Amazon's ancient rain
Contrary to suggestions that the Amazon Basin was a sea of arid savanna during
the Pleistocene, researchers have found that lowland tropical rainforest dominated
the region just as it does today. Consequently, a relatively stable wet tropical
climate similar to today's must have also existed. Such findings have serious
implications for a widely accepted hypothesis explaining why the Amazon Basin
exhibits such high levels of species diversity.
Goñi of the University of South Carolina in Columbia and Thomas Kastner,
now of Anadarko Petroleum in Texas, analyzed organic matter in sediment cores
from the Amazon deep-sea fan, an accumulation of sediment along the continental
margin. The organic matter eroded from the basin and traveled in the Amazon River
to the edge of the continental shelf between 70,000 and 10,000 years ago, a time
when sea level was much lower than today.
Goñi and Kastner looked at carbon isotopes and two sets of molecular biomarkers
that indicate the type of vegetation that was the source of the organic matter.
"If you look at a river that drains arid grasslands, you get a very different
signature in the materials eroded from the vegetation and the soils than if you
look at a river that erodes a tropical or temperate rainforest," Goñi
Researchers work on a sediment core
while on board Leg 155 of the JOIDES Resolution drill ship off the coast
During photosynthesis, temperate plants fractionate isotopes of carbon differently
than arid plants. The distinct ratios of carbon-13 to carbon-12 that result leave
an isotopic signature fixed in the organic matter.
The Amazon deep-sea fan deposits had an average carbon isotope value of negative
27.5 per mil (or thousand), which indicates the organic matter came from temperate
and tropical plants. "Those are extremely negative values and they are exactly
the same as you see today in the Amazon," Goñi says. Had the organic
matter come from savanna or arid grassland, the researchers would have expected
carbon isotope values between negative 12 per mil and negative 15 per mil.
The Refuge Hypothesis, proposed in 1969 by German ornithologist Jurgen Haffer,
attributes the evolution of present-day biodiversity to plants and animals having
been geographically isolated in the past. He reasoned that a cool, dry climate
during Pleistocene glacial periods would have allowed vast expanses of arid grasslands
to fragment the previously continuous tropical rainforest. Populations trapped
in pockets of rainforest, or refuges, would accumulate genetic variations and
eventually diverge into new species. Based on Darwinian ideas, the hypothesis
caught on quickly despite sparse geologic and climatic data from the Amazon, a
region that is still difficult to study.
However, by sampling deep-sea fan sediments cored off the coast of Brazil during
Leg 155 of the Ocean Drilling Program, scientists have now taken advantage of
the natural tendency of a drainage basin to collect and concentrate basin-wide
Previous studies had analyzed Amazon lakebed cores for grass pollen, but found
little that could confirm the presence of vast grasslands during the Pleistocene.
Some of these studies were conducted by ecologist Paul Colinvaux of the Ecosystems
Center at the Marine Biological Laboratory in Woods Hole, Mass. "This is
the first paper which has a proxy that is unequivocally a measure of whether it
was forest or savanna. And they've got nothing but forest all the way through,"
The conclusion that forest vegetation requiring a warm, wet climate prevailed
throughout the late Pleistocene could supply evidence to overturn Haffers
ideas about the origins of species diversity in the Amazon.
"In fact, maybe one of the reasons for such extensive biodiversity there,"
Goñi says, "is that the climate didn't change that much."
The Kastner and Goñi study also examined lignin, a cellulose-binding compound,
and cutin, a waxy coating that prevents dehydration. Differences in the way various
plant groups produce these biomarkers make it possible to identify the compound's
origin angiosperm vs. gymnosperm, or monocot (grasses) vs. dicot (trees).
The values of lignin and cutin were also similar to values found in suspended
sediment being exported by the Amazon River today. "Empirically, we see very
little difference," Goñi says.
The conclusion follows that present-day vegetation has predominated for the past
70,000 years, Goñi explains. "That is not to say that small changes
could not have occurred, but nothing basin-wide, otherwise we would have seen
it recorded in the sediments."
Geotimes contributing writer