Rossignol-Strick. (1983) relates sapropel formation in the .... ity for Water Resources, Oman) and Dan Danesh,. (Cansult Oman Limited) facilitated field work in ...
QUATERNARY
RESEARCH
33, 320-336 (1990)
Paleoclimatic Reconstruction in Northern Oman Based on Carbonates from Hyperalkaline Groundwaters IAN D. CLARK' AND JEAN-CHARLES FONTES Laboratoire
d’Hydrologie
et de Gkochimie Isotopique, UniversitC Paris&d, Received May 11, 1989
91405 Orsay Cedex, France
A paleoclimatic reconstruction for the past 35,000 years for northern Oman is based on an unusual approach using travertines and fracture calcites associated with hyperalkaline springs. High-pH groundwaters (pH up to 11.9) discharge from the mantle sequence of the Oman Ophiolite as the product of modem, low-temperature serpentinization. Under arid climatic conditions, hyperalkaline discharge occurs at the surface. Uptake of atmospheric CO, precipitates characteristic laminated travertines, accompanied by strong kinetic depletion of “C and “0. Pluvial climates supporting a shallow bicarbonate-groundwater flow system and vegetation are recorded by fracture calcites with equilibrium stable isotope contents and calcite-replaced roots and stems. All such carbonates have modem initial r4C contents, allowing radiocarbon dating and paleoclimatic reconstruction for the late Pleistocene and Holocene. Our reconstruction shows a dominantly wet late Pleistocene up to 19,000 yr B.P., when a phase of climatic deterioration began, leading to a period of hyperaridity which dominated from ca. 16,300 to 13,000 yr B.P. The early Holocene pluvial occurred from 12,500 to ca. 6500 yr B.P. and was followed by renewed climatic deterioration and the current phase of hyperaridity. Comparison of this paleoclimatic reconstruction with that for lacustrine deposits from the A’Rub al Khali of central Saudi Arabia and the summer insolationdriven monsoon record of east Africa and the Arabian Sea is remarkably good. 8 IWOUniversity of Washington.
reports fossil lake beds and fauna1 remains typical of a savanna grassland. The study of Pleistocene lake deposits as paleoclimatic indicators in Oman is severely hampered by the predominance of Tertiary carbonate bedrock and the incorporation of detrital carbonate material in lacustrine sediments. However, in northern Oman a series of carbonate deposits associated with hyperalkaline springs, the product of a low-temperature serpentinization of ultramafic rock, were investigated as potential paleoclimatic indicators (Fig. I). The hyperalkaline groundwaters (pH values up to 11.9), carbonate-free at the point of discharge, immediately begin neutralizing by incorporation of CO, and/or bicarbonate in the discharge zone, with the subsequent precipitation of calcite. These deposits provide a novel approach to paleoclimatology, based on two inherent characteristics: (i) their form and stableisotope contents vary as a function of the
INTRODUCTION
The Sultanate of Oman is centrally situated in the northern equatorial desert belt which stretches some 9300 km from eastern North Africa to the Thar desert in India. With a mean annual rainfall on the order of 100 mm for most of the country and summer temperatures regularly exceeding 45°C the occurrence of water resources and their recharge frequency are of considerable concern. However, evidence shows that paleoclimatic conditions in Oman were considerably more humid. Hydrogeological studies document major aquifer systems hosting fossil groundwaters (10,000 to 30,000 yr B.P.; Clark, 1987) and within the neighboring sand seas of the A’Rub al Khali (Empty Quarter) in Saudi Arabia, McClure (1976) ’ Present address: Department of Geology, University of Ottawa, Ottawa, Canada KIN 6N5. 320 0033-5894190 $3.00 Copyright All rkhts
0 1990 by the University of Washington. of rwmduction in al” form reserved.
PALEOHYDROLOGY
321
OF OMAN
GULF
OF
OMAN 24N-
ciitli KI LOMETRES
.
