ORIGINAL PAPER

Paleolimnological records of recent glacier recessionin the Rwenzori Mountains, Uganda-D. R. Congo

J. Russell Æ H. Eggermont Æ R. Taylor ÆD. Verschuren

Received: 11 November 2007 / Accepted: 14 May 2008 / Published online: 10 June 2008

� Springer Science+Business Media B.V. 2008

Abstract The status of tropical glaciers is enor-

mously important to our understanding of past,

present, and future climate change, yet lack of

continuous quantitative records of alpine glacier

extent on the highest mountains of tropical East

Africa prior to the 20th century has left the timing

and drivers of recent glacier recession in the region

equivocal. Here we investigate recent changes (the

last 150–700 years) in lacustrine sedimentation, gla-

cier extent, and biogeochemical processes in the

Rwenzori Mountains (Uganda- Democratic Republic

of Congo) by comparing sedimentological (organic

and siliciclastic component determined by loss-on-

ignition; LOI) and organic geochemical profiles

(carbon and nitrogen abundance, ratio, and isotopic

composition of sedimentary organic matter) from

lakes occupying presently glaciated catchments

against similar profiles from lakes located in catch-

ments lacking glaciers. The siliciclastic content of

sediments in the ‘glacial lakes’ significantly

decreases towards the present, whereas ‘non-glacial

lakes’ generally show weak trends in their siliciclas-

tic content over time, demonstrating that changes in

the siliciclastic content of glacial lake sediments

primarily record fluctuations in glacier extent. Radio-

metric dating of our sediment cores indicates that

prior to their late 19th-century recession Rwenzori

glaciers stood at expanded ‘Little Ice Age’ positions

for several centuries under a regionally dry climate

regime, and that recession was underway by 1870

AD, during a regionally wet episode. These findings

suggest that the influence of late 19th century

reductions in precipitation in triggering Rwenzori

glacier recession is weaker than previously thought.

Our organic geochemical data indicate that glacier

retreat has significantly affected carbon cycling in

Afroalpine lakes, but trends in aquatic ecosystem

functioning are variable among lakes and require

more detailed analysis.

Keywords Alpine glaciers � East Africa �Climate forcing � Loss-on-ignition �Global warming

Introduction

Tropical alpine glaciers exist today in the mountains

of South America, New Guinea, and East Africa, but

in all three regions they are rapidly retreating and

J. Russell (&)

Department of Geological Sciences, Brown University,

Box 1846, Providence, RI 02912, USA

e-mail: james_russell@brown.edu

H. Eggermont � D. Verschuren

Limnology Unit, Department of Biology, Ghent

University, K. L. Ledeganckstraat 35, Ghent 9000,

Belgium

R. Taylor

Department of Geography, University College London,

Gower Street, London WC1E 6BT, UK

123

J Paleolimnol (2009) 41:253–271

DOI 10.1007/s10933-008-9224-4

many will likely disappear within the next several

decades (Kaser and Osmaston 2002; Thompson et al.

2006; Vuille et al. in press). Glacier recession in

tropical East Africa has been particularly acute:

Glaciers on each of the highest mountains in the

region (Mt. Kenya in central Kenya, Kilimanjaro in

northeastern Tanzania, and the Rwenzori, on the

border between Uganda and the D. R. Congo), have

lost over 80% of their surface area since 1900

(Hastenrath and Kruss 1992; Thompson et al. 2002;

Taylor et al. 2006a). The widespread loss of tropical

glaciers has become a powerful symbol of the effects

of human-induced global warming (Thompson et al.

2006), yet the chronology of tropical glacier fluctu-

ations during the past several centuries in East Africa

is poorly known, complicating their climatic inter-

pretation (Kaser 1999). The current glacier recession

in tropical Africa is widely considered to have begun

following a regional wet phase that ended at about

1880 AD (Hastenrath and Kruss 1992; Hastenrath

2001; Molg et al. 2003), yet scientific observations of

glaciers on Kilimanjaro began only in 1887, and on

Mt. Kenya and the Rwenzori after 1900 AD (Has-

tenrath 1984), well after the proposed initiation of

their recent recession. Similarly, although all of East

Africa’s mountains are purported to have had

expanded glacier areas at the end of the Little Ice

Age (LIA) (Hastenrath 1984; Kaser 1999), in most

cases moraine deposits or other archives that docu-

ment these recent glacier fluctuations have not been

directly dated. Thus the timing, causes, and signifi-

cance of recent East African glacier recession in the

context of regional climate history remain enigmatic.

Unraveling the recent history of tropical African

glaciers is vitally important for understanding the

stability and resilience of tropical mountain glaciers

and ecosystems, the impacts of human-induced

global warming and natural climate variability in

tropical alpine environments, and the climatic con-

trols on tropical glacier extent. Paleolimnological

records from alpine lakes have provided valuable

information on the history of mountain glaciers in

north temperate regions (Leonard 1986; Benson et al.

1996), in the tropical Andes (Seltzer et al. 2002;

Polissar et al. 2006), and in East Africa during the

late Pleistocene and Holocene (e.g. Karlen et al.

1999; Olago et al. 1999; Barker et al. 2001). The

sedimentary record of Africa’s alpine lakes must also

contain valuable and largely untapped records of

recent fluctuations of its glaciers; however, unravel-

ing the history of tropical alpine glaciers from

paleolimnological records poses several challenges.

Foremost, changes in lacustrine sedimentology,

chemistry, and biology recorded in paleolimnological

records driven directly by climatic change can

resemble the variability driven by glacier advances

and recessions that are themselves the product of

climate variability (e.g. Polissar et al. 2006). Distin-

guishing between responses to climate and responses

to climate-driven glacier fluctuations in paleolimno-

logical records therefore requires careful evaluation

of the sensitivity of both the lake systems and

individual indicators of glacial processes.

Alpine glaciers in the Rwenzori, Ptolemy’s leg-

endary Mountains of the Moon, have steadily shrunk

from an area of *7.5 km2 in 1906 to \1 km2 in

2003, and glaciers on the lowest three peaks

(Mt. Luigi di Savoia, Gessi, and Emin) of the

Rwenzori’s six major peaks have now disappeared

completely (Kaser and Osmaston 2002; Taylor et al.

2006a). Although the Rwenzori contain dozens of

lakes whose sediments chronicle the history and

ecological impacts of this glacier recession, political

difficulties have until recently limited paleolimno-

logical research in the Rwenzori to palynological

studies of the three most accessible lakes on the

Ugandan side (Lakes Mahoma, Bujuku, and Lower

Kitandara; Livingstone 1967). Interest in the impacts

of human-induced climate change on highly sensitive

mountain glaciers, hydrology, and ecosystems has

spurred new initiatives to investigate past, present,

and future climate change in tropical mountains,

including the Rwenzori. For instance, the loss of

tropical glaciers has severely affected the hydrology

of the tropical Andes, with adverse effects on

water resources for human populations downstream

(Bradley et al. 2006). The impending loss of the

Rwenzori’s glaciers may not dramatically alter river

discharge at low elevations in western Uganda

(Taylor et al. in prep); however, the loss of the

Rwenzori glaciers will certainly affect aquatic and

terrestrial ecosystem processes in the Rwenzori’s

high elevation lakes (Panizzo et al. 2008).

