N 18O in mollusk shells from Pliocene Lake Hadar and modern ...

N18O in mollusk shells from Pliocene Lake Hadar and modern

Ethiopian lakes: implications for history ofthe Ethiopian monsoon

Million Hailemichael a;�, James L. Aronson b, Samuel Savin a,Michael J.S. Tevesz c, Joseph G. Carter d

a Department of Geological Sciences, Case Western Reserve University, Cleveland, OH 44106, USAb Department of Earth Sciences, Dartmouth College, Hanover, NH 03755, USA

c Department of Biological, Geological and Environmental Sciences, Cleveland State University, Cleveland, OH 44115, USAd Department of Geological Sciences, University of North Carolina, Chapel Hill, NC 27599, USA

Received 19 January 2001; accepted 7 June 2002

Abstract

Two of the five lacustrine intervals in the largely fluvial Hadar Formation, Afar, Ethiopia, occur in the SidiHakoma Member deposited 3.4^3.2 Ma. In a perspective of the N

18O of 11 modern Ethiopian lakes and their shells,the N

18O of the Hadar fossil shells provide a snapshot of the nature of ancient Lake Hadar and Ethiopia’s climate inthe Pliocene. Ethiopia’s modern lakes both in the Rift and on the Western Plateau are fed by drainage of Plateau rainwith its well established barely negative N

18OSMOW of 31.3x. Except for the man-made Lake Koka reservoir, allother Ethiopian lakes are isotopically quite positive ranging from +5.4 to +16.0x, indicating how significantevaporation is in their water budget. Shells from lakes with extant mollusk populations are mostly in isotopicequilibrium with the N

18O and temperature of their lake water. The upper transgressive interval in the Sidi HakomaMember is the largest one in the Formation beginning at its base with the ‘Gastropod Beds’ beach deposits. Mollusksfrom shell beds other than the ‘Gastropod Beds’ show more positive and more variable N

18O between shells, withinternal variations within shells as much as 7x. At these times the site must have been underlain by a shallowpartially isolated embayment of Lake Hadar which underwent rapid expansions and then contractions byevaporation, within the few year lifetimes of the individual mollusks. The results from the ‘Gastropod Beds’ are ofmost significance for interpreting the overall paleoclimate at Hadar. Their uniformly negative N

18OPDB shell valuesthat average 36.7x represent a much less evaporated stage of Lake Hadar when its N18OSMOW was 8x lower thanthe spectrum of modern lakes in Ethiopia, and indeed even 3x or more lower than average modern Plateau rain. Toexplain such negative values we hypothesize that the Atlantic-derived air mass component to the Ethiopian monsoonwas persistently strengthened during Pliocene summers, which intensified the amount and the negative isotopiccharacter of rainfall onto both the Afar and the Ethiopian Plateaus that drained to Lake Hadar. A similarphenomenon characterized the brief periodic pluvial episodes of the Quaternary, including the latest in the earlyHolocene, known as the African Humid Period. In contrast to the hot semi-desert steppe conditions of today’s

0031-0182 / 02 / $ ^ see front matter C 2002 Elsevier Science B.V. All rights reserved.PII: S 0 0 3 1 - 0 1 8 2 ( 0 2 ) 0 0 4 4 5 - 5

* Corresponding author.E-mail addresses: [email protected] (M. Hailemichael), [email protected] (J.L. Aronson), [email protected]

(S. Savin), [email protected] (M.J.S. Tevesz), [email protected] (J.G. Carter).

PALAEO 2906 29-8-02

Palaeogeography, Palaeoclimatology, Palaeoecology 186 (2002) 81^99

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western Afar, the diverse abundant terrestrial fossil fauna at Hadar, including the early hominid Australopithecusafarensis, is explained by the wetter, and probably cooler, summers that persisted throughout the LatePliocene. C 2002 Elsevier Science B.V. All rights reserved.

Keywords: Hadar; mollusk shell ; oxygen isotope; Ethiopia; paleoenvironment; hominid site; monsoon

1. Introduction

The Pliocene sedimentary strata of the EastAfrican Rift System are rich in vertebrate fossils.Among these, the sites of Hadar (Fig. 1) and theMiddle Awash in the western Afar of Ethiopia arenotable for the remarkable amount and quality oftheir hominid fossils. The 180-m-thick Hadar For-mation has produced over 90% of the known fos-sils of the early hominid Australopithecus afaren-sis, including the partial skeleton, ‘Lucy’. Theformation accumulated in the late Pliocene 3.4^2.3 Ma mostly as the £ood plain and channeldeposits of a major meandering river that wasancestral to the modern Awash River. Relativedown-dropping of the central Afar since the Plio-cene has caused the present Awash River and itstributaries to have cut down through and exposedits ancient deposits at the site of Hadar (Aronsonand Taieb, 1981). During the Pliocene, Hadar wasthe distal and delta plain reach of the river nearits entrance to a major lake we refer to as LakeHadar. Lake Hadar expanded and transgressedover Hadar laying down intervals of laminatedlacustrine muds and beach sands with scatteredto abundant shells of mollusks (Fig. 2). The lasttransgression of the lake is well dated at 2.95 Maand was followed by the sculpting of a majordisconformity (Fig. 2) (Aronson et al., 1996). Re-newed deposition of the uppermost 15% of theformation includes no record of the existence ofthe lake, and instead conglomerates appear thatwere deposited by steep transversely £owingbraided rivers from the Western Plateau. Hadaris only 40 km east of the present-day 2-km-highescarpment, along whose faults the Afar has beendropped.In contrast to the hot, dry and mostly sterile

setting of today’s Hadar in the western Afar, thevery high diversity and abundance of the terres-trial vertebrate fossil fauna of the Hadar Forma-

tion together with much fewer conglomeratesbeneath the disconformity have led to the hypoth-esis that the present-day Western Escarpmentmay have only been at a nascent stage in thelate Pliocene, 3 Ma. By this thinking western-most Afar, including Hadar, may have been amarginal tectonic block of the plateau that onlysince the Pliocene has descended along the presentescarpment fault to become part of the presentlyhot and arid Afar block (Aronson and Taieb,1981; Bonne¢lle et al., 1987; Aronson et al.,1996). Alternatively, the climate in the Afar mayhave been wetter than today. Our study addressesthis latter possibility via isotopic study of fossilshells of mollusks that lived in Lake Hadar about3.2 Ma.In their extensive, but brie£y documented iso-

topic study of the Hadar Formation, Hillaire-Marcel et al. (1982) included measurements ofshells from most or all of the ¢ve lacustrine inter-vals, along with carbonate nodules of unspeci¢edorigins. They reported that the shells they ana-lyzed were aragonite and unaltered. Our experi-ence mainly in the central sector of the site showsthat diagenesis has extensively a¡ected shells inthe beds of the lowermost and the upper threelacustrine intervals by recrystallizing the shellsand ¢lling them with spar calcite. This contrastswith the unaltered conditions of the shells in thetwo lacustrine intervals of the Sidi Hakoma mem-ber on which we report here.To better interpret the shell isotopic data in

terms of the lake environments represented, wealso examined the isotopic relationships of mod-ern shells and waters in some Ethiopian lakes onthe Plateau and in the Rift. Many of the fossiland modern shells were analyzed serially via mi-crosamples along the growth direction of theshells to determine how environmental conditionschanged seasonally during an individual mollusk’slife.

