Oxygen isotopic composition of opaline phytoliths: Potential for terrestrial climatic reconstruction

Geochimica et CosmochimicaActa, Vol. 60, No. 20, pp. 3949-3953, 1996 Copyright © 1996 ElsevierScienceLtd Printed in the USA. All rights reserved 0016-7037/96 $15.00 + .00

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Oxygen isotopic composition of opaline phytoliths: Potential for terrestrial climatic reconstruction RUTH SHAHACK-GRoSS,I ALDO SHEMESH,2 DAN YAKIR2"* and STEVE WEINER t ~Department of Structural Biology, Weizmann Institute of Science, Rehovot 76100, Israel 2Department of Environmental Sciences and Energy Research, Weizmann Institute of Science, Rehovot 76100, Israel (Received June 17, 1996; accepted in revised Jorm June 28, 1996)

A b s t r a c t - - O p a l i n e mineralized bodies are produced by many terrestrial plants and accumulate in certain soils and archaeological sites. Analyses of the oxygen isotopic compositions of these so-called phytoliths from stems and leaves of wheat plants grown in a greenhouse showed a linear relationship with stem and leaf water isotopic compositions and hence, indirectly, rain water isotopic composition. Analyses of wheat plants grown in fields showed that stem phytoliths isotopic composition directly reflects the seasonal air temperature change, whereas leaf phytoliths isotopic composition reflects both temperature and relative humidity. Temperature and the oxygen isotopic composition of stem phytoliths were related by an equation similar to that proposed for marine opal. Oxygen isotopic compositions of fossil phytoliths, and in particular those from stems, could be valuable for reconstructing past terrestrial climate change. 1. INTRODUCTION

ment is inversely correlated with relative humidity (Farris and Strain, 1978). An early attempt to calibrate the dependency of the 6180 values of leaf phytoliths from Gramineae and Tabashir (bamboo) plants on the 6 ISOjwand ambient temperature was hampered by the complex interactions between effects of temperature, humidity, and 6~80~w. The study showed that humidity was the main factor controlling the 6 ~sO values of leaf phytoliths and ambient temperature, therefore, could not be directly reconstructed (Bombin and Muehlenbachs, 1980). The 6~O of stem water (glOOm,w) is normally not influenced by humidity changes (Farris and Strain, 1978), and hence the isotopic composition of stem phytoliths is expected to be affected only by changes in 6~80 of stem water and by temperature. In this study we, therefore, extracted phytoliths and tissue water from leaves and stems of wheat plants ( Triticum aestivum) grown in a greenhouse and in two fields, and tested the possibility that variations in the 6LsO of the opaline phytoliths (6 ~80~) reflect variations in ambient temperature and 6 ~SO~w.

Understanding current global climate change is dependent, in part, on the ability to reconstruct past changes in climatic parameters from both the marine and terrestrial environments. Climatic reconstructions based on oxygen isotopic composition of carbonates (Shackleton, 1987), phosphates (Longinelli and Nuti, 1973; Shemesh et al., 1988), cellulose (Yakir et al,, 1994), and most recently marine diatomaceous opal (Shemesh et al., 1992) were developed when direct relations were demonstrated between the O~sO values (for definition see Methods) of the material measured and climatological factors. Biogenic opal (SiO2-nH20) is also deposited as microscopic bodies in a variety of land plants. These so-called opaline phytoliths are produced in large quantities, and their potential for terrestrial climatic reconstruction based on the isotopic composition of their nonexchangeable oxygen atoms is investigated here. Opaline phytoliths are the products of polymerization of soluble monosilicic acid (Si (OH)4), which enters the plant through the roots, migrates through the water conducting system and is deposited as discrete bodies within cells, cell walls, and intercellular spaces (Piperno, 1988). During the polymerization process three climatic factors may influence the 6 ~O values of opaline phytoliths. The first is the plant's source water isotopic composition (6 t~O~,~), which is essentially rain water. Rain isotopic signature is determined mainly by the temperature at which the rain was formed (Gat, 1980). The second is the influence of temperature on the fractionation factor between the opal and the water from which it forms (Shemesh et al., 1992; Kita et al., 1985). Thirdly, humidity is known to affect the ~ 8 0 of leaf water (6~sO~). Leaf water is significantly enriched in ~80 due to evapotranspiration during the day. The extent of this enrich-

