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 DOI: 10.1177/0959683607076452

2007 17: 517The HoloceneHenry F. Lamb, Melanie J. Leng, Richard J. Telford, Tenalem Ayenew and Mohammed Umer

record of the last 2000 yearsOxygen and carbon isotope composition of authigenic carbonate from an Ethiopian lake: a climate

  

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The Holocene 17,4 (2007) pp. 517–526

© 2007 SAGE Publications 10.1177/0959683607076452

Introduction

Knowledge of the range of natural rainfall variability is vital tothe management of water resources, especially in semi-aridAfrica, but instrumental data seldom provide a sufficiently longrecord to encompass the probable extremes. Natural archives ofpast climate can extend and supplement the instrumental record,but archives of sufficient length and resolution are scarce. The1100-yr record from Lake Naivasha, Kenya, is an importantexception (Verschuren et al., 2000). The record of salinity and

lake-level variation at this equatorial lake shows drier climatethan today from AD 1000 to 1270 (the European ‘MediaevalWarm Period’), followed by wetter conditions during the ‘LittleIce Age’ (AD 1270–1850). Three severe droughts up to 80 years induration interrupted the ‘Little Ice Age’ humid interval. The aridphases coincide with periods of high solar radiation, suggestinga causal link. If solar variability does contribute to long-termvariation in water balance, a similar temporal pattern of lakeresponse to climate should be evident from other sites in EastAfrica. Here we present a 2000-yr record of moisture variabilityinferred from the oxygen and carbon isotope composition ofsedimentary carbonates in a core from Lake Hayq in northernEthiopia, a region that is especially prone to drought. Pollen data

Oxygen and carbon isotope compositionof authigenic carbonate from anEthiopian lake: a climate recordof the last 2000 yearsHenry F. Lamb,1* Melanie J. Leng,2 Richard J. Telford,3Tenalem Ayenew4 and Mohammed Umer4

(1Institute of Geography and Earth Sciences, University of Wales, Aberystwyth SY23 3DB, UK;2NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham NG125GG, UK and School of Geography, University of Nottingham, Nottingham NG7 2RD, UK;3Bjerknes Centre for Climate Research, Allégaten 55, 5007 Bergen, Norway; 4Department of EarthSciences, P.O. Box 3434, University of Addis Ababa, Addis Ababa, Ethiopia)

Received 27 February 2006; revised manuscript accepted 3 November 2006

Abstract: The oxygen and carbon isotope composition of sedimentary carbonates formed in Lake Hayq,a closed-basin lake in northern Ethiopia, suggests marked variation in the balance between precipitationand evaporation during the last 2000 years. Diatom stratigraphy shows that the lake remained deep enoughto support planktonic taxa, and was never saline. The record suggests that rainfall was higher than that ofthe twentieth century during most of this time, especially during two short intervals centred on AD 700 andAD 1300. Climate was drier only around AD 800, and from AD 1750–1900. Similar, but slightly moister cli-mate than today, with high interdecadal variability, prevailed from AD 800 to AD 1200, equivalent to theEuropean ‘Mediaeval Warm Period’. A period of high effective precipitation followed, from AD 1200 to AD

1700, during the ‘Little Ice Age’. Pollen evidence suggests that increased forest cover from AD 1400 to AD

1750 was a response to higher rainfall; forest cover may have increased evaporative loss from the catch-ment, contributing to a subsequent trend of increasing lakewater �18O values.The record shows similari-ties in the timing and direction of changes in proxy-climatic records from Lake Naivasha (Kenya) and fromthe Nile.

Key words: Palaeolimnology, palaeoclimatology, stable isotopes, oxygen, carbon, diatom stratigraphy,East Africa, Ethiopia, late Holocene.

*Author for correspondence (e-mail: henry.lamb@aber.ac.uk)

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from the same core (Darbyshire et al., 2003) show markedchanges in forest cover, so the palaeoclimatic data help deter-mine the extent to which those vegetation changes are driven byclimate or by human impact. Conversely, land cover changes, ifat a sufficiently large spatial scale, may provide a positive feed-back on climate (Claussen and Gayler, 1997).