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M. Hailemichael et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 186 (2002) 81^9982

2. Samples and methods

Paired samples of lake water and modern mol-lusk shells from 11 lakes and from the AwashRiver at Kereyu National Park were analyzed.

The N18O of water samples from the 11 lakes

and other relevant physical and chemical informa-tion are presented in Table 1. All of the watersamples were collected in glass bottles by wadingout from the lakeshores during the rainy months

Fig. 1. Location of the 11 modern Ethiopian lakes studied, and the Afar site of Hadar. Numbers 1^11 indicate lakes studied inthe rift and plateau: (1) Lake Gamari; (2) Lake Hayk; (3) Lake Tana; (4) Lake Hora; (5) Lake Metehara (Beseka); (6) LakeKoka; (7) Lake Zway; (8) Lake Langano; (9) Lake Abiata; (10) Lake Shala; (11) Lake Awasa. The great early Holocene Afri-can Humid Period (AHP) expansions of the four lakes of the Zway^Shala Basin in the Main Ethiopian Rift and the four lakesin the Gamari^Abbe series in the Central Afar are indicated. The approximate present-day location of the Inter-Oceanic Con£u-ence (IOC), summer front between Atlantic and Indian Ocean derived air masses is shown (after Rozanski et al., 1996). Thisfront is proposed to have shifted several 100 km eastward over the western Afar, persistently during the Pliocene and periodicallyduring the Quaternary.

PALAEO 2906 29-8-02

M. Hailemichael et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 186 (2002) 81^99 83

Fig. 2. Composite stratigraphic section of the Hadar Formation; and detailed section of the Sidi Hakoma Member. Four of the¢ve lacustrine intervals are shown, of which the two in the Sidi Hakoma member were examined isotopically in detail here. Justafter the 2.95-Myr-old BKT-2 tu¡ was deposited, a major disconformity formed at Hadar, after which there is no evidence ofthe existence of Lake Hadar. An additional lacustrine interval exists at the top of the Basal Member (BM).

PALAEO 2906 29-8-02


of March and June, 1997. The bottles were plastictaped, para⁄n sealed and refrigerated until anal-ysis.The mollusks from lakes Awasa, Zway, Tana

and the Awash River were collected alive. Exceptfor two Lymnaea (Radix) peregra gastropods fromLake Zway, all modern and most fossil shells an-alyzed belong to one of the gastropod speciesMelanoides tuberculata, Bellamya unicolor, or thebivalve order Unionoida, which are the mostcommonly used mollusk shells in African lake iso-tope studies for which no ‘vital e¡ect’ inter-speciesisotopic fractionation has been noted. Microsam-ples were obtained by shallow scratching with adrill along the external growth band.For the Pliocene, the N

18O values of 37 micro-samples from 14 fossil shells from the Hadar For-mation are presented in Table 3. The taxa ana-lyzed are the gastropod Melanoides tuberculata,Bellamya unicolor, Cleopatra bulimoides, and theunionoid and Corbicula bivalve species.Because mollusk shell aragonite is metastable

and readily alters to calcite, the absence of calciteindicates the shell’s isotopic composition is likelyto be pristine. Powdered samples of all shellsunderwent X-ray di¡raction, calibrated for calcitesensitivity using mixtures of aragonite and calcitedown to 1% calcite where the calcite 104(hkl)peak is still detectable. The absence of this peakin all our shells indicates they are essentially un-altered. The least fresh mollusk shells in the studyare fossils from the two of three associated shellcoquina marker limestone beds known as the‘Gastropod Beds’. In those two beds the shellsare always naturally bleached white, in contrastto shells from the intermediate bed, from whichour HS samples come, which have a thin sur¢cialpinkish brown layer. Because the isotope resultsof the ‘Gastropod Beds’ shells turn out to be im-portant for interpreting the isotopic nature of pa-leo-Lake Hadar, we assessed them further for al-teration. Broken surfaces were scrutinized underthe scanning electron microscope to see if anysecondary recrystallization had altered the physi-cal character of the aragonite biostructure. Theseobservations, photographed in Hailemichael(2000), reveal minor areas (6 1% of the totalarea scanned) where textural replacement has oc-

curred in the naturally bleached shells. No reor-ganization was observed at all in the unbleachedshells of the intermediate HS layer of the ‘Gastro-pod Beds’, nor in any of the shells of the 13A bed,all of which preserve a nacreous luster. As ampli-¢ed in 5. Discussion, this low degree of alterationof these least fresh samples is within the limits ofacceptability for isotopic evidence.All shells were pretreated with sodium hypo-

chlorite, ground and vacuum roasted at 200‡Cfor 1 h and digested in H3PO4 at 25‡C accord-ing to McCrea (1950). The 13C/12C and 18O/16Oratios of the evolved CO2 were related to thePDB standard through repeated analyses of theSolenhofen limestone NBS Isotopic Standard 20with Craig’s (1957) assumed N

18O=34.14 andN13C= 31.06x.The N

18O values of water samples were mea-sured according to Epstein and Mayeda (1953)relative to Standard Mean Oceanic Water(SMOW). The equation of Coplen et al. (1983)was used to relate the SMOW and PDB scales.To assess if modern aragonite shells have grownin equilibrium with existing conditions, we usedGrossman and Ku’s (1986) isotopic equilibriumequation modi¢ed by Dettman (1994) to relatethe measured N

18Oaragonite values to the valuesof N

18Owater and temperature where thearagonite could have precipitated. The equationused is: 1000 ln Karagonite3water = (166.623T‡C)/4.784, where Karagonite3water = (N18Oaragonite+1000)/(N18Owater+1000).

3. Isotope meteorology of modern-day Ethiopia

The complex meteorology of Ethiopia, andEast Africa in general, is poorly understood be-cause it is related to the seasonal passage of airmass convergence zones across a varied plateau^rift topography spanning up to 4 km in relief(Gri⁄ths, 1972; Nicholson, 1996). Rainfall onthe Western Plateau and most of Ethiopia is mon-soonal, with about 75% falling in the mainsummer rainy season (the Ethiopian monsoon)when the Inter-Tropical Convergence Zone(ITCZ) is north of Ethiopia and the Inter OceanCon£uence (IOC) is over the Western Plateau

PALAEO 2906 29-8-02


(Fig. 1). Today these Atlantic- and Indian Ocean-derived air masses that are drawn to these con-vergence zones descend adiabatically 2 or morekm of elevation into the Afar where they becomehot and dry. The other 25% of Plateau rainfalloccurs in the springtime rains when the unstableITCZ is passing overhead. The mean annual tem-perature of the Western Plateau is about 15‡Ccompared to about 28^30‡C in western and cen-tral Afar (unpublished data from National Mete-orological Services Agency of Ethiopia, 1996;Gri⁄ths, 1972). The potential for evaporation isvery high, reaching to s 300 cm in the westernAfar (Taieb, 1974).Today’s rain on the Western Plateau is isotopi-

cally variable, but well characterized by the 35-year isotopic record of rainfall kept by the Inter-national Atomic Energy Agency (IAEA) for theAddis Ababa station (IAEA, 1996). The weightedmean N

