Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 110–119
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Palaeogeography, Palaeoclimatology, Palaeoecology
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o
Measurement of 10Be from Lake Malawi (Africa) drill core sediments and
implications for geochronology
L.R. McHargue a,⁎, A.J.T. Jull b, A. Cohen a
a
b
Department of Geosciences, University of Arizona, 1040 E. Fourth Street, Tucson, Arizona 85721, United States
NSF Arizona AMS Laboratory, University of Arizona, 1118 E. Fourth Street, Tucson, Arizona 85721, United States
a r t i c l e
i n f o
Article history:
Received 19 August 2008
Received in revised form 29 January 2010
Accepted 4 February 2010
Available online 12 February 2010
Keywords:
Beryllium-10
Cosmogenic radionuclide
Lake sediments
Lake Malawi
Radiometric dating
a b s t r a c t
The cosmogenic radionuclide 10Be was measured from drill core sediments from Lake Malawi in order to
help construct a chronology for the study of the tropical paleoclimate in East Africa. Sediment samples were
taken every 10 m from the core MAL05-1C to 80 m in depth and then from that depth in core MAL05-1B to
382 m. Sediment samples were then later taken at a higher resolution of every 2 m from MAL05-1C. They
were then leached to remove the authigenic fraction, the leachate was processed to separate out the
beryllium isotopes, and 10Be was measured at the TAMS Facility at the University of Arizona. The 10Be/9Be
profile from Lake Malawi sediments is similar to those derived from marine sediment cores for the late
Pleistocene, and is consistent with the few radiocarbon and OSL IR measurements made from the same core.
Nevertheless, a strong correlation between the stable isotope 9Be and the cosmogenic isotope 10Be suggests
that both isotopes have been well mixed before deposition unlike in some marine sediment cores. In
addition, the correlation of beryllium isotopes to a proxy of lake level TOC (Total Organic Matter) from Lake
Malawi indicates that the concentrations of 10Be in the lake sediments result from the combined effects of
global and local climates on lake level, local hydrology, and sediment transport in the Lake Malawi basin
rather than as a direct response to its production in the atmosphere modulated by the intensity of the Earth's
dipole. Therefore, a direct correlation of the 10Be/9Be to a chronology derived from the paleomagnetic
variations measured from marine sediments was not possible. Nevertheless, a comparison of the 10Be/9Be
chronology, allowing for decay, at Lake Malawi to that of the global marine paleomagnetic record suggests
that the bottom of core MAL05-1B is no more than 750 ka in age.
Published by Elsevier B.V.
1. Introduction
We investigated using the cosmogenic radionuclide 10Be to date
sediments from Lake Malawi because its half life duration is
appropriate for the time frame in which the lake sediments were
deposited, as well as the possibility of tying this record to that of
marine sediments. More than 600 m of the core have been obtained
from Lake Malawi (Scholz et al., 2006, and Scholz et al., 2011-this
issue), located in the southern part of the East African Rift Valley.
Sediments from Lake Malawi contain an important record for the
study of the tropical paleoclimate. However, most of the sediments of
the core from Lake Malawi are well beyond the approximate 50 kyr
range of radiocarbon dating. In addition, paleomagnetic data are
difficult to interpret because of the low latitude of the drill site
coupled with the fact that the core is unoriented. Lake Malawi lies in
the western branch of the African rift valley and is the second largest
(22,490 km2) and second deepest (∼700 m) of the African rift lakes
⁎ Corresponding author. Fax: + 1 520 621 9619.
E-mail address: mchargue@physics.arizona.edu (L.R. McHargue).
0031-0182/$ – see front matter. Published by Elsevier B.V.
doi:10.1016/j.palaeo.2010.02.012
(Johnson and Ng'ang'a, 1990). The lake's long, narrow and deep
morphology, as well the positions of bathymetric deep areas and
sediment depocenters are all controlled by its origin as a series of
linked half-graben basins (Rosendahl, 1987; Scholz and Rosendahl,
1990). Multiple reflection seismic surveys of the lake have shown a
very thick accumulation of rift sediments within the basin, in places
exceeding several kilometers (Scholz, 1989). The lake itself is
probably at least early Pliocene in age, based on both the sediment
thickness inferred from seismic data and outcrop exposures of
lacustrine deposits along the lake's margin (Betzler and Ring, 1995).
