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12C isotope ratios in landsnail shells
Isotope Geoscience, 1 (1983) 243-255 Elsevier Science Publishers B. V., Amsterdam - Printed in The Netherlands
243
ANNUAL CYCLE OF 180jl60 and l3C/IZC ISOTOPE RATIOS IN LANDSNAIL SHELLS
MORDECKAI MAGARITZ and JOSEPH HELLER
Isotope Department, The Weizmann Institute of Science, Rehouot (Israel) Zoology Department, The Hebreui Uniuersiiy, Jerusalem (Israel) (Received November 17,1982; revised and accepted May 25, 1983)
ABSTRACT Magaritz , M. and Heller, J., 1983. Annual cycle of landsnail shells. Isot, Geosci., 1; 243-255.
18
0 / 160 and 13C/12C isotope ratios in
Monthly data on the isotopic composition of carbon and oxygen in shells of the land snail, Theba pisano, were collected from two localities near Ashqelon, Israel. Seasonal trends in the isotopic data from the two areas were similar. However, significant monthto-month and locality-to-locality differences in the 6 13C_and 6 lBO-values of the snail shell carbonate indicate that microenvironmental diversity did indeed exist. We suggest that the factors responsible for the microclimatological diversity, such as plant coverage and primary productivity, control the degree of evaporation of "interface water" at the soil-air boundary. We used the oxygen isotopic composition of snail shell carbonate to calculate the isotopic composition of this "interface water", and the s 13C-values of snail shell carbonate to monitor the amount of heterotrophic CO 2 production in the soil.
ZOOLOGICAL AND ENVIRONMENTAL BACKGROUND
Landsnails are abundant in many continental loess and soil sequences (Fink and Kukla, 1977; Heller and Tchernov, 1978) and in archaeological sites (Evin et al., 1980; Magaritz and Heller, 1980). It has been suggested by Yapp (1979) and Magaritz and Heller (1980) that the isotopic record of the shell carbonate can be used to determine the palaeoenvironment. In order to better define the magnitude of local effects which may result in an incorrect interpretation of the conditions, we studied the isotopic record found in snail shells in vivo of one species from two quite distinct nearby microenvironments. Theba pisana Miller, a Mediterranean land snail that is very abundant along the Coastal Plain of Israel, is a semelparous species with an annual life cycle (Heller, 1982). Young snails hatch in early winter, grow rapidly in spring, and by summer they reach first adult shell size and then the adult shell weight. Shell size is negatively related to density, suggesting that, as in some other land snails, there may be a density-dependent inhibition of growth.
0167-6695/83/$03.00
© 1983 Elsevier Science Publishers B. V.
244
Summer forces the snails into 3-4 months of aestivation. After the first winter rains the snails, awakened perhaps by a sudden drop in barometric pressure, emerge from aestivation, copulate, and lay eggs. In mid-winter the snails die. We sampled two populations of snails found near Ashqelon on the southern coast of Israel. One group of snails (designated SA) was taken near southern Ashqelon ~ 1 km from the Mediterranean coast line. The snails' habitat here consisted of a 22-km2 area of semi-stabilized sand dunes. Perennial shrubs (mainly Artemisia and Retama) covered ~ 78% of the ground. Most of the snails from this location had effectively-banded shells. The other group of snails (group CAl was taken from central Ashqelon, in a small area (0.2 km") of stable sand where perennial vegetation covered only 2% of the ground and annual vegetation consisting of weeds covered the rest. The total organic productivity in this area is much larger than in the SA area, as is shown by the fact that the sand here has a soil-A horizon whereas in southern Ashqelon there is none. Most of the snails in the CA group had effectively non-banded shells. The life cycles, the reproductive biology and the seasonal changes in morphology frequencies of these two snail populations have been described by Heller (1982) and Heller and Volokita (1982). METHODS
The snails described in this study were taken from the monthly samples collected by Heller (1982). The snails were killed by asphyxiation in boiled water within hours of collection, and the soft parts were removed. In shells with diameter >9 mm, only the last half of the ultimate whorl, i.e. the whorl deposited most recently, was analyzed; shells with diameter <9 mm were used intact. Shell samples were heated overnight at 70-80°C in 30% hydrogen peroxide solution to remove shell organic matter, and then dried at 70-80°C. Shell samples were treated in vacuum with H 3P04 (sp. gr. = 1.8) and the 13C/12C and lSO/160 ratios of the evolved CO 2 were determined mass spectrometrically using McCrea's (1950) methods and a Varian~ M 250 mass spectrometer. The results for both C and 0 are reported in the usual a notation as the permil (%0) deviation of the sample CO 2 from the PDB Standard (Craig, 1957): S 16 olSO (%0) = eSo/160)~Pl~:- O/ 0 )standard X 1000 ( 0/ 0 )standard The reproducibility of duplicate samples was 0.1%0 for both carbon and oxygen measurements. When discussing the isotopic composition of oxygen in gas or liquid phases we will use the SMOW Standard (Craig, 1957). The PDB and SMOW scales are related by the equation:
e
s 180SMOW
=
1.0307 (j 180PDB- 30.70
245 RESULTS
The meteorological parameters and the mean monthly values of both 8 IBO and 8 13C are given in Table I.
Large variations were found in the 8 lBO-values of the shell carbonate in each population. With the exceptions of the SA population in two months (February and July 1979), no significant correlation was observed between the size of the shell and the 8 lBO-value of its carbonate. The carbon isotopic composition of the shell carbonate shows a more regular size-dependent pattern. In the SA population we found a significant correlation between the 8 13C-values and shell diameter: the small snails in the population had less negative 8 13C-values than the large snails.
Southern Ashqelon population (SA) The results of the snails from the southern Ashqelon area show bimodality in the isotopic data. Therefore, the overall monthly mean values of 8 180 and 8 13C have less significance than the mean values for the two designated sub-groups of large shells and small shells (Table I). All the snails from January 1979 were large and actually belonged to the previous (1978) generation. In February 1979, small shells belonging to the new (1979) generation, were found in the field together with the old 1978
Mg
300
mm
t'c
16
26
12
22
8
18
................ ~
: ...
250
..
~ ...
/--
FMAMJJ
ASON
Months
Fig. 1. Southern Ashqelon population: mean shell size (fine line); mean shell weight of those snails in the population whose diameter is 170 mm (thick line); and mean noon temperatures (dotted line) (adapted from Heller, 1982).
