Mekonnen et al.
Progress in Earth and Planetary Science(2022) 9:14
https://doi.org/10.1186/s40645-022-00472-9

RESEARCH ARTICLE

Progress in Earth andPlanetary Science

Open AccessClimate, vegetation and fire history
during the past 18,000 years, recorded in highaltitude lacustrine sediments on the Sanetti
Plateau, Bale Mountains (Ethiopia)
Betelhem Mekonnen1,2* , Bruno Glaser1, Roland Zech3, Michael Zech1,4, Frank Schlütz5,6, Robert Bussert7,
Agerie Addis8, Graciela Gil‑Romera9, Sileshi Nemomissa10, Tamrat Bekele10, Lucas Bittner1,4, Dawit Solomon11,
Andreas Manhart12 and Wolfgang Zech12

Abstract
Low-altitude lakes in eastern Africa have long been investigated and have provided valuable information about theLate Quaternary paleohydrological evolution, such as the African Humid Period. However, records often suffer frompoor age control, resolution, and/or ambiguous proxy interpretation, and only little focus has been put on highaltitude regions despite their sensitivity to global, regional, and local climate change phenomena. Here we report on
Last Glacial environmental fluctuations at about 4000 m asl on the Sanetti Plateau in the Bale Mountains (SE Ethiopia),
based on biogeochemical and palynological analyses of laminated lacustrine sediments. After deglaciation at about
18 cal kyr BP, a steppe-like herb-rich grassland with maximum Chenopodiaceae/Amaranthaceae and Plantago existed.
Between 16.6 and 15.7 cal kyr BP, conditions were dry with a desiccation layer at ~ 16.3 cal kyr BP, documenting atemporary phase of maximum aridity on the plateau. While that local event lasted for only a few decades, concentra‑
tions of various elements (e.g. Zr, HF, Nb, Nd, and Na) started to increase and reached a maximum at ~ 15.8–15.7 cal kyr
BP. We interpret those elements to reflect allochthonous, aeolian dust input via dry northerly winds and increasingly
arid conditions in the lowlands. We suggest an abrupt versus delayed response at high and low altitudes, respectively,
in response to Northern Hemispheric cooling events (the Heinrich Event 1). The delayed response at low altitudes
might be caused by slow negative vegetation and monsoon feedbacks that make the ecosystem somewhat resilient.
At ~ 15.7 cal kyr BP, our record shows an abrupt onset of the African Humid Period, almost 1000 years before the onsetof the Bølling–Allerød warming in the North-Atlantic region, and about 300 years earlier than in the Lake Tana region.
Erica pollen increased significantly between 14.4 and 13.6 cal kyr BP in agreement with periodically wet and regionallywarm conditions. Similarly, intense fire events, documented by increased black carbon, correlate with wet and warm
environmental conditions that promote the growth of Erica shrubs. This allows to conclude that biomass and thus fuelavailability is one important factor controlling fire events in the Bale Mountains.
Keywords: Bale Mountains, High-altitude lacustrine sediments, Heinrich event 1, African humid period, Fire, Erica*Correspondence: betymekonnen19@gmail.com
1
Institute of Agricultural and Nutritional Sciences, Soil Biogeochemistry,
Martin Luther University Halle-Wittenberg, Halle, Germany
Full list of author information is available at the end of the article

1 Introduction
East African lakes have attracted scientific interest for
decades. This is because global atmospheric circulation
systems strongly influence this region and its hydrological dynamics (e.g. Thompson et al. 2002; Costa et al.
2014; Lamb et al. 2018). For instance, during the Last

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Mekonnen et al. Progress in Earth and Planetary Science(2022) 9:14

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Glacial Maximum (LGM; 23–18 kyr BP) the decrease oftropical sea surface temperatures resulted in glaciation
on East African Mountains (Mount Kenya, Kilimanjaro,
Ruwenzori, and Ethiopian highlands) (Osmaston et al.
2005; Mark and Osmaston 2008). In addition, the southward shift of the Intertropical convergence zone (ITCZ)
during ice rafting episodes throughout Heinrich event 1(H1) are considered as a major cause for the dry event innorthern and southeastern Africa around 16–17 cal kyr
BP (Tierney et al. 2008; Marshall et al. 2007; Stager et al.
2011; Mohtadi et al. 2014). During the African humid
period (AHP) maximum northern Hemisphere summer
insolation shifted the rain belt associated with the ITCZ
to the north (Bastian et al. 2021), generating increased
rainfall across Northern Africa which turned the Saharandesert into green savanna (Gasse 2000). Palaeoclimaticstudies suggest the southward extension of the AHP toeastern Africa (Tierney et al. 2008; Tierney and DeMenocal 2013; Costa et al. 2014).
Besides, lacustrine sediments (Tiercelin et al. 2008;
Bittner et al. 2020; and many others), tropical glaciers(Gasse 2000; Thompson et al. 2002), peat bogs (Bonnefille and Mohammed 1994; Brown et al. 2007) and marine
sediments (Camuera et al. 2021; Tierney and deMenocal 2013) have been serving as potential archives for thereconstruction of paleoenvironmental fluctuations ineastern Africa. However, the regional, topographic, andclimate complexities, together with dating inaccuracy
and lack of unambiguous proxies, lead to differences in
the reconstructed timing of the onset and termination ofmajor climatic events such as H1 and AHP across sites(Lamb et al. 2007; Tierney et al. 2011; Costa et al. 2014;
Bastian et al. 2021). Moreover, the impact of these drastic
climatic events on the migration of humans and on vegetation dynamics is yet not fully understood.
The highlands of the Bale Mountains in southeastern
Ethiopia are promising sites for studying paleoenvironmental fluctuations. Their unique exposition to main
atmospheric circulation systems (SW monsoon, SE
monsoon, NE trades, and NW disturbances) (Miehe andMiehe 1994; Hillman 1986; Camberlin 2018) contributeto their climate sensitivity. In addition, the special geomorphological features, characterized by high altitudes
up to 4377 m above sea level (asl), by moraines and small
glacial depressions, support the reconstruction of thelandscape evolution (Nicholson 2000). During MIS 3, theBale Mountains were one of the most extensively glaciated mountains in Ethiopia (Messerli 1980; Osmaston
et al. 2005; Groos et al. 2021). Groos et al. (2021) suggest that about 265 k­ m2 of the Bale Mountains were icecovered between 42 to 28 kyr BP, well before the onsetof the global LGM. This caused a temperature decrease
and a downward shift of Afroalpine vegetation. Umer

et al. (2007) and Ossendorf et al. (2019) suggested thatthe glaciers retreated from the Bale Mountains at ~ 15.9–
16.7 cal kyr BP. Geochemical and pollen results from
Lake Garba Guracha, in a north-exposed, deeply incisedtrough valley at 3950 m asl, and from peat deposits on
the Sanetti Plateau, inform about Late PleistoceneHolocene climate fluctuations (Tiercelin et al. 2008;
Bittner et al. 2020) and respective vegetation changes(Umer et al. 2007; Kuzmicheva et al. 2013, 2014; GilRomera et al. 2019). These studies suggest that the warmand humid conditions during the early Holocene led tothe expansion of Erica vegetation on the Sanetti Plateau, and dry climate and natural and/or human-induced
fires might be major causes for the current patches ofErica on the Plateau. A study by Ossendorf et al., (2019),
revealed human settlements in high elevation (3469 masl) already during 47–31 cal kyr BP. Gil-Romera et al.
(2019) reported that fire played a major role in determining the ecological dynamics of the Erica vegetation on
the Bale Mountains. However, the influence of humanson the vegetation remains unclear. Open questions are:(1) Did the high altitudes of the Sanetti Plateau experience similar climatic fluctuations in intensity and timing
as the lower regions in northern and eastern Africa during the Late Glacial? (2) Did the highlands of the Sanetti
Plateau deglaciate at the same time as the north exposed
trough valleys? (3) When and to what degree did Erica
occupy the high altitudes of the Sanetti Plateau, where atpresent only patches can be found? (4) Are these patchesof Erica relics documenting climate deterioration or
human-induced fire?
In order to address these questions, and to better
understand the paleoecological evolution of these highaltitudes above the upper timberline, we analysed laminated lake sediments in high resolution, deposited ina glacial depression on the Plateau in about 4000 m asl
using a multi-proxy approach. The depression has no
noteworthy outflow and its catchment is not densely vegetated, thus being highly sensitive to local and regional
environmental fluctuations.

