Palaeogeography, Palaeoclimatology, Palaeoecology 186 (2002) 81^99
www.elsevier.com/locate/palaeo
N18O in mollusk shells from Pliocene Lake Hadar and modern
Ethiopian lakes: implications for history of
the Ethiopian monsoon
Million Hailemichael a; , James L. Aronson b , Samuel Savin a ,
Michael J.S. Tevesz c , Joseph G. Carter d
a
c
Department of Geological Sciences, Case Western Reserve University, Cleveland, OH 44106, USA
b
Department of Earth Sciences, Dartmouth College, Hanover, NH 03755, USA
Department of Biological, Geological and Environmental Sciences, Cleveland State University, Cleveland, OH 44115, USA
d
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 Sidi
Hakoma Member deposited 3.4^3.2 Ma. In a perspective of the N18 O of 11 modern Ethiopian lakes and their shells,
the N18 O of the Hadar fossil shells provide a snapshot of the nature of ancient Lake Hadar and Ethiopia’s climate in
the Pliocene. Ethiopia’s modern lakes both in the Rift and on the Western Plateau are fed by drainage of Plateau rain
with its well established barely negative N18 OSMOW of 31.3x. Except for the man-made Lake Koka reservoir, all
other Ethiopian lakes are isotopically quite positive ranging from +5.4 to +16.0x, indicating how significant
evaporation is in their water budget. Shells from lakes with extant mollusk populations are mostly in isotopic
equilibrium with the N18 O and temperature of their lake water. The upper transgressive interval in the Sidi Hakoma
Member is the largest one in the Formation beginning at its base with the ‘Gastropod Beds’ beach deposits. Mollusks
from shell beds other than the ‘Gastropod Beds’ show more positive and more variable N18 O between shells, with
internal variations within shells as much as 7x. At these times the site must have been underlain by a shallow
partially isolated embayment of Lake Hadar which underwent rapid expansions and then contractions by
evaporation, within the few year lifetimes of the individual mollusks. The results from the ‘Gastropod Beds’ are of
most significance for interpreting the overall paleoclimate at Hadar. Their uniformly negative N18 OPDB shell values
that average 36.7x represent a much less evaporated stage of Lake Hadar when its N18 OSMOW was 8x lower than
the spectrum of modern lakes in Ethiopia, and indeed even 3x or more lower than average modern Plateau rain. To
explain such negative values we hypothesize that the Atlantic-derived air mass component to the Ethiopian monsoon
was persistently strengthened during Pliocene summers, which intensified the amount and the negative isotopic
character of rainfall onto both the Afar and the Ethiopian Plateaus that drained to Lake Hadar. A similar
phenomenon characterized the brief periodic pluvial episodes of the Quaternary, including the latest in the early
Holocene, known as the African Humid Period. In contrast to the hot semi-desert steppe conditions of today’s
* 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).
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
PALAEO 2906 29-8-02
82
M. Hailemichael et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 186 (2002) 81^99
western Afar, the diverse abundant terrestrial fossil fauna at Hadar, including the early hominid Australopithecus
afarensis, is explained by the wetter, and probably cooler, summers that persisted throughout the Late
Pliocene. 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 East
African Rift System are rich in vertebrate fossils.
Among these, the sites of Hadar (Fig. 1) and the
Middle Awash in the western Afar of Ethiopia are
notable for the remarkable amount and quality of
their hominid fossils. The 180-m-thick Hadar Formation has produced over 90% of the known fossils of the early hominid Australopithecus afarensis, including the partial skeleton, ‘Lucy’. The
formation accumulated in the late Pliocene 3.4^
2.3 Ma mostly as the £ood plain and channel
deposits of a major meandering river that was
ancestral to the modern Awash River. Relative
down-dropping of the central Afar since the Pliocene has caused the present Awash River and its
tributaries to have cut down through and exposed
its ancient deposits at the site of Hadar (Aronson
and Taieb, 1981). During the Pliocene, Hadar was
the distal and delta plain reach of the river near
its entrance to a major lake we refer to as Lake
Hadar. Lake Hadar expanded and transgressed
over Hadar laying down intervals of laminated
lacustrine muds and beach sands with scattered
to abundant shells of mollusks (Fig. 2). The last
transgression of the lake is well dated at 2.95 Ma
and was followed by the sculpting of a major
disconformity (Fig. 2) (Aronson et al., 1996). Renewed deposition of the uppermost 15% of the
formation includes no record of the existence of
the lake, and instead conglomerates appear that
were deposited by steep transversely £owing
braided rivers from the Western Plateau. Hadar
is only 40 km east of the present-day 2-km-high
escarpment, along whose faults the Afar has been
dropped.
In contrast to the hot, dry and mostly sterile
setting of today’s Hadar in the western Afar, the
very high diversity and abundance of the terrestrial vertebrate fossil fauna of the Hadar Forma-
tion together with much fewer conglomerates
beneath the disconformity have led to the hypothesis that the present-day Western Escarpment
may have only been at a nascent stage in the
late Pliocene, 3 Ma. By this thinking westernmost Afar, including Hadar, may have been a
marginal tectonic block of the plateau that only
since the Pliocene has descended along the present
escarpment fault to become part of the presently
hot and arid Afar block (Aronson and Taieb,
1981; Bonne¢lle et al., 1987; Aronson et al.,
1996). Alternatively, the climate in the Afar may
have been wetter than today. Our study addresses
this latter possibility via isotopic study of fossil
shells of mollusks that lived in Lake Hadar about
3.2 Ma.
In their extensive, but brie£y documented isotopic study of the Hadar Formation, HillaireMarcel et al. (1982) included measurements of
shells from most or all of the ¢ve lacustrine intervals, along with carbonate nodules of unspeci¢ed
origins. They reported that the shells they analyzed were aragonite and unaltered. Our experience mainly in the central sector of the site shows
that diagenesis has extensively a¡ected shells in
the beds of the lowermost and the upper three
lacustrine intervals by recrystallizing the shells
and ¢lling them with spar calcite. This contrasts
with the unaltered conditions of the shells in the
two lacustrine intervals of the Sidi Hakoma member on which we report here.
To better interpret the shell isotopic data in
terms of the lake environments represented, we
also examined the isotopic relationships of modern shells and waters in some Ethiopian lakes on
the Plateau and in the Rift. Many of the fossil
and modern shells were analyzed serially via microsamples along the growth direction of the
shells to determine how environmental conditions
changed seasonally during an individual mollusk’s
life.
PALAEO 2906 29-8-02
M. Hailemichael et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 186 (2002) 81^99
83
Fig. 1. Location of the 11 modern Ethiopian lakes studied, and the Afar site of Hadar. Numbers 1^11 indicate lakes studied in
the rift and plateau: (1) Lake Gamari; (2) Lake Hayk; (3) Lake Tana; (4) Lake Hora; (5) Lake Metehara (Beseka); (6) Lake
Koka; (7) Lake Zway; (8) Lake Langano; (9) Lake Abiata; (10) Lake Shala; (11) Lake Awasa. The great early Holocene African Humid Period (AHP) expansions of the four lakes of the Zway^Shala Basin in the Main Ethiopian Rift and the four lakes
in the Gamari^Abbe series in the Central Afar are indicated. The approximate present-day location of the Inter-Oceanic Con£uence (IOC), summer front between Atlantic and Indian Ocean derived air masses is shown (after Rozanski et al., 1996). This
front is proposed to have shifted several 100 km eastward over the western Afar, persistently during the Pliocene and periodically
during the Quaternary.
2. Samples and methods
Paired samples of lake water and modern mollusk shells from 11 lakes and from the Awash
River at Kereyu National Park were analyzed.
The N18 O of water samples from the 11 lakes
and other relevant physical and chemical information are presented in Table 1. All of the water
samples were collected in glass bottles by wading
out from the lakeshores during the rainy months
PALAEO 2906 29-8-02
84
M. Hailemichael et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 186 (2002) 81^99
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. Just
after the 2.95-Myr-old BKT-2 tu¡ was deposited, a major disconformity formed at Hadar, after which there is no evidence of
the existence of Lake Hadar. An additional lacustrine interval exists at the top of the Basal Member (BM).
PALAEO 2906 29-8-02
M. Hailemichael et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 186 (2002) 81^99
of March and June, 1997. The bottles were plastic
taped, para⁄n sealed and refrigerated until analysis.