246 TABLE I Meteorological and isotopic data: the mean /) 18 0 _ and /) 13C-values of the shell carbonate (carb.) from the central Ashqelon and southern Ashqelon areas. divided into small and large subpopulations; also given are the calculated oxygen isotopic composition of the water and the carbon isotopic composition of the CO 2 (gas) which were in isotopic equilibrium with the mean /) 18 0 _ and /) 13C-values of the shell carbonate; the mean monthly temperature and the humidity at 20h0om; and the evaporation rate from a type-A pan Month Temper- Humidity ature (%)
(C)
Evaporation rate (mm day-I)
Central Ashqelon (CA) small snails
/)0'8 0
( 10 . )
1{79 2{79 3{79 4{79 5/79 6/79 7/79 8/79 9/79 10/79 11/79 12/ 79 1/80 2/80
14.2 16.1 17.3 20.0 20.7 24.8 26.1 26.5 25.7 24.2 21.6 14.8 13.4 13.9
72 66 68 64 76 76 74 74 75 68 66 69 71 72
2.0 3.3 3.9 6.2 6.0 6.9 7.5 7.1 5.8 4.7 3.4 2.5 2.3 2.4
large snails calculated from equilibrium
mean
-1.6 -1.7 -.().7
car b .
s:3 Ccar b . (
log)
-10.1 -11.2 -11.1
/)0180
( 100 )
H 20
-2.6 -2.3 -0.9
s:3 CC0 2 (
too)
mean /)180 /)13C o carbo 0 carbo ( log) ( 10 0 )
-21.7 -22.8 -22.6 +0.1 +0.6 -0.5 -1.1 -0.5 -0.4 -1.1
-9.4 -9.2 -9.6 -11.3 -10.7 -10.6 -10.5
generation. By March 1979 most of the southern Ashqelon population consisted of small snails (7.5-9.8 mm) belonging to the new generation (1979). During April and May the snails grew very rapidly (at a mean rate of 0.12 mm day"), After May, the growth in shell diameter diminished but the snails continued to gain weight (Fig. 1). The biological significance of this acceleration of CaC0 3 buildup may be related to the preparation, before the aestivation period, of a thick shell to protect the snail against the severe heat. Machin (1967) demonstrated experimentally that increased shell thickness reduces water loss, and should be considered a morphological adaptation in arid conditions. Another biological significancy may be the storage of shell components so that they will be available for activity immediately at the end of the aestivation period. In September some of the snails emerged from aestivation and became active whilst others were still aestivating. The large variability in the isotopic record in the period October-January may reflect addition of another variable which causes changes in the shell. This variable relates to the fact that some CaC0 3 of the shell is taken by the snail to deposit it in the eggs as a storage for building up embryonic shell of the next generation (Wilbur and Tompa, 1979). During this extraction, some recrystallization processes of the mother shell presumably take place. The degree of dissolution or
247
Southern Ashqelon (SA) large sn ail s
small snails calculated from equilibrium (, ls OH
(%0> '
0 (, 13C
CO
(%0)
calculated from equilibrium
m ean
(,13Ccar b (, u O H 0 o O car b · 0 ' 0 2.
(,l s
•
(,13CCO 0 '2
( /00>
( /00>
( /00>
( /00>
-1.6 +1.2 +0.2 +1.1 +0.6
-8.1 -9.2 -9.2 -7.7 -8.2 -8.2 -8 .7
-2.2 -1.1 +0 .3 +1.8 +1. 6 +2 .8 +2 .4
-19.7 -20.7 -20.6 -19.1 -19.4 -19.3 -19.8
+0 .4
- 9.0
+1 .7
-20.2
+0.1
- 9 .3 -8.2 - 8 .3
~.7
-20.9 -19.9 -20.0
(, I S
(, 13 o 0 car b • 0 Ccarb •
( /00>
~ .6
~.9
+0.5 +1.1 +0.9 +0.5 +1.3 +1. 2 +0.2
-20.8 - 20 .6 -20.8 -22.4 -21.8 - 21.8 -21.8
~.1
~.3 ~.4
-1.4 - 1. 4
calculated fr om equilibrium
m ean
~.8
+0.9 +0.8 +0.2 ~.1
+0 .3 0 .0 0 .0 ~.2 ~.1 ~.4
( /00>
s0IS O H '20 s013Ceo '2 ( /00>
( /00>
- 9. 0 - 8 .7
1.5 - 1 .3
- 20.7 -20.3
-9.3 -10.2 -10.2 -9.8 - 1 0. 0 - 9. 9 - 1 0 .4
+1.3 +1.4 +1.6 +1.6 +2.1 + 1.6 + 1.3
-20.7 - 2 1. 6 -21.4 - 20. 9 - 21.1 -21.1 -21.6
- 9 .2 - 10 . 4 -9.7
- 1. 0 -1.2 -1.4
-20.8 - 2 2. 1 - 21.4
recrystallization may be expected to vary from one individual to another. In January and February 1980 we again found a mixed population consisting partly of large snails from the 1979 generation and partly of the new small snails from the 1980 one . Central Ashqelon population (CA) The snails in the central Ashqelon population differ from those found at southern Ashqelon to the extent that almost no overlap of generations from year to year could be found. From January to March only small snails were found. The shells then grew rapidly during the spring months. From this period onwards only large shells were found. GENERAL DISCUSSION
The standard deviations around most of the mean 0 ISO-values (ranging from 0.3 to 0.8%0) are significantly larger than the analytical error. The larger variation found in 0 l3C-values from the southern Ashqelon population reflects the isotopic differences between the two size groups. The range of the isotopic data (-7 to -11.5%0) indicates that the factors af-
248
fecting the isotopic composition of the precipitating shell carbonate were highly variable even within the same environment. Because the area sampled was small in the two localities (not more than 100 m in diameter, both in southern Ashqelon and in central Ashqelon), the differences in [j l3C_ and o180-values may be related to the average monthly position of the snail on the plant. For example, a snail from the shaded parts of the plant may have a different isotopic record than a snail from the exposed part. In the following discussion we will concentrate on the average differences (Fig. 2) between the two localities, and place special emphasis on differences between the small « 10 mm) and large snails from southern Ashqelon. The discussion will be based on the monthly average isotopic data shown in Table 1. The fact that, amongst the small snails from southern Ashqelon, those collected during summer differ isotopically from those collected during the winter (Fig. 2) suggests that the CaC0 3 in the shell of the small snails does not remain unchanged all year round, although the shell size does not increase. Two explanations for the data can be put forward: (a) the shell of the snail re-equilibrates isotopically with the summer conditions because of processes of dissolution-rreprecipitation; (b) although the shell does not become larger, it adds CaC0 3 during the summer months to such an extent that a clear shift in [j 180-values from winter to summer can be observed. The fact that the small shells also differ from the large ones collected at the same month is not related to the method with which we selected the CaC0 3 for analysis (the whole shell of the small shells was collected and only the ultimate whorl of the large ones was used for analysis). If most of the shell of the summer small snail represent carbonate deposited during the winter months, one should observe less negative 0 lBO-values (similar to the olBO-values measured for the winter small snails). Nevertheless, the 0 180 _ values of the summer small snails are the most enriched in lBO of all the studied populations. To examine the causes of the isotopic differences between the various snail groups we followed the procedures given by Magaritz et al. (1981) to calculate: (1) the oxygen isotopic composition of the water in isotopic equilibrium with the shell carbonate; and (2) the carbon isotopic composition of the CO2 with which the snails were in isotopic equilibrium. Note that in this calculation we do not indicate the origin of the water, nor that of the CO2, The monthly mean temperatures, humidities and evaporation rates measured by the Israel Meteorological Service nearby (near Gaza) (Table I) were taken to be similar for the two size groups of southern Ashqelon and the population of central Ashqelon. We used the mean monthly temperature at 20hOOm because snails are most active mainly in the evening and at night. The data from the shells were corrected for calcite-aragonite fractionation, both in lBOjl60 and l3Cjl2C calculations (Robinson and Clayton, 1969; Tarutani et al., 1969). The relationships between oIBOwater-carbonate and temperature are shown in Fig. 3, that between
249
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b: 'Sf
-9.0
e! !2"g
-100
- 11.0
1.0 05 CD
~
.
