Biology Reference
In-Depth Information
around 54 percent of their current extent at the LGM. Maslin and Burns ar-
gue that “the oxygen isotopic composition of planktonic foraminifera recov-
ered from a marine sediment core in a region of Amazon discharge shows
that the Amazon Basin was extremely dry during the Younger Dryas [a rela-
tively minor cold interval compared to the several glacial maxima] with the
discharge reduced by at least 40% compared to that of today” (2000, 2285).
The argument has shifted over the years from the contention that (1) there
was no major climatic change in the Amazon Basin, to (2) it was colder but
not drier, to (3) it was colder and drier but not enough to disrupt a virtually
continuous rain forest over the basin throughout the Pleistocene, to (4) it
was cold and dry and the lowland rain forest was signifi cantly disrupted. As
the scaffolding around objections to the refugium theory weakens, a tipping
point is approaching, or has been reached, whereby some updated form of
the theory seems plausible as one mechanism for generating biodiversity in
the lowland neotropical rain forest.
As an example of the more intricate causal mechanisms being proposed
for climate changes in the Amazon Basin, Hooghiemstra and van der Ham-
men (1998) note that the precession cycle shifts the boundaries of the Inter-
tropical Convergence Zone. This means that the northern and southern
edges fl uctuate between moist and dry conditions with the meandering of
the system, while the central part remains more consistently moist. The mi-
gration of the ITCZ during the Holocene and associated dryness in the Am-
azon Basin and surrounding areas, for example, the Patí Valley of Colombi;
(González-Carranza et al. 2008), is further demonstrated from changes in
titanium and iron concentrations in the Cariaco Basin offshore from Ven-
ezuela (Haug et al. 2001). The record can be measured with a precision of
decades or less. The concentrations were reduced during the Younger Dryas
(dryness), increased during the thermal maximum, and reduced during the
Little Ice Age.
The palynological evidence for and against the theory has been re-
viewed by Hooghiemstra and van der Hammen (1998), van der Hammen
and Hooghiemstra (2000), and Behling and Hooghiemstra (1998). I have
offered my own summaries as well (II, chap. 8; Graham 2010; Graham
et al., work in progress). Examples of vegetation changes in a presumed
interrefugium area (Carajás, Brazil) and within a refugium (Pata, Brazil)
are shown in fi gures 9.5 and 9.6). Carajás is in an area presently of moist
forest, but the diagram shows fl uctuations between arboreal pollen (AP)
and nonarboreal pollen (NAP). Pata, in a suggested refugium zone, shows
relatively little fl uctuation in AP and NAP pollen. The concept is still being