Study area and site description

Lake Hayq (11�21� N, 39�43� E; 1950 m a.s.l.) lies in a grabenformed in Tertiary basalts on the eastern margin of the north-central highlands in South Wollo, Ethiopia (Figure 1). The lakehas a surface area of 23 km2 and a catchment area of 65 km2. Itis a deep, steeply shelving lake, with mean and maximum depthsof 37.4 m and 88.8 m, respectively, (Baxter and Golobitsh, 1970).At the time of these measurements, in 1938, the ninth-centuryIstifanos monastery was located on an island near the westernmargin of the lake. By 1969, the island had become a peninsula,because of a fall in lake level. The maximum lake depth recordedin 2000 was 81 m, suggesting a 7.8 m fall over 60 years. A rise of~40 m would be required to cause the lake to overflow. TheAnkwarka river is the principal inflow to Lake Hayq, which hasno surface outlet. An apparent palaeochannel, now permanentlydry, indicates that the adjacent Lake Hardibo overflowed intoLake Hayq at some time in the past. A newly constructed irriga-tion canal allows occasional water flow between the lakes.Despite being topographically closed, Lake Hayq is fresh with aconductivity of 920 �S/cm, a pH of 9, and a surface water tem-perature of 22�C recorded in January 1969 (Baxter andGolobitsh, 1970), so groundwater outflow must play a significant

role in the lake’s hydrology. A water balance estimate for the lake(Table 1) suggests groundwater outflow of 1.4 �106m3/yr, or 3%of the total, and evaporative losses of 33 �106m3/yr (65%).However, the net unmeasured groundwater flux (groundwaterinflows minus outflows) suggests a further 15 �106m3/yr (29%)groundwater outflow. These estimates are derived from fieldmeasurement of outflow springs and the discharge of theAnkwarka river, and from rainfall and evaporation data for thenearby town of Hayq for the years 1965–1999 (NationalMeteorological Services Agency (NMSA), 2000). The surfacerunoff coefficient was estimated from field observations of landuse, topography and geology using the method of Tripathi andSingh (1993), which weights land use, soil type and slope classesby area. Six land use, five slope and two soil type units were used,and the values were weighted using areas derived fromGeographic Information System (GIS) overlay operations. Theopen-water evaporation estimate is based on seven yearsColorado Class-A pan evaporation record (1985–1991) at Hayqtown (NMSA, 2000), modified by a factor of 0.8 to account forthe lower radiation energy receipt for a lake (Demellie, 2000).

Water collected from Lake Hayq between 1975 and 2001 had�18O values from �7.1 to �9.1‰ (Table 2), suggesting consid-erable evaporative enrichment (by comparison with the meanannual precipitation value of �1.2‰; see below). In January1969, Baxter and Golobitsh (1970) noted the low phytoplank-ton biomass and high water clarity of Lake Hayq. A deepoxygenated zone was recorded, with a sharp decline in oxygenconcentration between 30 m and 40 m. By March 1989,phytoplankton biomass had increase greatly, associated with areduction in water clarity and a significant decline in oxygenconcentration below 10 m depth. These changes were attributedto the introduction in 1978 of the zooplanktivorous fish Tilapia

518 The Holocene 17 (2007)

Table 1 Water balance estimates for Lake Hayq (m3�106 per yr).Inflows are estimated from field measurements of Ankwarka andspring flows in February and March 2000, average rainfall for1966–1999 (NMSA, 2000), and a runoff coefficient of 0.3 (esti-mated by the method of Tripathi and Singh (1993), using areaweighting of six land-use, five slope and two soil-type units of thelake catchment). Evaporation is from Penman estimates combinedwith a seven-year Colorado Class-A pan evaporation record(1985–1991) measured at nearby Hayq town (NMSA, 2000)

Precipitation on lake 26 (52%)Catchment runoff 19 (37%)Groundwater inflow 5.9 (11%)Groundwater outflow 1.4 (3%)Evaporation 33 (65%)Abstraction 1.6 (3%)Net unmeasured ground water flux 15 (29%)

Figure 1 Map of the study site, showing the location of coreHYK99-1 in Lake Hayq

Table 2 Stable isotope composition of water from Lakes Hayqand Hardibo

�18O (� 0.1‰) �D (� 1‰)

Lake Hayq1975a �8.9 �53January 1997 �9.1 –November 2001 �7.1 �46May 2002 �8.7, �8.4 �55

Lake HardiboJanuary 1997 �8.0 –November 2001 �7.0 –May 2002 �8.2, �7.9 �55, �48

aFrom Schoel and Faber (1976).

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(Oreochromis niloticus). In recent years periodic deep mixingevents, mainly during the dry season, have brought anoxic watersto the lake surface, causing large fish kills (Elizabeth et al., 1992).