Here, we use sedimentological proxies to evaluate

the sensitivity of Rwenzori mountain lakes to hydro-

logical and ecological changes associated with recent

glacier retreat. We do this by comparing loss-on-

ignition (LOI, organic and siliciclastic component)

254 J Paleolimnol (2009) 41:253–271

123

and organic geochemical profiles (carbon and nitro-

gen abundance, ratio, and isotopic composition of

sedimentary organic matter) from lakes occupying

presently glaciated catchments against profiles from

lakes located in catchments lacking glaciers (hereaf-

ter refered to as ‘glacial’ and ‘non-glacial’ lakes,

respectively) spanning the last 150–700 years. Using210Pb and AMS 14C-dating of these sediment records,

this study also aims to document the timing of recent

glacier recession in the Rwenzori, thereby providing

new insights into the climatic controls of deglacia-

tion. Together, our data show that Rwenzori’s high-

elevation lakes are highly sensitive to alpine glaci-

ation and constitute a unique laboratory to assess

relationships between glacier extent, Afroalpine eco-

system processes, and long-term changes in central

African climate.

Regional setting: the Rwenzori

The Rwenzori straddle the equator along the border

between Uganda and the D. R. Congo, between Lakes

Edward and Albert in the western arm of the East

African Rift System (Fig. 1). The range is formed

from a NNW trending horst, and has a total area of

*3,000 km2 and a maximum elevation of 5,109 m

asl (Fig. 1). Although the glaciers of Mt. Kilimanjaro

are more famous, the Rwenzori range contains the

most extensive glacier and Afroalpine ecosystem in

Africa (Livingstone 1967; Osmaston 1989). Prior to

the 1950s, glaciers existed on all six of the Rwenz-

ori’s peaks, and fingers of ice protruded from the

highest peaks (Mounts Stanley, Speke, and Baker) to

as low as 4,200 m asl in adjacent valleys (Fig. 2;

Kaser and Osmaston 2002). The Rwenzori are

protected by national park status in both Uganda

and the D. R. Congo, and have UNESCO World

Heritage site status in recognition of their endemism

and biodiversity.

Rain falls heavily on the Rwenzori from October-

November and March-May during the twice-yearly

migration of the Intertropical Convergence Zone

(ITCZ) through the region (Osmaston 1965). Annual

rainfall can exceed 2.5 m/yr, with monthly values

ranging from 8 to 10 cm/month during the dry season

(June–August) to 40 cm/month during November.

Rainfall on the Ugandan side of the Rwenzori is

strongly dominated by the easterly monsoons, and

annual rainfall on the western slopes is less than in

the east. Annual temperatures average 2�C at 4200 m

elevation, and follow a lapse rate of about 0.67�C per

100 m elevation (Osmaston 1965).

The Rwenzori are the site of 30 named lakes, nine

of which are located in the D. R. Congo and 21 in

Uganda (Fig. 1). These lakes are all located near or

above 3,000 m asl, and range from\0.01 to 11.23 ha

in surface area, and 3.0 to 37 m in depth. They are

generally acidic (mean pH of 5.61), highly dilute

(mean conductivity of 20 lS/cm), and are dystrophic

to oligotrophic (Eggermont et al. 2007). The lakes

are all of glacial origin, with the exception of Lake

Bujuku which formed within the last millennium

behind a landslide from the slope of Mt. Baker

(Livingstone 1967).

Materials and methods

We collected water samples, bathymetric and water-

column data, and short surface gravity cores from 16

lakes in the Rwenzori over the course of three

expeditions from 2005 to 2007 (Eggermont et al.

2007). All but one of these lakes are located on the

Uganda side of the range between 2,990 and 4,054 m

asl. The exception is Lac du Speke, which is located

in the D. R. Congo *1 km from the Ugandan border

at 4,235 m asl beneath glaciers on western Mt. Speke.

Our analyses focused on comparisons of paleoenvi-

ronmental datasets from lakes that have or until

recently had glaciers within their catchments (here-

after termed ‘glacial lakes’: Bujuku, Lac du Speke,

Upper and Lower Kitandara, and Kopello) against

similar datasets from lakes that in recent centuries

have not had glaciers in their catchments (hereafter

‘non-glacial lakes’: Mahoma, East Bukurungu, Ba-

toda, Upper, Middle, and Lower Kachope, Nsuranja,

Bigata, Africa, Kanganyika, Katunda).

We estimated the catchment areas of the lakes near

the Rwenzori’s highest glaciated peaks (Lac du Speke,

Lake Bujuku, and Upper and Lower Kitandara) by

calculating the height of land from GIS data published

in Kaser and Osmaston (2002). The glaciated land

surface area within each lake’s catchment in 1905,

1955, and 1990 was also calculated from these GIS

data. Catchment areas for Kopello and all non-glacial

lakes were calculated by tracing the height of land

from 3rd edition topographic maps (Uganda Lands and

J Paleolimnol (2009) 41:253–271 255

123

Surveys Department 2000) using Adobe Photoshop v.

9.02TM and calculating surface area using ImageJ v.

1.37 (W. Rasband unpublished software). The catch-

ment area of Lake Kopello that was still glaciated in

1955 was obtained from Kaser and Osmaston (2002);

earlier observations are not available.

We retrieved sediments with a UWITEC gravity

corer from the deepest part of each lake, sectioned

cores upright in the field in 1 cm increments (Ver-

schuren 1993), and transferred samples to Whirl-Pak

bags for transport to Ghent (Belgium) and Brown

University (US). Organic matter and carbonate con-

tent were determined using loss-on-ignition (LOI;

Dean 1974) at contiguous 1-cm intervals for all cores

except cores from Lakes Mahoma and Middle Kac-

hope, which were analyzed at contiguous 2-cm

intervals. We recovered multiple cores from Lakes

Batoda, Speke, Mahoma, and Upper Kitandara and

cross-correlated cores by visually matching shared

features in their LOI profiles.

The low pH and conductivity of these lakes

precludes precipitation and preservation of carbonate

minerals, an inference confirmed by smear-slide

analyses of sediments from all lakes. Smear-slide

analyses further show that biogenic opal comprises

less than 5% of the sediment. Therefore, the fraction

of sediment remaining after combustion at 1,000�C is

primarily composed of terrestrial inorganic particu-

late material, and will hereafter be termed siliciclastic

content.

We analyzed the carbon and nitrogen abundance,

ratio, and isotopic composition of sedimentary organic

matter (C/N, d13Corg and d15Norg) in cores from a

subset of glacial (Lac du Speke, Upper and Lower

Kitandara) and non-glacial lakes (Mahoma, Batoda,

Upper Kachope). About 1 cc sediment samples taken

Fig. 1 Topographic map of

the central Rwenzori

mountain range showing the

location of the 16 lakes

analyzed for this study

(modified from Eggermont

et al. 2007). Lakes situated

in glaciated catchments are

shaded in gray, other lakes

in black. 1 = Bujuku,

2 = Upper Kitandara,

3 = Lower Kitandara,

4 = Lac du Speke,

5 = Kopello, 6 = Batoda,

7 = Upper Kachope,

8 = Middle Kachope,

9 = Lower Kachope,

10 = East Bukurungu,

11 = Mahoma,

12 = Nsuranja,

13 = Bigata, 14 = Africa,

15 = Kanganyika,

16 = Katunda. The location

of the Rwenzori in Africa is

marked with an asterisk in

the inset map

256 J Paleolimnol (2009) 41:253–271

123

Fig. 2 Map showing the

areal extent of glaciers on

Mts. Speke, Stanley, and

Baker within the

catchments of Lac du

Speke, Lake Bujuku, and

Upper and Lower Kitandara

J Paleolimnol (2009) 41:253–271 257

123

at 1–2 cm intervals were freeze-dried, ground with a

mortar and pestle, and analyzed using a Carlo Erba

Elemental Analyzer interfaced to a Finnigan Delta

Plus XL isotope-ratio mass spectrometer through a

Conflo II interface. Analytical precision on internal

standards was 0.3 % for d15Norg and 0.07 % for

d13Corg. All results are reported relative to air for

d15Norg and to VPDB for d13Corg.