2 Site information and methods/experimental
2.1 Study area

The Bale Mountains covering an area of 2200 k­m2
include the Sanetti Plateau, which is the largest alpine
ecosystem in Africa (Fig. 1). The area is widely perceived
as natural (Miehe and Miehe 1994; Kidane et al. 2012).
The Plateau (Fig. 1) extends between ca. 3800 to 4100 m
asl, surrounded by the peaks of Tullu Konteh (4050 masl) and Tullu Dimtu (4377 m asl), the second-highest
peak of the country (Hillman 1988; Messerli and Winiger 1992). Erratic boulders, moraines, small lakes present
on the plateau and in trough valleys are clear indicators

Mekonnen et al. Progress in Earth and Planetary Science(2022) 9:14

Page 3 of 19CBA
7°4'30"N

Lake Tana
Addis
Ababa

Bale Mountains

6°53'30"N
Sanetti Plateau

6°42'30"N

LegendGerba Guracha
B4
6°31'30"N

BMNP boundary

39°5'0"E

039°27'0"E

4.25 8.51739°49'0"E

25.5

34

Km

40°11'0"E

Fig. 1 Location map of the study area. A Location of Ethiopia in Africa and regional location of the Bale Mountains National Park (BMNP) and Lake
Tana. B Location of B4 depression on the Sanetti Plateau and location of Lake Garba Guracha. C Photo of B4 depressionof former glaciations (Osmaston et al. 2005). The parent rocks comprise Oligo-Miocene basalts and Quaternary rhyolites with trachytes, which weather to brown orbrownish-black silty loams (Billi 2015). Muddy Gleysols
are developed in depressions (Yimer et al. 2006).
Due to the annual migration of the ITCZ between 10°
North and South, the climate of the Bale Mountains ischaracterized by a pronounced rainfall seasonality with ashort dry (November to February) and a long rainy season (March to October) (Levin et al. 2009; Costa et al.
2014). The rainfall pattern is bimodal, with a peak from
July to October, followed by a second peak from March
to June (Kidane et al. 2012). The rain-bearing air masses
derive from the Indian Ocean and the Atlantic Ocean
via the Congo Basin (Gasse 2000; Tierney et al. 2011;
Costa et al. 2014; Lemma et al. 2020), whereas northerly winds dominate during the dry season. In Dinsho(3170 m asl), the mean maximum temperature is 11.8 °C
while the mean minimum temperature ranges from 0.6 to
10 °C, with frequent frost occurring in the high mountain areas during winter season (November to January)(Hillman 1986; Tiercelin et al. 2008). The highest precipitation and humidity occurs in the southern part ofthe mountains with 1000–1500 mm/a, while the northern part exhibits annual rainfall ranging between 800 and1000 mm/a (Miehe and Miehe 1994; Umer et al. 2007).
The south to north rainfall gradient and the altitudinal temperature gradient stratify the vegetation into theAfromontane forest, the Ericaceous belt, and Afroalpine
zone (Hedberg 1951; Friis 1986). The Afromontane forest spans from ~ 1450 to 3200 m asl in southern exposition, and from ~ 2800 to 3300 m asl along northernslopes. The southern declivity comprises Podocarpus gracilior associated with Syzygium guineense and Aningeria

adolfi-friederici, whereas the northern slopes are mainly
dominated by Juniperus procera, Hagenia abyssinica,and Hypericum revolutum (Friis 1986; Bussmann 1997).
The Ericaceous belt covers ~ 90,000 ha between ca. 3200
and 3800 m asl and is dominated by Erica arborea L. andErica trimera (Engl.) (Hailemariam et al. 2016). Whilethe lower boundary of the Ericaceous belt (3300–3500 masl) is covered with Erica-dominated Hagenia-Hypericum
forests, the central part comprises monotonous Erica
trimera stands, which continues to the upper limit (ca.
3800 m asl) of the boundary. The Afroalpine vegetation
above 3800 m asl is open and rich in Tussock grasses
and mainly dominated by Helichrysum splendidumAlchemilla haumannii dwarf-scrubs and Giant Lobelia(L. rhynchopetalum) (Yineger et al. 2008), accompanied
by patches of Erica, growing between big boulders along
steep slopes (Miehe and Miehe 1994).
2.2 Sampling

We investigated a depression, located in an extended
plain on the Sanetti Plateau, above the upper limit of
the Erica belt at 3970 m asl (Fig. 1; 6° 53.3433′ S and
39° 54.5217′ E). A pit was dug down to the glacial boulders in a depth of 255 cm to prepare a profile named“B4” (Fig. 2). Subsequently, sediments were described(color, texture, structure, ect.) and sampled every 2 cmfrom 55 to 255 cm, representing humic-rich lacustrine,
partly laminated material, and every 5 to 10 cm from thebrownish upper part, rich in reddish mottles. Samples
were air-dried and stored in plastic bags. In addition, 10
sediment samples were collected from the pit for radiocarbon dating. The air-dried samples were analyzed forelemental and mineral compositions, electrical conductivity (EC), pH (­H2O), stable C isotopes, alkanes, pollen,

Mekonnen et al. Progress in Earth and Planetary Science(A)(2022) 9:14

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(B)

0

~~~
~~~
~ ~
50 ~
~~
~ ~~
65 ~
~
71

70 cm

Depth (cm)

0 cm

100
110
120
135

150
160

clayey sandy silt,
light brown, with
Age cal kyr BP
gleyic proper es
desiccation surface 10.2
grey clayey silt,
partly laminatedplant remains, swamp

grey clayey silt,
partly laminateddesiccation surface

250 cm

250
255

13.6
15.2
15.4
15.7
16.3

unit 2
H1

16.6

175

200

unit 3dark grey silty clay,
laminateddeglaciation,basalt boulders

unit 1

18.2

Fig. 2 Stratigraphy and photo of the studied profile. A Photo of the B4 profile; the red pocketknife (10 cm) serves as scale. B Stratigraphy of thesediments in profile B4, divided in units 1–3, based on morphological features and calibrated radiocarbon ages (see Fig. 3; Table 1; YD = Younger
Dryas, H1 = Heinrich Event 1)

and black carbon (BC). Plant leaves (Alchemilla sp., Helichrysum sp., Pennisetum sp., Potamogeton thunbergii,
and Ranunculus trichophyllus) and algae (Pediastrum sp.)were collected from the surrounding area to compare thestable isotopes composition of the sediment organic matter with its potential parent material.
2.3 Radiocarbon analysis

For the establishment of a reliable chronology, radiocarbon dates were obtained by dating alkali-soluble organic
matter extracted from carbonate-free bulk samples using
accelerated mass spectrometry (AMS) in the Radiocarbon Laboratory of the University Erlangen, Germany(Table 1). We established a Bayesian age-depth modelusing the IntCal13 calibration curve implemented in the
package Bacon (Blaauw and Christen 2011) in R software(R Core Team 2013).
2.4 Sediment analyses
2.4.1 Physical properties

Texture analyses were performed quantitatively in thelaboratory on 52 air-dried and 2 mm sieved samples. Clay(< 6.3 μm) and silt (6.3–63 μm) fractions were quantified
with a Mastersizer S (Malvern Instruments) after treating the samples with ­H2O2 and HCl. The sand fraction(63–2000 μm) was determined by wet sieving. We areaware that the fraction < 6.3 μm used in this study does
not quantitatively correspond to the clay fraction definedby the pipette method as the fraction < 2 μm is underestimated by the Mastersizer (Antoine et al. 2009).
2.4.2 Inorganic geochemistry

For X-Ray Fluorescence (XRF) analyses, ground aliquotsof 103 samples were dried at 105 °C. Major and minor
elemental composition was determined using a Philips
2404 X-Ray Fluorescence Spectrometer. Given that the
samples were carbonate-free, results were corrected
for soil organic matter and water contents according to
Eq. (1)
100%Xcorrected = %Xmeasured ×
100 − %GV1000 − %H2 O(1)
X is the content of an element in percent and %GV1000 is
the mass loss upon ignition at 1000 °C.
In order to evaluate the intensity of chemical weathering, the chemical proxy of alteration (CPA) was calculated according to Buggle et al. (2011) (Eq. 2).