The climate of the Hayq area is subhumid with a meanannual temperature of 18�C and a mean annual rainfall of1158 mm (Demellie, 2000). Temperature shows a large diurnalrange (8–23�C) but small seasonal variation. Rainfall in north-ern Ethiopia is highly seasonal, with spring and summer mon-soon rains bringing moisture from the Indian and Atlanticoceans (Gamachu, 1977). Most (~80%) of the annual totalrainfall occurs between April and September. Warm ENSOevents in the Indian and Pacific oceans tend to coincide withdroughts in Ethiopia (Seleshi and Demaree, 1995). The oxygenisotope composition of rainfall at Addis Ababa (2360 m alti-tude) since 1961 shows that spring and summer rains have mean�18O values of �0.6‰ (�0.1‰ weighted mean) and �1.7‰(�0.7‰ weighted mean), respectively, reflecting their differentsynoptic origins. Weighted mean annual values range from�0.2‰ in 1984 to �2.7‰ in 1975, with an overall weightedmean of �1.2‰ of rainfall at Addis Ababa (InternationalAtomic Energy Agency (IAEA),1990, 1992).

Methods

A 161 cm long sediment core (HYK99-1) was retrieved from41 m water depth in the southwestern part of Lake Hayq inJanuary 1999, using a micro-Kullenberg corer. Whole-core vol-ume magnetic susceptibility was measured with a BartingtonInstruments MS2 meter and type MS2C sensor. Sr, Ca and Fecontents of the uppermost sediments were logged using anItrax XRF core scanner. Sediment composition was estimatedusing the smear-slide method (Aaby and Berglund, 1986).Organic and carbonate contents were estimated from weightloss on ignition at 550�C and 950�C, respectively (Bengtssonand Enell, 1986). Samples from the top of the core (0–9 cm)were analysed for 210Pb content at the St Croix WatershedResearch Station, Minnesota, using the beta-counting method.AMS radiocarbon ages were obtained on bulk sediment, fol-lowing smear-slide examination to confirm that algal organicsform the dominant components. The samples were subjected toacid wash pretreatment prior to analysis by Beta Analytic Inc.The radiocarbon ages were calibrated with CALIB v5.0 (Stuiverand Reimer, 1993), using the calibration dataset of Reimer et al.(2004). Weighted averages were used to obtain central point esti-mates of the calibrated ages (see Telford et al., 2004).

Diatom preparation followed standard techniques (Battarbee,1986) and were mounted in naphrax. In general, 300 valves werecounted per level, but low diatom abundance in some levels neces-sitated a reduced count. Diatom taxonomy follows Gasse (1986)and Krammer and Lange-Bertalot (1986, 1988, 1991). Diatom-inferred conductivity was calculated from the salinity data set pro-vided by the European Diatom Database (EDDI), using aweighted-average transfer function with inverse deshrinking.

For stable isotope analysis, sediment samples were disaggre-gated in 5% sodium hypochlorite solution (10% chlorox) for24 h to oxidize reactive organic material. Samples were thenwashed three times in distilled water and sieved at 85 �m. The85 �m fraction was filtered through quartz microfibre filterpaper, dried at 40�C, and ground in agate. The isolated materialwas reacted with anhydrous phosphoric acid in vacuoovernight at a constant 25�C (McCrea, 1950). The CO2 thusliberated was separated from water vapour and collected foranalysis. Water samples for 18O/16O and 2H/1H analysis werecollected in leak-tight polyethylene bottles, further sealed withPVC tape. They were analysed using the equilibration method for

oxygen and Zn reduction method for hydrogen. Measurementswere made either on a VG Optima or Micromass Sira massspectrometer. Isotope ratios (18O/16O, 13C/12C, 2H/1H) areexpressed in delta units, �18O, �13C, and �D (‰, parts permille), and defined in relation to the international standards(VPDB for carbonates, VSMOW for water). Analytical preci-sion is typically �0.1‰ for carbonates (O and C), �0.05‰ forwater �18O, and �2‰ for water dD.

Results and interpretationLithology and chronologyThe sediments of the Hayq core HYK99-1 are variably lami-nated in grey shades at a millimetre scale, with some of the lam-inations made distinct by white carbonate layers. From 161 to94 cm depth, the laminations are irregular and contorted, pos-sibly as a result of the coring process. From 94 to 62 cm, andfrom 43 cm to the surface, the laminations are horizontal andmore distinct, with white layers at intervals of 1 cm to less than1 mm. Between 62 cm and 43 cm, the laminations are indistinctand less frequent, with no white carbonate layers. An XRF corescan of the top 23 cm of the core, taken at 200 �m intervals toshow the nature of the laminations, shows 24 calcium peaksspaced from 3 to 24 mm apart, with an average spacing of8.8 mm (Figure 2). Similar laminations occur in nearshore sed-iments, in a core taken at 12 m water depth. The sediments havean organic content of 15–20%, and a carbonate content of10–30%, by weight loss on ignition. Magnetic susceptibility israther uniform throughout the core, with the highest values,from 5 to 15 cm depth, probably because of increased clasticinflux to the lake, resulting from soil erosion associated with arecent increase in catchment cultivation.