The mollusks from lakes Awasa, Zway, Tana
and the Awash River were collected alive. Except
for two Lymnaea (Radix) peregra gastropods from
Lake Zway, all modern and most fossil shells analyzed belong to one of the gastropod species
Melanoides tuberculata, Bellamya unicolor, or the
bivalve order Unionoida, which are the most
commonly used mollusk shells in African lake isotope studies for which no ‘vital e¡ect’ inter-species
isotopic fractionation has been noted. Microsamples were obtained by shallow scratching with a
drill along the external growth band.
For the Pliocene, the N18 O values of 37 microsamples from 14 fossil shells from the Hadar Formation are presented in Table 3. The taxa analyzed are the gastropod Melanoides tuberculata,
Bellamya unicolor, Cleopatra bulimoides, and the
unionoid and Corbicula bivalve species.
Because mollusk shell aragonite is metastable
and readily alters to calcite, the absence of calcite
indicates the shell’s isotopic composition is likely
to be pristine. Powdered samples of all shells
underwent X-ray di¡raction, calibrated for calcite
sensitivity using mixtures of aragonite and calcite
down to 1% calcite where the calcite 104(hkl)
peak is still detectable. The absence of this peak
in all our shells indicates they are essentially unaltered. The least fresh mollusk shells in the study
are fossils from the two of three associated shell
coquina marker limestone beds known as the
‘Gastropod Beds’. In those two beds the shells
are always naturally bleached white, in contrast
to shells from the intermediate bed, from which
our HS samples come, which have a thin sur¢cial
pinkish brown layer. Because the isotope results
of the ‘Gastropod Beds’ shells turn out to be important for interpreting the isotopic nature of paleo-Lake Hadar, we assessed them further for alteration. Broken surfaces were scrutinized under
the scanning electron microscope to see if any
secondary recrystallization had altered the physical character of the aragonite biostructure. These
observations, photographed in Hailemichael
(2000), reveal minor areas ( 6 1% of the total
area scanned) where textural replacement has oc-
85
curred in the naturally bleached shells. No reorganization was observed at all in the unbleached
shells of the intermediate HS layer of the ‘Gastropod 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 alteration
of these least fresh samples is within the limits of
acceptability for isotopic evidence.
All shells were pretreated with sodium hypochlorite, ground and vacuum roasted at 200‡C
for 1 h and digested in H3 PO4 at 25‡C according to McCrea (1950). The 13 C/12 C and 18 O/16 O
ratios of the evolved CO2 were related to the
PDB standard through repeated analyses of the
Solenhofen limestone NBS Isotopic Standard 20
with Craig’s (1957) assumed N18 O = 34.14 and
N13 C = 31.06x.
The N18 O values of water samples were measured 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 grown
in equilibrium with existing conditions, we used
Grossman and Ku’s (1986) isotopic equilibrium
equation modi¢ed by Dettman (1994) to relate
the measured N18 Oaragonite values to the values
of N18 Owater and temperature where the
aragonite could have precipitated. The equation
used is: 1000 ln Karagonite3water = (166.623T‡C)/
4.784, where Karagonite3water = (N18 Oaragonite +1000)/
(N18 Owater +1000).
3. Isotope meteorology of modern-day Ethiopia
The complex meteorology of Ethiopia, and
East Africa in general, is poorly understood because it is related to the seasonal passage of air
mass convergence zones across a varied plateau^
rift topography spanning up to 4 km in relief
(Gri⁄ths, 1972; Nicholson, 1996). Rainfall on
the Western Plateau and most of Ethiopia is monsoonal, with about 75% falling in the main
summer rainy season (the Ethiopian monsoon)
when the Inter-Tropical Convergence Zone
(ITCZ) is north of Ethiopia and the Inter Ocean
Con£uence (IOC) is over the Western Plateau
PALAEO 2906 29-8-02
86
M. Hailemichael et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 186 (2002) 81^99
(Fig. 1). Today these Atlantic- and Indian Oceanderived air masses that are drawn to these convergence zones descend adiabatically 2 or more
km of elevation into the Afar where they become
hot and dry. The other 25% of Plateau rainfall
occurs in the springtime rains when the unstable
ITCZ is passing overhead. The mean annual temperature of the Western Plateau is about 15‡C
compared to about 28^30‡C in western and central Afar (unpublished data from National Meteorological Services Agency of Ethiopia, 1996;
Gri⁄ths, 1972). The potential for evaporation is
very high, reaching to s 300 cm in the western
Afar (Taieb, 1974).
Today’s rain on the Western Plateau is isotopically variable, but well characterized by the 35year isotopic record of rainfall kept by the International Atomic Energy Agency (IAEA) for the
Addis Ababa station (IAEA, 1996). The weighted
mean N18 OSMOW of 31.3x at Addis is con¢rmed
for other areas of the eastern margin of the Western Plateau by our isotopic measurements of individual rains and springs sampled during the
spring and summer rains of 1997 (Hailemichael,
2000), and by earlier measurements by Schoell
and Faber (1976) on spring and well waters in
the same region.
Though not well characterized, rainfall in the
rift system is isotopically more positive. The
IAEA set of six measurements from Awasa and
Zway in the Main Ethiopian Rift over the
summer rainy season of 1995 averaged 30.9 and
31.9x respectively (IAEA, 1996). In addition to
two rain samples collected in 1997 (April and
June) from Awasa (5.50 and 32.88x), we measured ¢ve samples from Dilla and Shashemene
(June) in the Main Ethiopian Rift (MER). The
average N18 OSMOW of rain from the total of these
13 MER rains is 30.4 U 2.3x.
Unfortunately the only isotopic data for the
Afar are our own spot measurements of intense
night-time rains we experienced in the western
Afar during the summer of 1997 at Hadar
(+1.98x) and Aditu (30.46x) and in the central Afar for two spring rains at Tendaho (+3.47
and +2.19x) and for a single summer night
shower at Asaita in the central Afar with a
N18 OSMOW of +8.66x. The more positive charac-
ter of the rift system rain compared to Plateau
rain is readily explained by: (1) the lower amount
of rain in a given storm, the inverse of ‘the
amount e¡ect’ (Craig, 1965; Dansgaard, 1953);
and (2) more evaporation of the rain while falling
through the hotter and drier air.
Between the Atlantic-derived and the Indian
Ocean-derived air mass sources of rain for the
Ethiopian Summer monsoon, the generally held
notion has been that the Atlantic’s Gulf of Guinea is the major source, particularly for the Western Plateau, with the moist Atlantic-derived air
masses being drawn all the way across Africa by
the Indian Sub-continental Low trough in atmospheric pressure (see discussions in Rozanski et
al., 1993; Telford and Lamb, 1999; Lamb et al.,
2000). But isotopic studies of modern rainfall produce a dilemma for this notion (Joseph et al.,
1992). For such a long path over which much
rain-out clearly occurs, Rayleigh distillation
should deplete considerable amounts of 18 O
from Ethiopian summer rain. Yet the well characterized 35-year weighted annual mean N18 O of
rain at the IAEA station in Addis Ababa is a
barely negative value averaging 31.3x. Joseph
et al. (1992) explained this anomaly by proposing
that it is Indian Ocean-derived air masses that are
the main moisture source for today’s Ethiopian
Summer monsoon, not the Atlantic ones. Recent
dynamic observations of satellite cloud patterns
support this and suggest a high proportion of
Ethiopian summer rain, perhaps in some years
greater than 50%, comes from air mass sources
from the direction of the Indian Ocean (personal
communication, 1999, Tesfaye Gissela, Ethiopian
Meteorology O⁄ce).
4. Results
4.1. Isotopic compositions of modern Ethiopian
lake and river waters
The N18 O values of water samples from
through-£owing Lake Tana on the Western
Plateau (N18 OSMOW = +5.63x in March and
+5.79x in April 1997) are higher than those of
rain water samples at Bahir Dar (+5.08x in the
PALAEO 2906 29-8-02
M. Hailemichael et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 186 (2002) 81^99
small rains of March and 33.31x in the large
summer rains of July, 1997, Hailemichael, 2000)
and indicate evaporative enrichment of 18 O has
occurred in Lake Tana.
The N18 OSMOW value of a water sample collected from Lake Hayk on the eastern margin of
the Western Plateau in July, 1997 was +9.34x
(Table 1), close to the value (+8.68x) reported
for the same lake by Schoell and Faber, 1976.