00
,
Ul
>
J l!..-
-0 5
\
~o
zo
s
..~
"
\ !
'ti,
- 1.0
\
\
\
"~
- 1.5
I 2 3 4 5 6 7 8 9 10 II 12 I 2 3
79
80 month
Fig . 2. The monthly mean 6 130 _and 0 IJC-values in the central Ashqelon population (CA ) and the small (8) and large (L) size groups of southern Ashqelon (SA). Note: (a) the mean 0180- value of small snails from both the central and southern Ashqelon areas were almos t the same during the winter months ; (b) from t he spring months onwards th e ol3O-values of the small snails from the southern Ashqelon areas were enriched in 18 0 relative to the large snails from southern and central Ashqelon, with the except io n of the June and winter samples (December 1979 and January/February 1980) when the small and large snails from the southern Ashqelon SA area had the same mean 0 180 value; (c) the carbon isotope pattern was similar to that of 0 180 in that the mean 0 13C-values from th e central Ashqelon population were more negative most of th e year relative to the two southern Ashqelon size groups ; and (d) the small snails from the southern Ashqelon area were less depleted in 13C relative to the large snails collected at the same months. (j 13CC01 and temperature are given in Table I. The (j 180water-carbonate-values show a general correlation for the three groups with temperature. One month, May, shows an extreme variation from the regular trend for the three groups. The calculated values for the isotopic composition of the waters are depleted in 180 in the central Ashqelon area relative to that of the southern
250
r=O.93
30
7 8
2.0
3;: 0
/
5
6
~
(f)
4
vi >
~
/"
0.0 ,/
,
c
,.
D o
2! 0
0
-1.0
N
,/
4Y , /
0
J
,/
.........
,,'
"
, ,9"8
......... 0
~
J
.--;,
/'
5
5
1.0
0
......... r=O.g)
8/
.........
-: "
-:
".b
- ' r=O.B7
'
7
0
,,'
"",/
"/3,-'
/"
c-: «> ,?I .> ~ -:
;"
J
~
G()
-2.0
,/
2
SAS(5)-SAL(2) - - CA(2)-----
-3.0
Fig. 3. The calculated Ii lBO-values of the water in isotopic equilibrium with the shell carbonate, using the mean monthly temperature at 20"00 m . The numbers represent 1979 months, D = December 1979; J = January 1980; and F = February 1980. Note that also these calculated waters are significantly correlated with the mean monthly temperature.
Ashqelon area. The water used by the large snails is apparently also depleted in 180 compared to that used by the small ones (mainly in the summer months (Table I, Fig. 3). A difference of - 0.5%0 usually exists between each of the groups but sometimes it is larger than 1%0' In the following discussion we will try using the stable-isotope results to identify the environmental reservoirs with which the extrapallial fluid (from which the shell CaC0 3 precipitates) is isotopically equilibrated. One has to realize that it is possible that only partial isotopic equilibration will occur between the outer environment and the inner shell fluid; and that this is not necessarily the water that the snails actually take up. Rather, these values reflect the overall biochemical and physical processes that take place before shell carbonate is precipitated. Hence, we used the term "interface water" when referring to our calculated values. "Interface water" must be related to H20 vapour and to calculate the isotopic composition of air moisture we used the empirical relation given by Tzur (1971):
o180vapour-water =
O.12T air -14.2
The use of this equation is justified because the moisture in the Coastal Plain of Israel is not controlled by the local land environment but by interactions between the Mediterranean Sea water and the air masses above it.
251
Therefore, we used a calibrated equation derived for a location further north on the Coastal Plain (30 km to the NE). The calculated air moisture o180-values were converted to 0 180-values of condensed water (dew) using Majoube's (1971) equation. Agreement between the two calculated waters (from the carbonate and from the air moisture) is shown in Fig. 4. The water calculated using both equations for the winter months for the central Ashqelon population are exactly the same, whereas the shell-water 0 18 0 _ values from southern Ashqelon begin to show enrichment of 18 0 relative to dew even in the winter months. In the summer months the degree of 18 0 enrichment relative to condensed air moisture is smaller in the central Ashqelon population than in the large-snail and small-snail southern Ashqelon groups. This indicates that although the trends are similar and agree with the model suggested by Magaritz et al. (1981) local variations exist. The calculated 0 1 3Coo. -values (Table I) for the various populations do not correlate clearly with the monthly mean evening temperature or relative humidity. The only significant correlation which was found is between the mean Ol3CCO -values of the small-snail group and relative humidity (r = 0.72). The r~ge of the equilibrium 0 l3C-values of the CO2 (-19 to -23%0) was very similar to that observed in soil CO2 in several localities in the Coastal Plain of Israel (-16 to -21%0; M. Magaritz, unpublished data,
0
0~
'-
:J
4.0
8. c > I Q)
<; 0
~
3.0
~
.:.0
2.0
o
Q)
<;
<:: 0
-e0
1.0
U I
$ o
0
~
~ .:.0
0.0
SAS(5)-SAU 2 ) - - CA (2)------
-1.0
Fig. 4. The difference in Ii lBO-values between the isotopic equilibrium water calculated, based on the shell carbonate (Table I) and the condensed water (dew) calculated using Tzur's (1971) and Majoube's (1971) equations. Note that also these differences are significantly correlated with the mean monthly temperature.