Unsupported 210Pb activity in the core is relatively low, andpresent only within the top 10 cm of sediment (Table 3). Surfaceactivity of 6.4 pCi/g drops to near background (supported) lev-els below about 5 cm. The low unsupported activities and shal-low depth to background yield a 210Pb inventory of 4.68 pC/cm2,which is equivalent to a core-specific flux of 0.15 pCi/cm2 per yr.This small inventory suggests a low atmospheric flux of 210Pb,about one-third that of north temperate regions. Background(supported) 210Pb activity, as defined by the lowermost six sam-ples, is fairly constant (0.52 �0.03 pCi/g), which helps greatly indefining the low levels of unsupported 210Pb below 5 cm. Theanalyses give an age estimate of AD 1867 �44 at 9 cm, and anaverage sediment accumulation rate of 0.69 mm/yr.

Four AMS 14C analyses on bulk sediment from the LakeHayq core yielded ages from 2350 cal. BP to 1170 cal. BP (Table4). Linear regression of all four ages against depth results in anage model (y 0.696x – 0.6576) with a surface age of near zeroand a sediment accumulation rate that is very close to that esti-mated from the 210Pb ages. However, a more realistic age modelis based on linear interpolation between all dated levels, includ-ing both 210Pb and 14C ages (Figure 3). We chose to exclude thatat 50–55 cm, which is too old relative to other dated levels, andindistinguishable from that at 90–95 cm, suggesting either anerroneous date or almost instantaneous sedimentation over adepth interval of 40 cm. Although there is a lithological changefrom 62 cm to 43 cm, the presence of indistinct laminations inthis interval argues against rapid sedimentation or deposition ofreworked older material. This model gives a mean sedimentaccumulation rate of 0.76 mm/yr, so the 1-cm sample intervalfor isotope analyses corresponds to a time resolution of about13 years. Given this sediment accumulation rate, it appears thatthe laminations were deposited at intervals of 4–30 years (aver-age interval 12 years) over the last 320 years, unlike the annual

Henry F. Lamb et al.: Stable istopes in lacustrine sedimentary carbonate, Ethiopia 519

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520 The Holocene 17 (2007)

Figure 2 XRF scan data from the top 23 cm of the HYK99-1. (A) Ca data to show peaks corresponding to carbonate laminae. (B) Detailof scan 14–17 cm depth in core, showing Ca, Sr and Fe data

Table 3 Results of 210Pb analyses of Lake Hayq core HyK99-1

Sample Cum. dry Unsup. Error of Cum. activity Age (yr) Age error Date AD Sediment Error ofdepth mass activity unsup. act. below sample. (�s.d.) accum. sed. accum.(cm) (g/cm2) (pCi/g) (�s.d.) (pCi/cm2) (g/cm2 per yr) (�s.d.)

1 0.3743 6.4083 0.2131 2.2807 23.08 1.94 1976.3 0.0162 0.000722 0.7300 2.9225 0.1179 1.2412 42.62 3.05 1956.8 0.0182 0.001343 1.0309 1.1485 0.0666 0.8956 53.10 4.05 1946.3 0.0287 0.003324 1.2998 0.8951 0.0605 0.6549 63.15 5.40 1936.3 0.0268 0.004115 1.5692 0.5151 0.0397 0.5161 70.80 6.78 1928.6 0.0352 0.006976 1.8310 0.6899 0.0562 0.3355 84.63 10.27 1914.8 0.0189 0.005057 2.1298 0.3545 0.0478 0.2296 96.81 14.84 1902.6 0.0245 0.009808 2.4153 0.3574 0.0472 0.1276 115.67 26.45 1883.7 0.0151 0.009529 2.6896 0.1881 0.0439 0.0760 132.31 44.09 1867.1 0.0165 0.01793

Supported 210Pb: 0.5238 �0.0337 pCi/g Cum. Unsupported. 210Pb: 4.6793 pCi/cm2. Unsupported. 210Pb Flux: 0.153 pCi/cm2 per yr.