Sediment cores collected from one non-glacial lake

(Batoda, core BATODA05-1G) and two glacial lakes

(Upper Kitandara, KITANU05-1G; Lac du Speke,

SPEKE06-1G) were analyzed for 210Pb, 226Ra, and137Cs by direct gamma assay in the Liverpool

University Environmental Radioactivity Laboratory,

using Ortec HPGe GWL series well-type coaxial low

background intrinsic germanium detectors (Appleby

et al. 1986). Ages at depth in each core were

calculated using the constant rate of supply (CRS)210Pb dating model (Appleby and Oldfield 1978), and

compared with the 1963/64 time marker of peak

nuclear bomb testing determined from the 137Cs

record. Best chronologies for each core were deter-

mined using the procedures described in Appleby

(2001). We obtained AMS 14C dates on pretreated

bulk organic matter from near the base of cores from

Upper Kitandara, Lac du Speke, and Mahoma at the

National Ocean Sciences Accelerator Mass Spectom-

eter Facility at Wood’s Hole to estimate the age of

sediments below the unsupported 210Pb inventory.

The 14C dates were calibrated using Calib v. 5.0

(Stuiver and Reimer 1993; Reimer et al. 2004).

Results

In 1905, glaciers occupied between *23 and 26% of

the catchment area of Lac du Speke, Bujuku, and

Upper and Lower Kitandara (Table 1, Fig. 2). The

average glaciated surface area declined to 15.8% in

1955 and 7.7% in 1990. Glaciers occupied three

hectares (*1%) of the catchment area of Lake

Kopello in 1955 (Osmaston 1989), but had disap-

peared from this drainage by 1990 (G. Kaser pers.

comm.). The surface area of Kopello’s glaciers in

1905 is not known. Lake Kopello is the uppermost

lake of a chain of pater noster lakes in the Nyam-

ugasani River Valley, but we do not categorize

downstream lakes as glacial.

Core sedimentology and organic geochemical

data

The siliciclastic content of our sediment cores ranges

from an average of only 16.5% in Mahoma to an

average of 87.0% in Bujuku (Fig. 3). Cores from

glacial lakes have an average siliciclastic content of

Table 1 Studied Rwenzori

lakes (numbered as on map

in Fig. 1) with elevation,

catchment area, and the

surface area of glaciers

within their catchments

calculated from GIS data in

Kaser and Osmaston (2002)

Catchment areas for chains

of lakes (e.g. Upper,

Middle, and Lower

Kachope; lakes in the

Nyamugasani valley)

include the areas of

upstream lakes

Lake Catchment

area (km2)

Glaciated area

in 1990 (km2)

Glaciated area

(1955) (km2)

Glaciated area

(1905) (km2)

1. Bujuku 5.09 0.59 0.96 1.30

2. Upper Kitandara 3.64 0.23 0.51 0.96

3. Lower Kitandara 4.15 0.23 0.51 0.96

4. Speke 1.06 0.08 0.19 0.24

5. Kopello 2.69 0.00 0.03 ?

6. Batoda 2.51 0.00 0.00 0.00

7. Upper Kachope 2.03 0.00 0.00 0.00

8. Middle Kachope 2.57 0.00 0.00 0.00

9. Lower Kachope 2.74 0.00 0.00 0.00

10. East Bukurungu 0.79 0.00 0.00 0.00

11. Mahoma 0.21 0.00 0.00 0.00

12. Nsuranja 1.75 0.00 0.00 0.00

13. Bigata 3.36 0.00 0.00 0.00

14. Africa 5.45 0.00 0.00 0.00

15. Kanganyika 7.50 0.00 0.00 0.00

16. Katunda 7.96 0.00 0.00 0.00

258 J Paleolimnol (2009) 41:253–271

123

70.4% and range from 47.2% in Kopello to 87.0% in

Bujuku, while non-glacial lakes average 39.4% and

range from 16.5% in Mahoma to 57.3% in Middle

Kachope. More importantly, there are significant

(F-test, p \ 0.0001), secular trends in the siliciclastic

content of cores from the glacial lakes. Siliciclastic

content decreases from 90.4% at the core bottom to

81.1% at the core top in Bujuku, 71.6 to 52.6% in

Lower Kitandara, 84.3 to 55.4% in Lac du Speke,

88.0 to 54.7% in Upper Kitandara, and 57.2 to 41.6%

in Kopello. Cores from non-glacial lakes show

irregular variations or weak, insignificant trends in

siliciclastic content (Fig. 3), except for Africa, Mid-

dle Kachope, and Nsuranja in which siliciclastic

content exhibits statistically significant decreasing

trends. Both long-term secular trends as well as short-

term fluctuations in siliciclastic content are highly

reproducible in both glacial and non-glacial lakes

(Fig. 4).

The C/N ratio of organic matter ranged from 7.6 to

14.6 across the set of six lakes analyzed, with an

average value of 10.5 (Fig. 5). Crossplots of % C vs.

% N for each lake (not shown) have intercepts that

cannot be statistically distinguished from zero; hence,

inorganic nitrogen concentrations in these sediments

are minimal. We do not find marked differences in

downcore C/N trends between glacial and non-glacial

lakes. C/N ratios decrease toward the tops of the

cores in glacial lakes Lower Kitandara and Lac du

Speke but also in the non-glacial lake Mahoma; they

exhibit irregular variations and no consistent trends

with depth in Upper Kitandara, Batoda, and Upper

Kachope. d15Norg values range from -0.1 to 3.8% in

Upper and Lower Kitandara, Lac du Speke, Upper

Kachope, and Batoda, with an average value of 1.8

%. Average d15Norg values from Lake Mahoma are

-1.2%, considerably lower than the other lakes.

There is a weak negative upcore trend in d15Norg in

Upper Kitandara, but all other analyzed lakes show

irregular, short-term variability with no clearly dis-

cernible long-term trends.

Trends in the d13Corg of organic matter are

distinctly different in the glacial and non-glacial

lakes. d13Corg values from glacial lakes decrease

toward the core tops, from -23.9 to -27.0% in Upper

Kitandara, -25.1 to -26.9% in Lac du Speke, and

-25.6 to -27.3 % in Lower Kitandara. d13C values

from non-glacial Lakes Batoda and Upper Kachope

exhibit very weak trends with depth, but in Mahoma

d13C values vary initially between -27.8 and

-24.8% until they fall in the uppermost part of the

core to a minimum of -28.4 %. This shift of -2.5%is similar to the magnitude of changes observed in the

glacial lakes Upper Kitandara and Lac du Speke.