18OSMOW of 31.3x at Addis is con¢rmedfor other areas of the eastern margin of the West-ern Plateau by our isotopic measurements of in-dividual rains and springs sampled during thespring and summer rains of 1997 (Hailemichael,2000), and by earlier measurements by Schoelland Faber (1976) on spring and well waters inthe same region.Though not well characterized, rainfall in the

rift system is isotopically more positive. TheIAEA set of six measurements from Awasa andZway in the Main Ethiopian Rift over thesummer rainy season of 1995 averaged 30.9 and31.9x respectively (IAEA, 1996). In addition totwo rain samples collected in 1997 (April andJune) from Awasa (5.50 and 32.88x), we mea-sured ¢ve samples from Dilla and Shashemene(June) in the Main Ethiopian Rift (MER). Theaverage N

18OSMOW of rain from the total of these13 MER rains is 30.4 U 2.3x.Unfortunately the only isotopic data for the

Afar are our own spot measurements of intensenight-time rains we experienced in the westernAfar during the summer of 1997 at Hadar(+1.98x) and Aditu (30.46x) and in the cen-tral Afar for two spring rains at Tendaho (+3.47and +2.19x) and for a single summer nightshower at Asaita in the central Afar with aN18OSMOW of +8.66x. The more positive charac-

ter of the rift system rain compared to Plateaurain is readily explained by: (1) the lower amountof rain in a given storm, the inverse of ‘theamount e¡ect’ (Craig, 1965; Dansgaard, 1953);and (2) more evaporation of the rain while fallingthrough the hotter and drier air.Between the Atlantic-derived and the Indian

Ocean-derived air mass sources of rain for theEthiopian Summer monsoon, the generally heldnotion has been that the Atlantic’s Gulf of Guin-ea is the major source, particularly for the West-ern Plateau, with the moist Atlantic-derived airmasses being drawn all the way across Africa bythe Indian Sub-continental Low trough in atmo-spheric pressure (see discussions in Rozanski etal., 1993; Telford and Lamb, 1999; Lamb et al.,2000). But isotopic studies of modern rainfall pro-duce a dilemma for this notion (Joseph et al.,1992). For such a long path over which muchrain-out clearly occurs, Rayleigh distillationshould deplete considerable amounts of 18Ofrom Ethiopian summer rain. Yet the well char-acterized 35-year weighted annual mean N

18O ofrain at the IAEA station in Addis Ababa is abarely negative value averaging 31.3x. Josephet al. (1992) explained this anomaly by proposingthat it is Indian Ocean-derived air masses that arethe main moisture source for today’s EthiopianSummer monsoon, not the Atlantic ones. Recentdynamic observations of satellite cloud patternssupport this and suggest a high proportion ofEthiopian summer rain, perhaps in some yearsgreater than 50%, comes from air mass sourcesfrom the direction of the Indian Ocean (personalcommunication, 1999, Tesfaye Gissela, EthiopianMeteorology O⁄ce).

4. Results

4.1. Isotopic compositions of modern Ethiopianlake and river waters

The N18O values of water samples from

through-£owing Lake Tana on the WesternPlateau (N18OSMOW =+5.63x in March and+5.79x in April 1997) are higher than those ofrain water samples at Bahir Dar (+5.08x in the

PALAEO 2906 29-8-02


small rains of March and 33.31x in the largesummer rains of July, 1997, Hailemichael, 2000)and indicate evaporative enrichment of 18O hasoccurred in Lake Tana.The N

18OSMOW value of a water sample col-lected from Lake Hayk on the eastern margin ofthe Western Plateau in July, 1997 was +9.34x(Table 1), close to the value (+8.68x) reportedfor the same lake by Schoell and Faber, 1976.These values are very much higher than spring-time and summer rain from Dese (+1.96 to34.12x), about 50 km south, and spring water(32.55x) sampled at Tita, between Dese andHayk (Hailemichael, 2000). This high N

18O val-ue of the closed basin lake water is consistentwith intense evaporation during the prominentEthiopian dry season. Comparably high values(N18OSMOW =+8.03x) occur in Lake Ashenge,in a similar tectonic setting 100 km to the north(Schoell and Faber, 1976).Eight lakes in the MER were examined in this

study. Lake Awasa and Lake Zway are relativelydilute while Lake Abiata is 100 times more con-centrated. The N

18O values of lakes in the MERrange between +5.44 (Lake Zway) and +7.98x(Lake Abiata) and approximately correlate withmajor anion concentrations and with conductivity(Table 1). For example, Lake Zway has low con-ductivity (94 WS) and Cl3 content (11 ppm) and aN18O value of +5.44x, whereas Lake Abiata,with a conductivity of 20 500 WS and a Cl3 con-tent of 2840 ppm, has a N

18O value of +7.98x.Lakes Langano and Shala have intermediateN18OSMOW values of +6.84x and +7.66x re-spectively. Our N18O value (+7.06x) of the largecaldera Lake Awasa in July 1997 matches Leng etal.’s (1999) December 1995 values between +7.3and +7.4x (n=12).A divide separates the Zway^Shala basin on the

north from the Awash River, where it is dammedto form the large Lake Koka hydroelectric reser-voir. North of Lake Koka, in the northern sector

Table 1Physical, chemical and isotopic character of modern Ethiopian lakes and of the Awash River examined in this study

Lake Elevationa Deptha pH Watertemperature

F3 Cl3 SO234 Conductivity N

18O Samplingdate

(m) (m) (‡C) (ppm) (ppm) (ppm) (WS) (SMOW)

Plateau lakesHayk 2030 23 9.0 26 0.9 42 0 210 9.34 Jun 97Tana (sample 1) 1785 9 na na 0.2 3 2 685 5.63 Mar 97Tana (sample 2) 1785 9 na na na na na na 5.79 Apr 97Hayk 2150 23 na na na na na na 8.68b 1977Ashenge 2300 25 na na na na na na 8.03b 1977Rift Valley lakesAwasa 1675 10 8.4 24 8 27 0 184 7.06 Jul 97Shala 1540 250 9.4 26 213 3080 119 770 7.66 Jul 97Langano 1580 46 8.8 25 18 160 13 422 6.84 Jul 97Abiata 1580 14 9.6 28 211 2844 194 20500 7.98 Jul 97Zway 1637 4 8.1 25 15 11 2 95 5.44 Jul 97Koka 1590 9 8.3 27 3 25 10 184 1.55 Jul 97Hora 1770 85 8.7 24 0 233 10 370 7.41 Jul 97Afar lakesMetehara 750 na na na 6.9 114 480 3794 6.74 Jul 97Gamari (sample 1) 320 na 9.2 32 5.0 369 50 685 15.89 Jun 97Gamari (sample 2) 320 na na 29 na na na na 16.14 Jul 97Awash Riverbefore entering Koka 1590 na na na na na na na 32.41 Jul 97at Kereyu Park 1000 na na na na na na na 0.48 Jul 97at Mile 410 na na na na na na na 4.29 Jul-97at Asita 320 na na na na na na na 4.19 Jun 97

a Ethiopian Mapping Authority, 1988.b Data from Schoell and Faber, 1976.