These values are very much higher than springtime and summer rain from Dese (+1.96 to
34.12x), about 50 km south, and spring water
(32.55x) sampled at Tita, between Dese and
Hayk (Hailemichael, 2000). This high N18 O value of the closed basin lake water is consistent
with intense evaporation during the prominent
Ethiopian dry season. Comparably high values
(N18 OSMOW = +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
87
study. Lake Awasa and Lake Zway are relatively
dilute while Lake Abiata is 100 times more concentrated. The N18 O values of lakes in the MER
range between +5.44 (Lake Zway) and +7.98x
(Lake Abiata) and approximately correlate with
major anion concentrations and with conductivity
(Table 1). For example, Lake Zway has low conductivity (94 WS) and Cl3 content (11 ppm) and a
N18 O value of +5.44x, whereas Lake Abiata,
with a conductivity of 20 500 WS and a Cl3 content of 2840 ppm, has a N18 O value of +7.98x.
Lakes Langano and Shala have intermediate
N18 OSMOW values of +6.84x and +7.66x respectively. Our N18 O value (+7.06x) of the large
caldera Lake Awasa in July 1997 matches Leng et
al.’s (1999) December 1995 values between +7.3
and +7.4x (n = 12).
A divide separates the Zway^Shala basin on the
north from the Awash River, where it is dammed
to form the large Lake Koka hydroelectric reservoir. North of Lake Koka, in the northern sector
Table 1
Physical, chemical and isotopic character of modern Ethiopian lakes and of the Awash River examined in this study
Lake
Plateau lakes
Hayk
Tana (sample 1)
Tana (sample 2)
Hayk
Ashenge
Rift Valley lakes
Awasa
Shala
Langano
Abiata
Zway
Koka
Hora
Afar lakes
Metehara
Gamari (sample 1)
Gamari (sample 2)
Awash River
before entering Koka
at Kereyu Park
at Mile
at Asita
a
b
Elevationa
Deptha
Water
temperature
(‡C)
F3
Cl3
SO23
4
Conductivity N18 O
(m)
(m)
(ppm)
(ppm)
(ppm)
(WS)
(SMOW)
2030
1785
1785
2150
2300
23
9
9
23
25
9.0
na
na
na
na
26
na
na
na
na
0.9
0.2
na
na
na
42
3
na
na
na
0
2
na
na
na
210
685
na
na
na
9.34
5.63
5.79
8.68b
8.03b
Jun 97
Mar 97
Apr 97
1977
1977
1675
1540
1580
1580
1637
1590
1770
10
250
46
14
4
9
85
8.4
9.4
8.8
9.6
8.1
8.3
8.7
24
26
25
28
25
27
24
8
213
18
211
15
3
0
27
3080
160
2844
11
25
233
0
119
13
194
2
10
10
184
770
422
20500
95
184
370
7.06
7.66
6.84
7.98
5.44
1.55
7.41
Jul
Jul
Jul
Jul
Jul
Jul
Jul
750
320
320
na
na
na
na
9.2
na
na
32
29
6.9
5.0
na
114
369
na
480
50
na
3794
685
na
6.74
15.89
16.14
Jul 97
Jun 97
Jul 97
1590
1000
410
320
na
na
na
na
na
na
na
na
na
na
na
na
na
na
na
na
na
na
na
na
na
na
na
na
na
na
na
na
32.41
0.48
4.29
4.19
Jul 97
Jul 97
Jul-97
Jun 97
pH
Ethiopian Mapping Authority, 1988.
Data from Schoell and Faber, 1976.
PALAEO 2906 29-8-02
Sampling
date
97
97
97
97
97
97
97
88
M. Hailemichael et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 186 (2002) 81^99
of the MER, are the small closed basin lakes
Hora and Metehara (Beseka) fed by drainage
from the Western Plateau and with N18 O values
of +7.4x and +6.74x. Our 18 O-enriched Ethiopian lake waters match values for lakes in Kenya
ranging from +3.6 to +9.2x, except the highly
variable Lakes Elmenteita and Magadi (Cerling et
al., 1988).
The most arid area we were able to visit during
this study was Lake Gamari, at 320 m above sea
level and the ¢rst of four lakes located in series in
the center of the Afar Depression (Fig. 1). We
could not reach the other lakes in the series and
only gained access to the large partially isolated
shallow bay at the northern end of Lake Gamari.
The N18 OSMOW values and temperatures of the
water samples collected on June 23 and again
on July 3 1997 were +16.01x (water T = 32‡C)
and +15.87x (water T = 29‡C), the highest values so far reported from any East African lake.
Despite its much higher N18 O, the conductivity of
Gamari water in June was 685 WS, lower than
Lakes Abiata and Shala, but higher than the
rest of the lakes in the MER.
Because of its large headwater region and
through-£owing condition, Lake Koka has the
lowest N18 O (+1.55x, July 1997) value of all
Ethiopian lakes studied. It has an 18 O enrichment
of about 3x above that of the weighted mean
Plateau rainwater (31.3x) which falls in the
Awash’s headwater region. A sample of the
Awash River taken the same day just upstream
of Lake Koka has a N18 O of 32.4x (Table 1).
Following the Awash River through the Afar in
the early 1997 summer, we measured a strong
progressive increase in the 18 O from +0.48x below the Awash Falls near the southern juncture of
the Afar with the MER to +4.29x at Mile
Farms, about 40 km downstream of Hadar; and
+4.19x at Asaita, 150 km further downstream
near the river’s terminus with lake Gamari in the
central Afar.
Fig. 3. Measured N18 OPDB values of modern shells plotted
against the N18 OSMOW from the Ethiopian lakes in which
they grew. These modern shell values are contrasted with results of fossil shells from the ‘Gastropod Beds’ of the Hadar
Formation. For the modern shells the derived temperatures
on the equilibrium fractionation curves shown are broadly
reasonable. In detail, temperatures are projected to be too
warm for Lake Tana, this exception is discussed in the text.
Clearly the waters of Pliocene Lake Hadar at the time the
shells of the ‘Gastropod Beds’ grew were distinctly more depleted in 18 O than for any present-day lakes in Ethiopia.
4.2. Oxygen isotopic composition of modern
mollusk shells
shells of live snails (Bellamya unicolor) and unionoid bivalves are somewhat out of equilibrium for
precipitation of aragonite from lake water of the
measured N18 OSMOW (+5.7x) at a mean monthly
air temperature of 20‡C at Bahir Dar (unpublished data from National Meteorological Services
Agency of Ethiopia, 1996). As shown in Fig. 3, a
higher temperature of about 30‡C is calculated for
the precipitation of these shells from the +5.7x
water. Among possible explanations, these mollusks may have resided near a spring outlet of
water less enriched in 18 O.
Abundant fresh, gray, unbleached shells of
Melanoides on the sediment surface of Lake
Hayk showed uniformly high N18 O values between
+7.58 and +8.04x (Fig. 3) which are in isotopic
equilibrium with the measured N18 OSMOW of the
lake water (+9.34x) and temperature (26‡C).
4.2.1. Western Plateau Lakes
At Lake Tana, the N18 O (+3.3 to +4.2x) of
4.2.2. Main Ethiopian Rift Valley lakes
The N18 OPDB values of whole shells of live Me-
PALAEO 2906 29-8-02
M. Hailemichael et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 186 (2002) 81^99
lanoides collected from Lake Awasa in July, 1997
range from +5.75 to +6.28x, while two microsamples of another Melanoides shell are +6.24
and +6.36x (Table 2). These N18 O values are
in equilibrium with the measured water temperature (24‡C) and N18 O (+7.06x) (Fig. 3). These
data are also consistent with the mean N18 OPDB
89
value of +5.9x reported for modern Melanoides
from 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 N18 OPDB values of +5.08 and +5.49x, in
equilibrium with the water N18 O (5.44x) and
the mean air temperature (21‡C). We found no
Table 2
The N18 O and N13 C values of modern Ethiopian mollusk shells
Sampling locality
Genus species
Whole shell
Sample #
Western Plateau
Lake Tana
B.
B.
B.
B.
unicolor
unicolor
unicolor
unicolor
LTS2-1-W
LTS2-2-W
LTS2-3-W
LTS2-4-W
Mean
S.D.