CRUSTAL
SEQUENCE
MANTLE
SEQUENCE
HYPERALKALINE
SPRINGS
FIG. 1. Outcrop area of the Semail Ophiolite series showing both the mantle and crustal sequences, hyperalkaline springs (after Neal and Stanger, 1983), and the principal study area.
prevailing climatic regime, indicating formation under arid conditions such as exist currently, or under a humid climate which sustained a shallow groundwater flow system and (ii) initial 14C contents of the travertines are modern and so are directly datable. HYPERALKALINE SPRINGS AND TRAVERTINE FORMATION
Background
Geology and Hydrogeology
Late Cretaceous erogenic events in Oman emplaced the Semail Ophiolite Seties onto the flanks of the Northern Oman
Mountains, an autochthonous antiform of Mesozoic carbonate strata arcing over 800 km along Oman’s northeastern coastline (Glennie et al., 1974). Although the most pervasive phase of serpentinization (60-U%) in the mantle sequence occurred at high temperature during abduction (Gass et al., 1985), the process has been found to persist up to the present time as a lowtemperature phenomenon (Barnes and O’Neil, 1969; Neal and Stanger, 1983). Facilitated by low water/rock ratios, this process produces a hyperalkahne Ca-OHwater with a pH up to 11.9, a phenomenon documented at only a few sites worldwide
322
CLARK AND FONTES
(Barnes and O’Neil, 1969; Barnes ef al., 1978; Neal and Stanger, 1983; Launay and Fontes, 1985). Discharge of essentially carbonate-free groundwater into a CO,-charged surface environment (pCO,,,,,, of 10-3.4 and pCO2[soil] up to 10-‘.5) causes an immediate calcite supersaturation. Such CO, uptake will proceed forward in neutralizing hydroxide alkalinity until neutrality is reached, at which point back-exchange reactions become important. Similarly, mixing between discharging basic waters and shallow bicarbonate waters causes an immediate calcite supersaturation and precipitation of calcite. In both cases, the calcite so formed has a modern initial 14C activity, and thus can be dated, providing a record which traverses the Holocene and much of the latest Pleistocene. Description
of Study Sites
Hyperalkaline groundwaters occur throughout much of northern Oman in the outcrop area of the ultramafic mantle sequence (Fig. 1). However, as most hyperalkaline springs in Oman exist within the mountains, continual flushing by the often torrential runoff has precluded significant travertine development. By contrast, travet-tines in the Nizwa area are hydrologically isolated from the Oman Mountains by a major wadi, offering quiescent conditions for travertine buildup. Further, they occur over an outcrop area which hosts only a hyperalkaline groundwater flow system. No shallow bicarbonate groundwaters are found at the site under the modern arid climate. The travertines extend over 7 km along the southeast periphery of a 3OO-km2 area of low black hills of ultramafic rock: a desolate region lacking both vegetation and soil. Thicknesses are generally less than 1 m, although they may exceed 3 m on the sloping hillside, and occur from the elevation of the wadi bounding the outcrop up to about 10 m above wadi level. Samples were collected from four sites
within this locality, the principal site being Nizwa A’Raddah (NR) which offered the thickest section for sampling, Nizwa Jill (NJ), Nizwa Tower (NT), and Clinic (Figs. 2 and 3). A series of fracture carbonates was also collected from a site in Muscat where recent highway construction has cut sections through outcrops of the ophiolite mantle sequence. FORMATION OF TRAVERTINE ARID CONDITIONS
UNDER
The form and stable isotope signature of modem travertines forming under the arid climate in Oman today can be identified in certain fossil travertines. Their formation through uptake of atmospheric CO* by the hyperalkaline waters is unlike other carbonates in the area which have been precipitated from “normal” bicarbonate groundwaters. The occurrence of such aridtype travertines implies arid climatic conditions in the past, under which a shallow bicarbonate-groundwater flow system could not be sustained. Laminated
Travertine
In the modern natural environment at the Nizwa study site, hyperalkaline groundwaters discharge at the surface where they are neutralized by uptake of atmospheric CO, and calcite precipitation. Such travertines form dominantly as horizontal laminations on the bedrock surface and as stalagmites and stalactites on overhangs, giving the appearance of a flowing landscape (Figs. 4,5, and 6). In pools and a&# near the spring vents calcite crusts up to a millimeter thick form on the surface of the flowing water. Analysis of stable isotope composition of modern examples of these travertines shows that they are remarkably depleted in both 13C and 180 (Clark et al., 1990) and * Channels or tunnels dug below grade to intercept groundwater flow or constructed on surface to channel water. Surface a&j at this site are formed from precipitating calcite and have been used for over 2700 years (Table 1). Singular is f&j.