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of the MER, are the small closed basin lakesHora and Metehara (Beseka) fed by drainagefrom the Western Plateau and with N

18O valuesof +7.4x and +6.74x. Our 18O-enriched Ethio-pian lake waters match values for lakes in Kenyaranging from +3.6 to +9.2x, except the highlyvariable Lakes Elmenteita and Magadi (Cerling etal., 1988).The most arid area we were able to visit during

this study was Lake Gamari, at 320 m above sealevel and the ¢rst of four lakes located in series inthe center of the Afar Depression (Fig. 1). Wecould not reach the other lakes in the series andonly gained access to the large partially isolatedshallow bay at the northern end of Lake Gamari.The N

18OSMOW values and temperatures of thewater samples collected on June 23 and againon July 3 1997 were +16.01x (water T=32‡C)and +15.87x (water T=29‡C), the highest val-ues so far reported from any East African lake.Despite its much higher N18O, the conductivity ofGamari water in June was 685 WS, lower thanLakes Abiata and Shala, but higher than therest of the lakes in the MER.Because of its large headwater region and

through-£owing condition, Lake Koka has thelowest N

18O (+1.55x, July 1997) value of allEthiopian lakes studied. It has an 18O enrichmentof about 3x above that of the weighted meanPlateau rainwater (31.3x) which falls in theAwash’s headwater region. A sample of theAwash River taken the same day just upstreamof Lake Koka has a N

18O of 32.4x (Table 1).Following the Awash River through the Afar inthe early 1997 summer, we measured a strongprogressive increase in the 18O from +0.48x be-low the Awash Falls near the southern juncture ofthe Afar with the MER to +4.29x at MileFarms, about 40 km downstream of Hadar; and+4.19x at Asaita, 150 km further downstreamnear the river’s terminus with lake Gamari in thecentral Afar.

4.2. Oxygen isotopic composition of modernmollusk shells

4.2.1. Western Plateau LakesAt Lake Tana, the N

18O (+3.3 to +4.2x) of

shells of live snails (Bellamya unicolor) and union-oid bivalves are somewhat out of equilibrium forprecipitation of aragonite from lake water of themeasured N

18OSMOW (+5.7x) at a mean monthlyair temperature of 20‡C at Bahir Dar (unpub-lished data from National Meteorological ServicesAgency of Ethiopia, 1996). As shown in Fig. 3, ahigher temperature of about 30‡C is calculated forthe precipitation of these shells from the +5.7xwater. Among possible explanations, these mol-lusks may have resided near a spring outlet ofwater less enriched in 18O.Abundant fresh, gray, unbleached shells of

Melanoides on the sediment surface of LakeHayk showed uniformly high N

18O values between+7.58 and +8.04x (Fig. 3) which are in isotopicequilibrium with the measured N

18OSMOW of thelake water (+9.34x) and temperature (26‡C).

4.2.2. Main Ethiopian Rift Valley lakesThe N

18OPDB values of whole shells of live Me-

Fig. 3. Measured N18OPDB values of modern shells plotted

against the N18OSMOW from the Ethiopian lakes in which

they grew. These modern shell values are contrasted with re-sults of fossil shells from the ‘Gastropod Beds’ of the HadarFormation. For the modern shells the derived temperatureson the equilibrium fractionation curves shown are broadlyreasonable. In detail, temperatures are projected to be toowarm for Lake Tana, this exception is discussed in the text.Clearly the waters of Pliocene Lake Hadar at the time theshells of the ‘Gastropod Beds’ grew were distinctly more de-pleted in 18O than for any present-day lakes in Ethiopia.

PALAEO 2906 29-8-02


lanoides collected from Lake Awasa in July, 1997range from +5.75 to +6.28x, while two micro-samples of another Melanoides shell are +6.24and +6.36x (Table 2). These N

18O values arein equilibrium with the measured water temper-ature (24‡C) and N

18O (+7.06x) (Fig. 3). Thesedata are also consistent with the mean N

18OPDB

value of +5.9x reported for modern Melanoidesfrom Lake Awasa by Leng et al., 1999.At Lake Zway, two whole shells of live Radix, a

species not previously reported in isotopic studies,give N

18OPDB values of +5.08 and +5.49x, inequilibrium with the water N

18O (5.44x) andthe mean air temperature (21‡C). We found no

Table 2The N

18O and N13C values of modern Ethiopian mollusk shells

Sampling locality Genus species Whole shell Microsamples

Sample # N18O N

13C Sample # location N18O N

13C

Western PlateauLake Tana B. unicolor LTS2-1-W 3.40 36.21 LTS2-5-A apex 3.44 34.02

B. unicolor LTS2-2-W 3.34 33.98 LTS2-5-B intermediate 3.11 33.92B. unicolor LTS2-3-W 3.38 34.70 LTS2-5-C aperture 3.58 32.49B. unicolor LTS2-4-W 3.41 35.23 Mean 3.38 33.48

Mean 3.38 35.03 S.D. 0.24 0.86S.D. 0.03 0.94

LTS1-1-A umbo 3.90 33.42unionoid LTS1-1-B mid shell 4.83 32.84

LTS1-1-C mid shell 4.35 33.45LTS1-1-D margin 3.65 31.89

Mean 4.18 32.90S.D. 0.52 0.73

Lake Hyke M. tuberculata LHS1-1-W 8.04 1.60M. tuberculata LHS1-2-W 7.58 31.91M. tuberculata LHS1-3-W 7.56 1.68M. tuberculata LHS1-4-W 7.87 30.29

Mean 7.76 0.27S.D. 0.23 1.71

Rift ValleyLake Awasa M. tuberculata LAWS1-1-W 5.75 30.27 LAWS1-5-A apex 6.36 0.29

M. tuberculata LAWS1-2-W 5.81 31.71 LAWS1-5-B aperture 6.24 30.92M. tuberculata LAWS1-3-W 6.25 30.01 Mean 6.30 30.32M. tuberculata LAWS1-4-W 6.28 0.26 S.D. 0.08 0.86

Mean 6.02 30.43S.D. 0.28 0.88

Lake Zway Radix LZS1-1-W 5.49 30.50Radix LZS1-2-W 5.08 31.80

Mean 5.29 31.15S.D. 0.29 0.92

Awash River M. tuberculata ARS1-2-W 30.16 34.23 ARS1-3-A apex 0.51 32.96‘Kereyu Park’ Bivalve MICS2-W 30.15 34.24 ARS1-3-B intermediate 30.06 33.09

ARS1-3-C aperture 30.32 33.35Mean 0.04 33.13S.D. 0.42 0.20

Awash River unionoid ARS1-1-A umbo 30.87 34.08‘Kereyu Park’ ARS1-1-B mid shell 30.72 35.17

ARS1-1-C mid shell 31.84 35.57ARS1-1-D margin 30.03 35.27

Mean 30.87 35.02S.D. 0.75 0.65

Values reported are as measured (aragonite) and are not corrected to calcite values.

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live mollusks at Lakes Langano, Shala, Abiata,and Gamari and no shells at all at Lakes Koka,Hora and Metehara.

4.3. Fossil shells from the Sidi Hakoma Member,Hadar Formation

Fig. 2 shows the stratigraphic position of thetwo lacustrine intervals in the Sidi Hakoma Mem-ber of the Hadar Formation whose shells are pris-tine enough for isotopic analysis.

4.3.1. Lower lacustrine intervalThe lowermost shell-bearing bed is associated

with the thin Kada Meha Tu¡ (KMT), 25 mabove the 3.40-Myr-old Sidi Hakoma Tu¡ (Wal-ter and Aronson, 1993). The pelecypod shells witha nacreous luster occur at the base of the tu¡. Theisotopic variability of microsamples taken withina given shell was only 0.5x for both 18O and13C, but the N

18O di¡erence between the averagesof each shell is +2.92x. This indicates either thetwo individuals washed in from separate nearbymicro-environments, or they lived at di¡erenttimes in an environment whose isotopic characterevolved with time.