Non-Glacial Lakes Non-Glacial Lakes

20406080 Bigata

20406080

100EastBukurungu

20406080 Lower

Kachope

020406080

100

0 5 10 15 20 25 30 35 40 45

UpperKachope

20406080

100Kanganyika

20406080

100Katunda

20406080

100Nsuranja

20406080

100

MiddleKachope

20406080

100Africa

100

Depth (cm)

% S

ilici

clas

tic

Batoda

Mahoma

20406080

100

020406080

100

Depth (cm)%

Sili

cicl

asti

c%

Sili

cicl

asti

c

1870 AD

1090 AD

1996 AD20406080

100

LowerKitandara

20406080

100

Bujuku

20406080

100UpperKitandara

20406080

100

Speke

Glacial Lakes

20406080

100Kopello

Depth (cm)

% S

ilici

clas

tic

1360 AD1870 AD

1260 AD1879 AD

0

00 10 15 20 25 30 35 40 45

Depth (cm)

100

5

0 10 15 20 25 30 35 40 455

0 10 20 30 40 50 60

Fig. 3 Siliciclastic content

versus depth determined by

LOI for gravity cores from

16 Rwenzori lakes, with

trends determined by simple

linear regression. Arrows

mark key transitions, dated

by AMS 14C and 210Pb (see

text). Note the consistent

positive slopes with depth

in all lakes with glaciated

catchments, in contrast to

lakes in non-glaciated

catchments

J Paleolimnol (2009) 41:253–271 259

123

Core chronology

Radiometric data (210Pb and 137Cs) for cores from

Lac du Speke, Upper Lake Kitandara, and Lake

Batoda are shown in Fig 6. At Lac du Speke and

Batoda the 210Pb inventories are comparable to

calculated fallout values, but at Upper Kitandara the

inventory is significantly lower. All three cores

contain a reasonably well-defined subsurface peak

in 137Cs activity that identifies the depth of the

1963–64 fallout maximum from atmospheric testing

of nuclear weapons.

Total 210Pb activity in core SPEKE06-1G from

Lac du Speke reaches equilibrium with supporting226Ra at a depth of 15 cm. Unsupported 210Pb activity

varies irregularly with depth in the top 7 cm of the

core, whereas below 7 cm it declines more or less

exponentially. The 137Cs record has a rather well-

defined peak at 6–8 cm, but the 137Cs/210Pb ratio,

sometimes a better guide to the 1963–64 depth

(Appleby 2001), peaks at 7–9 cm. Our best estimate

for the 1963–64 depth is therefore 7.5 ± 1.5 cm.210Pb dates calculated using the CRS model place

1963 slightly lower at c. 8 cm. The 210Pb chronology

suggests a relatively uniform sedimentation rate of

0.086 cm yr-1 on average from 1930 until the late

1970s, and a sedimentation rate averaging

0.06 cm yr-1 before that time. An AMS 14C date

from 24.5 cm depth in SPEKE07-1G yielded an age

of 1250 AD ±30 years (770 ± 30 14C yr BP;

Table 2). After cross-correlating the LOI profiles of

SPEKE06-1G and SPEKE07-1G (Fig. 4) to transfer

ages, and interpolating from the base of our 210Pb-

dated interval, we estimate a mean sedimentation rate

for the basal sediments of SPEKE07-1G core of

0.02 cm yr-1.

Total 210Pb activity at the base of core BA-

TODA05-1G (27.5 cm long) from Lake Batoda does

not reach equilibrium with the supporting 226Ra,

indicating that the entire core falls within the 210Pb-

dating horizon. Unsupported 210Pb activity varies

irregularly with depth, with significant non-mono-

tonic features at 2–5 and 16–23 cm that coincide with

increased siliciclastic content, suggesting local dilu-

tion of fallout 210Pb. 137Cs activity has a well-

resolved peak at 8–10 cm recording the 1963–64

fallout maximum. However, since the 137Cs/210Pb

ratio, sometimes a better guide to the 1963/4 depth

(Appleby 2001), peaks at 10–11 cm, we place the

1963/4 depth at 9.5 ± 1.5 cm. The 210Pb calculations

suggest a fairly uniform 20th century sedimentation

rate apart from two episodes of rapid siliciclastic

accumulation in the 1920s or 1930s and during the

1990s. The mean sedimentation rate during the period

of uniform sedimentation from 5 to 15.5 cm is

0.18 cm yr-1. Through correlating LOI profiles of

BATODA05-1G with the longer core BATODA06-

1G (Fig. 4) and extrapolating the 210Pb-derived

sedimentation rate estimated for the base of the

Fig. 4 Siliciclastic content determined by LOI for duplicate

cores collected from Lakes Upper Kitandara, Lac du Speke,

and Lake Batoda. Note the consistency between duplicate cores

260 J Paleolimnol (2009) 41:253–271

123

05-1G core to the longer 06-1G core, we estimate a

basal age of 1873 ± 20 yr AD for the oldest

sediments we recovered from Lake Batoda.

Total 210Pb activity in core KITANU05-1G from

Upper Lake Kitandara reaches equilibrium with the

supporting 226Ra at a depth of about 10 cm. Unsup-

ported 210Pb activity declines exponentially with

depth, suggesting slow but relatively uniform sedi-

mentation rates. The 137Cs activity has a maximum

value at 3–4 cm that probably records the 1963–64

fallout maximum. Again the 210Pb dates place

1963–64 slightly lower, at 4.25 cm. Reasons for this

discrepancy could include a small loss of sediment

from the top of the core, lack of resolution of the137Cs record, or downward diffusion of fallout 210Pb.

Since downward diffusion of 210Pb is unlikely, a best

chronology has been calculated assuming a small loss

from the top of the core and placing the 1963–64

depth at 3.9 cm. 210Pb dates calculated using the CRS

model indicate a relatively uniform sedimentation

rate of 0.058 cm yr-1 in the upper core section. A

single 14C age determined on bulk sediment from the

base of KITANU05-1G returned an age of 1360

AD ± 70 years (585 ± 30 14C yr, Table 2). Interpo-

lating from the base of our 210Pb profile, the mean

sedimentation rate for the basal 13 cm of this core is

0.027 cm yr-1, similar to the rate of 0.03 cm yr-1

calculated for the lowermost portion of the 210Pb-

dated interval and to the basal section of core

SPEKE07-1G from Lac du Speke.