Mekonnen et al. Progress in Earth and Planetary Science(2022) 9:14

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Table 1 Radiocarbon ages using Accelerated Mass Spectrometry of alkali soluble organic matter; calibrated according to Blaauw andChristen (2011)
Lab number

Sample depth(cm)

Dated material

Erl-5563

27

Bulk sedimentErl-5564

40

Bulk sedimentErl-5173

63

Bulk sedimentErl-5092

72

Bulk sedimentErl-5175

101

Bulk sedimentErl-5173

123

Bulk sedimentErl-5093

160

Bulk sedimentErl-5172

207

Bulk sedimentErl-5565

233

Bulk sedimentErl-5568

255

Bulk sediment

CPA =




Al2 O3× 100
Al2 O3 + Na2 O

14

C age (uncalibrated
a BP)

(2)

Qualitative mineral identification was carried out on bulk
samples (n = 100) using X-ray diffraction. The powderedsamples were packed into aluminum sample holders
and scanned in a Philips PW 1710 diffractometer from
3 to 80°2θ with Cu Kα radiation generated at 50 kV and
30 mA, at 0.02°2θ step size and 2.5 s step time. Phases
identified in the samples are quartz, feldspar, pyroxene(probably mainly augite), clay minerals, hematite, pyrite,
gypsum, and amorphous silica. No special clay mineral
analysis was performed, yet characteristic peaks of clay
minerals between ~ 6–12.5 and ~ 60–62°2θ show a wide
range of clay minerals, including smectite, chlorite, illite,
kaolinite and mixed-layer minerals.
2.4.3 Organic geochemistry

Total organic carbon (TOC) and total nitrogen (N) of
homogenized samples (n = 103) were quantified by dry
combustion on a Vario EL elemental analyser (Elementar, Langenselbold, Germany). In order to complement
the organic matter source identification and preservation
status, n-alkanes and hydrogen index (HI) were analysedin 34 samples (Talbot and Livingstone 1989). n-Alkanes
were extracted by soxhlet extraction using dichloromethane (DCM) and methanol (1:1) as solvents for 24 h (Zechand Glaser 2008). After adding 50 µl of 5α androstane as
an internal standard, the excess solvent was removed byrotary evaporation and transferred to aminopropyl columns. Subsequently, n-alkanes were eluted with 3 ml ofhexane DCM/MeOH (1:1). Finally, n-alkanes were quantified using gas chromatography (SHIMADZU, GC-2010,
Kyoto Japan) equipped with an SPB-5 columns of 30 m
length, 0.25 mm ID, 0.25 µm film thickness, and detectedusing a flame ionization detector (FID). Here, we used

Cal. (INTCAL13, 2 Sigma
ranges)4650 ± 55

5078–5532

9047 ± 108

14

C age (calibrated,
median cal year BP)
5396

7718 ± 72

8225–8678

8481

9603–10,472

10,052

11,856 ± 86

13,436–13,969

13,667

12,696 ± 96

14,372–15,241

14,876

13,214 ± 95

15,155–15,867

15,51813,525 ± 106

15,979–16,568

16,276

14,094 ± 119

16,916–17,458

17,193

14,485 ± 81

17,472–17,958

17,701

14,681 ± 92

17,879–18,653

18,173the n-alkane ratio Paq ­(C23 + ­C25)/ ­(C23 + ­C25 + ­C29 +­C31) to identify the source of organic matter. HI was analysed by pyrolysis of dried (40 °C) and fine powdered bulk
sediments (Talbot and Livingstone 1989).
We measured the natural abundance of δ13C (n = 101)by dry combustion of a 40 mg homogenized aliquot witha Fision 1108 elemental analyser coupled to a Delta S
gas isotope ratio mass spectrometer (EL-IRMS) with a
Conflow III interface (Thermo Finnigan MAT, Bremen,
Germany). Sucrose (ANU, IAEA, Vienna, Austria) and­CaCO3 (NBS 19, TS limestone) were used as calibration standards for δ13C. The precision of δ13C measurement was 0.2‰. In addition to bulk stable isotopes, wemeasured compound-specific carbon isotopes (δ13C)of long chain terrestrial n-alkanes ­C27, ­C29 and ­C31 tocomplement on the paleovegetation reconstruction (C3vs C4). Compound-specific δ13C analyses of long chain
n-alkanes ­(C27, ­C29 and ­C31) were carried out and measured as described above.
Black carbon (BC) refers to polycondensed aromatic
moieties formed during thermochemical combustion.
While charcoal is usually detected visually using a microscope, BC is usually detected through chemical extraction, which enables to quantify the complex signature ofpyrogenic carbon more precisely, especially when charcoal is not visible anymore. Thus, BC allows the reconstruction of fire events even if soot or char are not visible.
Moreover, its polycyclic aromatic structure resists chemical and biological degradation in soil (Glaser et al. 1998;
Wang and Li 2007; Wiedemeier et al. 2015) and persists
for a long period of time in soil and sediments (Kuzyakov et al. 2014). We analysed BC (n = 72) using benzene
polycarboxylic acids (BPCAs) as molecular markers following Glaser et al. (1998) and modified according to
Brodowski et al. (2005). Five hundred mg of each sample
was hydrolyzed with 10 ml 4 M TFA for 4 h at 105 °C.

Mekonnen et al. Progress in Earth and Planetary Science(2022) 9:14

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The hydrolyzed samples were filtrated on glass fiber filters and rinsed several times with de-ionized water toremove polyvalent cations. Subsequently, the samples
were digested with 65% nitric acid for 8 h at 170 °C in
a high-pressure digestion apparatus. The solution was
passed through Dowex 50 W resin columns (200 to 400
meshes) to remove polyvalent cations. After derivatization, BPCAs were separated using GC and detected using
FID with an injection temperature of 300 °C.

71 cm depth, is a grey, partly laminated silty clay, deposited between 16.6–13.6 cal kyr BP. A stiff sandy-silty lightlayer, preserved between 167 and 160 cm, likely documents intensive desiccation of the B4 depression at about
16.3 cal kyr BP. From 120 to110 cm (~ 15.4–15.2 cal kyrBP) plant root relics occur displaying swampy, shallowwater conditions. The upper lithostratigraphic unit 3 isabout 70 cm thick, deposited during the last 10 cal kyr BP.
It comprises light brown, weakly clayey, sandy silt with
red mottles and bleached aggregate surfaces, showing
water-logging during the rainy season. Unit 3 is strongly
influenced by gleysation, which makes paleoenvironmental interpretations and regional comparisons ambiguous.
Therefore, in the following, we present geochemical andpalynological results only from units 1 and 2 in detail.

2.4.4 Palynomorph analysis

For palynological analyses, samples of 1 ml were prepared using standard procedures, including Lycopodium
spores as exotic markers, acetolysis, HF treatment, andultrasonic sieving (5 µm), and storage in glycerol (Stockmarr 1971). Microscopic analyses took place under
400 × magnifications, backed by oil immersion (1250 ×).
For pollen identification, we used an existing referencecollection of ~ 5000 slides (in Goettingen) and relevant
literature (Gosling et al. 2013; Schüler and Hemp 2016).
The nomenclature of the common types follows Beug(2004). Detailed analyses were carried out on 38 samples
from the lower part of the profile (251–69 cm) with pollen counting sums of about 250 to 300 per sample. Pollen
influx (grains c­ m−1 ­a−1) was calculated using exotic Lycopodium spore markers. To estimate humidity conditions,
we calculated the A/C (Artemisia/Chenopodiaceae) ratio(Van Campo and Gasse 1993; Herzschuh 2007; Li et al.
2010) by dividing the number of Artemisia by that ofChenopodiaceae/Amaranthaceae. Poaceae pollen weresize differentiated between small (< 37 µm) and large(> 37 µm) diameter grains. Counts of the larger grains
allow an approximate estimate of C4 grasses, which
produce larger pollen grains than C3 Poaceae (Jan et al.
2015). Diagrams were plotted with the software C2 (Juggins 2007).

3 Results
3.1 Age‑depth model and sedimentation rates

B4 profile records sedimentation during the last 18.2 cal
kyr BP. Our age-depth model (Fig. 3) illustrates thatthe sedimentation rate was 0.05 and 0.01 cm per year
between 18.2 and 13.6 cal kyr BP and from 10.5 cal kyr
BP to present, respectively. Between 13.6 and 10.5 cal kyr
BP, a sedimentation hiatus is indicated.
3.2 Lithostratigraphy and interpretationof the depositional environment

The B4 sediments can be stratified into three units(Fig. 2). The lowermost unit 1 from 255 to 175 cm (18.2–
16.6 cal kyr BP) is a dark-grey, laminated, silty clay withlow sand content. It was deposited above a thin greyish
sandy silt layer and basalt boulders. Unit 2, from 175 to