Although Lake Malawi today is deep and freshwater, with an outlet
to the Shire River, many lines of evidence show that this has not
always been the case. Early studies of seismic stratigraphic data from
Lake Malawi indicated that extraordinary lake level fluctuations had
occurred in the lake's past (Scholz and Rosendahl, 1988), although
from that data alone the timing of such events could not be
constrained. Detailed studies of some of these lake level drops of up
to 550 to 600 m below modern lake level, documented by the Lake
Malawi drill cores attest to extraordinary climate variability in terms
of moisture balance (Scholz et al., 2007; Cohen et al., 2007; Brown
et al., 2007). This history is crucial in addressing questions about the
L.R. McHargue et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 110–119
climate and environmental change of eastern Africa. For example,
how have climate variables such as moisture and wind direction
changed over the glacial/interglacial and orbital cycles, how are these
changes tied to global climate, and how has the response of Lake
Malawi to these changes influenced local and regional flora and
fauna? And lastly, how has this extraordinary climate variability
influenced human evolution and expansion?
Geochronology is central to addressing these questions. The
cosmogenic radionuclide 10Be has advantages that may be applicable
to dating the sediments of Lake Malawi. 10Be is always being produced
in the atmosphere and removed within a year by dry and wet
precipitations. Thus, there is a constant supply of 10Be to the waters of
Lake Malawi, within which the 10Be is removed to the sediments
below. Hence, 10Be can be measured from all the sediments
throughout the length of the core. The presumed age of the oldest
sediments at the bottom of the core is less than the half life of 10Be
(1.34 × 106 yrs, Nishiizumi et al., 2007). In addition, the production of
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10
Be with time varies significantly. This variance in production is
random in time due to the randomness in the intensity of the dipole
field that modulates it. Thus, a 10Be record possibly could be correlated
to the paleomagnetic record and would be unique. The record of the
intensity of the dipole field has been progressively pushed back to
approximately 2 Ma from stacked marine sediment core records over
the last decade (Valet et al., 2005) which would cover the entire
duration of sedimentation expected for the Lake Malawi drill cores.
However, variations in 10Be concentrations due to the impact of
climate on lake levels, sedimentation rate, and sediment composition
need to be addressed and may make the method inapplicable.
In 2005 the Lake Malawi Drilling Project (LMDP) recovered
approximately 600 m of high quality drill core from two sites in
Lake Malawi, the southernmost of the African Rift Valley great lakes
(Scholz et al., 2006,) (Fig. 1 map). Lake Malawi was drilled in order to
obtain a high quality and continuous record of paleoclimate variability
in the African continental tropics. Sedimentological indicators such as
Fig. 1. Locations of the two sites in Lake Malawi and bathymetry. Site 2, in the northern Lake Malawi is at a water depth of 361 m, and site 1 (cores B & C) is at a water depth of 593 m.
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a dominance of sublittoral to deep water mud (in large part
laminated) and limited evidence of paleosols or other evidence of
prolonged exposure show that deposition in the core record was very
continuous and, compared to similar duration marine drill core
records, very rapid. Recent measurements of 10Be were made from
test samples from the recently acquired drill core from Lake Malawi,
East Africa (Scholz et al., 2006).
The longest of the Lake Malawi cores, MAL05-1B extended to
382 m below the lake floor and a water depth of 593 m, with an
average core recovery of 92% (Fig. 1 core stratigraphy). Based on initial
geochronometric information (paleomagnetic reversal and excursion
data) this core was initially estimated to extend back to 1.5 Ma from
estimated reversal picks, which would have made it by far the longest
continuous core record yet obtained from the continent of Africa.
However, a firm chronology based on these methods alone is
uncertain and other unpublished data now suggest a much younger
age for the base on the 1B core. Uncertainties about the interpretation
of the paleomagnetic record, the temporal range limitations of both
the OSL and 14C methods, and the uncertainty of success in applying
other dating methods because of limited amounts of appropriate
materials (e.g. Ar/Ar, and U-series dating) led us to explore how 10Be
might be used for dating the older parts of the Malawi core record.
The variations in the concentration of the cosmogenic radionuclide
10
Be in marine (Robinson et al., 1995; Frank et al, 1997; Carcaillet et al.,
2004) and lake sediments (Horiuchi et al., 1999; Ljung et al., 2007;
Belmaker et al., 2008) can serve as a proxy for the cosmic-ray flux.