Discussion

Sedimentary proxies for glacier recession

in the Rwenzori

Total bedrock erosion rates by wet-based mountain

glaciers such as those in the Rwenzori are propor-

tional primarily to glacier size (Karlen 1976; Leonard

1986). Rates of rock-flour production by glacial

erosion should therefore rise and fall as glaciers

Fig. 5 Variations in atomic

C/N ratio, d13C, and d15N

versus depth for

sedimentary organic matter

in lakes Lower Kitandara,

Upper Kitandara and Lac du

Speke (glaciated

catchments: left panels) and

Lakes Batoda, Mahoma and

Upper Kachope (non-

glaciated catchments: right

panels)

J Paleolimnol (2009) 41:253–271 261

123

expand and recede, causing the siliciclastic content of

sediments in downstream lakes to wax and wane

accordingly. However, the LOI profiles of alpine

lakes can be influenced by a variety of processes not

related to glacier dynamics, including changes in lake

level, surface runoff, river inputs, aquatic

productivity, and sedimentary processes such as

turbid flows and slumping (Dearing 1997; Shuman

2003). Moreover, the controls on glacial outwash

sedimentation can be complex, and can vary signif-

icantly depending on the hydrology, size, and

morphometry of individual lake catchments and

Fig. 6 Date/depth models,137Cs-activity profiles, and210Pb-activity profiles for

Lac du Speke, Batoda and

Upper Kitandara

Table 2 AMS 14C dates on

gravity cores from Lake

Mahoma, Lac du Speke,

and Upper Kitandara

Lake Depth (cm) NOSAMS # 14C Age ±

error

Calibrated

age (AD)

Calibrated age

range AD (2 sigma)

Upper Kitandara 22.5 OS-61947 585 ± 30 1360 1291–1427

Mahoma 54.5 OS-61983 955 ± 30 1090 1023–1155

Speke 24.5 OS-64643 770 ± 30 1249 1217–1281

262 J Paleolimnol (2009) 41:253–271

123

basins. Thus, not all lakes will be sensitive to glacier

dynamics. In wet, humid tropical mountains such as

the Rwenzori, erosion rates and sediment delivery

from surface runoff may exceed the flux of sediments

derived from glacier erosion, particularly for lakes

occupying large catchments. In such lakes changes in

siliciclastic content driven by glacier fluctuations

may be hard to discern from variations driven by

fluctuating surface runoff.

In light of these complexities, we employed a

comparative paleolimnological approach using glacial

and non-glacial lakes (as here defined) in order to

detect sedimentological changes caused specifically

by changes in glacier extent. The siliciclastic content

of surface sediments from the studied glacial lakes

averages 22% higher than that in the non-glacial

lakes, indicating that glacier erosion strongly affects

modern lacustrine sedimentation. More importantly,

in recent centuries as Rwenzori glaciers have receded,

the siliciclastic content of glacial lake sediments has

declined by about 30% in Upper Kitandara, 40% in

Lac du Speke, and by 10–15% in Lower Kitandara,

Bujuku, and Kopello. Regression analyses show that

the decreasing trends in siliciclastic content are highly

significant in each of these lakes (p \ 0.0001). In

contrast, in most non-glacial lakes such trends are not

statistically significant (p [ 0.05). In most of our non-

glacial lakes (Batoda, Bigata, East Bukurungu, Upper

Kachope, Kanganyika, Katunda, Lower Kachope, and

Mahoma), siliciclastic content remains fairly constant

or increases towards the present. Only in Lakes

Africa, Middle Kachope, and Nsuranja do we find

up-core decreases, as observed in glacial lakes.

However, siliciclastic content in Africa rises gradually

from 41% at the base of the core to 49% at 17 cm

depth and then declines gradually to 39% at the core

top. The trend in Middle Kachope is primarily driven

by two samples from the base of the core that may

result from debris flows or turbidites. Similarly, in

Lake Nsuranja the decreasing trend is primarily driven

by a peak in siliciclastic content near the bottom of the

core; exclusion of this peak causes the trend to

become insignificant. Thus, although the decreases in

siliclastic content in these three non-glacial lakes are

significant, its temporal patterns are qualitatively very

different from the secular shifts to low values

observed in the glacial lakes.

The majority of the cores from non-glacial lakes

have not been dated, complicating direct comparisons

of sedimentologic data from the two sets of lakes.

However, cores from the two non-glacial lakes for

which we have age control, Lakes Mahoma and

Batoda, both show very weak, positive trends in

siliciclastic content. The basal age of the Mahoma core

(*1090 AD; Table 2) is compatible with average

Holocene sedimentation rates for this lake determined

by Livingstone (1967). The 210Pb-derived basal age of

*1873 AD for our Batoda core implies a much higher

sedimentation rate there than in any of the other lakes

for which we have age control. Nevertheless, the dated

sequences in both lakes span the interval over which

Rwenzori glaciers have receded and changes in

siliciclastic content in glacial lakes are the largest.

Consequently, the distinct differences in trends in

siliciclastic content between glacial and non-glacial

lakes strongly support the hypothesis that the progres-

sive decrease in siliciclastic content in glacial-lake

sediments mainly reflects glacial recession.

Although glacier activity thus exerts considerable

influence on sedimentation in the Rwenzori lakes, we

observe clear differences in the amplitude and trends

of the response to glacial recession among glacial

lakes that reflect differing sensitivities of each lake to

glacial processes and/or basin-specific complexities in

the sedimentation of glacial outwash. For instance,

large (30–40%) changes in siliciclastic content in

cores from Upper Lake Kitandara and Lac du Speke

during the past two centuries contrast with much

smaller changes (10–15%) in Lower Kitandara,

Bujuku, and Kopello. The amplitude of this change

is not related to changes in either the total or

percentage area occupied by glaciers in each lake’s

catchment (Table 1). The modest change observed in

Lower Kitandara is almost certainly due to the

trapping of glacially eroded material from Mts.

Stanley and Baker by Upper Kitandara, located about

1 km upstream. However, the Lake Bujuku catchment

has the largest glaciers of any lake on the Ugandan

side of the Rwenzori, has no upstream basins that

might trap glacially-eroded material, and would

therefore seem an ideal site at which to investigate

glacier history, yet its decrease in silicilastic content is

the smallest of any of the glacial lakes studied.

Panizzo et al. (2008) also note weak changes during

the last 150 years in Lake Bujuku LOI profiles,

although they ascribe the cause of decreased LOI to

increased primary productivity as opposed to

decreased glacier erosion. Lake Bujuku has one of

J Paleolimnol (2009) 41:253–271 263

123

the largest catchments of any of the lakes analyzed in

this study, and its southern shore abuts large alluvial

fans descending from the rocky slopes of Mt. Baker

that undoubtedly contribute significantly to siliciclas-

tic inputs to the lake. These qualitative observations

suggest that Lake Bujuku’s sedimentation may be

rather insensitive to glacier retreat because of high

siliciclastic inputs from other parts of its catchment. In

contrast, the 15% shift in siliciclastic content observed

in Lake Kopello is probably linked to the historical

recession and eventual disappearance of the East

Thompson glacier on Mt. Luigi di Savoia (Kaser and

Osmaston 2002). As glaciers occupied only 3 hectares

of the Kopello catchment in 1955, sedimentation in

Kopello must be extremely sensitive to the presence/

absence of glaciers. We do not know the proximate

factors controlling this sensitivity, but the marked

contrast between the amplitudes of sedimentological

changes in Bujuku and Kopello relative to the

glaciated area in their respective catchments clearly

demonstrates the variable sensitivity to glacier

dynamics among Rwenzori lakes.

The 30–40% reductions in siliciclastic content

towards the present in Upper Kitandara and Lac du

Speke attest to their sensitivity to changes in glacier

extent. Yet, this sedimentological change occurs as a

steady shift in Upper Kitandara, whereas Lac du

Speke exhibits two smaller, short-lived oscillations

toward higher siliciclastic content at about 1940 and

2004 AD superimposed on the long-term decrease.

As glaciers in the Speke catchment retreated almost

continuously during most of the 20th century

(Table 1; Osmaston 1989; Kaser and Osmaston

2002; Taylor et al. 2006a), we speculate that these

short, relatively modest (up to 7%) fluctuations

resulted from catchment slope processes or storm

deposits. Clearly, there is no simple linear relation-

ship between stratigraphic variation in siliciclastic

content and glacier extent that can be applied across

all lake basins or within lakes across all time scales.

