Rainfall in the Negev Desert during the middle Holocene, based on 13C of organic matter in land snail shells

QUATERNARY

Rainfall

RESEARCH

34, 186-197 (1990)

in the Negev Desert during the Middle Holocene, 13C of Organic Matter in Land Snail Shells

Based on

A. GOODFRIEND Isotope Department, Weizmann Institute of Science, 76 100 Rehovot, Israel GLENN

Received August 28, 1989 Analysis of stable carbon isotope ratios (‘3CI’2C) of organic matter in land snail shells is used to infer middle Holocene rainfall amounts in the Negev Desert by reconstructing the distribution of C, plants in the family Chenopodiaceae. The organics are derived from the diet of the snails, which consists of plant material, and are enriched in “C where C4 plants are present. A survey of modem plant communities indicates that in areas receiving 2300 mm mean annual rainfall, nearly all plant communities consist of C, species only (no C4 chenopodes), whereas in areas under ~230 mm rainfall, most plant communities contain one or more C, chenopode species. In between is a transition zone consisting of a mosaic of both pure C3 and mixed C3 + C,, plant communities. Isotopic results for fossil land snails indicate a consistent geographic pattern throughout the middle Holocene, from ca. 6500 to 3000 yr B.P., with the transition zone located ca. 20 km south of its present position. This implies a near doubling of rainfall within this region as compared to preSent.

0 1990 University

of Washington.

INTRODUCTION

The geographic distributions of plants having a C4 photosynthetic pathway are generally related to climatic conditions. C4 dicots (members of many families but especially the Chenopodiaceae) tend to predominate in drier areas (Teeri, 1979; Goodfriend, 1988), whereas C, monocots (various species of grasses) tend to predominate in hotter areas (Teeri and Stowe, 1976; Vogel et al., 1986). Because of this relationship to climate, past records of the presence or absence or the abundance of C, plants can be useful as paleoclimatic indicators. As a result of the large enrichment of i3C found in C, plants as compared to C, plants (average 6i3C .value’ for C3 plants is . ca. -27%0; for C, it IS ca. - 11%0), it is possible to detect records of C, plants by isotopic analysis of fossil plant material or organic matter in animal skeletons, which is derived from plant material in the diet of the animal. This approach of paleoclimatic reconstruction from isotopic records of C, ’ 6i3C is defined as the deviation of the ‘3C/‘2C ratio of a sample from that of the PDB standard, expressed in per mil.

plants has been applied to materials as diverse as soil organic matter (Krishnamurthy et al., 1982), rock varnishes (Dorn and DeNiro, 1985), mammal teeth (Vogel, 1983), and land snail shells (Goodfriend, 1988). In the present study, rainfall conditions in the Negev Desert of southern Israel during the middle Holocene are inferred from reconstruction of the past geographic distribution of C, plants (species in the Chenopodiaceae), based on analysis of organic matter in the shells of the land snail Trochoidea seetzeni. This represents an extension of a previous study (Goodfriend, 1988) which covered the period ca. 4000-3000 yr B.P. Additional data are presented for this period and the study is extended back in time to ca. 6500 yr B.P. In land snail shells, organic matter occurs in the form of an organic matrix, which is composed of proteins (Hare and Abelson, 1965) with some polysaccharides (Poulicek, 1982). The organic matrix is generally considered to serve as a site for initiation of calcification of the shell (Lowenstam and Weiner, 1989). As in all animals, organic carbon is derived almost entirely from the

186 0033-5894/90 $3.00 Copyright All rights

0 1990 by the University of Washington. of reproduction in any form reserved.