252
1980). A similar range of soil CO2 values is reported for other soils (Rightmire, 1978). In most of the months the calculated carbon isotopic composition of CO2 in the central Ashqelon area was more negative than that in the southern Ashqelon area (based on the large-snail group). The values calculated based on the small-snail group are the least depleted in 13C. The source of carbon that the snails use for shell formation is unknown. From the fact that the carbon isotopic composition of the shell CO~- varied both from one locality to another and within a specific month (within the same population), it seems that the source of shell carbon is neither a pure organic component such as the organic material in the snail itself, nor the organic matter that the snails digest with their plant food. We know as yet nothing as to the food preference of Theba pisana, but for other helicid species we have a certain amount of knowledge: from examining faeces of several species (Mason, 1970; Wolda et al., 1971; Heller, 1979) one may conclude that they feed mainly upon dead and/or decaying plant material, although many helicids may take green material occasionally. The carbon isotopic composition of the organic material in the soil should not vary seasonally; rather it should reflect the overall isotopic composition of the plant cover in the area. On the other hand, both the fluctuation in 0 13C_ values and also the similarities between soil CO2 and the CO2 in isotopic equilibrium with the shell carbonate may suggest that of the snails fluids are isotopically equilibrated with the "interface water" during precipitation of their carbonate minerals, they may also equilibrate with the dissolved carbonate in that water as a source for the calcium carbonate of their shell. We suggest that "interface CO2 '' is a mixture of catabolic CO2 and atmospheric CO2 , The o 13C-values of the interface CO2 and carbonate that are dissolved within the interface water will vary as a mixture of these endmembers. The variations from locality to locality and from month to month reflect differences in the relative contributions of the two sources. Other investigators have found that the shells of marine and freshwater molluscs are formed in approximate isotopic equilibrium with the environmental water and dissolved carbonate (e.g., Mook, 1971; Fritz and Poplawski, 1974). It is very interesting to find that in terrestrial land snails, the mechanisms of shell growth may be similar. DIFFERENCES IN THE CARBON AND OXYGEN ISOTOPIC COMPOSITION IN THE ENVIRONMENT
The deviation of o180water-carbonate from o180water-vapour is linearly correlated with temperature and evaporation rate (Fig. 4). Interface water can become enriched in 18 0 either by disequilibrium evaporation, or by mixing with highly 180-enriched water from the soil underneath. One of the most important differences between the localities that we examined here was the type of soil: central Ashqelon has a thin soil layer which is characterized by high amounts of organic matter. The central Ashqelon
253
area has further an extensive plant cover throughout the year; as a result of this this cover serves to reduce evaporation of interface water during the winter. The variations in lj 13 C-values during the year seem to be related to annual bursts of increased biological activity in the soil: the minimum lj l3C-values occur first in early spring which coincides with the peak growth of the vegetation; and again in July, perhaps due to increased bacterial decomposition of organic matter during the warmer months. In contrast, the southern Ashqelon area consists of sand; it has no soil and the retention of dew is minimal, so that a larger contribution of evaporated water to the interface from the sand would exist every month. It seems possible that some environmental stress in the southern Ashqelon area inhibited the growth of some of the snails. The isotopic record rules out the possibility that the snails remained inactive from the spring onward. If this were the case then throughout the year all small snails should have had the same carbon and oxygen isotopic composition. In fact, Fig. 2 shows that they, like the large shells, also follow an annual cycle, and that a significant portion of their shell carbonate must be added every month. This feature of their growth is reflected in the increased weight per size class. Although the snails may have been unable to increase their shell diameter, they obviously continued to precipitate or dissolve and reprecipitate their shells. A similar argument could be put forward for the period of summer dormancy. If precipitation of carbonate stopped during this period, then the shells should not increase in weight. As shown in Fig. 1, the shells gained weight also during the aestivation period. We found slight differences in lj 180 _ and lj l3C-values during the months of aestivation which also suggest that precipitation of shell carbonate was continuing through the aestivation period. These observations may indicate that throughout the aestivation months the snail may be active for short periods. IMPLICATION TO THE STUDY OF ANCIENT SNAIL POPULATIONS
When one analyzes fossil snails from buried strata, it will of course be impossible to reconstruct an exact yearly temperature curve from the data. But based upon the results of this work, several parameters and relationships can be used to determine environmental characteristics: (a) The relative range of ljl80-values in the small and large shells. In areas like central Ashqelon the small shells represent the winter months, and could be used to calculate the isotopic composition of the dew and winter plant activity. (b) Differences between areas of desert and moderate climate could be obtained, like those reported in Magaritz and Heller's (1980) study of early stages of shell formation of Levantina caesareana. (c) Large shells, enriched in 180 , will reflect climatic conditions during summer. Another case will be when the small snails have the same range of ljl80-values (or even enriched in 18 0 ) relative to the large snails. Such a situation reflects very dense populations.
254
Similarly, the carbon isotopic composition can be used to reconstruct the annual cycle of the CO2 in the interface zone (together with the (j 18 0 data) and this can be used to get information on the type of plants (annual or perennial) and the amount of total organic soil activity that were present in the snails' environment. CONCLUSION
At present no geochemical tool exists that can be used to monitor parameters such as the degree of evaporation of the interface water or plant coverage. We found from analyzing the isotopic composition of carbon and oxygen isotopes in recent land snails that even minor changes in environmental parameters, such as type of soil, extent of soil activity or type of plants, could be monitored. All these parameters are most valuable in the study of palaeoclimate in continental regions, where fossil shells of land snails may be the only prevailing record (Fink and Kukla, 1977; Heller and Tchernov, 1978). ACKNOWLEDGEMENTS
The authors acknowledge the technical assistance given by Mr. M. Feld, T. Yelin, Mrs. I. Morovinski and R. Silanikov in the preparation of the sampIes and the isotopic measurements and the Israel Meteorological Service for the supplied information. Financial support for this study was given by a grant from the Israel Commission for Basic Research.
REFERENCES Craig, H., 1957. Isotopic standards for carbon and oxygen and correction factors for mass-spectrometric analysis of carbon dioxide. Geochim. Cosmochim. Acta, 12: 133-149. Craig, H., 1965. The measurements of oxygen isotope paleotemperatures. Proc. Conf. on Stable Isotope in Oceanographical Studies and Paleotemperatures, Spoleto, Cent. Rech. Nucl. (C.N .R.), Pisa, pp. 3-24. Evin, J., Marechal, J. and Pachiaveli, C., 1980. Conditions involved in dating terrestrial shells. Radiocarbon, 22: 545-555. Fink, J. and Kukla, G.J., 1977. Pleisotocene climates in Central Europe: at least 17 interglacials after the Olduvai event. Quat. Res. (N.Y.), 7: 363-371. Fritz, P. and Poplawski, S., 1974. 18 0 and 13C in the shells of fresh water mollusks and their environments. Earth Planet. Sci. Lett., 24: 91-98. Heller, J., 1979. Distribution, hybridization and variation in the Israeli landsnail Levantina (Pulmonata: Helicidae). Zool. J. Linn. Soc., 67: 115-148. Heller, J., 1982. Natural history of Theba pisana in Israel. J. Zool., London, 196: 475487. Heller, J. and Tchernov, E., 1978. Pleistocene landsnails from the Coastal Plain of Israel. Isr. J. Zool., 27: 1-10.