2. MATERIALS AND METHODS Leaves and stems from whole cultivated wheat plants (Triticum aestivum) were sampled on a monthly basis from two commercial wheat fields in Southern Israel (var. Shaphir, at Kiryat-Gat and var. Nirit, at Gilat near Be'er-Sheva). Sampling began at December 1994 and ended at May 1995. Temperature and relative humidity at the sites for the sampling period were obtained from the Israeli Meteorological Service, Beit-Dagan. Another set of leaves and stems was obtained from four groups of T. aestivum (vat. Beit-Ha'shita) grown in the same greenhouse on campus. Planting was on January 15, 1995 and harvest on April 23, 1995. Standard and identical horticultural practices were used for all plants except for the 61~O value of the irrigation water which was different for each group of plants ( - 18.2%o, - 10.3%v, -5.1%o, and +0.4%v, +_0.3%~).During the developmental stage, temperatures of soil, stems, and leaves were measured on a separately greenhousegrown plants over a 24 h period with copper-constantant thermocou-

* Author to whom correspondence should be addressed. 3949

R. Shahack-Gross et al.

3950 Table 1:

G r e e n h o u s e m e a s u r e d 8180 of s o u r c e (irrigation) water, p l a n t water, p l a n t opal a n d p l a n t cellulose, a n d t h e calculated a p p a r e n t fractionation factors b e t w e e n t h e s t e m opal a n d s t e m water. Source water

Plant water

Plant phytoliths

8a80

8180

~180si

STEMS

cP opal-water

Plant cellulose

0.4

1.7

36.3

1.035

-5.1

-2.8

34.9

1.038

-10.3

-7.1

30.7

1.038

29.2

-18.2

-5.9*

29.1

1.035

30.5

32.4

1.036±0.002

Average±std

LEAVES

0.4

19.7

43.1

-5.1

14.6

40.5

31.9 30.1

-10.3

12.7

39.2

29.0

-18.2

11.2"

38.2

28.9

*Extremely e n r i c h e d d u e to e v a p o r a t i o n of t h e soil w a t e r b e c a u s e of b a d drainage.

ples and a data logger (Campbell 21 × ). Precision of the measurements was _+0.1°C. Sampling of both field and greenhouse materials was done at midday and in each case sampling was done into sealed test tubes for water and cellulose extractions, and into plastic bags for drying and phytoliths extraction. Tissue water was distilled (80°C) directly from the sealed tubes under vacuum and equilibrated with CO2 for

45/

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•

/

9

40

•

O/

9,

6o

30

25. . . . . , .... -10 -5

, .... 0

, .... , .... 5 10

, ..... 15 20

Plant water 81so

Fig. 1. The relations between phytolith 6 ~sO~,and the plant water 6t80 in the greenhouse experiment. A regression line was fitted to the stem data (squares): 6tsO~ = 35.5 + 0.79 (_+0.27).61~O~,~ (r 2 = 0.811, p = 0.099). The 6180~ values of leaf phytoliths are plotted vs. measured 6 ~80~wvalues (closed circles ) and vs. calculated daily mean 6 ~801,,.values (open circles). Using the calculated values, the leaf data fall on the regression line obtained for the stems (extrapolated with a broken line).