3.3 Grain size and geochemical results
3.3.1 Unit 1: 255–175 cm (18.2–16.6 cal kyr BP)

According to Fig. 4 and Additional file 1, clay and silt
contents are high (partly up to 60% and 40%, respectively) and weakly fluctuating, whereas the sand contents
are very low. TOC values, being low at the bottom of thecore, constantly increase from ~ 0.7 to ~ 7% in 239 cm;
then high values (~ 7%) persist until the top of unit 1.
Low TOC/N values at the bottom of the profile increase
to an average value < 12 (Fig. 4). HI values range between
107 and 266, with low values at the bottom of the profile.
Our results show an average δ13C value of − 14.3‰
with a slight decrease from the bottom to the top ofunit 1. Analyses of water plants (Potamogeton thunbergii, Ranunculus trichophyllus) and algae (Pediastrum), sampled from nearby shallow lakes, resulted
in δ13C values of − 18.6, − 15.5, and − 17.3‰, respectively. Modern terrestrial plants growing around the B4
depression are characterized by δ13C values between− 30.1‰ (Helichrysum argyaranthum) and − 26.9‰(Alchemilla fischeri). Compound-specific analysis of long
chain n-alkanes ­C27, ­C29, and ­C31 revealed δ13C valuesof − 27, − 25.6, and − 25.1‰, respectively. Paq values
increase from 0.27 at 244.5 cm to 0.44 at 175 cm. Despitehigh value at the bottom of the profile, BC values werevery low in this unit (around 10 g ­kg−1 TOC) increasing between 195 and 175 cm (~ 16.9–16.6 cal kyr BP) toaround 30 g ­kg−1 TOC.
Lithogenic elements such as Zr, Hf, Nb, and N­ a2O show
minimum values except for a slight increase at 210 cm
and ~ 190 cm (~ 17.2 and ~ 17.0 cal kyr BP) (Fig. 5).
Besides primary minerals such as quartz, feldspar, andpyroxene, unit 1 sediments contain traces of pyrite andgypsum. CPA values do not show fluctuation except aslight decrease at 17.2 and ~ 16.9 cal kyr BP.

0.0

0.4
0.8
Memoryhiatus.max: 10000

Frequency
0e+00 6e−04

mem.strength: 4
mem.mean: 0.7
58 4.4cm sections

0 1 2 3 4

0.000

0
400
800
Acc. rate (yr/cm)

Page 7 of 19

0 4000 10000
Hiatus size (yr)

100
150
250

200

Depth (cm)50

00 2000 5000
Iteration(2022) 9:14

acc.shape: 1.5
acc.mean: 50

0.015

Log of Objective−90−75

Mekonnen et al. Progress in Earth and Planetary Science

0

5000

cal yr BP

10000

15000

Fig. 3 Bayesian age-depth model for profile B4 according to Blaauw and Christen (2011). Top left: The log-likelihood of the model fit for the saved
iterations of the model, top middle: prior and posterior distribution of accumulation rate, and top right: prior and posterior distribution of the
autocorrelation in accumulation rates (memory) and the time gap (hiatus size) at the end. Bottom panel shows the calibrated 14C dates (transparent
blue) and the age-depth model (darker greys) indicate more likely calendar ages; grey stippled lines show 95% confidence intervals; red curveshows single ‘best’ model based on the mean age for each depth

3.3.2 Unit 2: 175–71 cm (16.6–13.6 cal kyr BP)

Unit 2 starts at 175 cm depth with slightly increasing
values of the lithogenic proxies (Fig. 5). Most striking
is a pronounced sand maximum and a TOC minimumin ~ 160 cm, corresponding to ~ 16.3 cal kyr BP (Fig. 4).
Strongly increasing values of the lithogenic compounds
such as Zr, N­ a2O, ­K2O, Hf, Nd, and Nb and minimum
CPA values are recorded between 160 and 135 cm(Fig. 5). Noteworthy is that these elements do not show
the drastic excursion of TOC and sand in ~ 160 cm depth.
Moreover, strong positive correlations (R = ~ 0.8–0.9;
Additional file 2) are recorded between N­ a2O, ­K2O, Zr,
Hf, Nb, and Nd in this unit. This constellation changed
abruptly at ~ 15.7 cal kyr BP, documented by the abrupt
decrease of most lithogenic elements and their ratios.
However, some proxies such as sand content, TOC, HI,and δ13C do not show such an abrupt change except Paq
values, which decrease between 140 and 120 cm (Fig. 4).
BC which was partly very low in unit 1, increased
between 15.7 and 15.2 cal kyr BP (Fig. 4). The uppermost
part of unit 2, (deposited between ~ 14.4 and 13.6 cal kyr
BP (90–71 cm) is characterized by a distinct decrease in

TOC, HI, varying sand contents, absence of pyrite, andhigher contents of terrestrial n-alkanes and BC (Fig. 4).
3.4 Palynological results

The pollen record is divided into five local pollen zones(LPZ 1–5; Figs. 6, 7) based on changes in the pollen
spectra. No pollen was detected in unit 3 due to pedogenetic processes that disturbed pollen preservation.
About 75 pollen taxa have been identified in the record.
The green algae Botryococcus and Pediastrum attributed
to the intermittent presence of a water body in the B4
depression, thus their palynological record reflects local
conditions.
Poaceae < 37 µm dominate the pollen spectra with values around 35%. Other relatively common non-arboreal
taxa are Artemisia, Chenopodiaceae/Amaranthaceae,
Poaceae > 37 µm, and Plantago. The sum of arboreal pollen (AP) makes up around 15% in LPZ 1–4 (from 250.5 to
90 cm; 18.1–14.4 cal kyr BP) and increased to 38% during
LPZ 5 (from 90 to 70 cm; 14.3–13.6 cal kyr BP). The green
algae Botryococcus is most common in the lowest part of
the record. Pediastrum increases with parallel decreases

Mekonnen et al. Progress in Earth and Planetary Science(2022) 9:14

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Fig. 4 Analytical properties of the B4 sediments, with depth profiles of the sand fraction (63–2000 μm), total organic carbon (TOC), organiccarbon-to-nitrogen ratio (TOC/N), Hydrogen Index (HI), δ13C values, Black Carbon (BC) and Paq. The light orange color shows dryer events whileorange color indicates desiccation layers developed during Heinrich Event 1(H1). Blue color indicates the African Humid Period (AHP) andother humid episodes while dark green color indicates the event characterized as humid and arid according to pollen and geochemical results,
respectivelyof Botryococcus. Pollen of modern water plants, Potamogeton thunbergii and Ranunculus trichophyllus are notpresent in the fossil pollen record.
LPZ 1 (250.5–204 cm; 18.1–17.2 cal kyr BP) is characterized by Botryococcus maxima, which abruptly
decreases at 17.3 cal kyr BP and is replaced by highamounts of Pediastrum pollen (Fig. 7). Moreover, highpercentages of Poaceae, herbs (non-arboreal pollen,
NAP), Artemisia, and Chenopodiaceae/Amaranthaceae
are recorded in LPZ1. Plantain (Plantago) has been very
common. Arboreal pollen of Ericaceae, Podocarpus,
Myrica, Juniperus, and Olea are found from the beginning of the archive.
LPZ 2 (204–194 cm; 17.2–16.9 BP) records an increasein Chenopodiaceae/Amaranthaceae and Plantago andstrongly reduced occurrence of Artemisia and Pediastrum. Similarly, pollen percentage and pollen influx ofHagenia, Podocarpus, Myrica, Juniperus, and Olea showlow values between 17.2 and 16.9 cal kyr BP.
LPZ 3 (194–110 cm, 16.9–15.2 cal kyr BP) starts
with a subsequent increase of Podocarpus, Juniperus, and Olea from 194 to 177 cm (16.9–16.6 cal kyrBP) and a higher presence of Pediastrum. The afroalpine element, Alchemilla, which is highly underrepresented in the pollen record, is sporadically present
from now on. The influx of Erica, Pediastrum, Hagenia,
and Botryococcus pollen decrease between 165 and155 cm (16.3 and 16.1 cal kyr BP), while Chenopodiaceae/Amaranthaceae pollen increase. Subsequently,
higher Pediastrum values, more frequent fern spores,

Mekonnen et al. Progress in Earth and Planetary Science(2022) 9:14

Page 9 of 19

Fig. 5 XRF results of B4 sediments. Depth profiles of Zirconium (Zr), Sodium oxide (­ Na2O), Potassium oxide ­(K2O), Hafnium (Hf ), Niobium (Nb),
Zirconium/Rubidium (Zi/Rb), Zirconium/Silica oxide (Zr/SiO2), Zirconium/Titanium oxide (Zr/TiO2) and of the chemical proxy of alteration(CPA = ­Al2O3/(Al2O3 + ­Na2O), inversed scale. The light orange color shows dryer events while orange color indicate desiccation layers developed
during Heinrich Event 1(H1) and the hiatus. Blue color indicates the African Humid Period (AHP) and other humid episodes while dark green color
indicates the event characterized as humid and arid according to pollen and geochemical results, respectively

and Sparganium-type are recorded between 155 and135 cm (16.1 and 15.7 cal kyr BP). A lower occurrenceof Erica pollen and the montane forest tree Hagenia
recorded from 15.4 to 15.2 kyr BP, while absolute valuesof montane forest taxa like Podocarpus and Juniperus
increase. By contrast, Pediastrum, Plantago-type, andGrass pollen influx show high values.
LPZ 4 (110–90 cm, 15.2–14.4 cal kyr BP) records themost abundant Ericaceae and Podocarpus. Similarly, thesum of all arboreal pollen (AP) increases but does notexceed their former values. The same holds true for Artemisia. In contrast, the ratio of Chenopodiaceae/Amaranthaceae falls as low as never before. In consequence, theA/C ratio increases strongly.
LPZ 5 (90–71 cm, 14.4–13.6 cal kyr BP) is marked bya strong expansion of Erica and Podocarpus. The previously significant elements Chenopodiaceae/Amaranthaceae and Plantago disappeared nearly completely,
and Artemisia reduced strongly. Besides Poaceae, awide range of Asteraceae (Senecio, Matricaria-type,
Cichorioideae), which was present since the beginningof the record, became the most important vegetation
constituents.