Nevertheless, the principal long-term trends toward

lower siliciclastic content in recent glacial lake

sediments clearly records glacier recession, and can

be used to infer glacier history during past time

intervals when the lakes’ sediment stratigraphies

display comparable fluctuations.

In sum, during the recent glacier recession the

siliciclastic content of sediments in lakes occupying

glaciated Rwenzori basins has fallen, as predicted if

these lakes’ siliciclastic content is mainly controlled

by glacier extent and erosion rates. In contrast, lakes

occupying non-glaciated basins generally show weak

trends in siliciclastic content. Although the configu-

ration of each lake’s catchment can affect its

sensitivity to glacier changes, our data concur with

observations in both temperate and other tropical

alpine lakes (Leonard 1986; Matthews and Karlen

1992; Karlen et al. 1999; Polissar et al. 2006) that

glacier dynamics causes large fluctuations in silici-

clastic content of glacial lake sediments.

The timing and significance of recent glacier

recession in the Rwenzori

Recent research on the history of tropical mountain

glaciers has spurred considerable debate over the

relative importance of air temperature and hydrocli-

matic variables such as precipitation and humidity in

controlling East African glacier dynamics (e.g. Kaser

et al. 2004; Molg et al. 2006; Taylor et al. 2006a, b).

In these discussions, it is widely assumed that glacier

recession in East Africa commenced at 1880 AD (e.g.

Hastenrath 2001; Kaser et al. 2004), a timing signifi-

cant for two reasons. First, high lake levels in central

East Africa immediately before 1880 AD suggest that

a decades-long interval of high rainfall came to an

end at about that time (Nicholson 1998; Nicholson

1999; Hastenrath 2001). Lake Victoria stood approx-

imately 3 m higher than at present until about 1880

AD when it began to fall rapidly, indicating a

reduction in precipitation (Nicholson 1998). Decreas-

ing Mg concentration in authigenic calcite deposited

in Lake Edward, situated immediately south of the

Rwenzori, also indicates the occurrence of a wet

interval from about 1855 to 1885 AD (Russell and

Johnson 2007). The flooding of desiccated crater

lakes in western Uganda during the mid-19th century

further supports the timing of the start of this wet

period (Bessems et al. 2008). Coupled with observa-

tions of glacier recession from the earliest visits to

Africa’s glaciated mountains in the late 1880s, these

data suggest a potentially important role for precip-

itation in controlling East African glacier extent

(Hastenrath 2001). Moreover, tropical alpine glaciers

in both New Guinea and South America began to

retreat around 1850 AD (Hope et al. 1976; Allison

and Kruss 1977; Vuille et al. in press), several

decades before the supposed start of glacier recession

264 J Paleolimnol (2009) 41:253–271

123

in East Africa. As horizontal temperature gradients

high in the tropical troposphere are extremely weak,

in contrast to gradients in hydroclimatic parameters

such as rainfall, humidity, and cloud cover that vary

substantially at regional to continental spatial scales

(Sobel et al. 2001), asynchronous glacier recession

among the tropical continents would suggest that

factors other than air temperature, such as changes in

cloudiness and precipitation, controlled the onset of

tropical glacier recession.

Considering the importance of unraveling the

climatic controls on African glacier dynamics, we

suggest that the evidence supporting an 1880 onset of

the current East African glacier recession is incon-

clusive. The first quantitative documentation of

glacier status in the Rwenzori was made by an

expedition led by the Duke of the Abruzzi in 1906

(Abruzzi 1907). Subsequent field (Temple 1968;

Osmaston 1989) and recent satellite-based observa-

tions (Taylor et al. 2006) document progressive

glacier recession during the 20th and 21st centuries.

Similarly, early maps of Mt. Kenya produced by

Gregory (1894) and later observations document

glacier recession throughout the 20th century

(Hastenrath and Kruss 1992), but no data exist that

document changes in glacier extent during the 19th

century. Glaciers on Mt. Kilimanjaro have the longest

documented history of any in East Africa, with

photographs of glaciers on the eastern side of

Kilimanjaro’s highest peak available from as early

as 1887 (Meyer 1887). Although there is abundant

evidence for progressive glacier recession since

Meyer’s earliest observations (Hastenrath 1984;

Kaser et al. 2004), quantitative observations prior to

1887 are lacking. To quote Hastenrath (1984), there is

clear evidence for ‘‘a drastic and continued glacier

recession since the earliest observations at the end of

the 19th century,’’ yet we lack information on when

this recession actually started.

Further, it is generally assumed that glaciers began

retreating in 1880 from terminal positions established

during the Little Ice Age (LIA), an interval of glacier

advance defined in Europe between about 1400 and

1800 AD (Kaser 1999; Lamb 1977). However, East

African moraine deposits presumed to record

expanded glaciers during the LIA have rarely been

directly dated. In the Rwenzori, Lac Gris stage

moraines located ca. 200 meters below the current

glacier terminus on Mt. Stanley were estimated to

date between 1250 and 1850 AD based upon

lichenometry and the degree of plant colonization

and soil development (de Heinzelin 1953). Bergstrom

(1955) used lichenometry to assign ages of 1750 and

younger to four sets of moraines below the Elena

Glacier. However, rates of lichen growth, plant

colonization and succession, and soil development

are poorly known for tropical mountains (Osmaston

1989). Paleolimnological study of Hausberg tarn,

Mt. Kenya, suggests enlarged glaciers between about

1350 and 1550 AD (Karlen et al. 1999), but chrono-

logic control for sediments spanning the past

millennium in Hausberg Tarn is not adequate to

resolve the precise timing of this event. These data

constitute coherent evidence that African glaciers

were enlarged relative to today during the past

several centuries. However, in light of the consider-

able decade- to century-scale climate variability in

tropical East Africa during this period (e.g. Verschu-

ren 2004; Russell and Johnson 2007), and evidence

that tropical glaciers elsewhere fluctuated consider-

ably at sub-centennial time-scales during the Little

Ice Age (e.g. Polissar et al. 2006), we cannot assume

that African glaciers attained a stable equilibrium

throughout the LIA prior to their recent recession.

Given current incomplete knowledge, we could

envision several possible scenarios for the history of

East African glaciers during the past few centuries.

First, high regional rainfall from ca. 1860 to 1880 AD

(Nicholson 1998) may have delayed the retreat of

East African glaciers from the lower equilibrium

altitudes they had reached for several centuries

during the Little Ice Age, while glaciers in New

Guinea and South America had already begun to

retreat in response to warming at the end of the LIA

(Hastenrath 2001). Second, by analogy to glaciers in

Venezuela (Polissar et al. 2006), East African gla-

ciers may have expanded and receded multiple times

during the past millennium in response to decade- to

century-scale climatic variability. In this scenario, the

condition of East Africa’s glaciers observed in the

late 19th century by early explorers reflected a

decades-long accumulation of glacier mass driven

by abnormally wet conditions from *1860 to 1880

AD, and early-20th century glacier retreat merely

signaled a return to a ‘normal’ equilibrium altitude.

Third, the late-19th century episode of high rainfall

may have had relatively little influence on glacier

dynamics, and East African glacier retreat may have

J Paleolimnol (2009) 41:253–271 265

123

started well before 1880 AD, as in other tropical

regions. If so, early observers witnessed a stage of

that retreat rather than the glaciers’ maximally

extended positions.

