The Birth of the Mighty Amazon

Carina Hoorn

Looking down at the Amazon River from above, an observer cannot help noticing that water dominates the landscape even beyond the sinewy main channel. The river, which extends from the Pacific highlands of Peru some 6,500 kilometers to Brazil's Atlantic coast, swells out of its banks and inundates vast swaths of forest during the rainy seasons, and myriad lakes sprawl across its floodplains year-round.

All told, the river nurtures 2.5 million square kilometers of the most diverse rain forest on earth. Until recently, however, researchers had no idea how long this intimate relation between river and forest has actually existed. The inaccessibility of this remote region, now called Amazonia, meant that long-held theories about the early days of the river and surrounding forest were speculative at best.

In the past 15 years new opportunities to study the region's rock and fossil records have finally enabled investigators to piece together a more complete picture of Amazonian history. The findings suggest that the birth of the river was a complicated process lasting millions of years and that the river's development greatly influenced the evolution of native plants and animals. Indeed, many researchers now contend that the incipient river nourished a multitude of interconnected lakes in the continent's midsection before forging a direct connection to the Atlantic Ocean; this dynamic wetland produced ideal conditions for both aquatic and terrestrial creatures to flourish much earlier than previously thought. The new interpretations also explain how creatures that usually live only in the ocean - among them dolphins - now thrive in the inland rivers and lakes of Amazonia.

Telltale Sediments

Understanding how and when the Amazon River came to be is essential for uncovering the details of how it shaped the evolution of life in Amazonia. Before the early 1990s geologists knew only that powerful movements of the earth's crust forged South America's Andes and towering mountain peaks elsewhere (including the Himalayas and the Alps) primarily between about 23 million and five million years ago, an epoch of the earth's history known as the Miocene. Those dramatic events triggered the birth of new rivers and altered the course of existing ones in Europe and Asia, and the experts assumed South America was no exception. But the specific nature and timing of such changes were unknown.

When I began exploring this mystery in 1988, 1 suspected that the best records of the ancient Amazonian environment were the massive deposits of mud, sand and plant debris stored in the trough that the mighty river now follows to the Atlantic. But getting to those sediments - long since solidified into mudstone, sandstone and other rocks - posed considerable challenges. A jungle big enough to straddle nine countries with differing laws does not yield its secrets easily. And the rocks forming the trough, which poke aboveground only rarely, usually do so along nearly inaccessible tributaries and tend to be covered by dense vegetation.

Along the hundreds of kilometers of waterways my field assistant and I surveyed in Colombia, Peru and Brazil, we encountered only a few dozen sizable outcrops. And often we had to wield a machete to cut away the foliage - once surprising a giant green anaconda and another time exposing the footprints of a jaguar. Even then, we could reach only the uppermost layers of the thick rock formation, which extends almost a kilometer below the surface in some locations.

Once the initial fieldwork was complete, my first conclusion was that the Amazon River did not exist before about 16 million years ago, the start of what geologists call the ‘Middle’ Miocene. Most of the rocks we found that dated from earlier times consisted of the reddish clays and white quartz sand that clearly had formed from the erosion of granites and other light-colored rocks in the continent's interior. This composition implied that the region's earlier waterways originated in the heart of Amazonia. I inferred - and other researchers later confirmed - that during the Early Miocene, rivers flowed northwest from low hills in the continental interior, and some eventually emptied into the Caribbean Sea.

The Amazonian landscape altered significantly soon thereafter when a violent episode of tectonic activity began pushing up the northeaster Andes. By about 16 million years ago in the rock record, the red and white sediments disappear. In their place we found intriguing alternations of turquoise-blue, gray and green clays, brown sandstone and fossilized plant matter called lignite. It was obvious that the dark particles of mud and sand were from a source other than light-colored granites. And distinctive layered patterns in the fossilized sediments indicated that the water that deposited them was no longer flowing north; instead it flowed eastward. My guess was that the rising mountains to the west shifted the drainage pattern, sending water east toward the Atlantic.

In support of this idea, later analysis of the sediment at Wageningen University in the Netherlands proved that many of the brown sand grains were indeed fragments of the dark-colored schists and other rocks that began washing away as the newborn Andes rose up. What is more, some of the pollen grains and spores I found in the clays and lignites came from conifers and tree ferns that could have grown only at the high altitudes of a mountain range. This pollen contrasted with that in the older Miocene sediments, which came from plants known to grow only in the low-lying continental interior. Drill cores of Miocene rocks in Brazil, which provided the only complete sequence of the change from reddish clays to the blue and brown sediments, further corroborated my conclusions.

Finally, scientists had undeniable evidence for when the budding Amazon River was born. But it soon became clear that the river did not establish its full grandeur until much later. In 1997 David M. Dobson, now

at Guilford College in Greensboro, N.C., and his colleagues discovered that the Andean sand grains I found in Amazonia first began accumulating along the Brazilian coast only about 10 million years ago.

That timing means the river took at least six million years to develop into the fully connected, transcontinental drainage system of today. Research into the geologic changes that occurred in that transition period has now illuminated the origins of the region's enigmatic, present-day fauna.

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The main emphasis in this book will be on …	Основное внимание в данной книге будет сосредоточено на …
My discussion will be centered on (the following issues) …	Мой доклад будет сосредоточен на … (следующих вопросах) …
The paper presents (a comparative analysis of …).	В работе дан (сравнительный анализ…).
The problem is …	Проблема заключается в том, что …
The problem raised (studied, considered, discussed) here is …	Проблема, поднятая (изучаемая, рассматриваемая, обсуждаемая) заключается в …
The present book is aimed at … The present book aims at …	Целью настоящей книги является …
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On this question there is an enormous literature.	По данному вопросу существует обширная литература.
This question has been widely discussed in the literature.	Этот вопрос широко обсуждался в литературе.
The question of … was repeatedly raised in the literature.	Вопрос о … неоднократно поднимался в литературе.
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It has become customary to think that …	Стало привычным рассматривать, что …
According to this view …	Согласно данной точке зрения …
On the basis of the view it is possible to …	Исходя из данной точки зрения, возможно …
Some hold that … while others hold that …	Одни считают так, другие считают иначе.
The view that is characterized by … is …	Точка зрения, изложенная … является …
This point of view was stressed by …	Эта точка зрения была подчеркнута …
The view that has been widely held in recent years is …	Точка зрения, которой широко придерживались в последние годы, является …
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This supports the concept of …	Это подтверждает понятие …
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