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diet. In the case of T. seetzeni, the diet consists of dead plant parts which have dropped onto the ground or senescent plant material still attached to the plant. They apparently eat only dicots (not monocots, such as grasses) (Goodfriend, 1988). In the northern Negev, all the C, dicots are species of Chenopodiaceae (see below). The relationship of the 613C values of organic matter of shells of T. seetzeni to the presence or absence of C, Chenopodiaceae was evaluated by analysis of 18 live-collected populations from pure C, communities and from mixed C, + C4 communities which included a variety of C, species (Goodfriend, 1988). Based on this calibration, corrected for the change in atmospheric i3C due to the industrial effect (Stuiver et al., 1984), and allowing an uncertainty of *0.5%0, fossil snail organic values of Si3C < -22.2%0 are taken to indicate pure C, communities, while values > - 2 1.2%0 are taken to indicate the presence of some C, plants. Samples having intermediate values (-22.2 to - 21.2%0) are considered ambiguous and are therefore disregarded in the reconstruction of plant community distributions. The Negev Desert is a zone of strong rainfall gradient (Fig. 1); it is the transition zone between the hyperarid SaharanArabian desert belt to the south and the more humid Mediterranean climate to the north. This gradient is the result of the existence of a high pressure cell in the south, which tends to deflect rain-bearing systems from the Mediterranean eastward, away from the Negev. Because of this strong gradient, precise information on paleorainfall in the region can be obtained only from strictly local paleoclimatic indicators; those that integrate climate over too broad an area (e.g., lake levels and possibly pollen records) will obscure the patterns of change. It is for this reason that the organic matter of land snail shells, which reflects the vegetation of the immediate area, was chosen for study.

IN

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MATERIALS

AND METHODS

Specimens of T. seetzeni were collected from road cuts, wadi cuts, and various pits and trenches and were obtained from archeological excavations. Most of the samples came from rodent burrow middens or from wadi deposits but some samples from colluvial deposits were also analyzed. The rodent burrow midden samples consist of snails that were collected by rodents, carried inside their burrows, and eaten (see Goodfriend (1987a) for further details and photograph). The rodent species responsible for these middens is not known but in one midden, some bones belonging to a Gerbillus sp. were found (identified by Dr. E. Tchernov, Department of Zoology, Hebrew University). Rodent midden samples were the preferred material in this study, since the shells are of uniform age (there are no problems of possible redeposition of shells) and were collected by the rodent within a very small area (not more than tens of meters). Dating of the shells was carried out by 14C analysis of the shell carbonate of bulk samples (ca. 15-40 shells, weighing IO-25 g). All shells were first thoroughly cleaned of secondary deposits (mostly sediments cemented by soil carbonates) by a combination of ultrasonication, mechanical cleaned using a motorized tool with various dental cleaning attachments (at 7X magnification under a stereo microscope), and were given a final light dip in dilute HCl (see Goodfriend (1987a) for further details of procedures). Before selection for analysis, most rodent midden samples and all other samples were checked for age uniformity by amino acid epimer analysis (Dalloisoleucine/L-isoleucine, or A/I) of six individual shells from the samples. A/I ratios of Holocene land snails from the Negev show good age-predictive ability (Goodfriend, 1987b). Rodent middens nearly always showed uniform A/I ratios, whereas other deposits sometimes showed variable

GLENN A. GOODFRIEND

188

Pill -,Os~“aO6-----

/ So’,

IO

1

Negev

km

/

l Cq onlv I FIG. 1. Map of the northern Negev Desert and the adjoining area to the north (between 100 and 400 mm mean annual rainfall) showing the modem distribution of pure C, plant communities (without C, Chenopodiaceae) and mixed C, + C, plant communities (with C4 Chenopodiaceae). Mean annual rainfall isohyets (dashed lines) and elevational contours (solid lines; areas of >600 m elevation shaded) are shown. The transition zone, consisting of a mixture of pure C, and mixed C, + C4 plant communities, is outlined by a dotted line. Insert shows position of the Negev Desert, at the northern edge of the Saharan-Arabian desert belt.

