Geoscience Reference
In-Depth Information
13
C values in cave speleothems are an indirect guide to the type of vegetation
growing above the cave, which will also influence the
The
13
C in soil carbonates. Given
that the type of vegetation growing in arid areas reflects the amount of precipitation as
well as the soil type, it can be used as an indirect measure of effective precipitation. In
a novel approach to the question of megafaunal extinctions in North America, Polyak
et al. (
2012
) used the
234
U values in speleothem calcite from Fort Stanton
Cave in southern New Mexico as a proxy for effective precipitation. Ideally, some
independent control involving pollen analysis, tree ring data or vegetation remains
found in packrat or stick-nest rat middens should be used to evaluate inferences
based solely on speleothem isotopic data. Such an approach can also yield additional
paleoclimatic insights. For example, Marino et al. (
1992
) used the
13
Cand
13
C changes
in the C
4
shrub
Atriplex confertifolia
from packrat middens in the western United
States to provide a record of glacial to interglacial changes in atmospheric carbon
dioxide (CO
2
). They found that the atmospheric CO
2
was isotopically lighter during
the last glacial period relative to interglacial periods, probably as a result of a reduced
terrestrial biomass and lower biological productivity in the polar oceans.
7.6 The strontium isotope record in desert lakes, rivers,
dust and volcanic ash
On occasion, the results from one line of analysis do not accord with what appears to
have been well-established. For example, radiocarbon dates obtained from freshwater
gastropod shells in alluvial sediments along the lower White Nile indicated that the
most recently abandoned flood-plain of that river began to form as far back as 14-
15 ka (Adamson et al.,
1980
; Williams and Adamson,
1980
; Adamson et al.,
1982
;
Williams and Adamson,
1982
). Pollen and glaciological studies from the Ugandan
highlands also indicated a postglacial rise in temperature and precipitation after 14-
15 ka, consistent with diatom and sedimentary evidence of overflow from Lakes
Victoria and Albert into the upper White Nile in southern Sudan (Livingstone,
1980
).
In a similar vein, the Blue Nile began to deposit clay across its flood-plain soon after
15 ka, after a long dry interval during the Last Glacial Maximum, when lakes were dry
throughout East Africa (Butzer,
1980
; Adamson et al.,
1980
; Williams and Adamson,
1982
; Adamson et al.,
1982
). This substantial body of work was ignored in a short
paper claiming that Lake Victoria had remained a closed basin until 7.2 ka (Beuning
et al.,
1997a
). This conclusion was inferred from
18
O/
16
O ratios in sediment cellulose
from Lake Victoria. It was contrary to three decades of research throughout the Nile
Basin and ran counter to what was already well-established in Nile Valley prehistoric
archaeology (Butzer and Hansen,
1968
; Butzer,
1980
).
Talbot et al. (
2000
) resolved to test this hypothesis and to determine more precisely
when Lake Victoria began to overflow after its long dry interval during the Last Glacial
Maximum. They used strontium isotopes as tracers to ascertain just when Lakes