4.3.2. Main lacustrine intervalIn the central sector of Hadar the largest lacus-

trine interval in the Hadar Formation is 23 mthick at the top of the Sidi Hakoma Member(Fig. 2). It thins to the west and thickens some-what to the east to about 30 m at Ounda Hadarin the direction of the permanent location of LakeHadar (Aronson and Taieb, 1981). The base ofthe interval begins with two to three thin prom-inent gastropod coquina limestone marker bedsknown as the ‘Gastropod Beds’, and ends at thetop with o¡shore laminated claystones that pre-serve several thin bentonite tu¡s originally re-ferred to as the ‘Triple Tu¡s’ (TT). Of these,TT-4 has feldspar crystals well dated at3.22U 0.04 Myr (Walter, 1993). This tu¡ and itsenclosing laminated claystones with abundant os-tracods and ¢sh scales are a marker horizon trace-able from east to west across the 12 km breadthof the site and represent the largest westwardtransgression of the lake across Hadar. About

2.5 m beneath TT-4 in the central sector of Ha-dar, a thin sand with planar shallow cross-bed-ding and abundant mollusks formed as a localbeach deposit between TT-1 and TT-4 (Fig. 2).This thin sand extends for about 1 km acrossthe Kada Hadar Wadi and provided the exqui-sitely preserved nacreous fossil shells of the 13Asample.

4.3.2.1. Base of main lacustrine interval: the ‘Gas-tropod Beds’. Samples AT, 25, and HS derivefrom these prominent beds, which represent a se-ries of two or three closely spaced beach coquinas.During deposition the shells were the coarsest ma-terials available for waves to have swept up andaccumulated at the shore. The lowest of the threebeds represents the initial area-wide transgressivebeach that formed as Lake Hadar expanded overthe low £at distal £oodplain. The succeeding bedssuggest that the initial transgression wavered backand forth before full lacustrine conditions occu-pied Hadar.Samples AT, M25 and B25 are collected from

the more extensive lower and upper beds wherethe robust shells are always entirely bleachedwhite. In contrast the unbleached HS shellsfrom the less extensive middle bed are not sopacked together in the sand matrix and are notbleached. We excavated the outcrop and handselected those HS shells which best preserved thepinkish brown exterior of the shell.The mean N

18O values for individual wholeshells and microsamples from all of the ‘Gastro-pod Beds’ are quite uniformly negative only rang-ing from 34.99 to 38.14x and averaging36.7U 1.0x (n=17). Standard deviations ofthe N

18O values of microsamples of individualshells only range from 0.34 to 1.09x. This indi-cates that isotopic conditions were fairly uniformduring the life of each shell from the ‘GastropodBeds’, and that the N18OSMOW of Lake Hadar wasmuch more negative than when the mollusks be-low and above this important unit were living.

4.3.2.2. Upper portion of main lacustrine interval(sample 13A). Five mollusk shells (17 micro-samples) of 13A were hand picked for analysisover a 100-m outcrop of the regressive beach de-

PALAEO 2906 29-8-02


posit near the top of this lacustrine interval (Fig.2). The 13A nacreous fossils include whole gastro-pod shells and large fragments of pelecypodshells. In contrast to the shells from the ‘Gastro-pod Beds’ the variability of N

18O values withinindividual 13A shells is large. The N

18O valuesof all the microsamples from all of the ¢ve indi-viduals span a remarkably large range of 8x(32.02 to +6.33x). A good portion of this rangeis encompassed within the shell of a single Mela-noides, ranging from 30.58 to +6.33x and with-in a single pelecypod shell from 30.26 to+5.92x (Fig. 4). The gastropod experienced aprogressive evaporative concentration of 7x inLake Hadar all within its approximate one year oflife which began in the summer wet season (lowN18O) and ended in the dry season (high N

18O). Apractically identical 18O enrichment is recordwithin the partial shell of the longer-lived pelecy-pod, but it survived a dry season and its shellcontinued recording a succeeding wet season. Ofthe 17 microsamples from the 13A collection, themean N

18OPDB value is +1.37 U 2.3x. This posi-tive mean value and the mean of the minor trans-gression of KMT are about 9x more positivethan the uniformly negative 36.7 U 1.0x valuesrecorded by the ‘Gastropod Beds’ at the begin-ning of this main lacustrine interval. The largeinternal isotopic variability of the 13A fossils

was not observed within any of the microsampledmodern shells from today’s large Ethiopian lakes,but such a large internal variation was observedby Abell and Williams (1989) for shells from thesmall modern ephemeral Lake Lyadu in the Afar.The large cyclic variation in N

18O of the 13Amollusk shells suggests that during the late stageof the main lacustrine interval, the site of Hadartemporarily became a shallow bay that was par-tially isolated from the main body of Lake Hadarto the east and was strongly isotopically a¡ectedby seasonal evaporation. It was probably similarto modern-day Lake Gamari’s NW bay. How-ever, very soon after the regressive 13A beachformed near the end of this major transgressiveinterval, Lake Hadar brie£y re-transgressed acrossthe entire Hadar site to deposit the ‘OstracodBeds’ marker claystone and the TT-4 tu¡.

5. Discussion

5.1. Signi¢cance of the low N18O in the ‘Gastropod

Beds’

The rule governing the N18O of East African

lakes is that extensive dry season evaporation re-sults in quite positive values for their waters andthe shells which grow in them. Thus the uniformly

Fig. 4. Plot of N18O of microsamples from individual Hadar fossil shells. The oldest and the youngest stratigraphic units (on theleft and on the right, respectively) have mostly isotopically positive N

18O values, with large internal variations within or betweenshells. These represent a stage when highly varied evaporation condition prevailed at Lake Hadar. By contrast, the middle plotfrom the ‘Gastropod Beds’ shows shells that recorded uniformly negative N

18O values, and represents a fresh unevaporated stageof Lake Hadar, when more rain from an isotopically depleted air mass source and more cloud cover prevailed.

PALAEO 2906 29-8-02


Table 3The isotopic compositions of fossil shell microsamples from the three stratigraphic lacustrine units of the Pliocene Hadar Forma-tion

Age interpolated Stratigraphic position Genus species Sample # Microsample location N18O N

13C(m)

3.22 Ma 54 B. unicolor 13A1-1 apex 32.02 32.3813A1-2 penultimate 31.02 31.0613A1-3 penultimate 30.13 32.6913A1-4 outer lip 0.81 30.92

Mean 30.59 31.76S.D. 1.21 0.90

54 M. tuberculata 13A2-5 apex 30.58 32.4313A2-6 penultimate 3.32 0.4113A2-7 outer lip 6.33 0.90

Mean 3.02 30.37S.D. 3.46 1.80

54 Cleopatra 13A3-8 apex 1.52 0.8913A3-9 body whorl 0.78 31.54

Mean 1.15 30.33S.D. 0.52 1.72

54 unionoid 13A4-10 outer shell 2.13 32.9013A4-11 outer shell 0.15 33.5513A4-12 outer shell 5.92 30.6513A4-13 outer shell 3.60 33.4413A4-14 outer shell 30.26 35.53