Microsamples
18
N O
3.40
3.34
3.38
3.41
3.38
0.03
13
Rift Valley
Lake Awasa
location
36.21
33.98
34.70
35.23
35.03
0.94
LTS2-5-A
LTS2-5-B
LTS2-5-C
apex
intermediate
aperture
Mean
S.D.
3.44
3.11
3.58
3.38
0.24
34.02
33.92
32.49
33.48
0.86
LTS1-1-A
LTS1-1-B
LTS1-1-C
LTS1-1-D
umbo
mid shell
mid shell
margin
Mean
S.D.
3.90
4.83
4.35
3.65
4.18
0.52
33.42
32.84
33.45
31.89
32.90
0.73
LAWS1-5-A
LAWS1-5-B
apex
aperture
Mean
S.D.
6.36
6.24
6.30
0.08
0.29
30.92
30.32
0.86
ARS1-3-A
ARS1-3-B
ARS1-3-C
apex
intermediate
aperture
Mean
S.D.
umbo
mid shell
mid shell
margin
Mean
S.D.
0.51
30.06
30.32
0.04
0.42
30.87
30.72
31.84
30.03
30.87
0.75
32.96
33.09
33.35
33.13
0.20
34.08
35.17
35.57
35.27
35.02
0.65
M.
M.
M.
M.
tuberculata
tuberculata
tuberculata
tuberculata
LHS1-1-W
LHS1-2-W
LHS1-3-W
LHS1-4-W
Mean
S.D.
8.04
7.58
7.56
7.87
7.76
0.23
1.60
31.91
1.68
30.29
0.27
1.71
M.
M.
M.
M.
tuberculata
tuberculata
tuberculata
tuberculata
LAWS1-1-W
5.75
LAWS1-2-W
5.81
LAWS1-3-W
6.25
LAWS1-4-W
6.28
Mean
6.02
S.D.
0.28
LZS1-1-W
5.49
LZS1-2-W
5.08
Mean
5.29
S.D.
0.29
ARS1-2-W
30.16
MICS2-W
30.15
30.27
31.71
30.01
0.26
30.43
0.88
30.50
31.80
31.15
0.92
34.23
34.24
Lake Zway
Radix
Radix
Awash River
‘Kereyu Park’
M. tuberculata
Bivalve
Awash River
‘Kereyu Park’
unionoid
N13 C
Sample #
unionoid
Lake Hyke
N18 O
N C
ARS1-1-A
ARS1-1-B
ARS1-1-C
ARS1-1-D
Values reported are as measured (aragonite) and are not corrected to calcite values.
PALAEO 2906 29-8-02
90
M. Hailemichael et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 186 (2002) 81^99
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 the
two lacustrine intervals in the Sidi Hakoma Member of the Hadar Formation whose shells are pristine enough for isotopic analysis.
4.3.1. Lower lacustrine interval
The lowermost shell-bearing bed is associated
with the thin Kada Meha Tu¡ (KMT), 25 m
above the 3.40-Myr-old Sidi Hakoma Tu¡ (Walter and Aronson, 1993). The pelecypod shells with
a nacreous luster occur at the base of the tu¡. The
isotopic variability of microsamples taken within
a given shell was only 0.5x for both 18 O and
13
C, but the N18 O di¡erence between the averages
of each shell is +2.92x. This indicates either the
two individuals washed in from separate nearby
micro-environments, or they lived at di¡erent
times in an environment whose isotopic character
evolved with time.
4.3.2. Main lacustrine interval
In the central sector of Hadar the largest lacustrine interval in the Hadar Formation is 23 m
thick at the top of the Sidi Hakoma Member
(Fig. 2). It thins to the west and thickens somewhat to the east to about 30 m at Ounda Hadar
in the direction of the permanent location of Lake
Hadar (Aronson and Taieb, 1981). The base of
the interval begins with two to three thin prominent gastropod coquina limestone marker beds
known as the ‘Gastropod Beds’, and ends at the
top with o¡shore laminated claystones that preserve several thin bentonite tu¡s originally referred to as the ‘Triple Tu¡s’ (TT). Of these,
TT-4 has feldspar crystals well dated at
3.22 U 0.04 Myr (Walter, 1993). This tu¡ and its
enclosing laminated claystones with abundant ostracods and ¢sh scales are a marker horizon traceable from east to west across the 12 km breadth
of the site and represent the largest westward
transgression of the lake across Hadar. About
2.5 m beneath TT-4 in the central sector of Hadar, a thin sand with planar shallow cross-bedding and abundant mollusks formed as a local
beach deposit between TT-1 and TT-4 (Fig. 2).
This thin sand extends for about 1 km across
the Kada Hadar Wadi and provided the exquisitely preserved nacreous fossil shells of the 13A
sample.
4.3.2.1. Base of main lacustrine interval: the ‘Gastropod Beds’. Samples AT, 25, and HS derive
from these prominent beds, which represent a series of two or three closely spaced beach coquinas.
During deposition the shells were the coarsest materials available for waves to have swept up and
accumulated at the shore. The lowest of the three
beds represents the initial area-wide transgressive
beach that formed as Lake Hadar expanded over
the low £at distal £oodplain. The succeeding beds
suggest that the initial transgression wavered back
and forth before full lacustrine conditions occupied Hadar.
Samples AT, M25 and B25 are collected from
the more extensive lower and upper beds where
the robust shells are always entirely bleached
white. In contrast the unbleached HS shells
from the less extensive middle bed are not so
packed together in the sand matrix and are not
bleached. We excavated the outcrop and hand
selected those HS shells which best preserved the
pinkish brown exterior of the shell.
The mean N18 O values for individual whole
shells and microsamples from all of the ‘Gastropod Beds’ are quite uniformly negative only ranging from 34.99 to 38.14x and averaging
36.7 U 1.0x (n = 17). Standard deviations of
the N18 O values of microsamples of individual
shells only range from 0.34 to 1.09x. This indicates that isotopic conditions were fairly uniform
during the life of each shell from the ‘Gastropod
Beds’, and that the N18 OSMOW of Lake Hadar was
much more negative than when the mollusks below and above this important unit were living.
4.3.2.2. Upper portion of main lacustrine interval
(sample 13A). Five mollusk shells (17 microsamples) of 13A were hand picked for analysis
over a 100-m outcrop of the regressive beach de-
PALAEO 2906 29-8-02
M. Hailemichael et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 186 (2002) 81^99
91
Fig. 4. Plot of N18 O of microsamples from individual Hadar fossil shells. The oldest and the youngest stratigraphic units (on the
left and on the right, respectively) have mostly isotopically positive N18 O values, with large internal variations within or between
shells. These represent a stage when highly varied evaporation condition prevailed at Lake Hadar. By contrast, the middle plot
from the ‘Gastropod Beds’ shows shells that recorded uniformly negative N18 O values, and represents a fresh unevaporated stage
of Lake Hadar, when more rain from an isotopically depleted air mass source and more cloud cover prevailed.
posit near the top of this lacustrine interval (Fig.
2). The 13A nacreous fossils include whole gastropod shells and large fragments of pelecypod
shells. In contrast to the shells from the ‘Gastropod Beds’ the variability of N18 O values within
individual 13A shells is large. The N18 O values
of all the microsamples from all of the ¢ve individuals span a remarkably large range of 8x
(32.02 to +6.33x). A good portion of this range
is encompassed within the shell of a single Melanoides, ranging from 30.58 to +6.33x and within a single pelecypod shell from 30.26 to
+5.92x (Fig. 4). The gastropod experienced a
progressive evaporative concentration of 7x in
Lake Hadar all within its approximate one year of
life which began in the summer wet season (low
N18 O) and ended in the dry season (high N18 O). A
practically identical 18 O enrichment is record
within the partial shell of the longer-lived pelecypod, but it survived a dry season and its shell
continued recording a succeeding wet season. Of
the 17 microsamples from the 13A collection, the
mean N18 OPDB value is +1.37 U 2.3x. This positive mean value and the mean of the minor transgression of KMT are about 9x more positive
than the uniformly negative 36.7 U 1.0x values
recorded by the ‘Gastropod Beds’ at the beginning of this main lacustrine interval. The large
internal isotopic variability of the 13A fossils
was not observed within any of the microsampled
modern shells from today’s large Ethiopian lakes,
but such a large internal variation was observed
by Abell and Williams (1989) for shells from the
small modern ephemeral Lake Lyadu in the Afar.
