Holocene dust records from the West African Sahel and their implications for changes in
climate and land-surface conditions
Helen E. Cockerton1, Jonathan A. Holmes2, F. Alayne Street-Perrott1, Katherine J. Ficken1
1. Department of Geography, College of Science, Swansea University, Swansea, SA2 8PP, UK.
2. Department of Geography, University College London, Gower Street, London WC1E 6BT, UK.
Text S1 study sites and materials, methods and supplementary references
Study sites and materials
During the SAHEL (Sub-Saharan Africa: Hydrogeology, Environment, Limnology) project,
between 1992-1995, several of the lakes in the Manga Grasslands were cored and provided long,
near-continuous records of Holocene environmental change [Waller et al., 2007; Street-Perrott et
al., 2000; Holmes et al., 1997, 1999; Salzmann and Waller, 1998]. This study is based on data from
two sites, Jikariya Lake (13°18.82’N, 11°04.62’E) and Kajemarum Oasis (13°18.30’N,
11°01.73’E), which are two small-sized depressions (≤ 0.1 km2 and 0.7 km2, respectively). Jikariya
Lake is persistently wet due to its large groundwater seepage line in relation to evaporation surface,
compared to larger depressions, such as Kajemarum Oasis, which are seasonal playas [Waller et al.
2007]. The lakes are bordered by fringes of reed-swamp and doum palm, which are sustained by the
seepage of fresh groundwater from the grass-covered dunes.
At Jikariya Lake a 30 cm-deep pit near the centre was sampled at 1 cm intervals and then extended
to 690 cm using a 5 cm-diameter modified Livingstone Corer. The sediments were composed
mostly of organic lake muds intercalated with silty and sandy silt units. There are two probable
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desiccation surfaces at the base of the core and close to the top. At Kajemarum Oasis, the upper 175
cm of sediment was taken from a pit in the centre of the dry lake bed, and extended to a depth of
393 cm further using a modified Livingston piston corer. The sequence comprises organic muds and
clayey silts at the base and very fine silty sand in the top 75cm. Chronologies for the two sequences
were based on bulk-organic-matter radiocarbon dates (Table S1) and, for Jikariya Oasis, a 137Cs
profile. Radiocarbon dates were calibrated using IntCal13 [Reimer et al., 2013] and classical agedepth modelling performed using Clam 2.2 [Blaauw, 2010].
For Jikariya Oasis, an age-depth profile based on six bulk-organic-matter radiocarbon dates, and a
137
Cs peak representing the 1963 thermonuclear peak is described by a third-order polynomial curve
(age = -0.000062depth3 + 0.04822depth 2 + 1.7845depth + 26.466, where age is in calendar years
600
500
Depth (cm)
400 300 200
100
0
Before Present (AD 1950) and depth in cm; r2 = 0.999) (Fig. S1).
12000
10000
8000
6000
cal BP
4000
2000
0
Figure S1. Age-depth curve for Jikariya, based on 6 radiocarbon dates and a 137Cs peak at 3.5 cm
depth. Shading represents 95% confidence intervals.
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Table S1. Radiocarbon dates for lake sediment cores from Jikariya Oasis and Kajemarum
Oasis
Lab Code
Core
AA30926*
AA30925
AA30924
AA30923
AA30922
AA30921
AA30920
AA17158
AA17653
AA17652
AA17651
AA17157
AA17156
AA17660
Jikariya
Jikariya
Jikariya
Jikariya
Jikariya
Jikariya
Jikariya
Kajemarum
Kajemarum
Kajemarum
Kajemarum
Kajemarum
Kajemarum
Kajemarum
Midpoint Depth
(cm)
Radiocarbon Age (BP)
8.5
109.5
188.5
286.5
334.5
537.5
667.5
15.5
36.5
62.5
80.5
135.5
174.5
293.5
255
1040
2150
3995
4765
8965
10025
385
720
1635
2185
3640
4845
9630
Error (1s) Calendar Age Range (2s)
70
45
45
60
50
80
70
45
45
45
45
50
55
70
-4 - 493
803 - 1058
2004 - 2307
4249 - 4788
5327 - 5596
9787 - 10249
11261 -11817
316- 511
562 - 731
1410 - 1686
2063 - 2328
3837 - 4135
5468 - 5711
10759 - 11191
*Regarded as too old based on the inferred 1963 137 Cs peak at 3.5 cm and therefore excluded from age model
For Kajemarum Oasis, an age-depth profile for the sequence is well described by a third-order
polynomial curve based on seven radiocarbon dates (Age = - 0.0002depth3 + 0.1227depth2 +
15.169depth + 98.753 where age is in calendar years Before Present (AD 1950) and depth is in cm;
r2 = 0.999) (Fig. S2). The only evidence for truncation, thought to be a result of deflation, occurs at
the top of the sequence indicated by the extrapolated surface age of ~100 cal. BP, and by the lack of
both a 210Pb-excess profile and a thermonuclear 137Cs peak [Holmes et al., 1997; Street-Perrott et
al., 2000].