Since the intensity of the cosmic-ray flux is inversely related to the
intensity of the geomagnetic field the history of the latter can be
derived. The geochemical cycle of 10Be on the Earth begins as high
energy cosmic-rays, primarily in the upper atmosphere, break oxygen
and nitrogen apart (small amounts of 10Be are also produced in situ in
materials on the surface of the earth and are of interest in
geomorphology). Consequently, the production of 10Be is proportional
to the cosmic-ray flux incident upon the Earth which, in turn, is a
function of the intensity of the primary galactic and solar cosmic-rays,
and their modulation by the solar and geomagnetic fields. After 10Be is
produced it is quickly scavenged by particulate matter, and within a
year is precipitated onto the surface of the earth. After deposition onto
the surface of the earth 10Be is transported by water, wind, or currents
within the sea. Most 10Be (that which has not decayed during
transport) is eventually sequestered in ice, soils, and lake and marine
sediments, though it may be recycled later (McHargue and Damon,
1991).
Initial studies of 10Be in Antarctic ice, (Raisbeck et al., 1987) showed
that the concentration varied with depth and was found to be
anomalously high at 35 and 60 ka. These wide variations in the
concentration of 10Be in time were shown to be global in extent by later
work in marine sediments (McHargue et al., 1995). This was shown,
for example, at DSDP Leg 64, site 480, a drill site lying on a slope of the
Guaymas Basin in the Gulf of California (660 m in depth) that was
chosen because of its high sedimentation rate (100 cm/kyr) and
varved sediments. At this site there was evidence for two geomagnetic
excursions, the Mono Lake and the Laschamp, in which the inclination
of the dipole field deviated far from normal (Levi and Karlin, 1989).
10
Be concentrations in sediments from the sections of the core near
these excursions were unusually high. In general, the variations in the
10
Be concentrations, except for the 10Be anomaly associated with the
Mono Lake, seemed consistent with what would be expected for
modulation of the dipole field as shown by the research on the
magnetization of marine and lake sediments (Tric et al., 1992;
Meynadier et al., 1992; Thouveny et al., 1993; Carcaillet et al., 2004;
Knudsen et al., 2008).
From sediments from the Blake Outer Ridge (site CH88-10P) 10Be
production was shown to vary inversely related to the geomagnetic
field (McHargue et al., 2000). 10Be data from this site was directly
compared to the paleointensity data that were measured from the
same sediment samples (Schwartz et al., 1998). A direct comparison
of variances best showed the relationship between 10Be and the
paleointensity data. This was done by converting the 10Be data to
variances, and the NRM/ARM paleointensity data (Natural Remanent
Magnetization/Anhysteretic Remanent Magnetization) to inverse
variances about linear regressions.
Previous work on sediments from the Gulf of California and the
Blake Outer Ridge showed that it was possible to obtain reliable
cosmic-ray records from 10Be from marine sediments. Nevertheless,
reliable isotopic and paleointensity measurements from sediments,
corals, or ice, are typically difficult to obtain. Many environmental
complications affect the deposition of 10Be (Kok, 1999; Frank, 2000).
Lake systems are even more sensitive to short-term environmental
influences than the ocean due to the sensitivity of their lake levels,
sediment supply, and water chemistry to regional climate. With these
problems in mind the earlier work on two marine sediment cores
from the Atlantic and the Pacific will be compared to the 10Be results
from Lake Malawi to provide context.
2. Methodology
The two cores obtained by the Lake Malawi Drilling Project in this
initial study were initially sampled at intervals of 10 m throughout the
80 m of core MAL05-1C, and in core MAL05-1B from 80 to 382 m.
Sediment samples were then later taken every 2 m in core MAL05-1C.
The sediment samples from Lake Malawi were cut in 1-cm thick
blocks, estimated to represent about an average of about 20 yrs of
sedimentation. A 1–2 g split of the each sample was processed
keeping the rest of the sample in reserve. Each split was leached in a
solution of 25% acetic acid and 0.04 M hydroxylamine-HCl to separate
the authigenic mineral fraction of the sediment from the terrigenous
fraction (Bourles et al., 1989). The composition of the authigenic
fraction of the sediment more closely reflects the composition of
seawater than the whole sediment (McHargue et al., 2000) and is
composed of exchangeable ions, carbonates, and iron–manganese
hydroyoxides. An aliquot of the leachate was diluted with 5% HNO3
and set aside for Inductively-Coupled Plasma Atomic Emission
Spectrometer (ICPAES) analysis of elemental composition (Fe, Al, Ca,
Mn, and Mg). The residue was washed, dried, and weighed. The
difference, the mass loss, between the initial mass and the residue is
the presumed authigenic fraction of the sediment. A 2-mg beryllium
carrier was added to the remaining solution. The beryllium was
separated and purified by solvent extraction, precipitation of Be(OH)2,
and then combusted to BeO.