Table 4 Radiocarbon chronology of the Lake Hayq core HYK99-1

Sample depth (cm) Laboratory number �13C (‰) Conventional Calibrated age, cal BP 2� Calibrated age, cal BP,radiocarbon age, range (relative area under central point estimate14C yr BP probability distribution) (to nearest 10 yr)

50–55 Beta - 163939 �19.3 1240 �40 1068–1270 (1) 1170900–95 Beta - 163940 �21.4 1220 �40 1058–1265 (1) 1160125–130 Beta - 163941 �22.5 1590 �40 1389–1557 (1) 1470156–160 Beta - 142244 �22.7 2320 �50 2155–2268 (0.263) 2350

2296–2487 (0.734)2645–2651 (0.003)

AMS radiocarbon ages were determined on bulk sediment samples following acid wash pre-treatment. Calibrated ages were determinedusing CALIB v5.0.1 (Stuiver and Reimer, 1993), and central point estimates are summed mid-points of the weighted averages of the 2-sigmaranges (Telford et al., 2004).

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laminations described from a crater lake in central Ethiopia(Lamb et al., 2002). Lamina formation in Hayq may be linkedto irregular occurrences of lake overturn: if productivity in theepilimnion is nutrient limited (as the clear waters reported byearly visitors would suggest) then deep mixing at overturnwould supply nutrients to the epilimnion, causing an algalbloom and increased photosynthetic deposition of carbonate.

DiatomsThe diatom stratigraphy (Figure 4) is dominated by the plank-tonic species Aulacoseira ambigua (Grun.) Simonsen, exceptfrom the uppermost sample, from which it is absent. Synedraulna and S. acus are important from 110 to 70 cm. Nitzschia fil-iformis dominates two levels, 95 and 25 cm. Above 90 cm thereis an increased abundance of epiphytic taxa, and the salineindicators N. frustulum and N. inconspicua, but these are neverabundant, except in the uppermost sample where they entirelyreplace A. ambigua.

The diatoms show that the lake has always been deep enoughto support planktonic taxa, and has never been saline. Diatom-inferred conductivity is generally about 100 �S/cm below120 cm, increasing to 200 �S/cm above this level. The largepeaks in diatom-inferred conductivity are not reliable as thesesamples do not have good analogues in the EDDI African sur-face sample data set: N. filiformis is only found in a few sitesand at low frequencies. The replacement of A. ambigua in thesurface sediment may reflect the known ecological changes fol-lowing the introduction of the fish Tilapia in 1978, or increasednutrient input linked to land-use changes.

Stable isotopesOxygen and carbon isotope values for contiguous 1 cm samplesfrom the core (Figure 5). �18O show marked variability, rangingfrom a minimum of �7.4‰ at 51 cm (AD 1320) to a maximumof �7.6‰ at 17 cm depth (AD 1760). �13C show a similar rangefrom �7.4‰ at 54 cm (AD 1300) to �8.0‰ at 68 cm (AD 1100).A five-point moving average of the �18O and �13C data revealstheir underlying structure. The basal samples (160–115 cm; 400BC–AD 590) are relatively uniform, with individual �18O valuesfrom �3.8 to �4.8‰. The average �18O values fall to a mini-mum of �0.9‰ at 101 cm (AD 710) before rising rapidly to�6.2‰ at 91 cm (AD 800). Above 60 cm, �18O values declinesteeply to a minimum of �4.0‰ at 52 cm (AD 1310), risingmore gradually thereafter to a peak of �6.9‰ at 18 cm (AD

1750). Although no mineralogical data are available, the rangeof oxygen isotope values is consistent with other calcite pro-ducing lakes in Ethiopia. Effects of changing mineralogywould be expected only in highly evaporated lakes (Leng et al.,2005). Carbon isotopes are strongly correlated with the oxygenisotope data (r20.89) and for that reason are not described indetail here.

The oxygen isotope composition of authigenic carbonates iscontrolled by the temperature and isotope composition of thelake water in which they were formed (Leng and Marshall,2004). A lake’s isotope composition depends on the balancebetween surface and groundwater inflows, outflows and evapo-rative loss, and their respective isotope compositions. Rainfallat Addis Ababa has a weighted mean �18O composition of�1.2‰, while groundwater values recorded in Ethiopia rangefrom �6.2‰ to �1.3‰ (table 2 in Lamb et al., 2000). It seemslikely that the oxygen isotope values for the lake surface water(up to �9‰ VSMOW) result from evaporative enrichment(confirmed by the hydrological balance, Table 1), which is alsorepresented in the high values for the carbonates in the surfacesediments (�18O �6.2‰ VPDB). Higher values in the sedi-mentary record are therefore interpreted as indicating surfacewater evaporation during years with lower summer rainfall andlonger dry seasons, while lower values indicate dilution of thesurface waters by higher summer rainfall or less evaporation.However, the hydrological relationship between Hayq andHardibo might complicate this simple model. If Hardibo over-flowed into Hayq via the now-dry palaeochannel during wetintervals, its water may have influenced the composition ofHayq water, dampening and slowing the dilution of Hayq byenhanced rainfall.

