Droughts - Extreme Events: A Physical Reconstruction and Risk Assessment - page 40

Oxygen isotope concentrations in fossil bison molar teeth found in Nebraska,
USA, suggest a serious drought may have caused the death of a large herd of
these animals about 9500 years BP. The Hudson-Meng bone bed in Nebraska is a
deposit of hundreds of bison that were killed by a catastrophic event. Gadbury
et al .(2000)examined the carbon and oxygen isotopes of the bison teeth to
ascertain whether an environmental signal was preserved that may give clues
to thenature of this catastrophic event. One of the measures was to determine
isotopic variations 'down tooth' which Gadbury et al .(2000)suggestisaprogres-
sive measure of the environmental conditions over the life of the animal. The
bison were approximately six years of age when they died so the isotopic varia-
tions down tooth record environmental conditions over that six year period. Two
of the molars examined showed down tooth variations in

18 O and 13 C. The

18 Oincreases down tooth or over the life span of the ancient animal
with lowermost value (being the highest value of

level of

18 O) representing the con-
centration in the tooth just prior to animal's death. Gadbury et al .(2000)regard
this trend toward increasingly positive

18 Ovalues as reflecting an increase in
isotopically positive precipitation, which in turn, is a reflection of drying condi-
tions. They suggest that the winter/early spring and/or late fall (autumn)/winter
precipitation decreased with time. The isotopic concentrations also suggest a
decrease in C3 vegetation over time which agrees with the interpretation of
progressively drying conditions. It is possible that this progressive increase in
drying of the environment resulted in a prolonged severe drought causing star-
vation, poor reproductive success, and finally the death of hundreds of these
animals.
Oxygen isotope records have also played a significant role in unravelling the
cause of the demise of the Mayan civilisation, which occupied a vast area of
Central America between 2600 BC and AD 1200. They became a very prosper-
ous people between AD 600 and 800, constructing thousands of architectural
structures and were relatively advanced in astronomy and mathematics. How-
ever, their civilisation declined substantially between the years AD 800 and 900.
Hodell et al .(1995)undertook oxygen isotope measurements on ostracods from
lakes on the Yucatan Peninsula, Mexico. The

18 O measurements show distinct
peaks, representing arid climatic conditions at AD 585, 862, 986, 1051 and 1391.
The first peak at AD 585 coincides with the 'Maya Hiatus', which lasted between
AD 530 and 630 and was marked by a sharp decline in monument carving, aban-
donment in some areas and social upheaval. The Mayan culture flourished from
AD 600 to 800, a period shown in Figure 2.10 as a period of wetter conditions.
The collapse of Classic Mayan civilisation occurred between AD 800 and 900
which corresponds to a peak in the

18 OatAD862. At about AD 1000, mean
oxygen isotope values decrease indicating a return to more humid conditions.