Beryllium-10 was measured on the Tandem Accelerator Mass
Spectrometer at the University of Arizona. In past measurements
the blank (10Be/9Be) for the beryllium carrier averaged less than
2 × 10− 14. Repetitive measurements of the NIST standard (SRM 4325)
have determined that the typical error for a 10Be measurement with
the tandem accelerator to be less than 2%.
3. Results and discussion
The sediment mass loss after leaching varied from less than 5% to
almost 30%. A plot of the summed cation concentration for the major
elements of the aliquot against the measured mass loss shows that,
within the error of analytical and physical measurement of the
samples, the relationship is close to linear. Most of the weight loss is
attributable to the leaching of iron and manganese, and to a lesser
extent, calcium and magnesium species. Aluminum is slightly
negatively correlated to the mass loss from leaching. The concentrations of beryllium in the leachate, particularly 10Be, are independent
of the amount of sediment loss. This may be due to the restriction of
beryllium in the pore water and exchangeable phases of the sediment
during accumulation of the iron/manganese hydroyoxide phases.
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We compared the data from Lake Malawi cores 1B and 1C to data
from the Gulf of California (DSDP site 64, core 480) in order to define
the differences between the depositional regimes of marine and lake
sediments. The sediments deposited in the Gulf of California are varved
and are composed of alternating variations in the concentrations of
clay from terrestrial sources and seasonally induced diatom blooms.
Iron versus the aluminum obtained from the leachate from sediment
samples of the Gulf of California in Fig. 2b are plotted, along with that of
the totally digested residue after leaching. The aluminum to iron ratio
of the leachate is significantly different from that of the clays and the
diatoms in the residue of the varved sediments. (The silicates are not
attacked by the mild leaching during removal of the iron-enriched
authigenic fraction of the sediments.)
A plot of beryllium and beryllium-10 measured from the same
samples shows that the 10Be content in the authigenic fraction of the
sediment is closer to its probable oceanic content than that in the
residue (Fig. 2b). The concentration of the terrigenous beryllium (9Be)
is higher in the clay-rich residue than in the more diatomaceous
residue. In the diatomaceous residue the beryllium isotopes are more
similar to that of the authigenic fraction than the terrigenous residue
as would be expected. Nevertheless, both the clay-rich and diatomaceous residues are on the same trend line indicating riverine source
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dependence between 9Be and 10Be. On the other hand, in the
authigenic fraction, the concentrations of beryllium and 10Be are
largely independent at site 480 due to the different origins of 10Be and
beryllium in the open sea.
The residue, that is the sediment remaining after leaching, was not
processed for the Lake Malawi sediments for the initial study.
Nevertheless, comparisons can be made between the marine
sediments from the Gulf of California and those from Lake Malawi.
There are significant differences in the composition of those
sediments deposited in an open ocean environment to those
deposited in a restricted lake basin. The aluminum and iron
concentrations in the residue for the ocean sediments (Fig. 2) are
similar to that of the terrigenous material, such as clays within the
sediments and diatoms. The aluminum to iron ratio of the authigenic
fraction of marine sediments from the Gulf of California is positively
correlated and is slightly under 0.2 similar to that of sea water of 0.25
(Martin and Whitfield, 1983). On the other hand, the aluminum to
iron ratio of the authigenic fraction of Lake Malawi sediments are not
correlated to one another (Fig. 3). The ratio of aluminum to iron
ranges from 0.01 to 0.1 probably reflecting significant variations in
lake and pore water chemistry with time affecting iron removal and
accumulation in the sediments.
Fig. 2. Measurements of aluminum, iron, and beryllium isotopes from two phases of the marine sediments, the leached and the residue, from the Gulf of California (DSDP Leg 64, site
480) are plotted as a comparison to the lake sediments from Lake Malawi in Fig. 4. (a) Aluminum as a function of iron from two phases of Gulf of California sediments, the leachate
(authigenic fraction) and the residue (terrigenous and diatomaceous fraction). (b) 10Be as a function of total beryllium from the leachate and residue of marine sediments from the
Gulf of California.