LOI profiles from both Upper Kitandara and Lac

du Speke show stable, high siliciclastic concentra-

tions for several centuries prior to the late 19th

century (Fig. 7). In Upper Kitandara values remain

between 80 and 92% from ca. 1360 AD until the late

19th century, with a slight decrease from higher to

lower values at ca. 1650 AD. Siliciclastic concentra-

tion in Lac du Speke is also high from about 1450 AD

until the late 19th century. Trends in sedimentation in

the two lakes differ prior to *1450 AD, with lower

values in Lac du Speke during an interval of

continued high values in Upper Kitandara. This

difference should not be overemphasized, as it may

result from a variety of sedimentological processes

internal to each lake, or from errors in the exploratory

radiocarbon dating of these cores. Nevertheless, the

interval of high and stable % siliciclastic between

*1450 and the late 19th century in both lakes

suggests relatively large and stable glacier masses

throughout this period, supporting the hypothesis that

Rwenzori glaciers stood at or near their mid-19th

century positions for several centuries prior to the

recent major recession.

Our 210Pb- and 14C-based age models suggest that

the principal shift from high and stable siliciclastic

content toward lower values began in both Lac du

Speke and Upper Lake Kitandara at about 1870 AD,

although in Upper Kitandara this shift is preceded by

a modest reduction dated to between 1800 and 1830

AD. Panizzo et al. (2008) also document falling

siliciclastic content concentrations in Lake Bujuku

beginning at about 1870 AD. Given the subtle

differences in the LOI stratigraphies among lakes

and realistic age errors of about 15 years at 1870 AD

for our 210Pb-based age models, we cannot unequiv-

ocally determine whether glacier recession in the

Rwenzori began at 1870 or 1880 AD. However, in all

three dated lake records now available, transitions

toward decreased siliciclastic content were already

underway by 1870 AD, roughly a decade earlier than

previously suggested (Hastenrath 2001) and in the

middle of the period of high rainfall inferred from

regional lake-level data.

Our current paleolimnological records do not

record all of the climatological variables that influ-

ence glacier mass balance, but they can contribute to

our understanding of the drivers of tropical glaciation.

First, the interval from *1400 to 1750, roughly

coincident with the main phase of the LIA in Europe,

was quite arid in western Uganda. Evidence for this

drought includes lithostratigrahic data from crater

lakes situated immediately southeast of the Rwenzori

(Russell et al. 2007; Bessems et al. 2008), and rising

and high %Mg in calcite values from Lake Edward

(Russell and Johnson 2007). Yet our Rwenzori

records show clear evidence for large and stable

glacier masses during the LIA. Enhanced precipitation

therefore cannot be invoked to explain enlarged

glaciers in the Rwenzori during this interval, unless

century-scale changes in rainfall differ dramatically at

high versus low elevations. East African highlands

can experience different rainfall regimes than adjacent

lowlands (e.g. Vincent et al. 1979), but no meteoro-

logical data are available from high elevations to

evaluate whether this is the case in western Uganda.

Thus, resolving whether Rwenzori high-elevation

precipitation history is adequately represented by

Fig. 7 Percent siliciclastic content in surface cores from

Upper Kitandara and Lac du Speke plotted with the % Mg in

calcite from Lake Edward (Russell and Johnson 2007) and the

lake-level record for Lake Victoria (Nicholson 1998). Rising

and high % Mg values in Lake Edward indicate drought, while

falling or low values indicate relatively wet conditions. The

interval of falling and low %Mg in Lake Edward from 1865 to

1885 marks a wet interval that overlaps in timing with the Lake

Victoria highstand within the uncertainty of Lake Edward210Pb age models. The dashed line in the Lake Victoria water

level curve indicates lake level reconstructed from historical

observations; the solid line indicates lake gauge data. The

shaded gray bar marks the hypothesized timing (1860–1880

AD) of wet conditions in East Africa

266 J Paleolimnol (2009) 41:253–271

123

low elevation records will ultimately require devel-

oping precipitation proxies from the Rwenzori lakes

themselves. However, currently available data from

lowland western Uganda, including paleolimnological

records from Lake Edward that receives runoff

directly from the Rwenzori, indicate a regionally

uniform pattern of negative water balance, providing

strong evidence that enlarged glaciers persisted in the

Rwenzori despite negative precipitation anomalies

during the LIA.

Second, if our revised timing for the onset of

Rwenzori glacier recession is correct, air tempera-

tures at high elevations in East Africa must have risen

enough by 1870 AD such that high precipitation

could no longer sustain the early to mid-19th century

glacier equilibrium lines, providing additional evi-

dence for our claim that the effects of enhanced

precipitation prior to 1880 AD on Rwenzori glacier

mass balance were weaker than previously thought.

Hydroclimatic variables such as precipitation, humid-

ity, and cloudiness are certainly important to glacier

mass balance. In fact, increased precipitation during

the extremely wet year of 1961 coincided with a

short-lived and modest (3–5 m) advance in the

terminal positions of valley glaciers in the Rwenzori

Mountains (Temple 1968). However, our data sug-

gest that the principal pattern in Rwenzori glaciers

during recent centuries is determined by changes in

air temperature rather than precipitation (Taylor et al.

2006a; Thompson et al. 2006).

Effects of glacier recession on the

biogeochemistry of Rwenzori lakes

Glacier recession may influence the ecological struc-

ture of downstream lakes by altering lacustrine heat

and water budgets (e.g. Livingstone et al. 1999). All

Rwenzori lakes studied here are located several km

downstream from the glaciers, hence glacier melt is

unlikely to directly influence the thermal structure of

these lakes as inflowing rivers will have thermally

equilibrated with the surrounding air and land surface

by the time they enter the lakes. Glacier recession may

also significantly influence lacustrine biogeochemical

processes through changes in the inputs of dissolved

and suspended solids. Indeed, lakes in the Rwenzori

receiving glacier melt today have a higher pH, higher

transparency, and higher phosphate and nitrate con-

centrations than non-glacial lakes. Non-glacial lakes

also are less transparent and have a much higher

dissolved organic carbon (DOC) content, implying

dystrophic conditions (Eggermont et al. 2007). These

differences are driven primarily by more abundant

vegetation and reducing soils in non-glaciated catch-

ments, so one possible effect of glacier recession on

the glacial lakes is a long-term transition toward more

dystrophic conditions, similar to the lake ontogenetic

sequences observed following recent deglaciation in

high-latitude regions (Engstrom et al. 2000).

Our LOI data (Fig. 3) show that glacier recession

has certainly altered the particulate fluxes of organic

and inorganic materials to glacial Rwenzori lakes.

Yet trends in atomic C/N ratios of organic matter

(Fig. 5), which are typically interpreted to indicate

the relative contributions of aquatic and terrestrial

organic matter (Meyers 1997; Meyers and Teranes

2001), do not show clear differences between glacial

and non-glacial lakes. The abrupt fall of C/N values

towards the present in Lac du Speke perhaps indicates

increased aquatic primary production, which could

benefit from increased nutrient imports as soils

developed in freshly deglaciated areas of the catch-

ment. A similar, albeit weaker trend is observed in

Lower Kitandara, but C/N ratios also fall in Lake

Mahoma (2900 m asl), where no glaciers have

occurred since the late Pleistocene. Moreover, C/N

ratios are constant in glacial Upper Kitandara, and in

non-glacial Batoda and Upper Kachope. Thus, recent

glacier recession does not seem to have caused

uniform changes in the sources of organic matter

sedimenting in glacial lakes.