Mean 2.31 33.21S.D. 2.55 1.75

54 B. unicolor 13A5-21 penultimate 30.20 32.8613A5-22 near outer lip 1.47 30.8713A5-23 body whorl 1.49 31.43

Mean 0.92 31.72S.D. 0.97 1.03

3.27 Ma 42.5 B. unicolor AT-1-15 apex 36.15 33.98AT-1-16 intermediate 36.17 34.32AT-1-17 intermediate 35.67 34.02AT-1-18 aperture 35.49 34.27

Mean 35.87 34.15S.D. 0.34 0.17

42.5 B. unicolor B25-1-1 apex 36.36 1.57B25-1-2 mid shell 35.83 0.32B25-1-3 aperture 37.69 31.30

Mean 36.63 0.20S.D. 0.96 1.44

42.5 M. tuberculata M25-1-1 apex 36.58 0.75M25-1-2 aperture 37.70 0.71

Mean 37.14 0.73S.D. 0.79 0.03

40 M. tuberculata HS-2-1 apex 38.14 35.78HS-2-2 middle whorl 37.39 34.30HS-2-3 aperture 36.91 33.60

Mean 37.48 34.56S.D. 0.62 1.11

40 B. unicolor HS-5 whole shell 36.68 35.7939.5 M. tuberculata HS-3-1 apex 36.29 34.60

HS-3-2 middle whorl 38.41 32.83HS-3-3 aperture 37.77 31.46

Mean 37.49 32.96

PALAEO 2906 29-8-02


quite negative N18OPDB values of the fossil shells

in the ‘Gastropod Beds’ are of utmost signi¢cancefor interpreting the paleoclimate of Hadar. Theyrecord a relatively rare, least evaporated stage ofLake Hadar, close to, but more positive than theN18O of rain on the Western Plateau source regionduring the Late Pliocene.The shells in two of the three layers in the

‘Gastropod Beds’ show slight textural evidence,albeit small, that 6 1% of their aragonite bio-structure has been texturally altered. Despitethat, the following two reasons make it improb-able that their shells have been isotopically resetby pedogenesis, the most common form of dia-genesis to have a¡ected the formation. First,being at the base of the thickest lacustrine intervalin the Formation, the ‘Gastropod Beds’ were pro-tected from pedogenic in£uence by being 35 me-ters below the next higher paleosol, beneath theDD-3 sandstone (Fig. 2). Secondly, the distinctlydi¡erent N13C of the shells (32.5U 2.4x, n=17;Table 3) compared to that of the soil carbonates(37.3 U 1.1x, n=19; Hailemichael, 2000) rulesout that the two have undergone isotopic ex-change.

5.2. The N18O of Lake Hadar at the time of the

‘Gastropod Beds’; and N18O of Plateau and Afar

rain during the Pliocene

The least evaporated condition of Lake Hadarrecorded by the ‘Gastropod Beds’ was 12x low-er in 18O than that recorded by the most positive

microsample we have observed among the 13Ashells (+6x) that grew later during the samelacustrine interval.It is logical that the lowest N18O values would

have occurred in the very initial stage of the larg-est lacustrine interval in the Formation. Theseshells grew when the lake was expanding rapidlywestward and the rate of input of un-evaporated,low N

18O river water to the lake most exceededthe rate of evaporation.The temperature of lake water is a necessary

input for determining the N18O of the water

from the N18O of the shell, using Dettman’s

(1994) equation. More intense cloud cover andrainfall which caused the more negative N

18O ofrain during the Pliocene would have also loweredthe Pliocene temperatures of the Afar relative totoday’s. If we assume the mean temperature forPliocene Lake Hadar to be about 5‡C less thantoday’s mean air temperature (30‡C), then theaverage N

18Oaragonite (36.7x) value in the ‘Gas-tropod Beds’ would indicate a N

18OSMOW of thelake water to be about 35x ; or if the temper-ature was 30‡C, then the lake water will have a34x value (Fig. 3). Considering the upstreamlocation of Lake Koka with a N

18O value of1.55x, we can take its 3x evaporative enrich-ment relative to the isotopic composition of mod-ern Plateau rain (31.3x) as a minimum whichwould have prevailed for the much further trans-port downstream in the Afar to Lake Hadar dur-ing the Pliocene. For example, we observe a 6xincrease in N

18O for the Awash River’s whole Afar

Table 3 (Continued).

Age interpolated Stratigraphic position Genus species Sample # Microsample location N18O N

13C(m)

S.D. 1.09 1.5739 B. unicolor HS-4 whole shell 34.99 34.96

3.34 Ma 25 bivalve KMT19-2A apex 0.88 34.43KMT19-2B margin 1.76 34.92

Mean 1.32 34.68S.D. 0.62 0.35

3.34 Ma 25 bivalve KMT19-2-1 ventral margin 2.92 36.85KMT19-2-2 mid-shell 2.88 37.21KMT19-2-3 umbonal area 2.16 37.73

Mean 2.65 37.26S.D. 0.43 0.44

Values reported are as measured (aragonite) and are not corrected to calcite values.

PALAEO 2906 29-8-02


route from the Plateau into the central Afar today(Table 1). Thus one may infer that Pliocene rainson the Western Plateau were at least 3x morenegative than the 35x value inferred from the‘Gastropod Beds’ for Lake Hadar during the Plio-cene, i.e., a N

18OSMOW of 38x. This value ismuch less than the 31.3x weighted mean valuefor Plateau rain today.The inferred N

18O (38x) for rain on the West-ern Plateau during the Pliocene compares with aN18O of 35x for Pliocene rain in the Afar itself,calculated with similar assumptions about temper-ature from the N

18O of paleosol carbonates in theHadar Formation by Hailemichael (2000). Thus,in a similar fashion as for Plateau rain, Afar rainin the Pliocene (35x) was much lower than to-day’s value of about +2x as approximated fromour few spot measurements of modern Afar rain.That is, both the Plateau and the Afar experi-enced rain about 6^7x lower during the Pliocenethan today in each region.

6. Origin of the low NN18O rain in the Pliocene

Ethiopia

Only about 1x of the 6^7x lowering of N18Oof Pliocene rain in Ethiopia from today’s valuecan be accounted by the formation of the lowN18O polar icecaps during the Quaternary. Theremaining 5x decrease in the N

18O value of thePliocene rain both in the Plateau and the Afarsettings compared to today’s can be accountedfor by some combination of these three factors:(1) increased proportion of rain derived from airmass moisture sources more depleted by Rayleighdistillation in Pliocene than today’s sources; (2) in-creased amounts of rain per storm, the ‘amounte¡ect’ (Dansgaard, 1953); and (3) reduced evap-oration potential. All three of these are arguedbelow to have occurred and to have synergistical-ly interacted, especially once (1) was broughtabout.To explain the negative isotopic character of

the Pliocene rain that supplied Lake Hadar duringthe time of the ‘Gastropod Beds’, it is helpful tolook at the best near-modern analog. This near-modern analog is in the very center of the Afar,

but not as it is today, rather as it was only 9^6 kaduring the early Holocene pluvial period knownas the African Humid Period (AHP) (deMenocalet al., 2000). Then the analog area matched notonly the sedimentology, but also the climate andthe isotopic meteorology of Pliocene Hadar. Itsexplainable meteorology can in turn be adoptedas an explanation for the meteorology of Hadarin the Pliocene.In the early Holocene, summer heating of the