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Fig. 3. Measurements of aluminum, iron, and beryllium isotopes from Lake Malawi sediments from the authigenic fraction only. The scale for Fig. 4a and b has been kept the same as
that in Fig. 3 for comparison to each other. (a) Aluminum as a function of iron from the authigenic fraction of the sediments from Lake Malawi. Note that the vertical scale is 1/25 that
of Fig. 3a. (b) 10Be as a function of beryllium from the authigenic fraction of the sediments from Lake Malawi. R is the correlation coefficient. Note that the vertical scale is 1/4 that of
Fig. 3b.
There is a significant difference between the concentrations of the
beryllium isotopes between the authigenic fraction of the marine and
lake sediments. The concentration of 10Be to beryllium is 50 to 80
times higher in the marine sediments of the Gulf of California than in
the lake sediments due to increased scavenging from vigorous
upwelling. The residence time of 10Be in the open sea is nearly
1000 yrs (McHargue and Damon, 1991), whereas that in the lake may
be in the order of a few yrs to a few decades depending on the river
input, lake levels, and sediment deposition.
More importantly the distribution of 10Be in the authigenic
fraction with respect to 9Be is significantly different. At DSDP site
480 10Be is not correlated to beryllium (r = 0). This is as a result of
10
Be and beryllium ultimately having different sources and not having
mixed before their removal to the sediments. Almost all 10Be
originates in the atmosphere (Lal, 1988). Only a small amount of
10
Be (b1%) is produced in situ, that is, within the surface materials of
the earth. Beryllium (9Be), the stable form of the isotope, instead is
largely derived from terrigenous materials. The two isotopes mix
during transport in the hydrological cycle. At site 480 the complete
independence of 10Be from 9Be indicates little to no mixing of the two
isotopes before deposition. Thus the deposition of 10Be at site DSDP
480 directly related to its production in the atmosphere. On the other
hand, 10Be and 9Be in the sediments at Lake Malawi are more strongly
correlated (R = 0.80) than the marine sediments from the Gulf of
California. A significant amount of the 10Be deposited in the sediments
at Lake Malawi has been recycled and mixed with 9Be. The variations
in the iron and aluminum data and the dependence of 10Be on 9Be
indicate that the interpretation of the 10Be data from Lake Malawi as a
representative primarily of cosmogenic deposition may be problematical. The extent to which 10Be that is produced in the atmosphere
just prior to precipitation to Lake Malawi can be distinguished from
that recycled through aeolian and hydrological processes is therefore
uncertain.
Fig. 4 shows the two isotopes of beryllium plotted separately for
both core C and B from the Lake Malawi sediments. As the strong
correlation in Fig. 3b indicates there are similarities between the two
plots of beryllium (Fig. 4a) and 10Be (Fig. 4b). Most notably at a depth
of 150 m there is a sharp increase in both beryllium and 10Be probably
due to a sharp decrease in the sedimentation rate. In addition, both
show a trend to lesser concentrations at depth in the core, beryllium
weakly (r = − 0.39) and 10Be more strongly (r = −0.62), perhaps due
to decay. Mixing of both isotopes in the watershed of the lake during
L.R. McHargue et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 110–119
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Fig. 4. Beryllium isotopes as a function of depth cores 1C and 1B together for the sediments of Lake Malawi. (a) The concentration of beryllium (ppm) from the authigenic fraction of
the sediments as a function of depth. The sampling changed from approximately every 2 m at depths shallower than 80 m to 10 m or more for the depth interval from 80 to 370 m.
(b) The concentration of 10Be (in atoms/g) from the authigenic fraction from the same sediment samples as in Fig. 5a.
soil formation, riverine and aeolian transportation of sediments, and
deep water deposition would be expected.