PALEOHYDROLOGY
0 .,
500m
TRAVERTINE
u
ULTRAMAFIC
m
ALLUVIUM
m
VILLAGE
._ --
N A
w
323
OF OMAN
ti
j
:‘:;;
WADI
FIG. 2. Outline of Nizwa travertine deposits, with local wadis, villages, and individual study areas. Site shown in Fig. 1.
are not in isotopic equilibrium with any natural source of CO2 or groundwater in Oman (Fig. 7). Examples of such depletion in fossil samples of laminated travertines dating to the late Pleistocene (discussed below) are shown in Fig. 8. The wide range of stable isotope values represents depletions in heavy isotopes in excess of 10% for ‘*O and up to 30!& for 13C with respect to equilibrium conditions. Although similar depletions have been documented at other sites where such high pH discharge occurs (O’Neil and Barnes, 1971; Launay and Fontes, 1985), the mechanisms involved remained poorly understood. As part of the current investigation, a series of laboratory experiments was un-
dertaken using hyperalkaline solutions and a controlled atmosphere (Clark, 1987; Clark et al., 1990). The combination of CO, diffusion through air over the hyperalkaline solution and aqueous kinetic effects during the carbonate-ion speciation favor the light isotopes throughout crystal growth. As such kinetic effects can occur only by calcite formation through CO, uptake from the atmosphere, laminated travertines indicate formation under arid conditions without the participation of a shallow, bicarbonategroundwater flow system. Carbonate Nodules Very low &values characterize calcite nodules sometimes found growing on the
FIG. 3. Diagram of principal study site (Nizwa A’Raddah). showing morphological diagram location. Trench in bedrock is a fi/aj, constructed for local water supply.
NR5
23,000FRACTURES
M
(HUMID)
NR 9 16,300-
35,000
TEXTURES YR
- 25,000
(HUMID) 6.R
(ARID) BP
YR
B.I?
TRAVERTINES
LAMINATED 13,000 YR NR 6 NON-LAMINATED 25.000 YR 0.R
19,000
SOIL
~
TEXTURES
B.P
BEDROCK
8.P
features and field relationships of the principal units. See Fig. 2 for
. NR 4
--
pp-
SOIL
ULTRAMAFIC
N R 21, 24,25,28 RECENT LAMINATED (ARID) 5500 YR-
I
x P
PALEOHYDROLOGY
OF
325
OMAN
FIG. 4. Nizwa Jill (NJ) travertines shown with the discharge of hyperalkaline groundwater from the f&j cut into fossil travertines and ultramafic rock in the background (shown in Fig. 3). Discharge at this site was measured at 112 m?day. For scale, reeds are about 1 m high.
surface of travertines, as small cryptocrystalline colliform features measuring from millimeters to several centimeters (Fig. 9). X-ray diffraction of some samples shows that a small amount of portlandite (Ca(OH),) may also be present. Two 14C measurements indicate activities of 92.9 and 102.6 pmC (measured at TAMS facility, Gif-Sur-Yvette, France; not normalized), showing them to be modem features and a product of arid conditions. Their delicate form precludes long-term preservation. These features represent perhaps the isotopically lightest continental calcites yet reported in the literature. 13C values as low as - 33.4%~ were measured with corresponding 6lsO values between - 12.9 and - 13.2%0 PDB (Fig. 7).
Nonlaminated Travertine
A third form showing kinetic isotope depletion is a sequence of travertines often characterized by a rather massive, nonlaminated texture. Their occurrence on a more level area of the sloping outcrop (Fig. 3) suggests that discharge of high-pH water may have occurred into a shallow pool, with travertine sediment accumulating as neutralization progressed. The heavyisotope depletions seen in these samples suggest formation under arid conditions by uptake of atmospheric COZ. FORMATION OF TRAVERTINE HUMID CONDITIONS
Unlike
the laminated
UNDER
travertines,
sam-
326
CLARK
AND
FONTES
FIG. 5. Nizwa A’Raddah travertines, series NR9 with overlying veneer of more recent laminated travertine. Section is about 3 m high.