A/I ratios, indicating that the deposit consisted of shells of mixed age (Goodfriend, 1987a). Deposits with a coefficient of variation (SD/m > ca. 20% were rejected. All radiocarbon dates presented below are corrected for the age anomaly characteristic of the shell carbonate of land snails living in

carbonate-rich areas (Goodfriend and Stipp, 1983). This anomaly results from the fact that some of the body fluid bicarbonate, from which the shell carbonate is precipitated, is derived from calcium carbonate ingested by the snails. In a previous study (Goodfriend, 1987~)) age anomalies

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IN

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189

were measured in four samples of livebased on the rainfall map of the Israel Mecollected T. seetzeni. They were found to teorological Service (1977), which is based average 1610 yr and have an among-sample on data from 79 stations within the study variation of 5230 yr. Subsequent measurearea. Sites showing signs of burning or of ment of an additional live-collected pre- heavy grazing were avoided. The sites sebomb sample (RT-910; Haluqim Plain, N. lected were generally 2 1 km apart, to avoid Negev; collected 14.X.1949) gave an ap- biasing the results by concentrating too parent 14C age of 1540 + 90 yr B.P. (calcu- many data in one area. The survey was limlated according to method of Goodfriend ited to silty and clayey substrates (the dune (1987~)). Combining all these data, the es- areas in the northwestern part of the Negev timate of the average age anomaly and its were not included), since all fossil snail variability becomes 1590 ? 190 yr; a samples came from such sediments. At rounded figure of 1500 yr has been used each site, an area of ca. 10 x 10 m was here to correct the ages and the reported surveyed. The predominant species, and all uncertainties of the ages have been calcu- C, chenopode species, were listed for each lated combining the reported laboratory er- site. Identification of plants as C4 species ror with the age anomaly variability (square was based on published information root of the sum of the two variances). (Shomer-Ilan et al., 1981; Ziegler et al., The snail shell organic matter analyzed 1981; Winter, 1981) as well as carbon isofor 13C was obtained from the same set of tope analysis of species in families having shells used for 14C analysis. The shells were C, species but for which there are no data reacted with 5 N HCl (6 ml/g shell) under in the literature. Two species analyzed, Pegvacuum. The reaction was allowed to pro- anum harmala L. (Zygophyllaceae; 1 km ceed until no more CO, was generated and IV of Beer Sheva; S13C = -26.6%0) and Sibth. and Sm. boiling ceased. The CO1 generated was Polygonurn equisetiforme used for 14C analysis. The remaining solu- (Polygonaceae; Nahal Shiqma at Tel Milha; tion was centrifuged to obtain a pellet of 6r3C = - 25.0%0), both turned out to be C, organic matter (the acid-insoluble fraction), species. which was washed with distilled water and PRESENT DISTRIBUTION OF C4 centrifuged (repeated three times), then DICOTS IN RELATION TO RAINFALL freeze-dried. The organic matter was converted to CO, by combustion with CuO in Within the region of the survey, all C4 sealed evacuated quartz tubes for 2 hr at dicots were found to be shrubs or semi820°C. Stable carbon isotopes were ana- shrubs of Chenopodiaceae. The predomilyzed on a Varian M250 mass spectrometer nant species include Noaea mucronata and measured against standards calibrated (Forssk.) Aschers. and Schweinf., Hamto NBS-19. Several CO2 samples were mada scoparia (Pomel) Iljin, Anabasis ursealed into borosilicate glass tubes and ticulata (Forssk.) Moq., and A. syriaca Ilmeasured at the Department of Earth and jin. Also locally abundant are Halogeton Space Sciences, University of California, alopecuroides (Del.) Moq., Suaeda asphalLos Angeles. These samples were calitica (Boiss.) Boiss., Chenolea arabica brated against a sucrose standard, which Boiss., Salsola vermiculata L. var. villosa was calibrated to NBS-19. Eig., and Atriplex leucoclada Boiss. (noThe present geographic distribution of C, menclature from Danin, 1983). No CAM chenopodes was determined by a survey of (crassulacean acid metabolism) plants were 189 sites in the northern Negev, between found in the area. the 400 and 100 mm mean annual rainfall The results of the plant community surisohyets. The mean annual rainfall at each vey (Fig. 1 and Table 1) indicate a very site was interpolated to the nearest 10 mm clear distribution of C4 plants in relation to