A higher resolution look of the trends for 10Be and beryllium for
core 1C is shown in Fig. 5a and b, respectively. Also shown in Fig. 5c
are the TOC (Total Organic Carbon) data for the same section for
comparison (Lyons et al., 2011-this issue). The TOC is a proxy for the
past lake depth of Lake Malawi (Lyons et al., 2011-this issue). During
lowstands of the lake the TOC is low when arid conditions (Cohen et
al., 2007) limit runoff and sediment to coarser grained inputs. During
highstands, along with the increased sediment supply in more finely
laminated intervals, there is an increase in the terrestrial organic
matter (Cohen et al., 2007). The depth intervals for transgressions
and regressions of the lake level in Fig. 5c are highlighted by the
dotted lines and correlated to the beryllium isotope data in Fig. 5a
and b. The concentration of the beryllium isotopes do not directly
correlate with the concentration of the organic matter in the
sediments. However, the regressions and transgressions of Lake
Malawi as reflected by the TOC data is also, in part, reflected in the
beryllium isotope data. For example, the lowering of the lake level
between 45 and 70 m is associated with a decrease in the
concentration of the beryllium isotopes over the same section. In
addition, a rise in the lake level interpreted from the sediments
between 85 and 70 m corresponds to an increase in the concentration
of the beryllium isotopes. Similarly, a rise in the lake level from 30 to
10 m, followed by a lowering for sediments less than 10 m of core
depth, correspond to an increase and then a decrease in beryllium
isotopes, respectively. On the other hand, a significant transgression
between 38 m and 30 m is not associated with an increase in
beryllium isotopes but a decrease.
For comparison to the Lake Malawi data the global production of
Be as a function of the averaged paleomagnetic field intensities from
several marine sediment cores is shown in Fig. 6b (Valet et al., 2005).
In Fig. 6a the concentration of 10Be/9Be in the core C at site 1 at Lake
Malawi core is plotted with depth. From the organic matter from the
upper most part of the core numerous radiocarbon dates are available
and the depth and time boundaries are plotted (Scholz et al., 2007,
Supporting Information). These depths of the youngest and oldest
radiocarbon-dated samples in Fig. 6a are correlated with their dates in
Fig. 6b. In addition, the oldest obtained OSL IR (Optically-stimulated
luminescence dating) result is correlated in the same manner thus
bracketing the sediments measured in time. Also shown for
comparison in Fig. 6b are the MIS (Marine Isotopic Stages) associated
with this time period. The increases in the 10Be/9Be ratio are roughly
consistent with the 10Be production rates as derived from the
paleomagnetic intensity record. The 10Be/9Be ratio is low in the
Holocene due to the more intense dipole fields. The maximum 10Be/
9
Be in MIS 3 may be associated with the Laschamp geomagnetic
excursion. A secondary peak of 10Be in MIS 4 is consistent with similar
increases in 10Be concentrations seen in marine sediment cores and
ice cores in Antarctica. A significant 10Be/9Be increase, and associated
perhaps with the Blake Event, is more sharply defined in the sediments
than would be expected from the 10Be production curve but is
consistent with the 10Be data from the Blake Outer Ridge (not shown).
The above interpretation assumes the excesses of 10Be to 9Be in the
10
Be/9Be ratio are due to increases in the production rate of 10Be in the
atmosphere. However, the excess of 10Be to 9Be may result from an
increased collection for 10Be precipitating from the atmosphere due to
a greatly expanded surface area of Lake Malawi during highstands
10
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Fig. 5. Beryllium isotopes and TOC for core 1C from late Pleistocene Lake Malawi sediments. (a) The concentration of 10Be from the authigenic fraction of the sediments as a function
of depth. (b) The concentration of beryllium from the authigenic fraction of the sediments as a function of depth. (c) The concentration of organic matter (TOC) from the sediments as
a function of depth (Lyons et al. 2011-this issue). Major long-term changes in the TOC are correlated with the beryllium isotopes by vertical dashed lines. Declines in lake level
correspond to decreases in TOC and lake level rises that of increases in TOC.
with respect to recycled 10Be and 9Be. Therefore, the above correlations are problematical.
The history of the dipole field has been extended to 2 million yrs by
stacking together the data from sediment cores from different areas of
the world (Valet et al., 2005). In Fig. 7 the averaged intensity of the
geomagnetic field by Valet et al (2005) (Fig. 7b) is compared to
the 10Be/9Be ratio for the entire core 1B from Lake Malawi (Fig. 7a).