255 Heller, J. and Volokita, M., 1982. Shell-banding polymorphism of the landsnail Xeropicta vestalis along the Coastal Plain of Israel. BioI. J. Linn. Soc., 16: 279-284. Machin, Y., 1967. Structural adaptation for reducing water-loss in three species of terrestrial snail. J. Zool., London, 152: 55-65. Magaritz, M. and Heller, J., 1980. A desert migration indicator - oxygen isotopic composition of land snail shells. Palaeogeogr., Palaeoclimatol., Palaeoecol., 32: 153-162. Magaritz, M., Heller, J. and Volokita, M., 1981. Land-air boundary environment as recorded by the '8 0 / ' . 0 and 13C/'2C isotope ratios in the shells of land snail. Earth Planet. Sci. Lett., 52: 101-106. Majoube, M., 1971. Fractionnement en oxygene et en deuterium entre l'eau et sa vapeur. J. Chim. Phys. Phys.-Chim. BioI., 68: 1423-1436. Mason, C., 1970. Food, feeding rates and assimilation in woodland snails. Oecologia, 4: 358-373. McCrea, J.M., 1950. On the isotopic chemistry of carbonates and a paleotemperature scale. J. Chem. Phys., 18: 849-857. Mook, W.G., 1971. Paleotemperatures and chlorinities from stable carbon and oxygen isotopes in shell carbonate. Palaeogeogr., Palaeoclimatol., Palaeoecol., 9: 245-264. O'Neil, J.R., Clayton, R.N. and Mayeda, T.K., 1969. Oxygen isotope fractionation in divalent metal carbonates. J. Chem. Phys., 51: 5547-5558. Rightmire, C.T., 1978. Seasonal variation in Peo, and 13C content of soil atmosphere. Water Resour. Res., 14: 691-693. Rubinson, M. and Clayton, R.N., 1969. Carbon-13 fractionation between aragonite and calcite. Geochim. Cosmochim. Acta, 33: 997-1022. Tarutani, T., Clayton, R.N. and Mayeda, T.K., 1969. The effect of polymorphism and magnesium substitution on oxygen isotope fractionation between calcium carbonate and water. Geochim. Cosmochim. Acta, 33: 987-996. Tzur, Y., 1971. Isotope effects in the evaporation of water into air. Ph.D. Thesis, The Weizmann Institute of Science, Rehovot, 109 pp. (in Hebrew). Vogel, J.C., Groates, P. and Mook, W.G., 1970. Isotopic fractionation between gaseous and dissolved carbon dioxide. Z. Phys., 230: 225-238. Wilbur, K.M. and Tompa, A.S., 1979. Physiological changes in gastropods during egg shell calcification, Part II. Malacologia, 18: 561-562. Wolda, H., Zweep, A. and Schuitema, K., 1971. The role of food in the dynamics of the landsnail Cepaea nemoralis. Oecologia, 7: 361-381. Yapp, C.J., 1979. Oxygen and carbon isotope measurements of land snail shell carbonate. Geocbim. Cosmochim. Acta, 43: 629-635.
243
ANNUAL CYCLE OF 180jl60 and l3C/IZC ISOTOPE RATIOS IN LANDSNAIL SHELLS
MORDECKAI MAGARITZ and JOSEPH HELLER
Isotope Department, The Weizmann Institute of Science, Rehouot (Israel) Zoology Department, The Hebreui Uniuersiiy, Jerusalem (Israel) (Received November 17,1982; revised and accepted May 25, 1983)
ABSTRACT Magaritz , M. and Heller, J., 1983. Annual cycle of landsnail shells. Isot, Geosci., 1; 243-255.
18
0 / 160 and 13C/12C isotope ratios in
Monthly data on the isotopic composition of carbon and oxygen in shells of the land snail, Theba pisano, were collected from two localities near Ashqelon, Israel. Seasonal trends in the isotopic data from the two areas were similar. However, significant monthto-month and locality-to-locality differences in the 6 13C_and 6 lBO-values of the snail shell carbonate indicate that microenvironmental diversity did indeed exist. We suggest that the factors responsible for the microclimatological diversity, such as plant coverage and primary productivity, control the degree of evaporation of "interface water" at the soil-air boundary. We used the oxygen isotopic composition of snail shell carbonate to calculate the isotopic composition of this "interface water", and the s 13C-values of snail shell carbonate to monitor the amount of heterotrophic CO 2 production in the soil.
ZOOLOGICAL AND ENVIRONMENTAL BACKGROUND
Landsnails are abundant in many continental loess and soil sequences (Fink and Kukla, 1977; Heller and Tchernov, 1978) and in archaeological sites (Evin et al., 1980; Magaritz and Heller, 1980). It has been suggested by Yapp (1979) and Magaritz and Heller (1980) that the isotopic record of the shell carbonate can be used to determine the palaeoenvironment. In order to better define the magnitude of local effects which may result in an incorrect interpretation of the conditions, we studied the isotopic record found in snail shells in vivo of one species from two quite distinct nearby microenvironments. Theba pisana Miller, a Mediterranean land snail that is very abundant along the Coastal Plain of Israel, is a semelparous species with an annual life cycle (Heller, 1982). Young snails hatch in early winter, grow rapidly in spring, and by summer they reach first adult shell size and then the adult shell weight. Shell size is negatively related to density, suggesting that, as in some other land snails, there may be a density-dependent inhibition of growth.
0167-6695/83/$03.00
© 1983 Elsevier Science Publishers B. V.
244
Summer forces the snails into 3-4 months of aestivation. After the first winter rains the snails, awakened perhaps by a sudden drop in barometric pressure, emerge from aestivation, copulate, and lay eggs. In mid-winter the snails die. We sampled two populations of snails found near Ashqelon on the southern coast of Israel. One group of snails (designated SA) was taken near southern Ashqelon ~ 1 km from the Mediterranean coast line. The snails' habitat here consisted of a 22-km2 area of semi-stabilized sand dunes. Perennial shrubs (mainly Artemisia and Retama) covered ~ 78% of the ground. Most of the snails from this location had effectively-banded shells. The other group of snails (group CAl was taken from central Ashqelon, in a small area (0.2 km") of stable sand where perennial vegetation covered only 2% of the ground and annual vegetation consisting of weeds covered the rest. The total organic productivity in this area is much larger than in the SA area, as is shown by the fact that the sand here has a soil-A horizon whereas in southern Ashqelon there is none. Most of the snails in the CA group had effectively non-banded shells. The life cycles, the reproductive biology and the seasonal changes in morphology frequencies of these two snail populations have been described by Heller (1982) and Heller and Volokita (1982). METHODS
The snails described in this study were taken from the monthly samples collected by Heller (1982). The snails were killed by asphyxiation in boiled water within hours of collection, and the soft parts were removed. In shells with diameter >9 mm, only the last half of the ultimate whorl, i.e. the whorl deposited most recently, was analyzed; shells with diameter <9 mm were used intact. Shell samples were heated overnight at 70-80°C in 30% hydrogen peroxide solution to remove shell organic matter, and then dried at 70-80°C. Shell samples were treated in vacuum with H 3P04 (sp. gr. = 1.8) and the 13C/12C and lSO/160 ratios of the evolved CO 2 were determined mass spectrometrically using McCrea's (1950) methods and a Varian~ M 250 mass spectrometer. The results for both C and 0 are reported in the usual a notation as the permil (%0) deviation of the sample CO 2 from the PDB Standard (Craig, 1957): S 16 olSO (%0) = eSo/160)~Pl~:- O/ 0 )standard X 1000 ( 0/ 0 )standard The reproducibility of duplicate samples was 0.1%0 for both carbon and oxygen measurements. When discussing the isotopic composition of oxygen in gas or liquid phases we will use the SMOW Standard (Craig, 1957). The PDB and SMOW scales are related by the equation:
e
s 180SMOW
=
1.0307 (j 180PDB- 30.70
245 RESULTS
The meteorological parameters and the mean monthly values of both 8 IBO and 8 13C are given in Table I.