190

GLENN

A. GOODFRIEND

TABLE

1. DISTRIBUTION OF NORTHERN NECEV PLANT COMMUNITIES HAVING ONLY H.SVING BOTH C, SPECIES AND C, CHENOPODES, IN RELATION TO MEAN ANNUAL

Mean annual raiofah (mm)

No. of sites with C, only

330-400 300-320 240-290 200-230 100-190

No. of sites with C, + C,

31 20 18 1 9

Total no. of sites

2 1

33 21 1

18 21 68

36 22 77 I

C, SPECIES AND RAINFALL % of sites with C, only (?SE)” 94 (23) 50 (t8) 10 (k3)

189 a Calculated

from

the binomial

distribution.

mean annual rainfall. C, plants are virtually absent from wetter areas (mean annual rainfall 300-400 mm)-94% of the sites have only C3 species (Table 1). Between 240 and 290 mm mean annual rainfall, C, plants are present at half the sites surveyed. From 100 to 230 mm, the great majority of the sites (90%) have C, species present. Thus, the pattern is one of plant communities composed almost entirely of C, species in areas with 3300 mm rainfall; a transition zone between 240 and 290 mm; and a zone in which most plant communities have both C4 and C, species at ~230 mm. These results agree well with those published previously for a broader area which included also the Judean Desert along the Dead Sea rift valley (Goodfriend, 1988). The present study, restricted to the Negev, includes 152 of the 173 sites surveyed in the previous study and an additional 37 new sites. The only difference found was that additional sampling around the 290-mm isohyet indicated that it should be considered within the transition zone rather than within the zone of predominantly C, plant communities (3 of 10 sites have C, species). Data for the northwestern part of the survey region (on the coastal plain) are sparse due to extensive cultivation in this area. N. mucronata is the northernmost C4 species and therefore the one whose distribution determines the boundary between the predominantly pure C, communities and the transition zone. This is also the C4 species which occurs occasionally within

the predominantly pure C, plant zone (three records between 300 and 400 mm rainfall). A. articuluta occurs within the transition zone (up to 270 mm rainfall) in the eastern part of the survey area. The other C, species occur only within the zone in which mixed C, + C, plant communities predominate. The northern part of the C, plant zone (200-230 mm) consists almost entirely of plant communities that have C, species (21 of 22 sites). Only further south do significant numbers of pure C, communities occur within this zone. This pattern appears to relate to edaphic conditions, rather than to rainfall per se. The southern area consists of limestone hills with a very thin sediment cover along the flanks and tops; however, the lower slopes are covered by colluvium or alluvium, and the hills are separated by varying expanses of flat alluvial plains. It is on these very thin sediments on the hills that pure C3 communities often occur. They consist predominantly of Zygophyllum dumosum and Artimesiu herba-albu, and to a lesser extent, Gymnocarpos decander, Reaumuria hirtella, and R. negevensis. On the thicker sediments of lower slopes and alluvial plains, C, plants (H. scopuriu and/ or sometimes Halogeton ulopecuroides, N. mucronuta, A. articulutu, etc.) are invariably present. In the northern area, little bedrock is exposed, due to a deep cover of loess (Bruins and Yaalon, 1979). Here also on these thick sediments, plant communities with C, species predominate.