Using Leng and Marshall’s (2004) expression of Kim andO’Neil’s (1997) equation, the surface sediment �18O value of�6.2‰ (core top taken in 1999) and �18O water of �8.1‰(average between 1997 and 2001, Table 2) suggests precipitationtemperatures of around 23�C, higher than the measured meanannual air temperature of 18�C. Assuming the lake surfacewaters have a similar mean temperature as the air (18�C), thenthe modern surface carbonates would have precipitated inwater with an oxygen isotope composition of about �7.1‰,which suggests that our assumptions are not entirely valid (thewater at time of precipitation of the carbonate had a highoxygen isotope ratio or was at a higher temperature) or thecarbonate precipitated out of equilibrium.

Henry F. Lamb et al.: Stable istopes in lacustrine sedimentary carbonate, Ethiopia 521

Figure 3 Age models for core HYK991, showing linear extrapolation between calibrated radiocarbon ages, and (inset) 210Pb dated levels

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The carbon isotope composition of authigenic carbonates inHayq are similar to other lakes in arid zones, in that they have alarge range in �13C and also show a strong covariance with �18O.Examples include Lake Tilo (Lamb et al., 2000), Bosumtwi(Talbot and Kelts, 1986), Malawi (Ricketts and Johnson, 1996),Turkana (Ricketts and Anderson, 1998) and also high latitudelakes in the Arctic (Anderson and Leng, 2004). In hydrologi-cally closed lakes, especially in a non-karstic catchments, thecovariance is likely to reflect equilibration of the dissolved inor-ganic carbonate in the lake water with atmospheric CO2(Talbot, 1990; Li and Ku, 1997). This occurs alongside prefer-ential evaporative loss of the 16O, thus providing a degree of

�18O versus �13C covariation. Under isotope equilibrium withthe atmospheric CO2, lake water �13C would have a value up to�3‰ (Leng and Marshall 2004). In Hayq values up to �8‰may result in part from preferential assimilation of 12C duringaquatic photosynthesis. In some organic-rich lakes these typesof values are ascribed to methanogenesis, as there is a largekinetic fractionation effect when methane is generated (�13CH4~ �60‰) causing an enrichment in the lake water �13C(Bottinga, 1969). In Lake Tilo, methane-producing bacteriawere thought to facilitate �13C values of up to �12‰ (Lambet al., 2000). Similar mechanisms may have acted in LakeBosumtwi as �13C values reach �10‰ (Talbot and Kelts, 1986).

Figure 5 shows five-sample (70- year) running means of the1 cm (14 year) resolution record for both oxygen and carbon iso-topes for the top 110 cm of core, with 5 cm resolution from 110to 160 cm. Strong variability amongst individual values mayrepresent interannual variability in the balance between precip-itation and evaporation (P–E), and seasonal differences in thetiming of carbonate deposition. A running mean was used toremove some of the high-frequency variability that is oftenencountered in data from climate-sensitive lakes. The oxygenand carbon isotope record suggests stable, slightly wetter condi-tions than present until about AD 650, but this interpretationrequires caution because of low sample resolution (5 cm)and possible sediment disturbance in this part of the core. P–E

522 The Holocene 17 (2007)

Figure 5 Oxygen and carbon isotope composition of coreHYK99-1: Individual values and 5-point running means. Theshaded area marks the range of twentieth-century values

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Figure 4 Diatom stratigraphy and inferred conductivity fromcore HYK99-1

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(rainfall) was apparently high from about AD 650 to 800, with amaximum at AD 700, followed rapidly by peak aridity just afterAD 800. Similar, but slightly moister climate than today, on aver-age, prevailed from AD 800–1200, although marked interdecadalvariability is apparent from the record, with many high valuessuggesting decades with drier climate than present. P–E againincreased sharply to a maximum value at AD 1300, followed bya variable but overall increase over the subsequent 450 years.The interval of lowest oxygen and carbon isotope values(62–43 cm; AD 1180–1430) coincides with the unlaminated sec-tion of the core, which also contains abundant microfossilremains of the green alga Pediastrum. Diatom concentrationsalso reach a peak in this part of the core. The absence of lami-nations suggests that the bottom waters were oxygenated as aresult of more frequent overturn, which may have enhancedepilimnetic productivity. Increased rainfall could have reducedlake salinity, marginally reducing the density difference acrossthe thermocline, favouring more frequent overturn. Similarly,rainfall events would lower lake surface temperature and aidmixing. If the palaeochannel linking Hardibo to Hayq formedat this time, inflow from Lake Hardibo may also have increasedHayq’s productivity, and altered its bottom-water circulation.Inflow of water from Lake Hardibo, which appears to beslightly more dilute than Hayq (Table 2) may have contributedto the lowest stable isotope values recorded in the Hayqsediments.