Large variations were found in the 8 lBO-values of the shell carbonate in each population. With the exceptions of the SA population in two months (February and July 1979), no significant correlation was observed between the size of the shell and the 8 lBO-value of its carbonate. The carbon isotopic composition of the shell carbonate shows a more regular size-dependent pattern. In the SA population we found a significant correlation between the 8 13C-values and shell diameter: the small snails in the population had less negative 8 13C-values than the large snails.
Southern Ashqelon population (SA) The results of the snails from the southern Ashqelon area show bimodality in the isotopic data. Therefore, the overall monthly mean values of 8 180 and 8 13C have less significance than the mean values for the two designated sub-groups of large shells and small shells (Table I). All the snails from January 1979 were large and actually belonged to the previous (1978) generation. In February 1979, small shells belonging to the new (1979) generation, were found in the field together with the old 1978
Mg
300
mm
t'c
16
26
12
22
8
18
................ ~
: ...
250
..
~ ...
/--
FMAMJJ
ASON
Months
Fig. 1. Southern Ashqelon population: mean shell size (fine line); mean shell weight of those snails in the population whose diameter is 170 mm (thick line); and mean noon temperatures (dotted line) (adapted from Heller, 1982).
246 TABLE I Meteorological and isotopic data: the mean /) 18 0 _ and /) 13C-values of the shell carbonate (carb.) from the central Ashqelon and southern Ashqelon areas. divided into small and large subpopulations; also given are the calculated oxygen isotopic composition of the water and the carbon isotopic composition of the CO 2 (gas) which were in isotopic equilibrium with the mean /) 18 0 _ and /) 13C-values of the shell carbonate; the mean monthly temperature and the humidity at 20h0om; and the evaporation rate from a type-A pan Month Temper- Humidity ature (%)
(C)
Evaporation rate (mm day-I)
Central Ashqelon (CA) small snails
/)0'8 0
( 10 . )
1{79 2{79 3{79 4{79 5/79 6/79 7/79 8/79 9/79 10/79 11/79 12/ 79 1/80 2/80
14.2 16.1 17.3 20.0 20.7 24.8 26.1 26.5 25.7 24.2 21.6 14.8 13.4 13.9
72 66 68 64 76 76 74 74 75 68 66 69 71 72
2.0 3.3 3.9 6.2 6.0 6.9 7.5 7.1 5.8 4.7 3.4 2.5 2.3 2.4
large snails calculated from equilibrium
mean
-1.6 -1.7 -.().7
car b .
s:3 Ccar b . (
log)
-10.1 -11.2 -11.1
/)0180
( 100 )
H 20
-2.6 -2.3 -0.9
s:3 CC0 2 (
too)
mean /)180 /)13C o carbo 0 carbo ( log) ( 10 0 )
-21.7 -22.8 -22.6 +0.1 +0.6 -0.5 -1.1 -0.5 -0.4 -1.1
-9.4 -9.2 -9.6 -11.3 -10.7 -10.6 -10.5
generation. By March 1979 most of the southern Ashqelon population consisted of small snails (7.5-9.8 mm) belonging to the new generation (1979). During April and May the snails grew very rapidly (at a mean rate of 0.12 mm day"), After May, the growth in shell diameter diminished but the snails continued to gain weight (Fig. 1). The biological significance of this acceleration of CaC0 3 buildup may be related to the preparation, before the aestivation period, of a thick shell to protect the snail against the severe heat. Machin (1967) demonstrated experimentally that increased shell thickness reduces water loss, and should be considered a morphological adaptation in arid conditions. Another biological significancy may be the storage of shell components so that they will be available for activity immediately at the end of the aestivation period. In September some of the snails emerged from aestivation and became active whilst others were still aestivating. The large variability in the isotopic record in the period October-January may reflect addition of another variable which causes changes in the shell. This variable relates to the fact that some CaC0 3 of the shell is taken by the snail to deposit it in the eggs as a storage for building up embryonic shell of the next generation (Wilbur and Tompa, 1979). During this extraction, some recrystallization processes of the mother shell presumably take place. The degree of dissolution or
247
Southern Ashqelon (SA) large sn ail s
small snails calculated from equilibrium (, ls OH
(%0> '
0 (, 13C
CO
(%0)
calculated from equilibrium
m ean
(,13Ccar b (, u O H 0 o O car b · 0 ' 0 2.
(,l s
•
(,13CCO 0 '2
( /00>
( /00>
( /00>
( /00>
-1.6 +1.2 +0.2 +1.1 +0.6
-8.1 -9.2 -9.2 -7.7 -8.2 -8.2 -8 .7
-2.2 -1.1 +0 .3 +1.8 +1. 6 +2 .8 +2 .4
-19.7 -20.7 -20.6 -19.1 -19.4 -19.3 -19.8
+0 .4
- 9.0
+1 .7
-20.2
+0.1
- 9 .3 -8.2 - 8 .3
~.7
-20.9 -19.9 -20.0
(, I S
(, 13 o 0 car b • 0 Ccarb •
( /00>
~ .6
~.9
+0.5 +1.1 +0.9 +0.5 +1.3 +1. 2 +0.2
-20.8 - 20 .6 -20.8 -22.4 -21.8 - 21.8 -21.8
~.1
~.3 ~.4
-1.4 - 1. 4
calculated fr om equilibrium
m ean
~.8
+0.9 +0.8 +0.2 ~.1
+0 .3 0 .0 0 .0 ~.2 ~.1 ~.4
( /00>
s0IS O H '20 s013Ceo '2 ( /00>
( /00>
- 9. 0 - 8 .7
1.5 - 1 .3
- 20.7 -20.3
-9.3 -10.2 -10.2 -9.8 - 1 0. 0 - 9. 9 - 1 0 .4
+1.3 +1.4 +1.6 +1.6 +2.1 + 1.6 + 1.3
-20.7 - 2 1. 6 -21.4 - 20. 9 - 21.1 -21.1 -21.6
- 9 .2 - 10 . 4 -9.7
- 1. 0 -1.2 -1.4
-20.8 - 2 2. 1 - 21.4
recrystallization may be expected to vary from one individual to another. In January and February 1980 we again found a mixed population consisting partly of large snails from the 1979 generation and partly of the new small snails from the 1980 one . Central Ashqelon population (CA) The snails in the central Ashqelon population differ from those found at southern Ashqelon to the extent that almost no overlap of generations from year to year could be found. From January to March only small snails were found. The shells then grew rapidly during the spring months. From this period onwards only large shells were found. GENERAL DISCUSSION
The standard deviations around most of the mean 0 ISO-values (ranging from 0.3 to 0.8%0) are significantly larger than the analytical error. The larger variation found in 0 l3C-values from the southern Ashqelon population reflects the isotopic differences between the two size groups. The range of the isotopic data (-7 to -11.5%0) indicates that the factors af-
248