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One apparent exception to the general pattern of plant community distribution occurs in an area in the extreme northeastern Negev, northwest of Arad (Fig. 1). Here in the transition zone, between the 240 and 290 mm isohyets, only pure C, communities were found. Elsewhere within this zone, N. mucronara is usually found. A more detailed survey of this area, which included four additional sites (not included in the general survey because the sites were
Analyses were carried out of ‘3C/‘2C ratios of organic matter from 40 samples of T. seetzeni, radiocarbon dated to between 6700 and 2800 yr B.P. (Table 2). The patterns of distribution of plant communities inferred from these analyses are examined for three time divisions within this period. Of the 16 samples falling within the period ca. 39OG2800 yr B.P., 13 have 6t3C values whose interpretation is unambiguous (Fig. 2 and Table 2). These show a consistent geographic pattern of pure C3 plant records in the north and the presence of C, plants at all the southerly sites, with the transition zone occurring in the vicinity of what is today the 150-mm isohyet. A very similar pattern is seen for the period ca. 5100-4300 yr B.P. (Fig. 3), with pure C3 records in the north, the presence of C4 plants in the south, and a transition zone around the modem 150-mm isohyet. This same pattern is seen also during the period ca. 6700-5300 yr B.P. (Fig. 4). Plotting the data for all periods together yields a geographically consistent picture (Fig. 5): (1) Around the present-day 150-mm isohyet there is a transition zone, consisting of a mixture of pure C, plant communities and communities having C, plants. (2) To the north of this transition zone

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TABLE 2. LIST OF LAND SNAIL (Trochoidea seetzeni) SAMPLES ANALYZED, WITH TYPE OF DEPOSIT, 14C AGE, AND 613C VALUE OF ORGANIC MATTER OF SHELLS

Site no. 111 168 (180 cm) 168 (290 cm) 174 177 190 202 211 215 227 247 254 216 279 301 318 329 362 369 380 390 392 397 401 418 421 425 473 476 482 485 494 526 560 1097 1145 A4264 A4570 G282 Ml03

14C age (*SD) (yr B.P.)

Type of deposit” Colluvium Alluvium Alluvium Colluvium Alluvium Alluvium ? Colluvium Colluvium ? Colluvium RM RM SM RM RM Alluvium RM RM Alluvium RM RM RM Alluvium RM RM RM SM RM Alluvium RM RM RM RM RM RM RM Alluvium A A A A

5680 4600 4740 4450 3660 6710 3010 6240 6560 5950 5100 4640 4690 3130 3640 4840 3370 6470 3380 3840 4320 3840 5500 2790 5360 2970 3990 3120 3530 4330 5520 3630 3170 5270 3950 4320 4380 3930 5490 4850

a RM, rodent burrow midden; midden of rodent; A, archeological

613C of organics (%d

(+260) (+310) (k310) (k260) (5250) (k260) (2260) (-c320) (-+220) (2250) (2250) (k250) (2230) (2270) (k220) (2280) (k280) (2250) (r250) (2210) (+-240) (2210) (2250) (2210) (2250) (5280) (2260) (2210) (2220) (k230) (k210) (k 250) (2200) (+-260) (2260) (2250) (2230) (k220) (2300) (2220)

-23.2 -20.3 -20.2 -21.0 -21.1 - 20.6 -24.1 -23.0 -22.7 -22.1 -22.1 -21.9 -23.7 - 22.4 -21.8 -22.3 - 17.2 -23.0 - 24.0 - 19.1 -21.4 -20.5 -21.3 -22.6 - 18.7 - 18.9 -21.3 -22.8 - 20.8 -21.2 - 22.2 -23.3 -21.9 - 18.7 - 19.7 -20.7 -21.7 -21.7 -22.9 -23.4

SM, buried excavation.

surface

there are records of only pure C, plant communities. At present nearly all the plant communities in this area have C4 species present. (3) To the south of the transition zone, the presence of C, plants is recorded at ev-

192

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FIG. 2. The distribution of pure C, and mixed C, + C, plant communities in the northern Negev, based on VC values of organic matter of land snail shells radiocarbon dated to 3900-2800 yr B.P. The shaded area represents the present position of the transition zone between pure C, and mixed C, + C, plant communities based on Figure 1

ery site. This area also supports mainly plant communities with C4 species at present. The middle Holocene distribution pattern of plant communities in the Negev is thus similar to the present, except that it is shifted ca. 20 km to the south. If any significant shifts in the position of the transition zone had occurred within the period ca. 650@-3000 yr B.P., this should be seen as a widening of the transition zone compared to the modern one. Because this is not observed, the distribution patterns of the plant communities must have been essentially stable during the whole period. PALEOCLIMATIC INTERPRETATION MIDDLE HOLOCENE PLANT COMMUNITY DISTRIBUTIONS

