Amount Effect

Figures 9.11-9.13 show the dependence of the isotopic composition on the amount of rain: heavier rain events, or greater monthly precipitation amounts, result in more negative dD and d18O values. Dansgaard (1964) proposed two major explanations for this amount effect:

• Lower ambient temperatures cause the formation of clouds with lighter isotopic composition (temperature effect, Fig. 9.9); lower temperatures also cause heavier rains.

• Falling raindrops undergo evaporation, enriching the falling rain in the heavy isotopes. This effect is less severe both when ambient

Fig. 9.10 Monthly mean d18O values in precipitation and monthly mean air temperatures from 1971-1978 for Swiss stations. The value of January (1) is shown twice to complete the cycle. The d18O values are seen to covary with the temperature, reflecting a pronounced temperature effect of 0.35-0.5% d18O/°C. The measurements, carried out at three stations of different altitudes, revealed an altitude effect, precipitation at higher altitudes having isotopically lighter compositions. (From Siegenthaler and Oeschger, 1980.)

Month

Fig. 9.10 Monthly mean d18O values in precipitation and monthly mean air temperatures from 1971-1978 for Swiss stations. The value of January (1) is shown twice to complete the cycle. The d18O values are seen to covary with the temperature, reflecting a pronounced temperature effect of 0.35-0.5% d18O/°C. The measurements, carried out at three stations of different altitudes, revealed an altitude effect, precipitation at higher altitudes having isotopically lighter compositions. (From Siegenthaler and Oeschger, 1980.)

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Monthly precipitation, mm

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IOO 200 300 400 Monthly precipitation, mm

Fig. 9.11 Two cases of isotopic compositions varying with the amount of precipitation (the amount effect), reported by Dansgaard (1964). Left, Binza (Leopoldville), Congo; right, Wake Island. The d18O precipitation values are for individual months. The amount effects were —2.2% d180/100 mm rain for Binza and —1.6% d180/100 mm rain for Wake Island.

Fig. 9.12 Amount effect. Monthly rain and d O values measured for three years at Rowen Boos, Haute Normandie, France. Dots, October 1974-December 1975; x, 1976 and 1977. Rainier months reveal isotopically lighter rain. (After Conrad et al., 1979.)

Fig. 9.13 Amount effect. Monthly rain and d O values of northeastern Brazil plotted as a function of sampling date. The curves are mirror images, revealing low d18O values at the high rain months. (From Salati et al., 1980.)

& u temperatures are low and when the amount of rain is large (as the air gets more saturated).

The amount of monthly rain varies during the year, causing a seasonal variation in the isotopic composition. This point is demonstrated in a case study from northeastern Brazil (Fig. 9.13).

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