4 Discussion
4.1 Radiocarbon dating

Our radiocarbon ages show that depression B4 became
ice-free at ~ 18 cal kyr BP and thus records the climate,vegetation, and fire history from the end of the LGM to
the Late Holocene. The timing of deglaciation (18.2 cal
kyr BP) seems reliable, as the sediments are free ofcarbonates.
4.2 Organic geochemistry

Low TOC contents at the bottom of the profile (Fig. 4)
mark cold conditions with low biomass production in,
and around, the lake. TOC/N ratios are used to distinguish between terrestrial and aquatic carbon sources.
Organic matter derived from algae usually shows
TOC/N values between 4 and 10, while vascular land
plants and terrestrial soil organic matter may have
TOC/N ratios > 20 (Meyers 1994) and ~ 15, respectively

Mekonnen et al. Progress in Earth and Planetary Science(2022) 9:14

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Cyperaceae

LPZ

Tribulus

Caryophyllaceae

Alchemilla

Matricaria-type

Cichorioideae

Thalictrum

Senecio-type

Rumex-type

Plantago-type

Chenopodiaceae/
Amaranthaceae (C)A/C ratio

Artemisia (A)Poaceae

Pistacia

Olea

Juniperus

Myrica

Podocarpus

Hypericum

HageniaErica

Age (cal kyr BP)
14

Arboreal Pollen (AP)

Depth (cm)80

Poaceae >37um

grasses and herbs (NAP)

montane forests

Non Arboreal Pollen (NAP)

trees and shrubs (AP)

5

14.5
100

4

15
15.5

150

AHP
3

16

H1

16.5
17

2

17.5

1

200

250

18

20

40

10

20

5

5

10

20

5

5

5

5

50

100

20

40

15

10

20

ratio

10

10

20

15

5

5

5

5

5

5

5

5

5

Pollen percentage

Fig. 6 Percentage diagram of selected pollen taxa referring to the counting sum of pollen grains of a sample and fivefold exaggeration

LPZ

Pediastrum

Sparganium-type
Botryococcus

Caryophyllaceae
Alchemilla
Tribulus
Cyperaceae

Matricaria-type

Cichorioideae

Thalictrum

Senecio-type

Rumex-typewater, algae

Plantago-type

Chenopodiaceae/
Amaranthaceae

Artemisia

Poaceae

Pistacia

Olea

Juniperus

MyricaHypericum
Podocarpus

Hagenia

Erica

Arboreal Pollen (AP)Age (kyr BP)

Depth (cm)80

Poaceae >37um

grasses and herbs (NAP)
montane forests

Non Arboreal Pollen (NAP)

trees and shrubs (AP)

5

14
14.5

100

4

15
15.5

150

AHP
3

16

H1

16.5
200

17

2

17.5
250

1

18

80

160

240 20 40 60

20 40

50

100

20

20 40

20

20

600

1200

280

560

90

180

90

180

80

160

60

120 20 40

20 40

20

20

20

20 40

15000

30000

1000

2000

Pollen influx (grains cm -1 a -1)

Fig. 7 Pollen influx diagram of selected taxa and algae from B4 depression, showing pollen accumulation rate in grains c­ m−1 ­year−1(Mekonnen et al. 2019). However, the use of TOC/N
ratios as organic matter source indicators is highly constrained by degradation and mineralization, which resultsin low TOC/N values. TOC/N values of the B4 depression range between 3.5 and 12.3, suggesting mainly primary lacustrine biomass production. According to Ficken
et al. (2000), Paq values, between 0.4 and 1 characterize
submerged and floating aquatic plants, while terrestrial
plants yield values lower than 0.1. Indeed, average HI values of ~ 250, together with elevated Paq values support
the interpretation that mainly lacustrine primary biomass was produced under medium to high water levels,

generating anoxic conditions in B4 depression. TOC
values substantially decrease at 160 cm (16.3 cal kyr BP)
accompanied by sand maxima. This likely documents an
extremely low stand of the water table and the development of a desiccation surface. Increased sand input fromthe depression margins may take place because of temporary heavy rains during dry phases when the sparsevegetation cover protected less against erosion. Decreasing Paq values between 140 and 120 cm shows higher
terrestrial input, likely due to higher rainfall intensities
and accelerated erosion at the margins of the depression. Nevertheless, water levels dropped again alreadyMekonnen et al. Progress in Earth and Planetary Science(2022) 9:14

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at ~ 15.4 cal kyr BP as evidenced by the formation of aswamp around 15.4–15.2 cal kyr BP (120–110 cm; Fig. 4).
Decreasing TOC and HI values and absence of pyrite insediments deposited between 14.4 and 13.6 cal kyr BP
indicate accelerated local desiccation, likely correlatedwith periodic sediment input. Varying sand contentssuggest that humid and dry phases alternated, leading to
deflation of small particles and organic matter, leaving
behind sand in the depression. Intensive deflation andcomplete desiccation are likely responsible for the hiatus
at 70 cm depth (13.6–10.2 kyr BP), coinciding with thetransition from Late Glacial to Holocene.
Stable C isotopes are frequently used in paleovegetation reconstructions due to their potential in discerning between C3 and C4 plants (Eshetu 2002; Glaser andZech 2005). Our bulk δ13C values and those of long-chain
n-alkanes show average values of −14.3‰ and –25.9‰,
respectively. This might show C4 vegetation (Ficken et al.
2002; Glaser and Zech 2005) growing at the margin of
the depression under relatively dry conditions during
the Late Glacial. However, methanogenesis, assimilationof 13C-enriched ­CO2 and H­ CO3− by water plants due tolong-lasting ice cover, especially during the early Late
Glacial, and the low ­CO2 concentration in the atmosphere (Monnin et al. 2001) might have contributed tosuch positive δ13C values (Conrad et al. 2007). In contrast, modern plants from the surroundings of the lakeshow values indicating C3 vegetation. This result agrees
with a study of Mekonnen et al. (2019), reporting that themodern vegetation in high altitudes of the Bale Mountains comprises only C3 plants.
BC produced by incomplete combustion of plant biomass is ubiquitous in lake sediments and glaciers (Glaser
1998). The weak BC maximum in the lowermost samples(~ 250 cm) presumably reflects BC storage in glacier ice
during the LGM, originating from atmospheric input. In
contrast, high BC between 16.9 and 16.6 cal kyr BP and
between 15.7 and 14.4 cal kyr BP provides evidence forfrequent fire events, likely due to increased amounts of
combustible biomass under regionally wetter than under
dry conditions.

of detrital and allochthonous input (Davies et al. 2015).
For instance, Zr and Zr based ratios (Zr/Ti, Zr/Rb, Zr/Al) are often used as proxies for aeolian input of Saharan dust (Jimenez-Espejo et al. 2014; Scheuvens et al.
2013; Moreno et al. 2006, and Hemming 2007), while
Si ratios (Ti/Si, Zr/Si) are used as indicators of biogenic
silica production during favorable seasons (Lamb et al.
2018; Davies et al. 2015; Brown et al. 2007). Furthermore,also K is used as an effective moisture fluctuation indicator (Foerster et al. 2012). In our B4 profile, lithogenic
elements and ratios such as Zr, Hf, Zr/SiO2, Zr/Rb, andZr/TiO2 show minimum values after deglaciation; theyslightly increased around 210 and 190 cm (~ 17.2 and16.9 cal kyr BP), while a continuous increase of theseelements and ratios is recorded from 175 to 135 cm(~ 16.6–15.7 cal kyr BP) (Fig. 5). In contrast, CPA values
show a weak minimum at ~ 210 and 190 cm and further
decrease to < 80 between 175 and 135 cm. These results
likely reflect intensified dry northerly winds (Brown et al.
2007; Marshall et al. 2011; Lamb et al. 2018; JimenezEspejo et al. 2014), which transported less weathered(low CPA, Na enriched, Fig. 5), allochthonous material
in the Sanetti Plateau. The occurrence of gypsum-bearing layers in ~ 210 cm (~ 17.2 cal kyr BP) and between
166 and 152 cm (16.4–16.1 cal kyr BP) show lower watertables and provide evidence for dry events. The latter isfurther confirmed by increasing values of ­K2O (Fig. 5),
which agrees with results from Chew Bahir (Foerster
et al. 2012). Moreover, increased values of ­K2O and­Na2O (Fig. 5) coincide with the high abundance of Feldspar in ~ 135 cm (Additional file 3), highlighting Feldspar
containing allochthonous input. The abrupt decrease of
most allochthonous elements and ratios (Fig. 5) at about
15.7 cal kyr BP indicates a drastic atmospheric reorganization with an abrupt decrease of dust inputs related tothe beginning of the AHP, attributed to the northwardmigration of the rain belt and the ITCZ (Bastian et al.
2021).