fecting the isotopic composition of the precipitating shell carbonate were highly variable even within the same environment. Because the area sampled was small in the two localities (not more than 100 m in diameter, both in southern Ashqelon and in central Ashqelon), the differences in [j l3C_ and o180-values may be related to the average monthly position of the snail on the plant. For example, a snail from the shaded parts of the plant may have a different isotopic record than a snail from the exposed part. In the following discussion we will concentrate on the average differences (Fig. 2) between the two localities, and place special emphasis on differences between the small « 10 mm) and large snails from southern Ashqelon. The discussion will be based on the monthly average isotopic data shown in Table 1. The fact that, amongst the small snails from southern Ashqelon, those collected during summer differ isotopically from those collected during the winter (Fig. 2) suggests that the CaC0 3 in the shell of the small snails does not remain unchanged all year round, although the shell size does not increase. Two explanations for the data can be put forward: (a) the shell of the snail re-equilibrates isotopically with the summer conditions because of processes of dissolution-rreprecipitation; (b) although the shell does not become larger, it adds CaC0 3 during the summer months to such an extent that a clear shift in [j 180-values from winter to summer can be observed. The fact that the small shells also differ from the large ones collected at the same month is not related to the method with which we selected the CaC0 3 for analysis (the whole shell of the small shells was collected and only the ultimate whorl of the large ones was used for analysis). If most of the shell of the summer small snail represent carbonate deposited during the winter months, one should observe less negative 0 lBO-values (similar to the olBO-values measured for the winter small snails). Nevertheless, the 0 180 _ values of the summer small snails are the most enriched in lBO of all the studied populations. To examine the causes of the isotopic differences between the various snail groups we followed the procedures given by Magaritz et al. (1981) to calculate: (1) the oxygen isotopic composition of the water in isotopic equilibrium with the shell carbonate; and (2) the carbon isotopic composition of the CO2 with which the snails were in isotopic equilibrium. Note that in this calculation we do not indicate the origin of the water, nor that of the CO2, The monthly mean temperatures, humidities and evaporation rates measured by the Israel Meteorological Service nearby (near Gaza) (Table I) were taken to be similar for the two size groups of southern Ashqelon and the population of central Ashqelon. We used the mean monthly temperature at 20hOOm because snails are most active mainly in the evening and at night. The data from the shells were corrected for calcite-aragonite fractionation, both in lBOjl60 and l3Cjl2C calculations (Robinson and Clayton, 1969; Tarutani et al., 1969). The relationships between oIBOwater-carbonate and temperature are shown in Fig. 3, that between
249
-8 0 CD
b: 'Sf
-9.0
e! !2"g
-100
- 11.0
1.0 05 CD
~
.
00
,
Ul
>
J l!..-
-0 5
\
~o
zo
s
..~
"
\ !
'ti,
- 1.0
\
\
\
"~
- 1.5
I 2 3 4 5 6 7 8 9 10 II 12 I 2 3
79
80 month
Fig . 2. The monthly mean 6 130 _and 0 IJC-values in the central Ashqelon population (CA ) and the small (8) and large (L) size groups of southern Ashqelon (SA). Note: (a) the mean 0180- value of small snails from both the central and southern Ashqelon areas were almos t the same during the winter months ; (b) from t he spring months onwards th e ol3O-values of the small snails from the southern Ashqelon areas were enriched in 18 0 relative to the large snails from southern and central Ashqelon, with the except io n of the June and winter samples (December 1979 and January/February 1980) when the small and large snails from the southern Ashqelon SA area had the same mean 0 180 value; (c) the carbon isotope pattern was similar to that of 0 180 in that the mean 0 13C-values from th e central Ashqelon population were more negative most of th e year relative to the two southern Ashqelon size groups ; and (d) the small snails from the southern Ashqelon area were less depleted in 13C relative to the large snails collected at the same months. (j 13CC01 and temperature are given in Table I. The (j 180water-carbonate-values show a general correlation for the three groups with temperature. One month, May, shows an extreme variation from the regular trend for the three groups. The calculated values for the isotopic composition of the waters are depleted in 180 in the central Ashqelon area relative to that of the southern
250
r=O.93
30
7 8
2.0
3;: 0
/
5
6
~
(f)
4
vi >
~
/"
0.0 ,/
,
c
,.
D o
2! 0
0
-1.0
N
,/
4Y , /
0
J
,/
.........
,,'
"
, ,9"8
......... 0
~
J
.--;,
/'
5
5
1.0
0
......... r=O.g)
8/
.........
-: "
-:
".b
- ' r=O.B7
'
7
0
,,'
"",/
"/3,-'
/"
c-: «> ,?I .> ~ -:
;"
J
~
G()
-2.0
,/
2
SAS(5)-SAL(2) - - CA(2)-----
-3.0
Fig. 3. The calculated Ii lBO-values of the water in isotopic equilibrium with the shell carbonate, using the mean monthly temperature at 20"00 m . The numbers represent 1979 months, D = December 1979; J = January 1980; and F = February 1980. Note that also these calculated waters are significantly correlated with the mean monthly temperature.
Ashqelon area. The water used by the large snails is apparently also depleted in 180 compared to that used by the small ones (mainly in the summer months (Table I, Fig. 3). A difference of - 0.5%0 usually exists between each of the groups but sometimes it is larger than 1%0' In the following discussion we will try using the stable-isotope results to identify the environmental reservoirs with which the extrapallial fluid (from which the shell CaC0 3 precipitates) is isotopically equilibrated. One has to realize that it is possible that only partial isotopic equilibration will occur between the outer environment and the inner shell fluid; and that this is not necessarily the water that the snails actually take up. Rather, these values reflect the overall biochemical and physical processes that take place before shell carbonate is precipitated. Hence, we used the term "interface water" when referring to our calculated values. "Interface water" must be related to H20 vapour and to calculate the isotopic composition of air moisture we used the empirical relation given by Tzur (1971):
o180vapour-water =
O.12T air -14.2
The use of this equation is justified because the moisture in the Coastal Plain of Israel is not controlled by the local land environment but by interactions between the Mediterranean Sea water and the air masses above it.