3
0
50
Depth (cm)
150
100
200
250
300
10000
8000
6000
cal BP
4000
2000
0
Figure S2. Age-depth curve for Kajemarum, based on 7 radiocarbon dates. Shading represents 95%
confidence intervals.
Methods
Dust extraction
Between 1 to 3 g of air-dried bulk sediment were weighed prior to analysis.
An additional step was added to the procedure by Rea and Janecek [1981] to remove organic
carbon as the original methodology was created for ocean sediments, which generally have
negligible concentrations of organic matter present [Harrison et al., 2001]. Samples were either
treated with concentrated hydrogen peroxide or heating at 550˚C for 2 hours 45 minutes to remove
organic carbon. The latter method was preferred for its rapidity, whereas removal of organic matter
using hydrogen peroxide took up to 2 weeks for some samples. Testing was carried out to determine
the comparability of the two methods, and it was found that reproducibility was 2.3% (1 σ, n = 8),
only slightly greater than the precision of the dust extraction method (see below).
Following removal or organic matter, each sample was placed in a conical flask to which 25% (vol)
acetic acid was added to remove carbonate. 70-100 mL of the acid solution were added slowly to
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samples, which where then placed on an automated shaker for 4 hours, after which samples were
centrifuged and rinsed with distilled water for 10 minutes at 2000 rpm, and the rinse stage was
repeated twice more. To remove amorphous iron and manganese oxides and hydroxides a sodium
dithionite-sodium citrate solution was used following the procedure described in Rea and Janecek
[1981]. The sample was then sieved at 63μm and any remaining biogenic silica was removed by
adding 40 mL 0.4N sodium carbonate solution to the washed sediment and placing in a 100˚C
waterbath for 20 minutes. The supernatant was rinsed and centrifuged three times, each for 10
minutes at 2000 rpm. Once washed, the remaining material, deemed to be the aeolian dust
component, was air-dried and weighed. Percentage dust concentration (wt. %) of each sample was
calculated using the initial dry weight of the bulk sample and the final weight of the dried dust
residue as follows:
Dust content (wt.(mg) %) = Final dry weight of dust after chemical digestion (mg)
x 100
Initial dry weight of sample (mg)
Analytical precision for dust concentration measurements, based on replicate determinations from
Kajemarum sample 117-188cm, was ±1.12 % (1 σ, n = 10).
The preparation methods employed remove biogenic silica, organic matter and carbonate, along
with the silicate fraction that is >63µm in size. All of these are also components of modern dust.
However, biogenic silica, organic matter and carbonate are all produced autochthonously within the
lakes of the Manga Grasslands [Street-Perrott et al., 2000] and >63 µm fraction may include sand
from the surrounding dunes: the removal of all of these components is therefore essential in order to
derive realistic estimates of dust content, although these estimates will be minimum values as a
result of the preparation method used.
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Dust flux calculation
In order to determine the input history of dust to the sediment record, the mass accumulation rate
(MAR), or flux, of the dust was calculated using the following equation:
Dust flux (f) (g cm-2 yr-1) = LSR (cm yr-1) x DBD (g cm-3) x wt. % dust concentration
Weight percentage of the dust component (expressed in mg), linear sedimentation rate (LSR) and
dry bulk density values (DBD). The values calculated using the relationship above are multiplied by
1000 to obtain fluxes in g cm-2 kyr-1 The mass accumulation rate takes in to account both temporal
and areal components allowing comparisons with other sediment records of the same variable [Rea
and Janecek, 1981]. The 95 % confidence intervals of the dust-flux calculations were calculated by
propagating uncertainties in the sediment accumulation rates determined during age modelling, the
percentage dust determinations referred to above and the dry bulk density measurements (precision
= ±0.03 g cm-3, n = 10)
Particle-size analysis.