4.3 Inorganic geochemistry

After deglaciation, the period between 18.1 and 16.6 cal
kyr BP started with the accumulation of electrolytepoor melting water in depression B4 (see EC in Additional file 1). The presence of pyrite at the bottom of
the B4 profile implies a sedimentary environment withmainly anoxic conditions and sufficient organic matter that acts as a reductant and energy source for pyrite
formation (Berner 1984). Due to their geochemical stability, lithogenic elements such as Zr, Hf, Nb, and Ti areoften used in paleoenvironmental studies as indicators

4.4 Palynology

The green algae Botryococcus is generally found in tropical freshwater lakes under very cold environmental conditions (Jankovská and Komárek 2000). Therefore, thehigh occurrence of Botryoccoccus at the bottom of the B4
profile indicates deglaciation and discharge of cold meltwater into the lake. The abundant presence of Botroyoccocus, which is usually characterized by high TOC/N
values of 18–36 (Last and Smol 2002; Bittner et al. 2020)and high δ13C values (Grice et al. 2001), may have contributed to the TOC/N increase recorded between 250
and 240 cm depth and positive δ13C values at the bottom of the profile (Fig. 4). This supports the interpretation of predominant lacustrine TOC production afterMekonnen et al. Progress in Earth and Planetary Science(2022) 9:14

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deglaciation. The decrease of Botryococcus and a parallel
increase of Pediastrum also show a relatively stable water
body in the investigated depression.
The high percentages of Poaceae and herbs (non-arboreal pollen, NAP) between 18.1 and 17.2 cal kyr BP documents open grass-land vegetation around the Sanetti
Plateau. The high number of Artemisia and Chenopodiaceae/Amaranthaceae pollen indicate that the vegetation
may have been steppe-like. The abundant presence ofPlantago suggests a sparse vegetation cover with less competition shortly after deglaciation. However, the presenceof arboreal pollen points to forests down the slopes of theBale Mountains during the LGM. An increase in Chenopodiaceae/Amaranthaceae and Plantago and a decrease in
Artemisia, Pediastrum, Hagenia, Podocarpus, Myrica, Juniperus, and Olea between ~ 17.2 and 16.9 cal kyr BP reflect a
reduction of the montane forests due to a ~ 300-year interval with dry and/or cold climate. This interpretation further agrees with the mineralogical results which recorded
a dry event between 17.2 and 16.9 cal kyr BP. The increaseof arboreal pollen and Pediastrum between about 16.9–
16.6 cal kyr BP marks a recovery of the montane forest andwetter local setting at the slopes of the Sanetti Plateau. At
around 16.5 cal kyr BP, low values of Pediastrum, Erica,
Hagenia, and Botryococcus indicate dry environmental
conditions. The decrease in the influx of Erica pollen andsubsequent increase of Chenopodeacea/Amaranthaceae
around 16.3 cal kyr BP related to the desiccation phaseof the B4 depression recorded by low TOC and high sandinput. After this desiccation phase, a trend to locally morehumid conditions is indicated by higher Pediastrum values,
more frequent fern spores, and Sparganium-type between
155 and 135 cm (16.1 and 15.8 cal kyr BP). This is in agreement with increasing TOC values and it documents thaton the SP the H1 stadial is characterized by a short-term
desiccation, in contrast to the lowland archives (Fig. 8). Dry
environmental conditions are also evidenced from 15.4 to
15.2 cal kyr BP by the decrease in Erica and Hagenia pollen, while tolerant montane taxa such as Podocarpus andJuniperus survived. Nevertheless, as indicated by constanthigh Pediastrum influx and high Paq values, the B4 depression was partly or periodically flooded. Changes in seasonality and longer dry seasons may have given way to swamp
formation. However, pollen types, including typical swamp
and water plants (Cyperaceae, Sparganium-type), show
no increase. The increasing Plantago-type includes theamphibious species Littorella uniflora, but this swamp species is not known from Africa today (Hoggard et al. 2003).
Possibly, the increased Poaceae influx marks the spreading of swamp grasses at the site. A temporary decrease in
the influx of arboreal pollen may be related to a short cold
event that also decreased the flourishing of grasses andherbs (NAP). The long-time trend in NAP influx reduction

can relate to a change in the vegetation from wind-pollinated plant groups (Poaceae, Chenopodiaceae/Amaranthaceae, Plantago-type, Rumex-type) to insect-pollinatedtaxa (Senecio-type, Caryophyllaceae), leading to an overall
reduced NAP pollen input. By contrast, the decrease in
Chenopodiaceae/Amaranthaceae and consequent increaseof A/C between 15.2 and 14.4 cal kyr BP indicate the mosthumid conditions regionally, whereas locally the B4 depression started to dry out due to increased silting up. A significant increase in Erica and Podocarpus pollen between 14.4
and 13.6 cal kyr BP substantiates the expansion of the Ericaceous belt and the lowland forests, together with a fundamental change in vegetation composition on the Sanetti
Plateau. This is in contrast to the sedimentological results
which document progressive local drying out. This discrepancy might be due to the potential of pollen assemblage toprovide regional environmental signals, while biogeochemical proxies (Fig. 4), mainly reflect the local conditions of
the B4 record.

5 Environmental implications and comparison
with other records
5.1 Deglaciation and climate fluctuations
14

C ages from the lower sediment of depression B4 suggest that the Sanetti Plateau became at least locally, icefree already at ~ 18.2 cal kyr BP. Surface exposure datingof erratic boulders in N-exposed valleys and on the Sanetti
Plateau suggest the onset of deglaciation at 15.2 ± 1.2 cal
kyr BP (Groos et al. 2021; Ossendorf et al. 2019). The
basal 14C-age of Garba Guracha sediments (15.9–16.7 cal
kyr BP) is almost in agreement with the age of deglaciation estimated from cosmogonic ages (Umer et al. 2007;
Tiercelin et al. 2008; Bittner et al. 2020). This discrepancy
might be due to the ice cover in the N-exposed trough valley of Lake Garba Guracha that might have been thicker
and thus longer lasting than on the Plateau around theB4 profile, which is strongly wind-exposed and has no ice
accumulating catchment (Fig. 1). Besides, methodologicaluncertainties (exposure dating versus radiocarbon analysis)
cannot be ruled out. Nevertheless, the deglaciation age of
B4 depression seems reliable.
Our palynological and biogeochemical results record
climate fluctuations on the Sanetti Plateau since the endof the LGM. Disregarding the initial phase around 18 cal
kyr BP, the environmental conditions during unit 1 canbe characterized as predominantly lacustrine, correlating with results from Lake Tanganyika, Challa, and Chew
Bahir, which also show predominantly humid conditions(Fig. 8). However, between ~ 17.2 and 16.9 cal kyr BP
partly fluctuating allochthonous elements (Fig. 5), correlating with increased deflation in the northerly dust
source areas, and an increase in Chenopodiaceae/Amaranthaceae and reduction in montane forest indicate