OF

The distribution pattern of the plant communities in the Negev during the period ca. 6500-3000 yr B.P. implies that during this

period, the 260-mm isohyet, which constitutes the middle of the present-day transition zone, was located ca. 20 km south of its present position, i.e., around the presentday 150-mm isohyet. This indicates a near doubling of rainfall in this area in the middle Holocene as compared to the present. If the climatic differences between these periods are the result of a simple southward shifting of the isohyet patterns during the middle Holocene as compared to present, then all of the northern part of the Northern Negev (between the present-day 150- and 300-mm isohyets) would have experienced substantially greater rainfall in the middle Holocene, as the rainfall gradient in this area is quite steep (Fig. 1). Further to the north the gradient is weaker and a 20-km shift in the isohyet positions would have had a smaller effect. Similarly, further south a less dramatic change would have been experienced. For example the Sde Boqer area

HOLOCENE

RAINFALL

IN

193

NEGEV

31°N

---

Plant community For

each

-I

:

site: -21.3

(3990)

39E I

1 FIG.

3. As in Figure

2 but covering

(Fig. l), which now receives 100 mm mean annual rainfall, would have received ca. 130 mm if the isohyets were shifted 20 km southward. A simple geographic shift in the position of the isohyets would occur if rainfall changes were due to shifts in the average position of the high pressure cell that sits over the eastern Sahara and Arabian deserts to the south (inset, Fig. 1) and which causes storm-bearing systems from the north, northwest, or west to be deflected toward the east. Alternatively, the increased middle Holocene rainfall could have been the result of increased frequency of arrival of rain-bearing systems in the region. Under this scenario, rainfall would have increased by a similar amount throughout the region. Alternative paleocological interpretations of the shift in the distribution of C4 plants could also be made. Although it was shown above that C, plant distributions relate closely to mean annual rainfall, there

51004300

Yr B.P.

are other climatic variables that are usually closely correlated with mean annual rainfall, such as the mean annual number of rain days. Possibly C, plant distributions are causally related to one such correlated variable rather than to mean annual rainfall per se. Grazing by domesticated animals can be another factor responsible for changes in plant distributions. However, during part of the period considered here, from Middle Bronze Age II through Late Bronze Age (ca. 3700-3200 14C yr B.P.; Weinstein, 1984), no human cultural remains are known from the northern Negev south of Beer Sheva (Cohen, 1985), whereas previously during the Chalcolithic and Early Bronze Ages (ca. 650&3900 yr B.P.), there were extensive human settlements across the northern Negev, including the ancient city of Arad. If grazing by the domesticated animals of these peoples had a significant impact on the distribution of C4 plants, it would in-

194

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FIG. 4. As in Figure 2 but covering 6700-5300 yr B.P.

deed be strange that for the next ca. 500 yr following abandonment of the Negev, the C4 plant distributions would have remained unchanged. Thus, although grazing may well have affected plant biomass and perhaps the relative abundances of species, it does not seem to have had an impact on the distribution of C4 plants. COMPARISON TO OTHER PALEOCLIMATIC RECORDS IN THE REGION

Several pollen diagrams-none welldated but covering part or all of the period under consideration-exist for the area of northern Israel. The Hula pollen diagram of Tsukada (van Zeist and Bottema, 1982) covers all of this period but there are no radiocarbon dates within this time interval itself. A general trend of decreasing deciduous tree pollen is seen between levels radiocarbon dated at 7400 f 160 and 2480 2 100 yr B.P. Horowitz’s (1979) pollen diagram for Hula borehole U.P.6, which dates