the isotopic analysis (Epstein and Mayeda, 1953 ). Dried plant material left in the vacuum tubes was used for cellulose extraction (Sternberg, 1989), followed by pyrolisis and conversion to CO2 (Santrock and Hays, 1987) which was used for the isotopic analysis. To obtain opal from the plant material, leaves and stems were separated and dried at 60°C for 72 h. Dried samples were ground in a mill and 3 g of powdered sample were equally divided into three 50 ml plastic centrifuge tubes. A 20 mL mixture of 70% HNO3 and 70% HCIO4 (1:1 by vol) was added to the tubes and the partially capped tubes were held at 65°C for 3 h. Double distilled water was added to the tubes before centrifuging for 2 min at 4500 rpm. The supernatant was removed and the washing and centrifuging repeated after the three sub samples were recombined. The acid treatment was repeated for 2 h followed by washing as above. Samples were then further treated with a 20 mL mixture of 70% HNO3 and 98% H2SO4 (4:1 by vol) for about 17 h, washed five times and dried at 60°C for 24 h. Total organic carbon of the purified samples did not exceed 0.4%. For isotopic determinations, 4 mg of purified opaline samples were equilibrated with water vapor of known isotopic composition under controlled conditions (6 h at 200°C) and reacted in vacuum with BrF5 at 600°C. The oxygen produced was reacted with hot graphite to produce CO2 which was used for the isotopic analysis (Shemesh et al., 1992). All isotopic determinations were carried out on a Finnigan M A T 250 ratio mass spectrometer and calibrated against the V-SMOW and NBS-28 standards. External precision was _+0.2%0 for water, _+0.5%0 for cellulose, and _+0.2%,, for silica. All isotopic data are reported as 6 I~O values where 6 ~sO(%,,) - (Rsample/Rstandard -- 1 )* 1000 and R is the 1~O/~60 ratio.

3. R E S U L T S AND DISCUSSION T h e 6 ~80 v a l u e s o f water, o p a l i n e p h y t o l i t h s , a n d c e l l u l o s e s a m p l e s f r o m the s t e m s a n d l e a v e s o f the g r e e n h o u s e g r o w n p l a n t s are r e p o r t e d in T a b l e 1. W e n o t e that t h e s t e m w a t e r isotopic c o m p o s i t i o n is e n r i c h e d in ~sO c o m p a r e d to that o f the s o u r c e p r o b a b l y d u e to s o m e e v a p o r a t i o n f r o m t h e soil or t h e g r e e n s t e m s . E x c e p t for o n e g r o u p ( s o u r c e w a t e r - 18.2%~), this effect w a s in the o r d e r o f 1 - 3 % 0 a n d t e n d e d to r e d u c e the r a n g e o f w a t e r isotopic c o m p o s i t i o n s u s e d in t h e e x p e r i m e n t . C o n t a i n e r s o f t h e p l a n t s irrigated with t h e

395l

O isotopic composition of opaline phytoliths Table 2: Seasonal changes measured in meteorological parameters and isotopic composition of plant water,

phytoliths and cellulose from two wheat fields in southern Israel (Gilat, Gat). Field

GIL A T

Month

Monthly average relative

Stem phytoliths

Stem water

Stem cellulose

Leaf phytoliths

Leaf

Leaf

water

cellulose

( *C )

h u m i d i t y (%)

8180

~180

~180

8180

6180

6180 27.4

December

11.4

70

34.1

- 3.2

27.0

35.3

14.8

January

11.9

69

33.9

- 3.5

28.9

36.7

9.6

29.4

March

15.3

61

33.3

- 4.5

28.3

37.4

16.9

29.5

April

18

55

32.7

- 2.7

30.3

38.7

25.4

29.6

May

21.6

60

32.6

-

36.2

-

Average

GAT

Monthly average temperature

15.6 + 4.3

- 3.5 +

0.8

December

12.2

72

- 3.3

27.3

33.4

11.9

January

13.2

73

35.6

- 1.9

29.9

37.9

6.5

29.5

March

15.6

65

33.7

- 2.4

28.8

39.4

14.4

28.5

28.5

39.1

19.3

26.9

April

17.7

59

33.7

- 1.4

May

21.6

62

32.6

-

Average

17.9 + 3.6

-18.2%c water had drainage problems resulting in this case with additional enrichment in the soil water and consequently stem water 6'sO value. The higher 6~80 values of the leaf samples reflect the mO enrichment of leaf water during the day and this affects the 6 ~+O~ values of the leaf phytoliths. Considering the relationships between the 6~sO values of plant water and phytoliths in stems and leaves, a linear correlation was apparent with samples from stems and leaves falling into two distinct groups (Fig. 1 ). Note, however, that the midday 6 ~sO~t,, and 6 IgOlw , plotted in Fig. 1, represent spot measurements during peak 180 enrichment. As it is likely that opal is more or less continuously deposited, leaf phytoliths are, therefore, more likely to record the daily mean r5~801,~values. As a first approximation, we substituted the mean 6~*O values between those of stem and midday leaf water as an estimate of mean diurnal values, for leaf water values plotted in Fig. 1. This shifts the 6 ~sO values of the leaves very close to the regression line obtained for the stem samples, as would be expected (Fig. 1). Temperature corrections were not required for this modification as the average temperature difference between leaves and stems of wheat plants grown separately in the same greenhouse was only 0.3°C + 0.4 (n = 72). The daily changes in the cSlSO~tw are negligible (cf. Farris and Strain, 1978; FOrstel, 1978) and the slope of the regression line was fairly close to unity (0.8 _+ 0.3), as would be expected for a simple equilibrium system. Assuming such an equilibrium system and realizing the experiment was not conducted at constant temperature, an apparent fractionation factor [oe[,p,~ ,~*~r = (1000 + 6~SO+0/(1000 + 6~+Ow~,~r)] was calculated for the stem data points (Table 1). The greenhouse experiment showed that the 6 ~sOs, values