Northern Hemisphere maximized to values of 8%more insolation than today due to cycles inEarth’s orbital parameters (Overpeck et al.,1996). More intense summer insolation deepenedthe East Saharan atmospheric low which in turnstrengthened the summer African Southwest mon-soon and brought Atlantic-derived moisture muchfurther north than today. The increased rainfallgreened the Sahara (Petit-Maire, 1990) and ¢lledthe lakes of the Nubian Paleo-lake Basin in whatis today’s hyperarid eastern Sahara of northwestSudan (Hoelzmann et al., 2000). These early Ho-locene rainwaters of undisputed Atlantic deriva-tion were clearly ¢ngerprinted (in lacustrine andriverine carbonates and in fossil groundwaters) bya distinctly negative N

18O values (Abell andHoelzmann, 2000; Rodrigues et al., 2000; Thor-weihe et al., 1990), as to be expected from theirfar transport and increased intensity.The great rise in the level of the lakes in the

Ethiopian Rift and the Afar (Gasse and Street,1978) is as equally a de¢ning episode of theAHP as the greening of the Sahara, but becauseof Ethiopia’s complex meteorology, its meteoro-logical causes have only been addressed peripher-ally. Our meteorological explanation for the in-creased early Holocene rainfall of both thePlateaus and Afar in Ethiopia is a simple exten-sion of what happened in the eastern Sahara. Justas a deepened Saharan Low of the early Holocenepulled moist Atlantic-derived air masses north-eastward toward the eastern Sahara and shiftedthe Sahelian rain belts northward (Ritchie andHaynes, 1987), it is logical that the even deeperTibetan Low (10 millibars lower today), respon-sible for strengthening the Indian Southwest mon-soon (Overpeck et al., 1996), pulled these samemoist, isotopically depleted, Atlantic-derived air

PALAEO 2906 29-8-02


masses east-northeastward over Ethiopia. As de-tailed below, the 18O-depleted early Holocene car-bonates in the Afar of various origins suggest theAtlantic-derived component of air masses £owedas far east as the Afar in higher proportions thantoday.

7. An Early Holocene environmental and isotopicanalog of Hadar

A depositional analog of the Hadar Formationis today’s low £at distal £oodplain and delta plainof the Awash River at its terminus with LakeGamari, the ¢rst of the series of four lakes inthe central Afar (Aronson and Taieb, 1981). Butthe hot and dry setting, the low ecological pro-ductivity and diversity make it an otherwise inap-propriate analog.Gasse (1977), Gasse et al. (1974), and Gasse

and Street (1978) have documented that duringthe early Holocene about 9^6 ka, the level ofthe central Afar lake system from Lake Gamarito Lake Abbe rose a remarkable 150 m higher inelevation than today’s surface of Abbe. All fourlakes coalesced into one great lake that expandedacross the Asaita Plain to an area 13-fold that oftoday (Gasse and Street, 1978) and several timesthe area of Hadar. Also the level of the Zway^Shala lakes in the MER rose 80 m, coalesced toover£ow the divide northward into the Awash(Gasse and Street, 1978), and further augmentthe discharge of Plateau rainfall into the Afar.Not only was the central Afar region an excel-

lent depositional and climatic analog for Hadarduring the AHP, but evidence also suggests thatthe increased rainfall on the Plateau and in theAfar during the AHP had an isotopically negativecharacter similar to that evidenced by the ‘Gastro-pod Beds’ for the Pliocene. This evidence comesfrom two previous isotopic studies on early Ho-locene mollusk shells from the Afar lake systems.Gasse et al. (1974) measured N

18O of 11 14C-datedshells from various stages of the expanded AHPAbbe lake system. Their N

18OPDB values average32.1x (n=11) with one sample as low as35.0x. Because evaporative concentration of18O is so prevalent in East African lakes, it isunlikely that any particular shell sample would

catch a paleo-lake at its freshest, least evaporativeand lowest N18O stage. At 25‡C the lowest obser-vation of 35.0x corresponds to a N

18OSMOW

value of about 33x for the lake water, far lowerthan present-day Lake Gamari’s +16x. The neg-ative isotopic compositions show that the in-creased AHP rainfall on the Ethiopian Plateauswhich fostered the lake expansions in the MERand Afar (Gasse and Street, 1978) was isotopi-cally more negative than today. Probably theevaporative potential of the Afar was less, as well.Abell and Williams (1989) also measured shells

with negative N18O from an early Holocene sedi-

ments at Lake Beseka in the southern Afar closeto the Western Escarpment and from small springdeposits at the base of the Southern Escarpmentof the Afar with the Eastern Plateau, both directlyfed by Plateau run-o¡. At about 25‡C, the result-ing N

18OSMOW of the water when these individualmollusks lived would have been 34 to 31x,much lower than the +6.7x of today’s Lake Be-seka (Table 1). Again, the enhanced AHP rainfallon the plateaus whose run-o¡ fed these two smalllakes was undoubtedly isotopically more negativethan today.Evidence presented elsewhere indicates even the

AHP rain in the Afar itself had a much morenegative N

18O than today’s approximate +2xvalue. This evidence comes from modern Afarsoils near Hadar whose soil calcite nodules give14C dates that indicate the nodules formed duringthe AHP (Hailemichael, 2000). These soil carbon-ates have a mean N

18OPDB value of 36.5x. At25‡C these value would translate to soil watersthat was about 34x (i.e., about 5^6x moredepleted in 18O than today). If plateau rain duringthe AHP was depleted in 18O by a comparableamount, going from a present-day value of about31.3x to about 36 or 37x in the early Ho-locene. This is close to the 38x that we inde-pendently deduced for Plateau rain during thePliocene from the ‘Gastropod Beds’ data.

8. Meteorological hypothesis of the causes for thelow NN

18O rain in the Pliocene

The intensi¢ed summer Indian Sub-continent

PALAEO 2906 29-8-02


Low of the early Holocene (Overpeck et al., 1996)can be hypothesized to have pulled both compo-nents of the Ethiopian monsoon eastward. Duringtoday’s summer, the Indian Ocean- and Atlantic-derived air masses converge in the northeasttrending IOC front, whose location (Fig. 1) isonly very approximately known to hover insummer over the western part of the Western Pla-teau (Nicholson, 1996). It seems logical that bypulling stronger on both components of theEthiopian monsoon that the intensi¢ed Indianlow would have shifted their IOC front eastward.We speculate a shift of perhaps a few hundredkilometers to bring the IOC over the westernAfar. The moist, far-traveled, Rayleigh-depletedAtlantic air masses would then have reached thehigh eastern shoulder of the Western Plateau, theescarpment with the Afar and the western Afaritself. This shift of the IOC would have resultedin dramatically increased amounts of rain bothalong the eastern shoulder of the Plateau thatdrained to the ancestral Awash River, and alsoon the western Afar itself. The con£uence of theAtlantic and Indian Ocean air masses in the Afarwould have forced the relatively less stable, morehumid, Atlantic air masses (Nicholson, 1996) tohave risen over the drier Indian Ocean air massesin storms self-perpetuated by the release of latentheat of precipitation. The increased proportionsof the negative N

18O ¢ngerprinted Atlantic com-ponent of rain falling in more intense storms onthe Plateau headwaters of the ancestral AwashRiver explains the low N