Particle size characteristics of the dust fraction were analysed using a Beckman Coulter Laser
Particle Sizer LS320. Approximately 0.1g of sample was dispersed in 5% calgon (sodium
hexametaphosphate and sodium carbonate) prior to analysis to ensure proper wetting and to prevent
agglomeration. Wherever possible several measurements were taken per sample to gain an
average/reproducible distribution. Particle size reproducibility was ±1.4 µm (1 σ, n = 14). Of the
149 samples, 3 did not produce the minimum amount of material required to create enough
obscurity (> 8%) in the detector unit to enable particle size to be carried out. Particle size
distributions were mapped and average measures for grain size (μm) were calculated for all 146
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samples. For calculation of the fine/coarse dust ratio, fine particles were defined as those with <20
µm median diameter and coarse particles those with >20 µm median diameter.
Supplementary references
Blaauw, M., (2010), Methods and code for 'classical' age-modelling of radiocarbon sequences. Quat. Geochron. 5, 512518.
Harrison, S. P., K. E. Kohfeld, C. Roelandt, and T. Claquin (2001), The role of dust in climate changes today, at the last
glacial maximum and in the future, Earth-Sci Rev., 54, 43-80.
Holmes, J. A., F. A. Street-Perrott, M. J. Allen, P. A. Fothergill, D. D. Harkness, D. Kroon, and R. A. Perrott (1997),
Holocene palaeolimnology of Kajemarum Oasis, Northern Nigeria: An isotopic study of ostracodes, bulk carbonate and
organic carbon, J. Geol. Soc., 154, 311-319.
Holmes, J. A., F. A. Street-Perrott, R. A. Perrott, S. Stokes, M. P. Waller, Y. Huang, G. Eglinton, and M. Ivanovich
(1999), Holocene landscape evolution of the Manga grasslands, NE Nigeria: evidence from palaeolimnology and dune
chronology, J. Geol. Soc., 156, 357-368.
Rea, D. K. (1994), The paleoclimatic record provided by eolian deposition in the deep-Sea - the geologic history of
wind, Rev. Geophys., 32, 159-195.
Rea, D. K., and T. R. Janecek (1982), Late Cenozoic changes in atmospheric circulation deduced from North Pacific
eolian sediments, Mar. Geol., 49, 149-167.
Reimer, P. J., E. Bard, A. Bayliss, J. W. Beck, P. G. Blackwell, C. B. Ramsey, C. E. Buck, H. Cheng, R. L. Edwards, M.
Friedrich, P. M. Grootes, T. P. Guilderson, H. Haflidason, I. Hajdas, C. Hatte, T. J. Heaton, D. L. Hoffmann, A. G.
Hogg, K. A. Hughen, K. F. Kaiser, B. Kromer, S. W. Manning, M. Niu, R. W. Reimer, D. A. Richards, E. M. Scott, J.
R. Southon, R. A. Staff, C. S. M. Turney, and J. van der Plicht (2013), Intcal13 and Marine13 radiocarbon age
calibration curves 0-50,000 Years Cal BP, Radiocarbon, 55, 1869-1887.
Salzmann, U., and M. Waller (1998), The Holocene vegetational history of the Nigerian Sahel based on multiple pollen
profiles, Rev. Palaeobot. Palyn., 100, 39-72.
Street-Perrott, F. A., J. A. Holmes, M. P. Waller, M. J. Allen, N. G. H. Barber, P. A. Fothergill, D. D. Harkness, M.
Ivanovich, D. Kroon, and R. A. Perrott (2000), Drought and dust deposition in the West African Sahel: A 5500-year
record from Kajemarum Oasis, northeastern Nigeria, Holocene, 10, 293-302.
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Waller, M. P., F. A. Street-Perrott, and H. Wang (2007), Holocene vegetation history of the Sahel: pollen,
sedimentological and geochemical data from Jikariya Lake, north-eastern Nigeria, Journal of Biogeography, 34, 15751590.
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