- 2.3 +

27.0

33.8

0.8

of wheat opaline phytoliths reflect the 6~80 values of the water in which they form. For the stems this is close to bulk 6~so~,~ and fulfills the first requirement for using opaline phytoliths to obtain a climatic record. At the two commercial wheat fields, climatic conditions varied significantly during the growing season. Mean daytime air temperatures increased by about 12°C and relative humidity decreased by about 10% (Table 2). The 61*O values of midday stem water remained essentially constant, and the 6 l+O values of midday leaf water increased by 15-20%0 during the growing season (Fig. 2a). The 6 ~8Osi values of the phytoliths also showed seasonal trends (Fig. 2b). Leaf phytoliths showed an increase of about 5%o in their 61sO+~ values, which was followed by a decrease in May when the plants dried up in the field. The seasonal trend in the 6 ~O~ values of leaf phytoliths reflected the changes in leaf water, except for the drying stage. On an absolute basis, however, the seasonal range in the isotopic compositions of the leaf phytoliths is much smaller than that of midday leaf water. This is consistent with the uncertainties regarding the actual 61SOlw associated with phytolith deposition (as discussed above), as well as with the contrasting effects of increasing temperature and decreasing relative humidity. The decrease of about 1.5%o in 6mO~ values of stem phytoliths in both fields is not accompanied by any seasonal change in 6 ~+O~,w.It results from a decrease in aopa,-wat+r, and strongly supports the idea that this trend reflects the seasonal change in ambient temperature. The increase in wheat plant biomass during the season is normally exponential (Kirby, 1985). The opal concentrations in the plant stems were relatively constant during the season ( 1.5 + 0.5 wt% from dry plant material). Thus, the

3952

R. Shahack-Gross et al. 30

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DEC

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JAN ,

FEB

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MAR i

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APR i

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MAY i

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[] []

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30 0

..... 30

JAN

,.. 60

FEB

MAR

APR

MAY

,.., . . , . 90 120 150

180

Days a f t e r s e e d i n g Fig. 2. Seasonal variation in isotopic compositions of opal phytoliths and plant tissue water from two wheat fields in southern Israel. Data for Gilat (near Be'er-Sheva) are represented by circles and for Gat (near Kiryat-Gat) by squares. (a) 6~sO of leaf water (open symbols) and stem water (closed symbols) throughout the season. (b) 6 ~sO~of leaf (open symbols) and stem (closed symbols) phytoliths. Both fields exhibit similar trends in the 6JsO~ and 6~sO of the water of the stems (solid line) and leaves (dashed line).