The large cyclic variation in N18 O of the 13A
mollusk shells suggests that during the late stage
of the main lacustrine interval, the site of Hadar
temporarily became a shallow bay that was partially isolated from the main body of Lake Hadar
to the east and was strongly isotopically a¡ected
by seasonal evaporation. It was probably similar
to modern-day Lake Gamari’s NW bay. However, very soon after the regressive 13A beach
formed near the end of this major transgressive
interval, Lake Hadar brie£y re-transgressed across
the entire Hadar site to deposit the ‘Ostracod
Beds’ marker claystone and the TT-4 tu¡.
5. Discussion
5.1. Signi¢cance of the low N18 O in the ‘Gastropod
Beds’
The rule governing the N18 O of East African
lakes is that extensive dry season evaporation results in quite positive values for their waters and
the shells which grow in them. Thus the uniformly
PALAEO 2906 29-8-02
92
M. Hailemichael et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 186 (2002) 81^99
Table 3
The isotopic compositions of fossil shell microsamples from the three stratigraphic lacustrine units of the Pliocene Hadar Formation
Age interpolated
Stratigraphic position
(m)
Genus species
Sample #
Microsample location
N18 O
N13 C
3.22 Ma
54
B. unicolor
13A1-1
13A1-2
13A1-3
13A1-4
54
M. tuberculata
13A2-5
13A2-6
13A2-7
54
Cleopatra
13A3-8
13A3-9
54
unionoid
13A4-10
13A4-11
13A4-12
13A4-13
13A4-14
54
B. unicolor
13A5-21
13A5-22
13A5-23
42.5
B. unicolor
AT-1-15
AT-1-16
AT-1-17
AT-1-18
42.5
B. unicolor
B25-1-1
B25-1-2
B25-1-3
42.5
M. tuberculata
M25-1-1
M25-1-2
40
M. tuberculata
HS-2-1
HS-2-2
HS-2-3
40
39.5
B. unicolor
M. tuberculata
HS-5
HS-3-1
HS-3-2
HS-3-3
apex
penultimate
penultimate
outer lip
Mean
S.D.
apex
penultimate
outer lip
Mean
S.D.
apex
body whorl
Mean
S.D.
outer shell
outer shell
outer shell
outer shell
outer shell
Mean
S.D.
penultimate
near outer lip
body whorl
Mean
S.D.
apex
intermediate
intermediate
aperture
Mean
S.D.
apex
mid shell
aperture
Mean
S.D.
apex
aperture
Mean
S.D.
apex
middle whorl
aperture
Mean
S.D.
whole shell
apex
middle whorl
aperture
Mean
32.02
31.02
30.13
0.81
30.59
1.21
30.58
3.32
6.33
3.02
3.46
1.52
0.78
1.15
0.52
2.13
0.15
5.92
3.60
30.26
2.31
2.55
30.20
1.47
1.49
0.92
0.97
36.15
36.17
35.67
35.49
35.87
0.34
36.36
35.83
37.69
36.63
0.96
36.58
37.70
37.14
0.79
38.14
37.39
36.91
37.48
0.62
36.68
36.29
38.41
37.77
37.49
32.38
31.06
32.69
30.92
31.76
0.90
32.43
0.41
0.90
30.37
1.80
0.89
31.54
30.33
1.72
32.90
33.55
30.65
33.44
35.53
33.21
1.75
32.86
30.87
31.43
31.72
1.03
33.98
34.32
34.02
34.27
34.15
0.17
1.57
0.32
31.30
0.20
1.44
0.75
0.71
0.73
0.03
35.78
34.30
33.60
34.56
1.11
35.79
34.60
32.83
31.46
32.96
3.27 Ma
PALAEO 2906 29-8-02
M. Hailemichael et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 186 (2002) 81^99
93
Table 3 (Continued).
Age interpolated
Stratigraphic position
(m)
Genus species
Sample #
3.34 Ma
39
25
B. unicolor
bivalve
HS-4
KMT19-2A
KMT19-2B
3.34 Ma
25
bivalve
KMT19-2-1
KMT19-2-2
KMT19-2-3
Microsample location
N18 O
N13 C
S.D.
whole shell
apex
margin
Mean
S.D.
ventral margin
mid-shell
umbonal area
Mean
S.D.
1.09
34.99
0.88
1.76
1.32
0.62
2.92
2.88
2.16
2.65
0.43
1.57
34.96
34.43
34.92
34.68
0.35
36.85
37.21
37.73
37.26
0.44
Values reported are as measured (aragonite) and are not corrected to calcite values.
quite negative N18 OPDB values of the fossil shells
in the ‘Gastropod Beds’ are of utmost signi¢cance
for interpreting the paleoclimate of Hadar. They
record a relatively rare, least evaporated stage of
Lake Hadar, close to, but more positive than the
N18 O of rain on the Western Plateau source region
during 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 biostructure has been texturally altered. Despite
that, the following two reasons make it improbable that their shells have been isotopically reset
by pedogenesis, the most common form of diagenesis to have a¡ected the formation. First,
being at the base of the thickest lacustrine interval
in the Formation, the ‘Gastropod Beds’ were protected from pedogenic in£uence by being 35 meters below the next higher paleosol, beneath the
DD-3 sandstone (Fig. 2). Secondly, the distinctly
di¡erent N13 C of the shells (32.5 U 2.4x, n = 17;
Table 3) compared to that of the soil carbonates
(37.3 U 1.1x, n = 19; Hailemichael, 2000) rules
out that the two have undergone isotopic exchange.
5.2. The N18 O of Lake Hadar at the time of the
‘Gastropod Beds’ ; and N18 O of Plateau and Afar
rain during the Pliocene
The least evaporated condition of Lake Hadar
recorded by the ‘Gastropod Beds’ was 12x lower in 18 O than that recorded by the most positive
microsample we have observed among the 13A
shells (+6x) that grew later during the same
lacustrine interval.
It is logical that the lowest N18 O values would
have occurred in the very initial stage of the largest lacustrine interval in the Formation. These
shells grew when the lake was expanding rapidly
westward and the rate of input of un-evaporated,
low N18 O river water to the lake most exceeded
the rate of evaporation.
The temperature of lake water is a necessary
input for determining the N18 O of the water
from the N18 O of the shell, using Dettman’s
(1994) equation. More intense cloud cover and
rainfall which caused the more negative N18 O of
rain during the Pliocene would have also lowered
the Pliocene temperatures of the Afar relative to
today’s. If we assume the mean temperature for
Pliocene Lake Hadar to be about 5‡C less than
today’s mean air temperature (30‡C), then the
average N18 Oaragonite (36.7x) value in the ‘Gastropod Beds’ would indicate a N18 OSMOW of the
lake water to be about 35x; or if the temperature was 30‡C, then the lake water will have a
34x value (Fig. 3). Considering the upstream
location of Lake Koka with a N18 O value of
1.55x, we can take its 3x evaporative enrichment relative to the isotopic composition of modern Plateau rain (31.3x) as a minimum which
would have prevailed for the much further transport downstream in the Afar to Lake Hadar during the Pliocene. For example, we observe a 6x
increase in N18 O for the Awash River’s whole Afar
PALAEO 2906 29-8-02
94
M. Hailemichael et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 186 (2002) 81^99
route from the Plateau into the central Afar today
(Table 1). Thus one may infer that Pliocene rains
on the Western Plateau were at least 3x more
negative than the 35x value inferred from the
‘Gastropod Beds’ for Lake Hadar during the Pliocene, i.e., a N18 OSMOW of 38x. This value is
much less than the 31.3x weighted mean value
for Plateau rain today.
The inferred N18 O (38x) for rain on the Western Plateau during the Pliocene compares with a
N18 O of 35x for Pliocene rain in the Afar itself,
calculated with similar assumptions about temperature from the N18 O of paleosol carbonates in the
Hadar Formation by Hailemichael (2000). Thus,
in a similar fashion as for Plateau rain, Afar rain
in the Pliocene (35x) was much lower than today’s value of about +2x as approximated from
our few spot measurements of modern Afar rain.
That is, both the Plateau and the Afar experienced rain about 6^7x lower during the Pliocene
than today in each region.