That said, it is worth questioning the reliability of

atomic C/N ratios in detecting possible transitions in

the source of organic matter to the Rwenzori lakes’

sediments. Sedimentary C/N ratios in the six lakes

analyzed average 10.5 (range 8–14), which equals the

average POC/PON ratio of suspended organic matter

in Rwenzori lakes (Eggermont et al. 2007). These

values are much closer to the average composition of

aquatic algae (*8) than that of vascular plants ([30;

Meyers and Teranes 2001). However, in light of

the lakes’ oligotrophic to dystrophic status, large

catchment areas, and high surface runoff rates, a

predominantly aquatic source for suspended and

sedimentary organic matter seems unlikely. The

C/N ratios of Afroalpine vegetation and the soil

organic matter that provide DOC to these lakes is

unknown, but observation of values less than 20 in

J Paleolimnol (2009) 41:253–271 267

123

many soils (Meyers and Teranes 2001) may limit the

utility of C/N in detecting terrestrial inputs. In sum,

although the C/N data imply that there have not been

major changes in the source of organic matter in the

lakes studied, additional environmental data are

needed to meaningfully interpret C/N ratios in this

environment, let alone the trends in C/N ratios in

relation to terrestrial and aquatic ecosystem succession.

d15Norg profiles from our lakes do not exhibit clear

differences between glacial and non-glacial lakes or

clear historical trends in the glacial lakes, indicating

that recent glacial recession does not appear to have

strongly affected the nitrogen cycle of Rwenzori

Lakes. In fact, the only clear feature in our d15Norg

data is the depleted values in Lake Mahoma relative

to the other lakes. This almost certainly indicates that

a dominant source of N in Mahoma is in situ

N-fixation, whereas the more enriched d15Norg values

in the other lakes likely reflect large inputs of N from

terrestrial sources (Talbot 2001).

The most consistent difference between glacial

and non-glacial lakes in the organic geochemical data

is a 3% decline in d13Corg in the glacial lakes,

observed most clearly in Lac du Speke and Upper

Kitandara beginning at about 1870 AD. d13Corg in

Lower Kitandara also declines by *1.4% over the

length of our core, similar in magnitude and sign to

shifts in d13Corg observed by Panizzo et al. (2008) in

Lake Bujuku. The modest shifts in Lower Kitandara

and Bujuku may not reflect changes in the carbon

dynamics of these lakes because the d13C of atmo-

spheric CO2 has changed by about 1.5% since 1900

AD (the Suess Effect; Keeling 1979; Verburg 2007).

However, the declines in d13Corg in Lac du Speke and

Upper Kitandara greatly exceed the Suess effect and

are substantially larger than changes in d13Corg

obeserved in non-glacial lakes other than Mahoma,

where values fall by about 3% near the top of the

core. The similar timing and large magnitude of the

negative shift in d13Corg in the two lakes most

sensitive to glacier input in our study, Lac du Speke

and Upper Kitandara, strongly suggests that glacier

recession is causing changes in carbon cycling in the

Rwenzori’s glacial lakes.

Fractionation during algal photosynthesis favors12C, thereby enriching the residual dissolved carbon

pool in 13C (Schelske and Hodell 1991). Thus, a

possible interpretation of the trends in d13Corg in Lac

du Speke and Upper Kitandara is that glacier

recession has decreased primary productivity, per-

haps due to decreased glacier erosion rates reducing

phosphorus supply to the lakes. Although productiv-

ity in these lakes does appear to be P-limited

(Eggermont et al. 2007), work by Panizzo et al.

(2008) indicates that primary productivity in Lake

Bujuku has increased in response to glacier recession

during the last century. Although the stable and low

C/N values in these two lakes suggest that the shift in

d13Corg in the two lakes did not result from a change

in the relative abundance of terrestrial versus aquatic

organic matter, if Rwenzori soils are characterized by

low C/N ratios it is possible that the changes in

d13Corg reflect increasing inputs of soil and terrestrial

plant input from glacial lake catchments. This

hypothesis requires further testing. It seems unlikely

that early diagenesis of organic matter, which

releases isotopically light C, would strongly affect

only a subset of the lakes. We therefore hypothesize

that changes in d13Corg in Lac du Speke and Upper

Kitandara must have been driven by factors affecting

carbon cycling in these lakes other than primary

productivity- presumably variations in respiration and

lake stratification. Although we cannot at present

distinguish between these processes, our data do

suggest substantial effects of glacier recession on the

biogeochemical cycle of carbon in the Rwenzori

lakes.

Conclusions

Through comparative paleolimnological studies of

virtually all of the lakes on the Ugandan side of the

Rwenzori Mountains, East Africa, we provide consid-

erable insight into the timing and limnological impacts

of recent glacier recession. Specifically, we show that:

1) The siliciclastic content of sediments in lakes

occupying glaciated basins in the Rwenzoris is

controlled by changes in glacier extent and erosion.

2) The magnitude of changes in siliciclastic

content can vary considerably between lake basins

despite similar magnitudes and rates of glacier

recession. In other words, lakes occupying glaciated

basins can differ dramatically in their sensitivity to

glacier fluctuations.

3) The Rwenzoris supported large glaciers for

several centuries prior to 1850 AD, including the

interval prior to 1750 AD marked by a prolonged

268 J Paleolimnol (2009) 41:253–271

123

drought in western Uganda. These data suggest that

precipitation variability does not appear to have been

the primary control on Rwenzori glacier advance

during the Little Ice Age.

4) Reductions in siliciclastic content show that

glacier recession in the Rwenzori began at about 1870

AD, slightly earlier than previously thought and

preceding the end of the late 19th century wet phase

recorded by climate-proxy data from lowland western

Uganda.

5) Recent glacier recession does not appear to have

strongly affected either the dominant source of

organic matter to sediments in glacial lakes in the

Rwenzori, nor their nitrogen cycle. Glacier recession

does appear to have caused changes in the carbon

cylce of these lakes, although the precise effects and

mechanisms cannot be unequivocally identified.

Acknowledgements The fieldwork was conducted under

Uganda NCST research clearance NS21 and Uganda Wildlife

Authority permit UWA/TBDP/RES/50, with logistic support

from the Rwenzori Mountaineering Services. We greatly thank

Ilse Bessems, Halewijn Missiaen and Kay Van Damme for

field assistance, and Angelica Alcantara, Sophie McCoy and

Marc Mayes for laboratory assistance. Georg Kaser is thanked

for providing GIS maps and data, and Benjamin Hudson is

thanked for GIS analyses. This research was sponsored by the

Salomon Fund of Brown University (US), US National

Geographic Society (grant 7999-06), the Fund for Scientific

Research of Flanders, the Leopold III-fund Belgium, and the

Stichting Ter Bevordering van het Wetenschappelijk

Onderzoek in Afrika (Belgium). H.E. is a postdoctoral fellow

with the Fund for Scientific Research of Flanders (FWO-

Vlaanderen). We thank J. C. Stager and an anonymous

reviewer for their insightful comments on an earlier version

of this manuscript

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