18O of the ‘GastropodBeds’. Further, the increased summer cloud cover,rainfall, and vegetation cover in the Afar itselfthat would have accompanied the eastward shiftof the IOC that would have reduced the Afarmean temperature from today’s high mean valueof about 30‡C, down perhaps to about 25‡C andreduced the potent ability of evaporation to in-crease the N

18O of Lake Hadar.Thus, as brie£y suggested by Hillaire-Marcel et

al. in their 1982 paper, one only has to go back afew thousand years to the AHP in the centralAfar to ¢nd an excellent depositional analog,and also an excellent climatic and isotopic analogfor the Hadar Formation. Although indeed it ispossible that the tectonic relief created by down-

dropping the Afar may have been less accentuated3 Ma, there is no need to invoke such accentuatedrelief to explain the aridi¢cation that has a¡ectedHadar since the Pliocene. Rather, we propose thePliocene climate at Hadar was very similar to thepluvial climate in the Afar during the early Holo-cene only 6^9 ka, when the tectonic situation wasno di¡erent than today’s.The abundant and diverse terrestrial vertebrate

fauna in the main part of the Hadar Formationbeneath the disconformity argues that an en-hanced monsoon was a persistent feature of thePliocene. However, one can speculate that the ¢vetransgressions of Lake Hadar may have beencaused by the cyclic peaking of the Earth’s orbitalfactors superimposed upon a persistently strongEthiopian Monsoon of the Pliocene. They alsomay have been tectonically induced.Despite the persistent appearance of wetter

summer at paleo-Hadar compared to today, thedry season must have been pronounced as shownby the cyclic variation to quite positive N

18O val-ues within the 13A shells, and by the accumula-tion of carbonate nodules in the formation’smany paleosols. Nevertheless, even in the dry sea-son the environment at Hadar would have been avery habitable refuge because of the year-roundpresence of river, lake, shore and wetland envi-ronments to store the ample summer water.

9. Other possible causes of the low NN18O in

Pliocene rain

There may have been other causes of the iso-topically depleted rain of the enhanced Ethiopianmonsoon during the Pliocene and the AHP thatacted instead of, or in concert with the increasedAtlantic-derived component hypothesized here.These include: (1) Hadar having possibly beenat a higher elevation (Bonne¢lle et al., 1989);and/or (2) a strengthened Indian Ocean-derivedcomponent to the Ethiopian monsoon. For exam-ple, Indian Ocean air mass sources may have beenpumped harder over Ethiopia toward an intensi-¢ed, relatively closer Saharan Low before beingdiverted east toward the even stronger atmospher-ic Tibetan Low. Such Indian Ocean sources of

PALAEO 2906 29-8-02


rainfall could have experienced lower N18O by the‘amount e¡ect’ and by Rayleigh distillation asthese air masses mounted the Western Escarp-ment with the Afar. This e¡ect would have beenreinforced in the Pliocene if the Hadar structuralblock was an elevated part of the Plateau that hadnot yet dropped into the Afar.As regards Pliocene Afar, the recent tectonic

analysis at the other end of the Indian Ocean byCane and Molnar (2001) suggests that higher seasurface temperatures (SST) may have existed forthe Indian Ocean then. They propose that warmsouth Paci¢c waters used to come into the IndianOcean and were closed o¡ about 4^3 Ma by thenorthward tectonic movement of New Guineainto the Indonesian seaway. Such Indian Oceanwarming, should it have occurred, may havebeen superimposed upon the already globallywarmed oceans of the pre-glacial world so as tofeed more Indian Ocean-derived, isotopically de-pleted, storms into the Afar and the plateaus. Thewarmer SSTs of either one or both of the Atlan-tic/Indian oceans would account for the long per-sistence of the wetter summer climate in Ethiopiaduring the Pliocene, compared to its brief episodicoccurrences in the Quaternary.

10. Conclusions

The late Pliocene Hadar Formation accumu-lated in the western Afar mostly as the distal£ood and delta plain sediments of the ancestralAwash River. Lake Hadar transgressed westwardacross the site ¢ve times. On the basis of the iso-topic results of the lacustrine shell zones of theSidi Hakoma Member in a context of the isotopichydrology of modern Ethiopia, we conclude thefollowing.(A) Evaporation strongly enriches the 18O con-

tent of the 11 modern lakes, except for thethrough-£owing Lake Koka.(B) The beach ‘Gastropod Beds’ laid down at

the start of the largest lacustrine interval captureda record of the least evaporated stage of LakeHadar equivalent to a N

18OSMOW value of35x. This water was derived from Pliocene Pla-teau rain of at least 38x, much lower than to-

day’s average (31.3x). This lower N18O is com-parable to that for rain in the Afar inferred fromisotopic studies of Hadar Formation paleosol car-bonates to be presented elsewhere.(C) Near the end of this lacustrine interval

when the site became a partially isolated shallowbay of the lake, shells have much more positiveN18O value with dramatic cyclic internal variationsto values as high as +6x. Evaporation in thePliocene dry season must have been pronouncedlike today. But the mosaic ecotone nature of thePliocene depositional setting included many wetsub-environments for annually storing the muchlarger summer supply of fresh water through thedry season. This explains the abundant, diversefossil vertebrates in the formation and the longstability and success there of the hominid Austra-lopithecus afarensis.(D) The best depositional, meteorological and

environmental analog to the Pliocene Hadar is theLake Gamari Plain in the central Afar itself, as itwas just 9^6 ka, during the AHP. The early Ho-locene expansions of the Ethiopian Rift and Afarlakes have been recognized as a de¢ning episodeof the AHP comparable to the ‘greening’ of theSahara. We argue that the cause of the Ethiopianpart of the AHP was due to the strengthened Ti-betan Low that pulled the moist isotopically de-pleted Atlantic-derived air mass component of thesummer Ethiopian Monsoon as far east as theAfar. By analogy the negative N

18O rainfall ofthe Plateau and Afar throughout the Pliocenecould have originated similarly. However, thehigh paleoecological productivity throughout the¢rst half million years of the Hadar Formationmeans that the Ethiopian Monsoon was persis-tently strong as opposed to periodically so inthe Quaternary.

Acknowledgements

We thank the Ethiopian Geological Survey forpermits and logistic support in collecting the mod-ern waters and mollusks. We are grateful to theDirector, Ketema Tadesse, for logistic supportand encouragement and for his dedication to im-proving knowledge of Ethiopia’s resources. For

PALAEO 2906 29-8-02


support at Hadar, we thank all our colleagues atthe Institute of Human Origins (IHO), TesfayeYemane, Carl Vondra, William Kimbel, Don Jo-hanson, Gerry Eck, and Kay Reid. We are espe-cially indebted to Robert Walter for his generoussharing of his knowledge of the Hadar Forma-tion. Linda Abel gave expert help in the isotopelab at CWRU. Tenesa Mamecha, Mes¢n Dubaleand Zerihun Tsegaye helped collect mollusks fromLake Tana, Lake Hayk and Lake Awasa. Finan-cial support for this study was provided by anexploratory grant to J.L.A. at CWRU from theNational Science Foundation (Anthropology).M.H. was generously supported as a graduate as-sistant by Geological Sciences at CWRU. Fortheir role in writing up this paper, J.L.A. andM.H. were supported by Dartmouth’s Earth Sci-ences Department. Taking full responsibility forthe interpretations presented, we are grateful forthorough constructive reviews by FrancXoiseGasse.

References

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