from ca. 5000 yr B.P. (lowest dated level is 4565 +- 75 yr B.P.) shows a similar overall trend but does not agree in detail. A core (KIND4) from Lake Kinneret analyzed by Baruch (1986) contains only one reliable radiocarbon date (a twig dated at 2750 + 150 yr B.P.; Thompson et al., 1985), the other dates being based on organics in sediments, corrected for hard water error according to the model of Thompson et al. (1985). The ca. 1 m of sediments lying below the 2750 yr B.P. date, which presumably includes some of the latter part of the period under consideration, shows a slight progressive decrease in arboreal pollen with time, due to a decrease in deciduous oaks. Comparison of these core pollen records with modem pollen spectra is problematic, as the vast majority of the forests in the north have been cut down during historical times. While these various pollen studies give a broadly consistent picture, their interpretation in terms of paleoclimate is problematic. The decrease in arboreal pollen might

HOLOCENE

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IN

195

NEGEV

3l”N

-\.---

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/

Plant community : 0 C3 only

0 35Y

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FIG. 5. Summary of middle Holocene (ca. 6500-3000 yr B.P.) distribution of pure C, and mixed C, + C4 plant communities in the northern Negev, based on data from Figures 2-4 and including only unambiguous points. The dotted lines outline the middle Holocene position of the transition zone between pure C, and mixed C, + C4 plant communities. The shaded area represents the present position of this transition zone.

have resulted from a general trend of drying or from deforestation and/or cultivation activities by man (Baruch, 1986). Dead Sea levels also mainly reflect conditions in the area of northern Israel, the main source of water within its drainage basin (Carmi et al., 1984). Limited evidence suggests that a high stand of the Dead Sea occurred about 6500 yr B.P. (Goodfriend et al., 1986). By 2800 yr B.P. the sea had fallen to near modern levels (Begin et al., 1985). However, it is not clear whether this was a gradual fall, resulting from progressively drier conditions, or whether it occurred rapidly at the beginning of this period, following the transition from a wetter early Holocene period (Goldberg and BarYosef, 1982; Danin, 1986; Magaritz and Goodfriend, 1987). Thus, the interpretation of rainfall during

the middle Holocene in northern Israel is ambiguous. It may have shown a progressive decrease over this period or it may have been relatively constant. It is also not clear whether the middle Holocene rainfall amounts in this region were significantly greater than at present. Consequently, it is not possible to determine the relationship between rainfall changes in southern and in northern Israel. Thus, an understanding of the mechanisms involved in climatic change in the desert boundary region of the south will have to await better paleoclimatic information from the north. CONCLUSIONS

The data presented here point to a stable position of the transition zone between pure C, and mixed C, + C, plant communities in the Negev during the middle Ho-

196

GLENN

A. GOODFRIEND

locene Cca. 65OCL3000 vr B.P.). This nosition is some 20 km south of its present position and implies significantly greater rainfall in the region at that time. This appreach of using the isotopic composition of land snail shell organic matter to reconstruct paleoclimates provides rather precise geographic detail, which is critical in regions such as the Negev where climatic gradients are very strong. While the method is potentially a powerful one for paleoclimatic work, caution should be exercised in applying it to older material where isotopic ratios may be diagenetically altered. ACKNOWLEDGMENTS The author is indebted to Dr. A. Danin for identification of plant material. Radiocarbon dates were provided by Mr. I. Carmi. Technical assistance was provided by Mrs. E. Negreanu, Mr. S. Shasha, Ms. K. Zuckerman, Mr. C. Watad, Ms. M. Wertheimer, and Ms. I. Tamir. Most of the mass spectrometric measurements were carried out by Mrs. R. Silanikov. Dr. J. Rounick also provided isotopic analyses of several samples and Mr. D. Winter (Department of Geology, UCLA) analyzed additional samples. Samples from archeological sites were provided by Dr. R. Amiran, Dr. Y. Gilad, and Dr. T. Levy. Dr. E. Tchemov provided identification of rodent bones. This research was supported in part by a grant from the Fund for Basic Research, administered by the Israel Academy of Sciences and Humanities.

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