ples of fracture calcites and calcite-replaced vegetation provide evidence of carbonate formation under humid conditions. Stable-isotope values for such forms show an equilibrium with “normal” bicarbonate waters, indicating the existence of a shallow bicarbonate-groundwater flow system at some time in the past. Owing to the shallow nature of the fracturing (generally 1 to 2 m depth), such groundwater flow would have been topographically controlled, with recharge occurring locally, within the area of ultramatic outcrop. Fracture Carbonate Throughout the outcrop area of the mantle sequence of the Oman Ophiolite, extensive fracturing and calcite infilling of the upper 1 to 2 m is evident (Fig. 10). These calcites are commonly coarsely crystalline (>l to several millimeters) with euhedral crystal habits, reflecting slow growth over long periods of stable groundwater circulation. Where fractures intersect the surface
of the outcrop, millimeterto centimeterthick deposits of very finely layered dense calcite coat the ultramafic rock. These “fracture spring” carbonates are evidence of discharge from fractures at the surface and, as will be shown by their stableisotope contents, are genetically related to the coarser fracture calcite. Fracture calcites such as these must be taken as evidence of formation under humid conditions with active circulation of both shallow bicarbonate groundwaters and the ascending hyperalkaline groundwaters taking place within the fracture network. Stable-isotope data in Fig. 8 show that these fracture minerals have indeed formed under equilibrium conditions with the groundwater. al80 values are consistently greater than the -6 to -5%0 threshold below which kinetic depletion effects are apparent, and vary up to as high as +3%0. Values greater than about O%Osuggest that in some cases the groundwater has experienced some evaporation.
PALEOHYDROLOGY
OF OMAN
327
FIG. 6. Laminated texture, seen here in a stalagmite formed by diffusion of atmospheric CO2 into high-pa water. Such laminated forms show strong stable isotope depletion effects and characterize travertine formation under arid conditions.
13C contents for the fracture calcites tend to fall into one of the two soil CO, equilibrium groups shown in Fig. 8. Given that pluvial conditions enhance soil development, soil CO1 dominates the carbonate system in shallow bicarbonate groundwaters. The distribution in Fig. 8 indicates isotopic equilibrium with a soil dominated by either a C3- or CCtype vegetation. This observation, discussed below, is significant in that it may signal variations in vegetation patterns in this region for the principal pluvial episodes.
solved bicarbonate in isotopic equilibrium with soil CO,. ‘*O values for these travertines also indicate formation by mixing between high-pH and bicarbonate groundwaters. Values vary between - 5 and -2%0 PDB (Fig. 8), suggesting equilibrium with groundwater at normal temperatures. According to the equilibrium fractionation data of O’Neil et al. (1969), a host groundwater for such travet-tine would have a al80 of -4 to - 1%0 SMOW at 20°C. Most groundwaters in northern Oman discharge with 6l*O values within this range, with occasional samples Soil Zone Travertines showing positive enrichment from evaporaA second travertine which bears evi- tion (Clark, 1987). al80 values of fossil dence of formation under pluvial conditions groundwaters from southern Oman range is that of calcite-replaced vegetation (stems from -3.5 to -6&L SMOW. and roots) and preserved soils (Fig. 11). 13C Carbonate replacement of vegetation is environdata for these forms (Fig. 8) do not show observed in travertine-forming ments where a high pCOZ is present, such the highly depleted, nonequilibrium isotope as the geothermal travertines from Tivoli effects seen in the arid regime travertines. Rather, these data fall into the range of dis- (Turi, 1986) and Mount Edziza (pC0, > 1
328
CLARK AND FONTES -0
E
1
-10
-
-12
-
1
I
l
0
0
02
0
-14
0 0
0
00
co
102.6 TAMS (1.7)
-
92.9
4B 94.8
@
(0.7)
e
(2.3) u3 0
-16
QQ.8 (1.2)
-
f
‘96.5
(0.7)
:!a
0
LAMINATED
@
STALAGMITE
0
NODULE
0
CaC03
CRUST
115.5 (0.9) TAMS -18a -34
-30
-26
-22
-18
-14
6 13C ‘loo PDB FIG. 7. Stable isotope contents of modem travertine in Oman. 14Cactivities and errors (in brackets) measured for some samples are labeled (TAMS: tandem mass accelerator spectrometry, Jif-SurYvette, France). Note that the crusts were forming at the time of sampling (Fig. lo), while the nodules and stalagmite were dry and no longer forming.