Mekonnen et al. Progress in Earth and Planetary Science(2022) 9:14

Page 13 of 19

a short dry event during this time. Possibly, this event
relates to a short weakening of the East-Asian monsoon
system as reported by Wang et al. (2001) for Asia around17 cal kyr BP as a precursor of Heinrich Event 1 (Hemming 2004). Also, Camuera et al. (2021) described suchan early H1 phase (refers to HS1b) between 17.2–16.9 cal
kyr BP in the Mediterranean. The ~ 300 years of climate
deterioration are also weakly documented in sedimentsof Chew Bahir (Fig. 8) and might be part of the megadrought postulated for the Afro-Asian monsoon system
between 17–16 cal kyr BP (Stager et al. 2011).
We interpret increasing input of allochthonous elements between 16.6 and ca.15.9 cal kyr BP (Fig. 5) as anindicator of the dominance of northerly wind, correlatedwith progressive ecosystem degradation in the lowlands,
and occurring contemporaneously with the North Hemispheric cooling during H1. Interestingly, on the Sanetti
Plateau TOC-minima and sand-maxima indicate only
short-term intensive desiccation at 16.3 cal kyr BP, andalready between ~ 16.1 and 15.7 cal kyr BP TOC and HI
increased again, whereas sand contents rapidly decreased(Fig. 4), reflecting that the environmental conditions
locally, on the Sanetti Plateau improved, in agreementwith the palynological results. This discrepancy between
the high altitudes of the Sanetti Plateau and the vast lowlands northerly might reflect that the Bale Mountains,located close to the Indian Ocean, react more rapidly toenvironmental changes, whereas the lowland ecosystems
are much more resilient, due to slow negative vegetationand monsoon feedbacks.
In many studies, arid conditions in tropical Africa
between ~ 16.8 and 15.4 cal kyr BP were alreadydescribed, coinciding with H1 (e.g. Bonnefille and Chalie
2000; Gasse 2000; Talbot and Lærdal 2000; Hemming
2004; Tiercelin et al. 2008; Tierney et al. 2008). Lambet al. (2007) also mentioned an “episode of shallow water”
in the Lake Tana basin between 16.7 and 15.7 cal kyr BP.
Based on δD values of higher plant leaf waxes from Lake
Challa (Fig. 8), Tierney et al. (2011) concluded that thesearid conditions in eastern Africa were due to reduced
moisture input from the Indian Ocean. Previous studies described that climatic fluctuations in eastern Africa
during the Late Quaternary were related to teleconnections between North Atlantic cooling events, recordedin Greenland ice cores and N-Atlantic sediments, andthe weakening of the Indian Summer Monsoon (Tierney et al. 2008; Marshall et al. 2011; Mohtadi et al. 2014;
Lamb et al. 2018). It is suggested that during these cooling events, caused by massive iceberg surges and cold
meltwater entering the North Atlantic, the ITCZ and theNorthern Hemisphere westerlies shifted abruptly southwards and abruptly northward at the end of the cooling, thus controlling deflation of lithogenic elements outof the northern source areas and their input in the surrounding areas (Tierney et al. 2008; Marshall et al. 2009;
Lamb et al. 2018). This might explain the high input of
allochthonous lithogenic elements (Fig. 5). A similar finding is presented by Brown et al. (2007), reporting elevatedinputs of e.g. Zr to Lake Malawi due to intensified northerly winds.
The allochthonous elements decreased abruptly atabout 15.7 cal kyr BP, likely correlating with the onset of
the AHP, and evoked by rapid northward migration of therain belt and the ITCZ in relation to precession-driven
insolation changes (Bastian et al. 2021). We assume
that this resulted in a relatively rapid establishment of adenser vegetation cover and a drastic reduction of dust
deflation from northern source areas (Figs. 5 and 8). The
timing of this event agrees with the results of Camuera
et al. (2021). They concluded that the AHP started in the
Mediterranean at 15.7 cal kyr BP after the termination of
HS1c. Also, Lamb et al. (2007, 2018), described such an
abrupt shift from Lake Tana, and Marshall et al. (2011)
dated this event to 15.3 cal kyr BP. In Lake Victoria, therapid refilling of the basin is dated to 14.5 cal kyr BP (Talbot and Lærdal 2000). One reason for this discrepancy
might be dating uncertainties. However, our data fromthe B4 profile let assume that locally, at the high altitudesof the Sanetti Plateau, the abrupt onset of humidity might
have started almost 1000 years before the onset of the
Bølling–Allerød warming in the North-Atlantic region(Alley and Clark 1999; Van Raden et al. 2013) and before
at least some hundred years earlier than in Lake Tanaregion, located 630 km northward. In contrast to the sedimentological and geochemical results, our pollen results(Figs. 6 and 7) do not show clear evidence for this abrupt
start of the AHP. Within this humid period, water levelsin the B4 record did not remain high but lowered alreadyaround 15.4 cal kyr BP, and a swamp developed, reflecting probably higher evaporation and less precipitation.

(See figure on next page.)
Fig. 8 A summary figure comparing palynological and biogeochemical results from unit 1 and unit 2 of the B4 sediments with relevant proxies
from Lake Tanganyika (Tierney et al. 2008), Lake Challa (Tierney et al. 2011) and Lake Chew Bahir (Foerster et al. 2012) sediments. Light orange
color indicate dryer events, orange color indicates a desiccation layer developed during Heinrich Event 1 (H1). Blue color characterizes the African
Humid Period (AHP) and other humid episodes while dark green color indicates the event characterized as humid and arid according to pollen andgeochemical results, respectively. δD and K values are in inversed scaleMekonnen et al. Progress in Earth and Planetary ScienceFig. 8 (See legend on previous page.)(2022) 9:14

Page 14 of 19

Mekonnen et al. Progress in Earth and Planetary Science(2022) 9:14

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A similar finding was reported from Lake Tana, where apapyrus swamp developed between 15.7 and 15.1 cal kyr
BP, due to lower lake levels and reduced rainfall (Lambet al. 2007). Slightly rising allochthonous proxies (e.g.
Zr, Nb, Hf, see Fig. 5) likely document progressive local
drying-up after about 14.4 cal kyr BP, whereas ascending arboreals indicate regional wetter conditions (Fig. 7).
The uppermost part of the unit 2 sediments, deposited
between ~ 14.4–13.6 cal kyr BP, is enriched in arboreal pollen, reflecting regional wetting; but fluctuating
sand contents and higher input of terrestrial alkanes let
assume that drying-up of the B4 depression by advanced
periodic sediment input continued. These processes
do not allow a reliable correlation of the environmental
dynamics affecting the Sanetti Plateau with those documented in sediments of Lake Tanganyika, Challa, andChew Bahir (Fig. 8); also an unambiguous relationshipwith climatic phenomena known from the North Atlantic region (e.g. Allerød) cannot be identified in the upper
part of unit 2.

deposits, Umer et al. (2007) and Kuzmicheva et al. (2014)
assume that Erica vegetation expanded on the Sanetti
Plateau above the actual upper limit of the Erica belt, only
during the early Holocene beginning at 11.2 cal kyr BP.
The dry climate during mid to Late Holocene and natural
and/or human-induced fires (Miehe and Miehe 1994) aresuggested as major causes for the decline of Erica which
at present exists above the upper line of the Erica belt
only in form of isolated fragments, mainly restricted toslopes rich in basalt boulders. Furthermore, Miehe andMiehe (1994) assumed that these Erica patches are relics that document an intensive extension of Erica specieson the Sanetti Plateau after deglaciation, which were lateron destroyed because of the invasion of hunter-gatherers
and pastoralists. However, since the upper timberline istemperature-controlled (Körner 2008), we hypothesize
these Erica patches on the plateau survive due to better
microclimate, generated by dark basalt boulders.

5.2 Vegetation

The B4 pollen record allows the reconstruction of vegetation changes from 18.1 to 13.6 cal kyr BP. Between
18.1 and 16.8 cal kyr BP, the vegetation is characterized
as herb-rich grassland with Chenopodiaceae/Amaranthaceae. Plantago maxima between 17.2–16.9 cal kyr BP
are related to dry and/or cold environmental conditions(see Section 4.4). These results are roughly in agreementwith Umer et al. (2007), reporting open grassland vegetation at 16 cal kyr BP around Lake Garba Guracha. Ourpollen record further shows high Arboreals such as Podocarpus, Hygenia, Erica, and Juniperus at 16.9 cal kyr BP,
indicating more favorable conditions, in agreement withthe sedimentological data. Increased aridity between
16.6 and 15.7 cal kyr BP, well documented by most biogeochemical proxies and interpreted to correlate with
H1, is not clearly revealed in the palynological data. The
desiccation surface developed around ~ 16.3 cal kyr BP
correlates with decreased contents of Erica pollen, Pediastrum, ferns, and Botryococcus, and elevated Chenopodiaceae/Amaranthaceae ratios, indicating a temporary
phase of less favorable, drier environmental conditions.
This possibly marks the time of maximum H1 dryness.
Similarly, the AHP, recorded by biogeochemical proxies,
is not obvious in the pollen results. Our data show that
Ericaceae pollen content considerably increased on the
Sanetti Plateau from 14.4 to 13.6 cal kyr BP, likely correlating with an increase in temperature, which might
be synchronous with the European Meiendorf interstadial starting at 14.4 varve years (Litt et al. 2001). According to pollen results from Lake Garba Guracha and peat