The mean paleomagnetic dipole field intensities (Valet et al., 2005)
have been converted to the production of 10Be by the method of Lal
(1988). In order to make comparisons of the 10Be data as measured
from the sediments in terms of concentration (atoms/g), or by the ratio
of 10Be to beryllium (10Be/9Be), it is necessary to take in account the
decay of 10Be (half life of 1.34 million yrs). The concentrations of 10Be
have not been corrected to their “original” concentrations because the
time scale has not been established. Therefore, the production of 10Be
has been modified to reflect the decay since those earlier periods of
time — the production of 10Be has not increased with time as might be
inferred from Fig. 6. The sediments below 80 m were taken every 10 m
so all the results down to 380 m have been compressed for comparison
against the plot of the geomagnetic field. The dotted lines show
possible correlations to broad features both in the 10Be/9Be data and
the geomagnetic data. There does seem to be a rough similarity
between the two data sets. For example, a significant decrease in 10Be/
9
Be at depths in the core deeper than 300 m and an increase from 200
to 300 m is reflected in the 10Be production curve for the time interval
older than 400 kyr. Also, the rate of the decrease in 10Be/9Be is
consistent with the decay of 10Be. The implication from Fig. 7 is that
the bottom of the core may be between 650 to 750 thousand yrs in
age.
However, as was discussed earlier, it is not likely that the 10Be
concentrations in the Lake Malawi sediments are directly tied to its
production in the atmosphere but is mostly recycled from several
sources. In addition, the changes in lake level affect the transport of
the beryllium isotopes to the lake, and the changing surface area of the
lake effects the collection of atmospheric 10Be. The correlation
between the two data sets may be either coincidence, or a secondary
correlation between climate and the geomagnetic field and then
between climate, lake level, and sediment deposition.
L.R. McHargue et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 110–119
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Fig. 6. The ratio of beryllium isotopes from the late Pleistocene Lake Malawi sediments, core 1C, are correlated to an averaged 10Be production curve derived from marine sediments.
(a) 10Be/9Be as a function of depth for Lake Malawi sediments from core 1C, site 1. Two dashed lines bracket the depths of the range of C-14 dates, and one dashed line corresponds to
the depth for the oldest OSL IR date. (b) 10Be averaged global production (Valet et al., 2005) with possible correlation ties as dotted lines to Fig. 5a. Marine Isotope Stages (MIS) 1
through 6 are shown.
4. Conclusion
Measurements of 10Be were made on the Lake Malawi drill core
MAL05-1C at intervals of every 2 m to a depth of 80 m and then every
10 m from core 1B to near the bottom at 380 m. 10Be and 9Be, in
addition to several major elements, indicate that a record of past lake
water chemistry can be obtained from the authigenic fraction of the
sediments. Comparisons between the results for 10Be/9Be and those
from a marine sediment core from the Gulf of California highlighted
some differences between the oceanic and lacustrine sedimentary
regimes. A profile for the 10Be/9Be ratio from Lake Malawi does show
correspondence, with some differences, with the global 10Be production for the late Quaternary. The bottom of the core is no older in age
than 750,000 yrs (perhaps as young as 650,000 yrs) rather than the
1.5 million yrs of original estimates. However, the correlation between
10
Be and 9Be in Lake Malawi sediments unlike that of the Gulf of
California suggests a significant recycled component of 10Be. Perhaps
more importantly the wide variations in the lake level affect the flux
of the beryllium isotopes to the lake sediments significantly. When
the lake level is high there is an increased flux of beryllium isotopes to
the lake from the surrounding drainage basin. In addition, the
increased surface area of the lake enhances the collection of 10Be
from the atmosphere with respect to that of the recycled beryllium
isotopes. When the lake level is low during times of aridity transport
of the beryllium isotopes to the lake from the local drainage basin is
restricted and direct precipitation of 10Be from the atmosphere is less
due to a much smaller lake surface area. Thus, any correspondence
between the 10Be/9Be profile and the known 10Be production profile
may reflect an indirect relationship between the dipole field and
climate. The effects of climate on lake levels, sedimentation rate, and
water chemistry will affect the concentration of 10Be within the
sediments, and climate, in part, may be influenced by the dipole field.
Unfortunately, the effects on the local climate on sedimentation and
lake levels may obscure even the effects of the secondary correlation,
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L.R. McHargue et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 110–119
Fig. 7. Beryllium isotopes from both cores 1C and 1B are correlated to an averaged 10Be production curve derived from Pleistocene marine sediments. (a) 10Be/9Be for Lake Malawi
sediments for cores 1C and 1B with dotted lines as possible correlations to Fig. 6b. (b) 10Be averaged Quaternary global production from Valet et al. (2005) modified by the decay of
10
Be.
and a potential chronology as well. Nevertheless, these interdependent effects may be better understood by further work on the
lacustrine 10Be geochemical cycle.
Acknowledgement
The beryllium-10 measurements for this work were made possible
by NSF grant EAR0622235.
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