Effective rainfall (P–E) apparently declined to a minimum atAD 1750, when conditions were drier than at any time in the pre-ceding two millenia, before increasing slightly to the present.Oxygen isotope values increased from 2.2‰ in AD 1936 to 6.2‰

in AD 1976, an interval during which lake level fell by nearly 8 m.If these values relate to the recorded lake levels, then the earlier,larger variations in stable isotope values must reflect far greaterextremes of lake level change. Assuming 18�C water tempera-tures, present-day lake water (�7.1 to �9.1‰) would precipitatecalcite with a �18O value of �6.2 to �8.1‰, the latter beinghigher than any values measured in the core. Overall, the recordsuggests that the climate of the area during the last 2000 yearshas generally been moister than present, especially at AD 700and AD 1300, with drought exceeding that of today only fromabout AD 1750 to 1880, and at about AD 800.

Discussion

The inferred (P–E) changes at Hayq show some links to vegeta-tional change inferred from pollen abundances in the same core(Darbyshire et al., 2003), despite strong human influence on thevegetation. The first, brief period of rainfall increase, peaking atAD 700, took place when the vegetation of the area was a bush-land of Dodonea, with abundant grasses, interpreted as a second-ary vegetation following earlier forest clearance (Figure 6).Although there is a slight increase in Afromontane trees (Podo-carpus, Cupressaceae, Olea, Celtis) at this time, the increasedmoisture was apparently insufficient to have a significant effecton the vegetation. From AD 1250 to 1500 when rainfall appar-ently reached maximum values, the pollen shows high grassabundance, and more Cyperaceae (sedges), together with amarked temporary increase in the planktonic lacustrine algaPediastrum duplex. Since these changes coincide with maximum

Henry F. Lamb et al.: Stable istopes in lacustrine sedimentary carbonate, Ethiopia 523

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rainfall, they may record increased extent of littoral marshvegetation associated with flooding of the lake margins,especially the lowlying land at its southern edge. This interpreta-tion assumes that some proportion of the Poaceae pollen isderived from marsh grasses (eg, Phragmites), as well as fromdry grasslands. The previous interpretation of the pollen data(Darbyshire et al., 2003), without the supporting isotope record,that the grass maximum indicated drought, is therefore incorrect.Tree pollen, especially Cupressaceae (Juniperus), increased sub-stantially from about AD 1400, about a century after the isotopedata suggest maximum rainfall. It seems reasonable to interpretthis increase in tree cover as a response to the moister climate,with the lag being due to the time taken for tree populations toincrease. Increased forest cover may have delayed surface runoff,increased evaporative water loss from the catchment, and thuscontributed to the trend of increasing lakewater �18O values fromAD 1400 to 1700. The decline in Cupressaceae since its maximumat AD 1700 may be a response to aridity, although it is likely thathuman impact was also a factor. Since rainfall apparentlydeclined during the period of increasing forest cover, these dataoffer no support for a hypothesis of positive feedback effects ofvegetation on rainfall, probably because the vegetation changesdid not occur over a sufficiently large area.

The marked changes in the isotope record from Hayq con-trast with muted variation in the diatom stratigraphy, suggest-ing that isotope data are much more sensitive than diatoms toclimate-driven hydrological change in this large freshwater lake.There is some evidence that catchment vegetation may havecontributed to this variation: the highest oxygen and carbonisotope values are recorded at a time of near-maximum forestcover, which would have delayed runoff to the lake, andincreased catchment evaporation. Comparison with other EastAfrican sites is one way of testing the validity of the Hayq

record, although such comparisons are imprecise because ofuncertainties in the various chronologies, and because lakesdiffer substantially in their hydrological response to climatechange. The isotope record from Lake Turkana (figure 6 inHalfman et al., 1994) shows two intervals of low values, at AD

550 and AD 1350 (1500 and 600 14C yr BP), probably equivalentto the AD 700 and AD 1300 events at Hayq. The lake level andsalinity record from Lake Naivasha, Kenya (Verschuren et al.,2000), which extends from AD 900 to the present, shows simi-larities in the timing and direction of changes at Hayq,although the amplitudes of the changes differ (Figure 7). Theperiod of relatively dry conditions from AD 800 to 1200 atHayq coincides with an interval of dry climate at Naivasha (AD

