Geoscience Reference
In-Depth Information
to change, may in fact show microbially induced alteration soon after death (Zazzo
et al.,
2004
). This caveat would also apply to claims that
18
O variations measured
on samples of dentinal hydroxyapatite collected from annual growth bands in the
tusks and teeth of late Pleistocene mastodonts and mammoths can be used to measure
seasons of death and seasonal paleoclimates (Koch et al.,
1989
).
Cerling et al. (
1991
) used the stable carbon isotopic composition of pedogenic
carbonate and organic matter from paleosols at a 14 Ma site in western Kenya with
abundant remains of fossil fauna, including hominoids, to test competing hypotheses
about the plant cover at that time. (The Hominoidea superfamily includes the apes
and ancestral humans, as discussed in
Chapter 17
). They were able to demonstrate
conclusively that C
3
plants dominated the local vegetation when the soils were form-
ing, so that the soils had probably developed under forest or woodland. Any of the
grasses in these soils identified by earlier workers were most likely transient features
of the landscape that had developed in the aftermath of volcanic eruptions but did not
persist for long and so had little impact on soil development.
Ambrose and Sikes (
1991
) studied the
13
C/
12
C ratios in soil organic matter in late
Holocene soils along an altitudinal transect in the central Rift Valley of Kenya. They
found that the forest-savanna boundary had advanced by more than 300 m in altitude,
an observation not evident from earlier studies of the regional pollen record.
Analysis of stable carbon isotopes in paleosols associated with 4.4Ma
Ardipithecus
ramidus
hominid fossils at Aramis in the Afar Desert of Ethiopia has been used to
deduce in what type of environment these early hominids might have lived (Wolde-
Gabriel et al.,
2009
). Additional isotopic investigations included analysis of oxygen
and carbon isotopes in mammalian tooth enamel, supplemented by pollen analysis
and phytolith abundance measurements (see Chapters
16
and
17
for more details).
Although WoldeGabriel et al. (
2009
) concluded that the habitat at Aramis consisted
of woodland and forest patches, a reappraisal of their primary data by Cerling et al.
(
2010
) reached a different conclusion - namely, that the vegetation comprised tree-
or bush-savanna, with 25 percent or less of woody canopy cover. White et al. (
2010
)
in turn offered a vigorous rebuttal of this conclusion and defended the original recon-
struction.
Seventy-four thousand years ago (74 ka), the entire Indian subcontinent was covered
in a thin layer of volcanic ash 10-15 cm thick derived from the highly explosive
eruption of Toba volcano in Sumatra, Indonesia. Akey and still hotly disputed question
relating to this eruption concerns its possible impact on regional vegetation and
climate. In order to help resolve this issue, the
13
C values in pedogenic carbonate
nodules in paleosols above and beneath the 74 ka ash layer were analysed across a
400 km transect in north-central India (Ambrose et al.,
2007a
; Williams et al.,
2009a
).
The results showed that before the eruption, this part of India was under woodland
or forest and was replaced by open woodland and grassland after the eruption. These
results were consistent with pollen analyses carried out on a marine core from the Bay
