Droughts - Extreme Events: A Physical Reconstruction and Risk Assessment - page 36

− 5

− 4

− 3

2
−

−

1
0
2
4000

3000 2000 1000 0 1000 2000

Year BC

Ye a r AD

13 Cprofile for peat cellulose in the Jinchuan core. Positive shifts
represent lower than average soil moisture and negative shifts represent higher soil
moisture or precipitation (from Hong et al ., 2001).

Figure 2.8.

been subject to episodes of severe drought that lasted several years leading to a
significant reduction of available water in the landscape.

Carbon isotopes

Stable carbon isotopes can be used to estimate the relative humidity of
past climates. Different photosynthetic processes divide plants, mainly grasses,
into C3 or C4 vegetation categories (Gadbury et al ., 2000). Plants that utilise
different ways of fixating CO 2 through photosynthesis have different ratios of
stable carbon isotopes such as

13 C. Variations in precipitation or soil water
content are reflected in C3 plants by varying levels of

13 C. In this fashion, fos-
sil C3 plants can be used as indicators of prehistoric changes in soil moisture
and precipitation. High values of

13 Careassociated with low soil moisture

13 Cvalues are associated with high soil mois-
ture or precipitation conditions when the plant was living (Hong et al ., 2001).
The abundance of C4 grasses tends to be more specifically related to the mini-
mum temperature during the growing season (Gadbury et al ., 2000).
Hong et al .(2001)usedthe

or precipitation, while low

13 C content of peat, in Jinchuan (see Fig. 2.8),
northeastern China, to estimate soil moisture or precipitation during prehis-
toric climatic regimes. The peat had formed mainly from Carex species, an
annual C3 species with a shallow root system, which makes it sensitive to and
dependent upon available soil moisture. Three main climate stages were iden-
tified after a radiocarbon chronology had been established. Low

13 Cvalues,
suggestive of high soil moisture or precipitation, were identified in samples
aged between 4000 and 2200 BC (6000--4000 year BP). Comparisons with

18 O
levels from samples within the same age bracket confirmed that this period
wasrelatively cold and wet. These results were also in agreement with the
findings from European lakes that had persistently high water levels at the
same time as glaciers were advancing in many locations globally. After 4800
years BP,

13 Cvalues showed a progressive increase suggesting that climate