1000–1270), equivalent to the European ‘Mediaeval WarmPeriod’. At Hayq, however, this interval is not significantlydrier than present. Rainfall increased at both Hayq andNaivasha at about AD 1300, the start of the ‘Little Ice Age’,although the relative response of Hayq oxygen isotope values isgreater, possibly because of inflow from Hardibo. It is possiblethat the intermediate peak in oxygen and carbon isotope valuesat Hayq (c. AD 1550) is equivalent to the second of the threedrought intervals that interrupted the generally wet ‘Little IceAge’ at Naivasha. Highest lake level at Naivasha is not paral-leled by minimal isotope values at Hayq. The third drought,marked by a sharp fall in the level of Naivasha at AD 1750,occurs about 75 years after the abrupt change to dry conditionsat Hayq, and the subsequent rise is not so marked at Hayq.Lamb et al. (2000) presented oxygen isotope data at a similartime resolution. The record from Lake Tilo, a saline crater lakein the Ethiopian Rift valley (Lamb et al., 2000) shows little sim-ilarity with that of Hayk, probably because Tilo is shallow(10 m) and influenced by hot springs. Legesse et al. (2002) inferhigh lake levels and moist climate during part of the eighteenthcentury AD at Lake Abiyata in the Ethiopian Rift, in agreementwith the data from Hatq. However, the timing of maximimdrought differs, being AD 1890 at Abiyata, rather than AD 1750as at Hayq. Hassan (1981) devised an index of the relativestrength of Blue and White Nile flows in the 1300-yr record ofNile flow at Cairo. Positive values of this index (Figure 7),denoting relatively low Blue Nile flows, occurred during thetwelfth century AD, matching the evidence for low rainfall atHayq, which lies at the eastern edge of the Blue Nile catch-ment. Negative values, denoting strong influence of the BlueNile, peak at about AD 1430, about 100 years after maximumrainfall inferred from Hayq. The dry conditions at Hayq afterAD 1750 are not matched by positive values of the Nile index,possibly because of low rainfall in the White Nile catchment atthe same time (as indicated by the Naivasha record). Overall,the broad similarities in timing and direction of the majorchanges in the Hayq, Naivasha and Nile records suggest that aclimatic interpretation of the record from Hayq is valid, andthat relatively dry conditions prevailed in northeast Africa atthe time of Europe’s ‘Mediaeval Warm Period’, followed byhigher effective rainfall during the ‘Little Ice Age’.

Conclusions

The oxygen and carbon isotope data from Lake Hayq representa reasonably well-dated decadal-scale record of effective precip-itation in a semi-arid region that is especially vulnerable todrought. A quantitative model of the isotope hydrology of LakeHayq is required before we can make a confident palaeoclimaticinterpretation of the sedimentary record. Nevertheless, therecord suggests that the arid conditions now characteristic ofnorthern Ethiopia have occurred only since about AD 1750; the

524 The Holocene 17 (2007)

Figure 7 A comparison of Nile (Hassan, 1981), Naivasha(Verschuren et al., 2000) and Hayq proxy-climate records

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climate of the ‘Mediaeval Warm Period’ (AD 900–1250) wassimilar but slightly wetter than present. Higher effective rainfall(precipitation–evaporation) than present is more typical of theregion. In particular, there were two intervals of more moist cli-mate, centred on AD 700 and AD 1300. The transitions betweenwet and dry intervals were rapid, taking place within decades.The change to relatively dry climate at about AD 800 occurred atabout the time of the fall of the Axumite empire of northernEthiopia (Butzer, 1981), suggesting a possible causal relation-ship. The record suggests that droughts of greater severity thanthose of the last few centuries are not characteristic of the nat-ural range of climatic variability, but it would be unwise toassume that this pattern will remain in the face of anthro-pogenic modification of the global climate. The Hayq data showsome similarities to the Nile discharge record, and to the recordsfrom lakes Turkana and Naivasha, suggesting regional climaticforcing, such as sea-surface temperature, or solar variability ashypothesized by Verschuren et al. (2000). More high-resolutionclimatic records for the last two millennia are required fromEast Africa before we can explore the true causes and effects ofrecent climatic variability in the region.

Acknowledgements

Fieldwork was funded by grants from the Leverhulme Trustand the Royal Geographical Society. Stable isotope analyseswere carried out by Joanne Green, and funded through theNatural Environment Research Council.

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