"
1
4-
I
I
0
ATMOSPHERIC C-3
TYPE
O-
P8
0 b)
-8 -
l 63 0.0.
lD 0 -12
0
0
-16
.
0 -22
-18
-14
-10
I -6
0
LAMINATED
8
NON-LAMINATED
A
FRACTURE
-
a
FRACTURE
0
CALCITE-REPLACEDVEGETATION
SPRING
1 -2
0
2
6 13C ‘/oo PDB FIG. 8. Stable isotope contents of fossil travertines and fracture carbonates sampled in this study. Fracture calcite forms include coarsely crystalline fracture fangs and dense, bedded deposits which form at the discharge outlet of fractures on the surface. Soil zone travertines include calcified vegetation (Fig. 11). The fields showing equilibrium with CO, are determined using “0 fractionation data of O’Neil et al. (1969), the “C fractionation data from Mook et al. (1974), a temperature of 2O”C, and a range of -4 to 0 for 6’8O in the hyperalkaline groundwaters.
PALEOHYDROLOGY
OF
329
OMAN
FIG. 9. Calcite nodules found occasionally on the surface of fossil travertines. With 613C values as low as - 33.4, these features are perhaps the most isotopically depleted travertines yet reported.
atmosphere, Clark er al., 1989). However, for the calcite-replaced vegetation seen in the Oman travertines, the observed replacement is probably due to the inundation of the soil zone at some point by the more deeply circulating hyperalkaline groundwaters according to the sequence of events: (i) Humid climatic conditions. Development of a shallow bicarbonate-groundwater circulation system; growth of fracture calcite crystals in zone of mixing between ascending hyperalkaline groundwaters and shallow bicarbonate groundwaters. (ii) Installation of vegetation patterns and development of soil zone. (iii) Climatic deterioration. Zone of mixing rises toward the surface in response to a decrease in recharge of shallow bicarbonate groundwaters. (iv) Renewed discharge of hyperalkaline groundwaters at the surface, initially within the soil zone. Inundation of the soil zone resulting in calcite replacement.
PALEOCLIMATIC
OBSERVATIONS
Forty-two samples of travertine and fracture carbonate, classified on the basis of their stable isotopes and textural features, were analyzed for their radiocarbon contents by benzene synthesis. Results are normalized to 613C = -25%0 according to e14C = 2.3 e13C; (Salibge and Fontes, 1984; Table 1) and are presented in the paleoclimatic reconstruction of Fig. 12 showing the climatic variations for the upper Pleistocene and the Holocene in Oman. In many studies of continental carbonates, radiocarbon dating is encumbered by the problem of assessing the origin of the carbonate and determining the 14C activity (A,,) at time of formation. However, as the carbonates studied in northern Oman have formed from modern sources of COz, no error by dilution with dead carbon is introduced. From these radiocarbon data, it is evident that the Holocene and late Pleistocene (to cu. 35,000 yr B.P.) were characterized
FIG. 10. Fracture carbonate seen in a section through the upper ultramak sequence, exposed by of Fig. 4 supplying the date palms. Note calcite crust forming on surface of water. Cut the qanatfalaj is about 2.5 m deep.