6. Origin of the low N18 O rain in the Pliocene
Ethiopia
Only about 1x of the 6^7x lowering of N18 O
of Pliocene rain in Ethiopia from today’s value
can be accounted by the formation of the low
N18 O polar icecaps during the Quaternary. The
remaining 5x decrease in the N18 O value of the
Pliocene rain both in the Plateau and the Afar
settings compared to today’s can be accounted
for by some combination of these three factors:
(1) increased proportion of rain derived from air
mass moisture sources more depleted by Rayleigh
distillation in Pliocene than today’s sources; (2) increased amounts of rain per storm, the ‘amount
e¡ect’ (Dansgaard, 1953); and (3) reduced evaporation potential. All three of these are argued
below to have occurred and to have synergistically interacted, especially once (1) was brought
about.
To explain the negative isotopic character of
the Pliocene rain that supplied Lake Hadar during
the time of the ‘Gastropod Beds’, it is helpful to
look at the best near-modern analog. This nearmodern analog is in the very center of the Afar,
but not as it is today, rather as it was only 9^6 ka
during the early Holocene pluvial period known
as the African Humid Period (AHP) (deMenocal
et al., 2000). Then the analog area matched not
only the sedimentology, but also the climate and
the isotopic meteorology of Pliocene Hadar. Its
explainable meteorology can in turn be adopted
as an explanation for the meteorology of Hadar
in the Pliocene.
In the early Holocene, summer heating of the
Northern Hemisphere maximized to values of 8%
more insolation than today due to cycles in
Earth’s orbital parameters (Overpeck et al.,
1996). More intense summer insolation deepened
the East Saharan atmospheric low which in turn
strengthened the summer African Southwest monsoon and brought Atlantic-derived moisture much
further north than today. The increased rainfall
greened the Sahara (Petit-Maire, 1990) and ¢lled
the lakes of the Nubian Paleo-lake Basin in what
is today’s hyperarid eastern Sahara of northwest
Sudan (Hoelzmann et al., 2000). These early Holocene rainwaters of undisputed Atlantic derivation were clearly ¢ngerprinted (in lacustrine and
riverine carbonates and in fossil groundwaters) by
a distinctly negative N18 O values (Abell and
Hoelzmann, 2000; Rodrigues et al., 2000; Thorweihe et al., 1990), as to be expected from their
far 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 the
AHP as the greening of the Sahara, but because
of Ethiopia’s complex meteorology, its meteorological causes have only been addressed peripherally. Our meteorological explanation for the increased early Holocene rainfall of both the
Plateaus and Afar in Ethiopia is a simple extension of what happened in the eastern Sahara. Just
as a deepened Saharan Low of the early Holocene
pulled moist Atlantic-derived air masses northeastward toward the eastern Sahara and shifted
the Sahelian rain belts northward (Ritchie and
Haynes, 1987), it is logical that the even deeper
Tibetan Low (10 millibars lower today), responsible for strengthening the Indian Southwest monsoon (Overpeck et al., 1996), pulled these same
moist, isotopically depleted, Atlantic-derived air
PALAEO 2906 29-8-02
M. Hailemichael et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 186 (2002) 81^99
masses east-northeastward over Ethiopia. As detailed below, the 18 O-depleted early Holocene carbonates in the Afar of various origins suggest the
Atlantic-derived component of air masses £owed
as far east as the Afar in higher proportions than
today.
7. An Early Holocene environmental and isotopic
analog of Hadar
A depositional analog of the Hadar Formation
is today’s low £at distal £oodplain and delta plain
of the Awash River at its terminus with Lake
Gamari, the ¢rst of the series of four lakes in
the central Afar (Aronson and Taieb, 1981). But
the hot and dry setting, the low ecological productivity and diversity make it an otherwise inappropriate analog.
Gasse (1977), Gasse et al. (1974), and Gasse
and Street (1978) have documented that during
the early Holocene about 9^6 ka, the level of
the central Afar lake system from Lake Gamari
to Lake Abbe rose a remarkable 150 m higher in
elevation than today’s surface of Abbe. All four
lakes coalesced into one great lake that expanded
across the Asaita Plain to an area 13-fold that of
today (Gasse and Street, 1978) and several times
the area of Hadar. Also the level of the Zway^
Shala lakes in the MER rose 80 m, coalesced to
over£ow the divide northward into the Awash
(Gasse and Street, 1978), and further augment
the discharge of Plateau rainfall into the Afar.
Not only was the central Afar region an excellent depositional and climatic analog for Hadar
during the AHP, but evidence also suggests that
the increased rainfall on the Plateau and in the
Afar during the AHP had an isotopically negative
character similar to that evidenced by the ‘Gastropod Beds’ for the Pliocene. This evidence comes
from two previous isotopic studies on early Holocene mollusk shells from the Afar lake systems.
Gasse et al. (1974) measured N18 O of 11 14 C-dated
shells from various stages of the expanded AHP
Abbe lake system. Their N18 OPDB values average
32.1x (n = 11) with one sample as low as
35.0x. Because evaporative concentration of
18
O is so prevalent in East African lakes, it is
unlikely that any particular shell sample would
95
catch a paleo-lake at its freshest, least evaporative
and lowest N18 O stage. At 25‡C the lowest observation of 35.0x corresponds to a N18 OSMOW
value of about 33x for the lake water, far lower
than present-day Lake Gamari’s +16x. The negative isotopic compositions show that the increased AHP rainfall on the Ethiopian Plateaus
which fostered the lake expansions in the MER
and Afar (Gasse and Street, 1978) was isotopically more negative than today. Probably the
evaporative potential of the Afar was less, as well.
Abell and Williams (1989) also measured shells
with negative N18 O from an early Holocene sediments at Lake Beseka in the southern Afar close
to the Western Escarpment and from small spring
deposits at the base of the Southern Escarpment
of the Afar with the Eastern Plateau, both directly
fed by Plateau run-o¡. At about 25‡C, the resulting N18 OSMOW of the water when these individual
mollusks lived would have been 34 to 31x,
much lower than the +6.7x of today’s Lake Beseka (Table 1). Again, the enhanced AHP rainfall
on the plateaus whose run-o¡ fed these two small
lakes was undoubtedly isotopically more negative
than today.
Evidence presented elsewhere indicates even the
AHP rain in the Afar itself had a much more
negative N18 O than today’s approximate +2x
value. This evidence comes from modern Afar
soils near Hadar whose soil calcite nodules give
14
C dates that indicate the nodules formed during
the AHP (Hailemichael, 2000). These soil carbonates have a mean N18 OPDB value of 36.5x. At
25‡C these value would translate to soil waters
that was about 34x (i.e., about 5^6x more
depleted in 18 O than today). If plateau rain during
the AHP was depleted in 18 O by a comparable
amount, going from a present-day value of about
31.3x to about 36 or 37x in the early Holocene. This is close to the 38x that we independently deduced for Plateau rain during the
Pliocene from the ‘Gastropod Beds’ data.
8. Meteorological hypothesis of the causes for the
low N18 O rain in the Pliocene
The intensi¢ed summer Indian Sub-continent
PALAEO 2906 29-8-02
96
M. Hailemichael et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 186 (2002) 81^99
Low of the early Holocene (Overpeck et al., 1996)
can be hypothesized to have pulled both components of the Ethiopian monsoon eastward. During
today’s summer, the Indian Ocean- and Atlanticderived air masses converge in the northeast
trending IOC front, whose location (Fig. 1) is
only very approximately known to hover in
summer over the western part of the Western Plateau (Nicholson, 1996). It seems logical that by
pulling stronger on both components of the
Ethiopian monsoon that the intensi¢ed Indian
low would have shifted their IOC front eastward.
We speculate a shift of perhaps a few hundred
kilometers to bring the IOC over the western
Afar. The moist, far-traveled, Rayleigh-depleted
Atlantic air masses would then have reached the
high eastern shoulder of the Western Plateau, the
escarpment with the Afar and the western Afar
itself. This shift of the IOC would have resulted
in dramatically increased amounts of rain both
along the eastern shoulder of the Plateau that
drained to the ancestral Awash River, and also
on the western Afar itself. The con£uence of the
Atlantic and Indian Ocean air masses in the Afar
would have forced the relatively less stable, more
humid, Atlantic air masses (Nicholson, 1996) to
have risen over the drier Indian Ocean air masses
in storms self-perpetuated by the release of latent
heat of precipitation. The increased proportions
of the negative N18 O ¢ngerprinted Atlantic component of rain falling in more intense storms on
the Plateau headwaters of the ancestral Awash
River explains the low N18 O of the ‘Gastropod
Beds’. Further, the increased summer cloud cover,
rainfall, and vegetation cover in the Afar itself
that would have accompanied the eastward shift
of the IOC that would have reduced the Afar
mean temperature from today’s high mean value
of about 30‡C, down perhaps to about 25‡C and
reduced the potent ability of evaporation to increase the N18 O of Lake Hadar.