tainty, to the opal-temperature relationships derived from marine diatoms ( S h e m e s h et al., 1992) and amorphous silica precipitated from geothermal waters (Kita et al., 1985 ). This suggests that all three processes involved in opal formation are basically similar. Furthermore, applying this equation to the greenhouse data yields an apparent mean daily temperature of about 16°C, consistent with temperature measurments in the greenhouse during the growing season (carried out on an irregular basis). The results show that the 6 ~ O values of plant phytoliths are clearly influenced by environmental conditions, and record different information in leaves and stems. In the stems, the 6 ~80~ values seem to predominantly record variations in temperature superimposed on the isotopic signature of source water. Thus stem phytoliths can potentially provide an invaluable research tool for reconstructing regional scale terrestrial paleoclimate. In the leaves, a similar signal must be greatly modified by effects of relative humidity, mediated by isotopic enrichment of leaf water ( Farris and Strain, 1978; FOrstel, 1978), and by marked, though poorly understood at present, changes that occur during the drying stage. This later change is quite considerable and clearly needs further research. Leaf phytoliths appear to be less suitable for climatic calibration at this stage. Notably, the 6180 values of cellulose extracted from the same plants showed little or no clear difference between leaves and stems or seasonal trends (Tables 1, 2). This is probably because cellulose contains photosynthetic intermediates formed and isotopically imprinted in the leaves before their translocation to the deposition sites in the base of the leaves and in the stems. This isotopic imprint is later modified by exchange with stem water. Thus, the isotopic signature of plant cellulose always represents a mixture of the leaf and stem (i.e., temperature and humidity) signatures (Yakir, 1992; Yakir et al., 1994).

20-

18-

& •

~9 o amount of newly added opal at each time interval must greatly exceed the already accumulated portion, and effectively imposes the new isotopic signature. Note that whole plants of increasing size were always sampled in this study. We, therefore, used the stem 6 lSOsi , the average seasonal 6 tsOsw of each field and the mean monthly measured temperatures to quantitatively evaluate the apparent temperature dependency of the 6t80 values of stem phytoliths. The best fit line to the data (Fig. 3) is

16-

14-

12

10

-5 t°C = 5.8 - 2.8(+0.7)*(61sO~i - b~80~,w - 4 0 ) , or in the a notation, 1 0 0 0 1 h a = - 1 . 6 5 + 3.14

+0.78)*(106/T:), ( r 2 = 0.76, p = 0.011).

These equations are similar, within the analytical uncer-

AA •

-4

-3

-2

-1

(~18Osi-~jlSOstw-40)

Fig. 3. Calibration of the temperature dependency on the isotopic fractionation between stem silica and stem water, using linear regression on data from both fields: t°C = 5.8 - 2.8 (_+0.7)*(stem 61sO~i - 6180~w - 40). The calibration is similar (within the analytical uncertainty) to the opal paleotemperature equation obtained from marine diatoms (Shemesh et al., 1992).

O isotopic composition of opaline phytoliths The application of the results presented above to terrestrial climatic reconstructions depends to a large extent on the availability of suitable fossil samples. W e note that in some archaeological sites stem phytoliths are separated from leaf phytoliths, as stem material was used in the production of mats, baskets, m u d bricks, and cordages (Rosen, 1987, 1992; Miller-Rosen, 1993). W e also note that preserved assemblages of stem phytoliths can be recognized by their distinctive morphologies as compared to leaf and inflorescence derived phytoliths (Rosen, 1992). Phytoliths are present in many archaeological sites and in certain soil sections (Kelly et al., 1991 ). Thus, potential samples do exist and could be used for climate reconstruction, provided that their isotopic signal has not been altered by diagenesis. Deep Sea opal seem to preserve its isotopic signal on time scales of at least 400 Ka ( S h e m e s h et al., 1995). W h e t h e r the same applies to continental materials has not been determined yet and will clearly require more research. " F o s s i l " 6t80~,~ could be determined by establishing a second paleotemperature equation using other plant produced minerals, such as calcite or calcium oxalate. This could also be achieved by combining opal and cellulose measurements, or by using measured isotopic compositions of dated fossil groundwater. Samples that have been heated by fire are unsuitable for paleoclimate reconstruction, as their opaline isotopic compositions are affected by increased temperatures (data not shown, cf. Labeyrie and Juillet, 1982). Isotopic measurements of wood ash phytoliths could, however, be used to prove fire use, and possibly even estimate the firing temperature. This study shows that the isotopic compositions of opaline wheat phytoliths, and in particular those from the stems, can potentially provide long-term integrated terrestrial climatic information from different time periods. Acknowledgments--We thank Gilat Experimental Farm, Minhor Farm of Hazera Co., and the Israel Meteorological Service for materials and information. The technical assistance of E. Negreano, Y. Bolakia-Cohen, and R. Silenikov, and the helpful advice of A. Miller-Rosen, are gratefully acknowledged. This study was supported by the Minerva Foundation, Munich, Germany. Editorial handling: J. D. Macdougall

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