by a dominantly pluvial climate with two principal arid episodes centered on the glacial maximum and the late Holocene. Comparative data are provided by McClure (1976) who dated lacustrine carbonates from the A’Rub Al Khali (shown in Fig. 1, inset). Late Pleistocene
Pluvial
Preservation of travertine deposited prior to the late Pleistocene pluvial is poor due to subsequent erosion during this period. Hence, paleoclimatic interpretations prior to about 35,000 yr B.P. are not clear. However, fracture calcite from the lower
slope area of Nizwa A’Raddah suggests that wet conditions prevailed around 35,000 yr B.P., and calcified roots in a wellpreserved red soil (Sample NT6f, Fig. 11) were dated to greater than 45,000 yr B.P. The extremely low 14C activity of this sample (45,000
‘T
- 10.0 -13.1 - 13.9 - 10.4 -11.5 - 15.8 - 10.4 -9.4 -2.6 -9.3 -3.5 -2.3 5.8 -5.5 -0.5 -2.3 0.2 0.7 -4.1 -2.2 -1.0 -6.0 -6.9 -4.2 -9.5 -6.2 -8.4 -9.9 -3.5 -4.5 -4.6 -4.5 2.8 -8.1 -4.8 -1.8 -7.9 -2.2 2.7 -4.7 -9.5 -5.8 -4.0 -5.7
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
a AU analyses were carried out at the LJniversitC de Paris-Sud with the exception of NR4b, NRlOc, NRKc, and NR16d (Institute for HydrologyKSF, Munich) and NR9c, NR9e, NRl6e, NR2la, and NRlOa (Environmental Isotope Laboratory, University of Waterloo).
Pleistocene humid phase in the northwestern Sahara, shown by 234U/230Th to be much older (ca. 80,000-100,000 yr B.P.) (Causse et al., 1988).
Latest
Pleistocene
Arid Phase
Late Pleistocene humidity shows signs of possible deterioration by ca. 19,000 yr B.P.
PALEOHYDROLOGY
333
OMAN
s 13C ‘/oo PDB
PALAEOCLIMATIC CHRONOLOGY
OF
-25
-*D
-10
-15
6 -5
0
-15
.
18
0 %o PDB
-10
l
-5
0
5
2 0 g
r8 a
AA
0 4
.Og 0.6 .*. 0
0
8
00
8
8
8
0 A
A
8 e
8
ii
A
A A 63 A
0 @e
8
A
n
n
Cl
PALAEOCLIMATE ARID
0 8
TRAVERTINE .
LAMINATED
8
NON-LAMINATED
a
FRACTURE
CALCITE
A
FRACTURE
SPRING
0
CALCITE-REPLACED
VEGETATION/SOIL
FIG. 12. Paleoclimatic chronology for Oman travertines and fracture carbonates. Stable-isotope data given for those samples measured for AL4C (Table 1).
However, by 16,300 yr B.P., hyperarid conditions dominated, and a thick uninterrupted sequence of laminated, isotopically depleted travertines was deposited on the hillslope at Nizwa A’Raddah until around 13,000 yr B.P. Further, as no fracture carbonates are found during this arid phase, shallow circulation of bicarbonate groundwaters must have been extremely restricted or did not exist. McClure (1976) demonstrates that hyperarid conditions were established in the
A’Rub al Khali by 17,000 yr B.P., as represented by termination of playa lake sedimentation and the deposition of red eolian sands. Early Holocene Pluvial A wet early Holocene has been well documented in North Africa and the Middle East (e.g., McClure, 1976; Street and Grove, 1979; Street-Perrott and Roberts, 1983; Fontes et al., 1985, 1987; Gasse and Fontes, 1990; Zouari, 1988), and is docu-
334
CLARK
AND
FONTES
mented here for northern Oman. A number of fracture calcites and fracture-spring deposits and replaced vegetation from the upslope area at Nizwa A’Raddah (Fig. 3) indicate that early Holocene pluvial conditions were established by about 12,500 yr B.P. and persisted until as recently as 6100 yr B.P. However, formation of laminated travertines indicates that climatic deterioration began as early as 6200 yr B.P. The overlap in ages of arid travertines vs humid types suggests a transitional period, although for only a period of a few hundred years. McClure (1976) shows a “subpluvial” with renewed although less extensive playa lake development between ca. 9000 and 6000 yr B .P., which he attributes to a temporary northward displacement of the southwest monsoon. Rossignol-Strick (1983) relates sapropel formation in the Mediterranean Sea to periods of high monsoonal activity in east Africa, stimulated by periods of high summer insolation, forced by Milankovitch orbital mechanisms during the late Pleistocene. Kutzbach and StreetPerrott (1985) place the most recent onset of monsoon strengthening at ca. 15,000 to 6000 yr B.P., reaching a maximum between 11,000 and 10,000 yr B.P. These dates correspond well with the Holocene pluvial in northern Oman (Fig. 12). The southwest monsoon is at present restricted to a small coastal region in the Dhofar highlands of southern Oman. The Sr3C values for fracture calcites from the Holocene period (Fig. 12) are considerably higher than those for fracture minerals from the late Pleistocene pluvial (ca. 25,000 yr B.P.), suggesting that tropical grassland vegetation (CCtype photosynthesis) dominated the bicarbonate in the groundwater. This agrees with McClure’s (1976) interpretation of a grassland vegetation for the A’Rub al Khali.
