincidence between Atlantic sea surface
temperature and the position of the ITCZ in South
America has been suggested as the control on aridity
in the northern portion of the South American
continent (Haug et al., 2001). Haug et al. (2001) also
speculate that the driving control on Atlantic Ocean
conditions and the position of the ITCZ may
ultimately lie in the equatorial Pacific. The timing of
53
the D zones in Lake Bosumtwi is not sufficiently
resolved in this study to differentiate North Atlantic
versus equatorial climatic forcing. However, this
study does provide additional evidence as to the
global distribution of abrupt climate change during
Heinrich events.
5.3. African Humid Period (AHP)
During the AHP, a prominent, black, organic-rich
sapropel lithology was deposited (Fig. 8). This
lithology corresponds to magnetic zone C (Figs. 8
and 9) and is characterized by fine-grained, SD, lowcoercivity (i.e., magnetite and maghemite) minerals
(Table 1; Figs. 4, 5, and 6C). The relatively high Sratio and low H cr are interpreted as a reduction in the
atmospheric transport of high-coercivity aerosols to
Lake Bosumtwi because of a greener and more humid
source region. This interpretation agrees with other
geologic records documenting greatly diminished dust
flux from the Sahara and Sahel (Gingele, 1996;
DeMenocal et al., 2000a; Swezey, 2001) and the
greening of the Sahara during the AHP (Street and
Grove, 1976; Giresse et al., 1994; Maley and Brenac,
1998; Abell and Hoelzmann, 2000; Arz et al., 2003).
The lower-coercivity mineralogy of SD size may
also result from bacterial magnetite production. An
increase in magnetotactic bacteria populations during
sapropel production has been described in the
Mediterranean Sea; however, these SD magnetite
grains are often lost by the process of reductive
diagenesis (Dinares-Turell et al., 2003). Diagenetic
magnetic mineral enhancement of iron sulfides has
also been associated with sapropel production (Roberts et al., 1999; Maher and Thompson, 1999).
However, unlike the D zones, the magnetic mineral
parameters do not indicate the presence of iron sulfide
minerals in zone C (Figs. 4, 6, 8, and 9)). Furthermore, evidence that the lake was filled and overflowing the crater rim during this time interval (Talbot
and Delibrias, 1980; Turner et al., 1996b) makes
evaporative concentration and iron sulfide formation
less likely to have occurred in comparison to earlier
periods of greatly restricted lake levels (Reynolds et
al., 1999).
The onset of the AHP (and zone C) is estimated to
have occurred at ca. 12,037 cal years ago with a sharp
decrease in the flux of aeolian material to the lake.
54
J.A. Peck et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 215 (2004) 37–57
However, this boundary is based upon a linear
regression between samples at 10,940 and 14,298
cal years BP and assumes a constant rate of
sedimentation over this interval (Fig. 7). Despite these
uncertainties in the timing of this transition, it is
apparent that the transition to humid conditions was
abrupt, occurring in less than 300 years.
The end of magnetic zone C and the interval of
organic-rich sapropel deposition ended just as
abruptly at ca. 3200 cal years ago. A sharp lithologic
contact marks the top of the sapropel and the
overlying dark gray, thinly laminated mud. The
transition from zone C to zone B is characterized by
an increase in magnetic concentration and in highcoercivity magnetic minerals (i.e., hematite and
goethite), which suggests an increase in aridity at this
time (Figs. 4, 8, and 9)). A climatic shift around 4 cal
ka towards increased aridity is supported by lake level
lowstands in other African basins at this time (Vincens
et al., 1998; Gasse, 2000).
Some well-dated records from elsewhere in Africa
do not agree with the timing of the onset and demise
of the AHP recorded in Lake Bosumtwi sediments.
Based upon decreased aeolian sedimentation adjacent
to West Africa, DeMenocal et al. (2000a) report the
abrupt onset of the AHP at 14.8 cal ka, in agreement
with the infilling of Lake Victoria (Stager et al., 2002).
Lake Bosumtwi does not record AHP-like magnetic
mineral characteristics prior to the YD (Fig. 8).
However, the infilling of other lakes in North Africa
also lagged behind the YD by 1–2 ka (Gasse, 2000).
DeMenocal et al. (2000a) also suggest that the AHP
ended abruptly with a return to more arid conditions at
5.5 cal ka, several thousands of years before changes
are recorded in Lake Bosumtwi.
The later onset and termination of the AHP in the
Lake Bosumtwi record as compared to the nearby
well-dated marine record may be a function of either
differences in age control or regional climatic differences. The Lake Bosumtwi region has a rainfall
maximum in June; a cooler, drier period in August;
and a secondary rainfall peak in October (Turner et al.,
1996a). Fig. 8 illustrates how the radiation at 68N for
June through October lags the June–August curve by
about 3 ka and provides a closer match to the Lake
Bosumtwi record. Hence, monsoon strength near Lake
Bosumtwi may be responding to insolation variation
that includes both rainfall peaks (Ruddiman, 2001).
The abrupt start and end of sapropel deposition
occurred at similar June–October insolation levels,
which are 2.8% greater than today’s insolation and 4.0
% greater than the insolation minimum (Fig. 8). This
level of enhanced insolation and the corresponding
strengthened summer monsoon may represent a
threshold above which effective moisture supply
greatly increased, Lake Bosumtwi transgressed
greatly, and sapropel deposition occurred.
6. Conclusions
The Lake Bosumtwi sedimentary record displays
clearly defined transitions in magnetic mineral character that relate to regional- and global-scale climate
dynamics over the past 26 cal ka. The glacial–
interglacial transition at 12 cal ka is represented by
a pronounced shift from high-coercivity magnetic
minerals during the glacial period to low-coercivity
magnetic minerals during the interglacial. These
results are interpreted as a shift from increased rates
of aeolian dust flux during the glacial to substantially
reduced dust flux during the AHP. The glacial period
is also characterized by zones (D1–3) of increased
concentrations of iron sulfide minerals (likely greigite) centered at 12,470, 17,290, and 22,600 cal ka
that correlate in timing and duration with the YD and
Heinrich events H1 and H2 as recorded in highlatitude paleoclimate records. The greigite-bearing D
zones likely formed during periods of intense aridity
in the Lake Bosumtwi region. Since the end of the
AHP at approximately 3.2 cal ka, more arid conditions have returned to Lake Bosumtwi as evidenced
by an increase in high-coercivity magnetic minerals.
From this study, it is clear that examination of
magnetic mineral changes within Lake Bosumtwi at
greater temporal resolution and spanning multiple
glacial–interglacial cycles may reveal significant
insights into equatorial climate in the context globalscale climate dynamics.
Acknowledgements
We wish to thank P. Amoako, the Ghanaian
Geological Survey, and the people living at Lake
Bosumtwi for their assistance with this study. The
J.A. Peck et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 215 (2004) 37–57
help with piston coring by N. Peters, P. Cattaneo, K.
Ford, and W. Wheeler is much appreciated. J.
Russel, K. Brooks, and R. Arimoto were extremely
helpful by providing radiocarbon ages, loss-onignition data, and aerosol samples, respectively. W.
Wheeler greatly aided us by providing freeze core
samples and helpful discussions concerning the
cores. We wish to thank M. Winklhofer for providing his MATLAB FORC program, and both C. Pike
and M. Winklhofer for helpful discussions regarding
the FORC diagrams. We thank Drs. J. Bloemendal
and N. Thouveny for their helpful reviews. This
research was supported by the US National Science
Foundation grant ATM-0117019.
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