251
Therefore, we used a calibrated equation derived for a location further north on the Coastal Plain (30 km to the NE). The calculated air moisture o180-values were converted to 0 180-values of condensed water (dew) using Majoube's (1971) equation. Agreement between the two calculated waters (from the carbonate and from the air moisture) is shown in Fig. 4. The water calculated using both equations for the winter months for the central Ashqelon population are exactly the same, whereas the shell-water 0 18 0 _ values from southern Ashqelon begin to show enrichment of 18 0 relative to dew even in the winter months. In the summer months the degree of 18 0 enrichment relative to condensed air moisture is smaller in the central Ashqelon population than in the large-snail and small-snail southern Ashqelon groups. This indicates that although the trends are similar and agree with the model suggested by Magaritz et al. (1981) local variations exist. The calculated 0 1 3Coo. -values (Table I) for the various populations do not correlate clearly with the monthly mean evening temperature or relative humidity. The only significant correlation which was found is between the mean Ol3CCO -values of the small-snail group and relative humidity (r = 0.72). The r~ge of the equilibrium 0 l3C-values of the CO2 (-19 to -23%0) was very similar to that observed in soil CO2 in several localities in the Coastal Plain of Israel (-16 to -21%0; M. Magaritz, unpublished data,
0
0~
'-
:J
4.0
8. c > I Q)
<; 0
~
3.0
~
.:.0
2.0
o
Q)
<;
<:: 0
-e0
1.0
U I
$ o
0
~
~ .:.0
0.0
SAS(5)-SAU 2 ) - - CA (2)------
-1.0
Fig. 4. The difference in Ii lBO-values between the isotopic equilibrium water calculated, based on the shell carbonate (Table I) and the condensed water (dew) calculated using Tzur's (1971) and Majoube's (1971) equations. Note that also these differences are significantly correlated with the mean monthly temperature.
252
1980). A similar range of soil CO2 values is reported for other soils (Rightmire, 1978). In most of the months the calculated carbon isotopic composition of CO2 in the central Ashqelon area was more negative than that in the southern Ashqelon area (based on the large-snail group). The values calculated based on the small-snail group are the least depleted in 13C. The source of carbon that the snails use for shell formation is unknown. From the fact that the carbon isotopic composition of the shell CO~- varied both from one locality to another and within a specific month (within the same population), it seems that the source of shell carbon is neither a pure organic component such as the organic material in the snail itself, nor the organic matter that the snails digest with their plant food. We know as yet nothing as to the food preference of Theba pisana, but for other helicid species we have a certain amount of knowledge: from examining faeces of several species (Mason, 1970; Wolda et al., 1971; Heller, 1979) one may conclude that they feed mainly upon dead and/or decaying plant material, although many helicids may take green material occasionally. The carbon isotopic composition of the organic material in the soil should not vary seasonally; rather it should reflect the overall isotopic composition of the plant cover in the area. On the other hand, both the fluctuation in 0 13C_ values and also the similarities between soil CO2 and the CO2 in isotopic equilibrium with the shell carbonate may suggest that of the snails fluids are isotopically equilibrated with the "interface water" during precipitation of their carbonate minerals, they may also equilibrate with the dissolved carbonate in that water as a source for the calcium carbonate of their shell. We suggest that "interface CO2 '' is a mixture of catabolic CO2 and atmospheric CO2 , The o 13C-values of the interface CO2 and carbonate that are dissolved within the interface water will vary as a mixture of these endmembers. The variations from locality to locality and from month to month reflect differences in the relative contributions of the two sources. Other investigators have found that the shells of marine and freshwater molluscs are formed in approximate isotopic equilibrium with the environmental water and dissolved carbonate (e.g., Mook, 1971; Fritz and Poplawski, 1974). It is very interesting to find that in terrestrial land snails, the mechanisms of shell growth may be similar. DIFFERENCES IN THE CARBON AND OXYGEN ISOTOPIC COMPOSITION IN THE ENVIRONMENT
The deviation of o180water-carbonate from o180water-vapour is linearly correlated with temperature and evaporation rate (Fig. 4). Interface water can become enriched in 18 0 either by disequilibrium evaporation, or by mixing with highly 180-enriched water from the soil underneath. One of the most important differences between the localities that we examined here was the type of soil: central Ashqelon has a thin soil layer which is characterized by high amounts of organic matter. The central Ashqelon
253
area has further an extensive plant cover throughout the year; as a result of this this cover serves to reduce evaporation of interface water during the winter. The variations in lj 13 C-values during the year seem to be related to annual bursts of increased biological activity in the soil: the minimum lj l3C-values occur first in early spring which coincides with the peak growth of the vegetation; and again in July, perhaps due to increased bacterial decomposition of organic matter during the warmer months. In contrast, the southern Ashqelon area consists of sand; it has no soil and the retention of dew is minimal, so that a larger contribution of evaporated water to the interface from the sand would exist every month. It seems possible that some environmental stress in the southern Ashqelon area inhibited the growth of some of the snails. The isotopic record rules out the possibility that the snails remained inactive from the spring onward. If this were the case then throughout the year all small snails should have had the same carbon and oxygen isotopic composition. In fact, Fig. 2 shows that they, like the large shells, also follow an annual cycle, and that a significant portion of their shell carbonate must be added every month. This feature of their growth is reflected in the increased weight per size class. Although the snails may have been unable to increase their shell diameter, they obviously continued to precipitate or dissolve and reprecipitate their shells. A similar argument could be put forward for the period of summer dormancy. If precipitation of carbonate stopped during this period, then the shells should not increase in weight. As shown in Fig. 1, the shells gained weight also during the aestivation period. We found slight differences in lj 180 _ and lj l3C-values during the months of aestivation which also suggest that precipitation of shell carbonate was continuing through the aestivation period. These observations may indicate that throughout the aestivation months the snail may be active for short periods. IMPLICATION TO THE STUDY OF ANCIENT SNAIL POPULATIONS
When one analyzes fossil snails from buried strata, it will of course be impossible to reconstruct an exact yearly temperature curve from the data. But based upon the results of this work, several parameters and relationships can be used to determine environmental characteristics: (a) The relative range of ljl80-values in the small and large shells. In areas like central Ashqelon the small shells represent the winter months, and could be used to calculate the isotopic composition of the dew and winter plant activity. (b) Differences between areas of desert and moderate climate could be obtained, like those reported in Magaritz and Heller's (1980) study of early stages of shell formation of Levantina caesareana. (c) Large shells, enriched in 180 , will reflect climatic conditions during summer. Another case will be when the small snails have the same range of ljl80-values (or even enriched in 18 0 ) relative to the large snails. Such a situation reflects very dense populations.
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Similarly, the carbon isotopic composition can be used to reconstruct the annual cycle of the CO2 in the interface zone (together with the (j 18 0 data) and this can be used to get information on the type of plants (annual or perennial) and the amount of total organic soil activity that were present in the snails' environment. CONCLUSION
At present no geochemical tool exists that can be used to monitor parameters such as the degree of evaporation of the interface water or plant coverage. We found from analyzing the isotopic composition of carbon and oxygen isotopes in recent land snails that even minor changes in environmental parameters, such as type of soil, extent of soil activity or type of plants, could be monitored. All these parameters are most valuable in the study of palaeoclimate in continental regions, where fossil shells of land snails may be the only prevailing record (Fink and Kukla, 1977; Heller and Tchernov, 1978). ACKNOWLEDGEMENTS
The authors acknowledge the technical assistance given by Mr. M. Feld, T. Yelin, Mrs. I. Morovinski and R. Silanikov in the preparation of the sampIes and the isotopic measurements and the Israel Meteorological Service for the supplied information. Financial support for this study was given by a grant from the Israel Commission for Basic Research.
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