laminated travertine giving the appearance of a flowing landscape (Figs. 3 and 5). Radiocarbon dates (Fig. 12) relate this phase of travertine formation to a mid-Holocene (ca. 6100 yr B.P.) climatic deterioration and the onset of the hyperarid regime of modern-day Oman.
Late Holocene Arid Phase The above-described sequences of travertine have been glazed over by a thin (10 to 20 cm) yet pervasive coating of “arid-type”
Although the beginning of the late Pleistocene pluvial in northern Oman is speculative, our data show that the pluvial persisted for at least 15,000 years, significantly
Erosion and Geomorphological Observations The climatic variations recorded by the textures and isotope contents of the travertines have also left a geomorphological imprint on these deposits. Surface flow in the small wadi draining the periphery of the ophiolite bedrock (Fig. 3) during pluvial phases has likely been responsible for the erosion of the local travertine deposits. The remnants of carbonate-preserved soils from the late Pleistocene on the crest of the slope (Fig. 3) suggest subsequent downcutting occurred, probably during the early Holocene pluvial. Similarly, the laminated travertines of the late Pleistocene hyperarid phase have been deeply incised by subsequent fluvial erosion. Accumulation of travertine during the current arid phase demonstrates that such erosional processes, despite a lack of vegetation cover, are not effective today and are thus additional evidence for pluvial periods. SUMMARY
The principal climatic episodes for the late Pleistocene and Holocene of northern Oman can be summarized as follows: Period (yr B.P.)
Climate
>33,ooo to 19,000
Pluvial, with brief arid episodes Climatic deterioration Hyperaridity Climatic improvement Pluvial Climatic deterioration Current arid phase
19,000 to 16,300 16,300 to 13,000 cu. 13,ooo 12,500 to 6500 6500 to 6100 6100 to Present
PALEOHYDROLOGY
longer than the early Holocene pluvial. Further, the genesis of this wet period may differ from that of the early Holocene pluvial, which clearly corresponds to increased monsoon activity. Rossignol-Strick (1983) and Kutzbach and Street-Perrott (1985) link a periodically strengthened southwest monsoon to Milankovitch orbital mechanics involving increased summer insolation and the northern tropical thermal gradient. However, their monsoon index and the sapropel evidence for such increased monsoon activity show only two periods of monsoon-triggered pluviosity in the late Pleistocene, ca. 82,000 and 11,000 yr B.P. A wet late Pleistocene in northern Oman may then be a function of lower evapotranspiration during the generally low orbital insolation which characterized the last glacial maximum. The coincidence of late Pleistocene hyperaridity (17,000 to 13,000 yr B.P.) with the end of the northern hemisphere glacial maximum is also instructive. At this time, the Arabian Gulf was dry and no longer a potential vapor source for the interior. Only by about 12,000 yr B.P. had the Arabian Gulf reoccupied at least the central portion of modem basin (Felber et al., 1978). However, by this time, a renewed southwest monsoon had already begun a period of climatic improvement. ACKNOWLEDGMENTS Annik Filly, Marc Massault, Ho Kim Ngan, Sophie Maurice, et tout I’equipe du Laboratoire d’Hydrologie et de Geochimie Isotopique sont remercies pour leur aide technique et leur antitie. Barghash bin Ghalib al Said, Marlin Lowry, and Digger Jones (Public Authority for Water Resources, Oman) and Dan Danesh, (Cansult Oman Limited) facilitated field work in February 1988. Research support to I. Clark was provided by Le Centre International des Etudiants et Stagiaires (France). Peter Fritz contributed substantially to the inception of this study and critical review of the manuscript.
REFERENCES Barnes, I., and O’Neil, J. R. (1969). The relationship between fluids in some fresh alpine-type ultramaIics and some possible modem serpentinization, western
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