Thus, as brie£y suggested by Hillaire-Marcel et
al. in their 1982 paper, one only has to go back a
few thousand years to the AHP in the central
Afar to ¢nd an excellent depositional analog,
and also an excellent climatic and isotopic analog
for the Hadar Formation. Although indeed it is
possible that the tectonic relief created by down-
dropping the Afar may have been less accentuated
3 Ma, there is no need to invoke such accentuated
relief to explain the aridi¢cation that has a¡ected
Hadar since the Pliocene. Rather, we propose the
Pliocene climate at Hadar was very similar to the
pluvial climate in the Afar during the early Holocene only 6^9 ka, when the tectonic situation was
no di¡erent than today’s.
The abundant and diverse terrestrial vertebrate
fauna in the main part of the Hadar Formation
beneath the disconformity argues that an enhanced monsoon was a persistent feature of the
Pliocene. However, one can speculate that the ¢ve
transgressions of Lake Hadar may have been
caused by the cyclic peaking of the Earth’s orbital
factors superimposed upon a persistently strong
Ethiopian Monsoon of the Pliocene. They also
may have been tectonically induced.
Despite the persistent appearance of wetter
summer at paleo-Hadar compared to today, the
dry season must have been pronounced as shown
by the cyclic variation to quite positive N18 O values within the 13A shells, and by the accumulation of carbonate nodules in the formation’s
many paleosols. Nevertheless, even in the dry season the environment at Hadar would have been a
very habitable refuge because of the year-round
presence of river, lake, shore and wetland environments to store the ample summer water.
9. Other possible causes of the low N18 O in
Pliocene rain
There may have been other causes of the isotopically depleted rain of the enhanced Ethiopian
monsoon during the Pliocene and the AHP that
acted instead of, or in concert with the increased
Atlantic-derived component hypothesized here.
These include: (1) Hadar having possibly been
at a higher elevation (Bonne¢lle et al., 1989);
and/or (2) a strengthened Indian Ocean-derived
component to the Ethiopian monsoon. For example, Indian Ocean air mass sources may have been
pumped harder over Ethiopia toward an intensi¢ed, relatively closer Saharan Low before being
diverted east toward the even stronger atmospheric Tibetan Low. Such Indian Ocean sources of
PALAEO 2906 29-8-02
M. Hailemichael et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 186 (2002) 81^99
rainfall could have experienced lower N18 O by the
‘amount e¡ect’ and by Rayleigh distillation as
these air masses mounted the Western Escarpment with the Afar. This e¡ect would have been
reinforced in the Pliocene if the Hadar structural
block was an elevated part of the Plateau that had
not yet dropped into the Afar.
As regards Pliocene Afar, the recent tectonic
analysis at the other end of the Indian Ocean by
Cane and Molnar (2001) suggests that higher sea
surface temperatures (SST) may have existed for
the Indian Ocean then. They propose that warm
south Paci¢c waters used to come into the Indian
Ocean and were closed o¡ about 4^3 Ma by the
northward tectonic movement of New Guinea
into the Indonesian seaway. Such Indian Ocean
warming, should it have occurred, may have
been superimposed upon the already globally
warmed oceans of the pre-glacial world so as to
feed more Indian Ocean-derived, isotopically depleted, storms into the Afar and the plateaus. The
warmer SSTs of either one or both of the Atlantic/Indian oceans would account for the long persistence of the wetter summer climate in Ethiopia
during the Pliocene, compared to its brief episodic
occurrences in the Quaternary.
10. Conclusions
The late Pliocene Hadar Formation accumulated in the western Afar mostly as the distal
£ood and delta plain sediments of the ancestral
Awash River. Lake Hadar transgressed westward
across the site ¢ve times. On the basis of the isotopic results of the lacustrine shell zones of the
Sidi Hakoma Member in a context of the isotopic
hydrology of modern Ethiopia, we conclude the
following.
(A) Evaporation strongly enriches the 18 O content of the 11 modern lakes, except for the
through-£owing Lake Koka.
(B) The beach ‘Gastropod Beds’ laid down at
the start of the largest lacustrine interval captured
a record of the least evaporated stage of Lake
Hadar equivalent to a N18 OSMOW value of
35x. This water was derived from Pliocene Plateau rain of at least 38x, much lower than to-
97
day’s average (31.3x). This lower N18 O is comparable to that for rain in the Afar inferred from
isotopic studies of Hadar Formation paleosol carbonates to be presented elsewhere.
(C) Near the end of this lacustrine interval
when the site became a partially isolated shallow
bay of the lake, shells have much more positive
N18 O value with dramatic cyclic internal variations
to values as high as +6x. Evaporation in the
Pliocene dry season must have been pronounced
like today. But the mosaic ecotone nature of the
Pliocene depositional setting included many wet
sub-environments for annually storing the much
larger summer supply of fresh water through the
dry season. This explains the abundant, diverse
fossil vertebrates in the formation and the long
stability and success there of the hominid Australopithecus afarensis.
(D) The best depositional, meteorological and
environmental analog to the Pliocene Hadar is the
Lake Gamari Plain in the central Afar itself, as it
was just 9^6 ka, during the AHP. The early Holocene expansions of the Ethiopian Rift and Afar
lakes have been recognized as a de¢ning episode
of the AHP comparable to the ‘greening’ of the
Sahara. We argue that the cause of the Ethiopian
part of the AHP was due to the strengthened Tibetan Low that pulled the moist isotopically depleted Atlantic-derived air mass component of the
summer Ethiopian Monsoon as far east as the
Afar. By analogy the negative N18 O rainfall of
the Plateau and Afar throughout the Pliocene
could have originated similarly. However, the
high paleoecological productivity throughout the
¢rst half million years of the Hadar Formation
means that the Ethiopian Monsoon was persistently strong as opposed to periodically so in
the Quaternary.
Acknowledgements
We thank the Ethiopian Geological Survey for
permits and logistic support in collecting the modern waters and mollusks. We are grateful to the
Director, Ketema Tadesse, for logistic support
and encouragement and for his dedication to improving knowledge of Ethiopia’s resources. For
PALAEO 2906 29-8-02
98
M. Hailemichael et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 186 (2002) 81^99
support at Hadar, we thank all our colleagues at
the Institute of Human Origins (IHO), Tesfaye
Yemane, Carl Vondra, William Kimbel, Don Johanson, Gerry Eck, and Kay Reid. We are especially indebted to Robert Walter for his generous
sharing of his knowledge of the Hadar Formation. Linda Abel gave expert help in the isotope
lab at CWRU. Tenesa Mamecha, Mes¢n Dubale
and Zerihun Tsegaye helped collect mollusks from
Lake Tana, Lake Hayk and Lake Awasa. Financial support for this study was provided by an
exploratory grant to J.L.A. at CWRU from the
National Science Foundation (Anthropology).
M.H. was generously supported as a graduate assistant by Geological Sciences at CWRU. For
their role in writing up this paper, J.L.A. and
M.H. were supported by Dartmouth’s Earth Sciences Department. Taking full responsibility for
the interpretations presented, we are grateful for
thorough constructive reviews by FrancXoise
Gasse.
References
Abell, P.I., Hoelzmann, P., 2000. Holocene Palaeoclimates in
northwestern Sudan: stable isotope studies on molluscs.
Global Planet. Change 26, 1^12.
Abell, P.I., Williams, M.J., 1989. Oxygen and carbon isotope
ratios in gastropod shells as indicators of paleoenvironments
in the Afar region of Ethiopia. Palaeogeogr. Palaeoclimatol.
Palaeoecol. 74, 265^278.
Aronson, J.L., Taieb, M., 1981. Geology and Paleogeography
of the Hadar Hominid Site, Ethiopia. In: Rapp, G., Vondra,
C. (Eds.), Hominid Sites and their Geological Settings.
AAAS Selected Symposium 63, pp. 165^195.