5.3 Fire dynamics on the Sanetti Plateau

Our black carbon results (Fig. 4) show that fire was acommon phenomenon in the Bale Mountains even during the early Late Glacial. However, pollen data indicatethat at this time Erica was not growing around the B4
depression (Fig. 8). Therefore, BC in the lower part ofthe B4 profile likely originates from other vegetation fires
or the Erica belt in lower altitudes. This interpretation is
supported by the fact that BC, even in the dry mode H1
sediments, is not clearly increased. Only with the beginning of the AHP around 15.7 cal kyr BP, BC started toincrease, correlating with warmer and more humid environmental conditions, and reaching a maximum at about
15 cal kyr BP coinciding with increasing Erica pollen. In
the uppermost part of unit 2 (70–80 cm) Erica pollen
further increased, again correlating with maximum BC at
about 13.6 cal kyr BP.
Up to now, despite the human invasion during 47–31 cal
kyr BP, there is no evidence that during the Late Glacial
hunter-gatherers were living on the plateau and burned
the Erica to facilitate hunting. However, we cannot ruleout that fires were triggered by the increase of easily combustible Erica biomass due to improved environmental
conditions. For a different time period, but in connection with the fuel-controlled fire process, the charcoal
record from Garba Guracha supports a strong correlation
between fire occurrence and heathland expansion during the Holocene (Gil-Romera et al. 2019). Elevated BC in
the early humid Holocene deposits of the B4 record (notshown here) supports our interpretation that the burningof Erica is mainly controlled by the amount of Erica biomass, being higher under warm and humid climates.

Mekonnen et al. Progress in Earth and Planetary Science(2022) 9:14

6 Conclusions
Our high altitude sedimentary archive provides a newolder deglaciation age of 18.2 cal kyr BP for the Sanetti Plateau. Biogeochemical results show that the Plateau was sensitive to local, regional and global climatechanges. We detected a severe local drought event on the
Sanetti Plateau at ~ 16.3 cal kyr BP with complete desiccation of the past B4 lake for some decades, related toH1. Between ~ 16.6 and 15.7 cal kyr BP allochthonous
elements like Zr, Hf, Nd, Nb, Na, presumably windblownby dry northerly winds during H1, accumulated increasingly in the B4 record, indicating continuous but delayed
degradation of the wind source areas in lower altitudes
but without documenting maximum dryness at 16.3 cal
kyr BP. This is in contrast to our high altitude B4 archiveand reflects the resilience of the lowland ecosystems.
The Abrupt change to humid conditions at ~ 15.7 cal kyr
BP indicates the onset of AHP in the Bale Mountains
some hundred years earlier than in the Lake Tana region
but in agreement with the termination of the HS1c phasein the Mediterranean.
The vegetation on the Sanetti Plateau was less sensitive to increased aridity during H1 and also to increased
humidity during AHP. Nevertheless, the Erica pollen
increased in the B4 sediments at ~ 14.4 cal kyr BP, correlating with a wet and warm regional climate. Despite thisincrease, it remains open whether the Ericaceous belt
expanded to the plateau during that time or during theearly Holocene, except for the currently existing isolated
patches on boulder-rich steep slopes. Our results indicatethat fire incidences mainly coincide with an expansionof the vegetation cover and less with dry periods. Most
likely, a warm and humid climate promotes biomass production of Erica, hence increasing the amount of fuel,which burns from time to time. This allows to conclude
that biomass and thus fuel availability is important factor
controlling fire events in the Bale Mountains.
Abbreviations
AHP: African humid period; A/C: Artemisia/Chenopodiaceae; AP: Arboreal
pollen; BC: Black carbon; BPCAs: Benzene polycarboxylic acids; CPA: Chemical
proxy of alteration; DCM: Dichloromethane; FID: Flame ionization detector;
GC: Gas chromatography; H1: Heinrich event 1; HI: Hydrogen index; LGM:
Last Glacial Maximum; NPA: Non-arboreal pollen; XRF: X-ray fluorescence; YD:
Younger Dryas.

Supplementary Information
The online version contains supplementary material available at https://​doi.​
org/​10.​1186/​s40645-​022-​00472-9.
Additional file 1. Grain size distribution (clay < 6.3 μm, silt 6.3–63 μm andsand 63-2000 μm), electrical conductivity (EC), pH, and depth profiles of­SiO2 of the B4 sediments. Light orange color indicates drier event whileorange color indicates desiccation during H1 and the hiatus. Blue colors
characterize the AHP and other humid episodes while dark green color

Page 16 of 19

indicate the event characterized as humid and arid according to pollen
and geochemical results, respectively.
Additional file 2. PCA biplot results based on geochemical compoundsof the B4 sediments in unit 1 and unit 2. In this figure 63.5% of varianceis explained (component 1 = 45.5% and component 2 = 18%). The two
ellipses show the cluster of our samples based on the stratigraphic units.
The arrows indicate each proxy and the distance between each arrow
exhibits the strength of the correlation between elements and com‑pounds. While ­SiO2, MnO, ­P2O5 and Ba contents are high in unit 1, unit 2
is characterized by high Zr, Hf, Sm, Nd, Ce, Y and ­Na2O values. However,
MgO, ­Al2O3, ­TiO2, Cu, Cr, Rb, Sr, Sc etc. are very low in both units.
Additional file 3. A table showing mineralogical results of selected
samples from the B4 profile.
Acknowledgements
We are grateful to the Bale Mountains National Park and Ethiopian Wild Life
Conservation Authority for permitting the scientific fieldwork and facilitating
access to sample materials used in this study. We express our gratitude to
the Department of Plant Biology and Biodiversity Management, Addis Ababa
University, for scientific collaboration. We also extend our sincere thanks to M.
Heider, M. Benesch, and H. Maennicke for their generous help during labora‑
tory work and J. Feilner for his help with the drawing. B. Mekonnen acknowl‑edges the support given by the Katholischer Akademischer Ausländer-Dienst(KAAD). We kindly thank three anonymous reviewers for their constructive
comments and suggestions that highly helped to improve our manuscript.
Authors’ contributions
WZ and BG proposed the topic, conceived and designed the study. Fieldwork(sample collection) was done by WZ, DS, RZ and AM. BM, FS, RB and AA did
the laboratory work and BM prepared the manuscript with the help of WZ, FS
and BG. All co-authors contributed to, read and approved the manuscript.
Funding
Open Access funding enabled and organized by Projekt DEAL. This work was
carried out within the DFG-funded Research Unit 2358, “The Mountain Exile
Hypothesis” (Grant Nos. GL327/18-1, ZE154/70-1).
Availability of data and materials
The datasets supporting the conclusions of this article are available in the fol‑lowing Zenodo repository: https://​doi.​org/​10.​5281/​zenodo.​47671​56.

Declarations
Competing interests
The authors declare that they have no competing interest.
Author details
1
Institute of Agricultural and Nutritional Sciences, Soil Biogeochemistry, Martin
Luther University Halle-Wittenberg, Halle, Germany. 2 Department of Urban
Agriculture, Misrak Polytechnic College, Addis Ababa, Ethiopia. 3 Instituteof Geography, Friedrich-Schiller-University Jena, Jena, Germany. 4 Instituteof Geography, Technische Universität Dresden, Dresden, Germany. 5 Lower
Saxony Institute for Historical Coastal Research, Wilhelmshaven, Germany.
6
Department of Palynology and Climate Dynamics, Albrecht‑Von‑Haller Insti‑
tute for Plant Sciences, Gottingen University, Göttingen, Germany. 7 Instituteof Applied Geosciences, TU Berlin, Berlin, Germany. 8 Department of Natural
Resource Management, Jigjiga University, Jigjiga, Ethiopia. 9 Departmentof Geo‑Environmental Processes and Global Change, Pyrenean Instituteof Ecology, CSIC, Zaragoza, Spain. 10 Department of Plant Biology and Biodiver‑sity Management, Addis Ababa University, Addis Ababa, Ethiopia. 11 Inter‑national Livestock Research Institute, Addis Ababa, Ethiopia. 12 Department
of Soil Science, University of Bayreuth, Bayreuth, Germany.
Received: 18 May 2021 Accepted: 9 February 2022

Mekonnen et al. Progress in Earth and Planetary Science(2022) 9:14

Page 17 of 19

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