Aronson, J.L., Vondra, C., Yemane, T., Walter, R., 1996.
Characteristics of the disconformity in the upper part of
the Hadar Formation. Geol. Soc. Am. Abstracts with Programs (N. Central Section) 28.
Bonne¢lle, R., Vincens, A., Buchet, G., 1987. Palynology, stratigraphy and paleoenvironment of a Pliocene hominid site
(2.9^3.3 m.y.) at Hadar, Ethiopia. Palaeogeogr. Palaeoclimatol. Palaeoecol. 60, 249^281.
Cane, M., Molnar, P., 2001. Closing of the Indonesian seaway
as a precursor to east African aridi¢cation around 3^4 million years ago. Nature 411, 157^162.
Cerling, T.E., Bowman, J.R., O’Neil, J.R., 1988. An isotopic
study of a £uvial-lacustrine sequence: The Plio-Pleistocene
Koobi Fora sequence, East Africa. Palaeogeogr. Palaeoclimatol. Palaeoecol. 63, 335^356.
Coplen, T., Kendall, C., Hopple, J., 1983. Comparison of stable isotope samples. Nature 302, 236^238.
Craig, H., 1965. The measurement of oxygen isotope paleotemperatures. In: Stable Isotopes in Oceanographic Studies
and Paleotemperatures, Spoleto, July 26^30.
Craig, H., 1957. Isotopic standards for carbon and oxygen and
correction factors mass-spectrometric analysis of carbon dioxide. Geochim. Cosmochim. Acta 12, 133^149.
Dansgaard, W., 1953. Comparative measurements of standards for carbon isotopes. Geochim. Cosmochim. Acta 3,
253^256.
deMenocal, P., Ortiz, J., Guilderson, T., Adkins, J., Sarnthein,
M., Baker, L., Yarusinsky, M., 2000. Abrupt onset and
termination of the African Humid Period: rapid climate
responses to gradual insolation forcing. Quat. Sci. Rev. 19,
347^361.
Dettman, D.L., 1994. Stable Isotope Studies of Fresh-water
Bivalves (Unionidea) and Ostracods (Podocopida): Implications for Late Cretaceous/Paleogene and Early Holocene
Paleoclimatology and Paleo-hydrology of North America.
Ph.D. Dissertation, University of Michigan, Ann Arbor,
MI.
Epstein, S., Mayeda, T., 1953. Variation of 18 O content of
waters from natural sources. Geochim. Cosmochim. Acta
4, 213^224.
Ethiopian Mapping Authority, 1988. National Atlas of Ethiopia. Addis Ababa.
Gasse, F., 1977. Evolution of Lake Abhe¤ (Ethiopia and
T.F.A.I.) from 70,000 B.P.. Nature 2, 42^45.
Gasse, F., Fontes, J.C., Rognon, P., 1974. Variations hydrologique et extensions des lacs Holocenes de Desert Danakil.
Palaeogeogr. Palaeoclimatol. Palaeoecol. 15, 109^148.
Gasse, F., Street, F.A., 1978. Late Quaternary lake-level £uctuation and environments of the northern rift valley and
Afar regions (Ethiopia and Djibouti). Palaeogeogr. Palaeoclimatol. Palaeoecol. 24, 279^325.
Gri⁄ths, J.F., 1972. Climates of Africa. In: Gri⁄ths, J.F.
(Ed.), World Survey of Climatology, V. Elsevier, Amsterdam, p. 10.
Grossman, E.L., Ku, T., 1986. Oxygen and carbon isotope
fractionation in biogenic aragonite: Temperature e¡ects.
Chem. Geol. 59, 59^74.
Hailemichael, M., 2000. The Pliocene Environment of Hadar,
Ethiopia: A Comparative Isotopic Study of Paleosol Carbonates and Lacustrine MOllusk Shells of the Hadar Formation and of Modern Analogs. Ph.D. Thesis, Case Western Reserve University, Cleveland, OH, 239 pp.
Hillaire-Marcel, C., Taieb, M., Tiercelin, J., Page, N., 1982. A
1.2 Mys record of isotopic changes in a late Pliocene Rift
Lake, Ethiopia. Nature 296, 640^642.
Hoelzmann, P., Kruse, H., Rottinger, F., 2000. Precipitation
estimates for the eastern Saharan palaeomonsoon based on
a water balance model of the West Nubian Palaeolake Basin. Global Planet. Change 26, 105^120.
IAEA, 1996. Database of Global Network of Isotopes in
Precipitation (GNIP), available at http://www.iaea.or.at/
programs/ri/gnip/gnipmain.html.
PALAEO 2906 29-8-02
M. Hailemichael et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 186 (2002) 81^99
Joseph, A., Frangi, P., Aranyossy, F., 1992. Isotope characteristics of meteoric water and groundwater in Sahelo-Sudan
zone. J. Geophys. Res. 97, 7543^7551.
Lamb, A.L., Leng, M.J., Lamb, H.F., Mohammed, M.U.,
2000. A 9000 year oxygen and carbon isotope record of
hydrological change in a small Ethiopian crater lake. Holocene 10, 167^177.
Leng, M.J., Lamb, A.L., Lamb, H.F., Telford, R.J., 1999.
Palaeoclimatic implications of isotopic data from modern
and early Holocene shells of the freshwater snail Melanoides
tuberculata, from lakes in the Ethiopian Rift Valley. J. Paleolimnol. 21, 97^106.
McCrea, J.M., 1950. On the isotopic chemistry of carbonates
and palaeo-temperature scale. J. Chem. Phys. 18, 849^857.
National Meteorological Services Agency of Ethiopia
(NMSA), 1996. Unpublished data.
Nicholson, S.E., 1996. A review of climate dynamics and climate variability in Eastern Africa, In: Johnson, T.C., Odada, E.O. (Eds.), The Limnology, Climatology and Paleoclimatology of the East African Lakes. Gordon and Breach,
Amsterdam, pp. 25^56.
Overpeck, J., Anderson, D., Trumbore, S., Prell, W., 1996.
The southwest Indian Monsoon over the last 18000 years.
Clim. Dyn. 12, 213^225.
Petit-Maire, N., 1990. Will Greenhouse green the Sahara? Episodes 13, 103^107.
Ritchie, J.C., Haynes, C.V., 1987. Holocene vegetation zonation in the eastern Sahara. Nature 330, 645^647.
Rodrigues, D., Abell, P.I., Kro«pelin, S., 2000. Seasonality in
the early Holocene climate of northwest sudan: interpreta-
99
tion of Etheria elliptica shell isotopic data. Global Planet.
Change 26, 181^187.
Rozanski, K., Araguas-Araguas, L., Gon¢antini, R., 1993.
Isotopic patterns in modern global precipitation. In: Swart,
P.K., Lohman, K.C., McKenzie, J., Savin, S. (Eds.), Climate
Change in Continental Isotopic Records, Am. Geophys.
Union, Washington, DC, pp. 1^36.
Rozanski, K., Araguas-Araguas, L., Gon¢antini, R., 1996.
Isotope patterns in the East African Rift. In: Johnson,
T.C., Odada, E.O. (Eds.), The Limnology, Climatology
and Paleoclimatology of the East African Lakes. Gordon
and Breach, Amsterdam, pp. 79^93.
Schoell, M., Faber, E., 1976. Survey on the isotopic composition of waters from NE Africa. Geol. Jahrb. D17, 197^
213.
Taieb, M., 1974. Evolution Quaternaire du Bassin de l’Awash.
Thesis, Universite¤ de Paris, 290 pp.
Telford, R.J., Lamb, H.F., 1999. Groundwater-mediated response to Holocene climatic change recorded by the diatom
stratigraphy of an Ethiopian crater lake. Quat. Res. 52, 63^
75.
Thorweihe, U., Brinkman, P.J., Heinl, M., Sonntag, C., 1990.
Hydrological and hydrogeological investigations of the Darfur area, western Sudan. Berl. Geowiss. Abh., Reihe A 120,
279^326.
Walter, R.C., 1993. Age of ‘Lucy’ and the ‘¢rst family’. Geology 21, 57^61.
Walter, R.C., Aronson, J.L., 1993. Age and sources of the Sidi
Hakoma Tu¡, Hadar Formation, Ethiopia. J. Hum. Evol.
25, 229^240.
PALAEO 2906 29-8-02