- The vast wetland that used to sit in the heart of where the Amazon lies today received a more recent pulse of seawater than previously thought, a new study confirms — a phenomenon that contributed to the region’s species richness, including its iconic river dolphins.
- The study also says the likeliest source of these marine incursions, some 23 million to 8.8 million years ago, was the Caribbean Sea, with the water surging inland down what is today the Orinoco River Basin in Venezuela.
- Researchers say investigating the distant past of the Amazon can yield clues about its near future, given that the late Miocene was a period of global warming, with temperatures far higher than the 2°C (3.6°F) rise that the Paris Agreement is trying to prevent.
- But the current rate of global warming is taking place on an exponentially shorter time scale, and combined with record rates of fires and deforestation, it gives animal and plant species no time to adapt, scientists say.
Scientists already knew that where the western Amazon rainforest sits today was once a vast wetland, almost four times the size of Texas and periodically flooded by pulses of seawater. Now, a new study posits that the source of these pulses was most likely the Caribbean Sea, and that there was another — later and much more significant — episode of marine incursions that contributed to the species richness of the region as we know it today, including its iconic river dolphins.
“This new paper goes to great lengths to confirm this link between the Caribbean and the Amazon,” Carina Hoorn, a researcher from the University of Amsterdam, who in the 1990s was the first to suggest the idea of marine incursions in the western Amazon, told Mongabay in a video call.
Previous research by Hoorn and other scientists dates these episodes to the early Miocene epoch (23 million to 16.3 million years ago) and the middle Miocene (14.9 million to 12.9 million years ago). Since then, these “sea invasions” have been the subject of several studies trying to delve into their timing, duration, intensity and origin.
The latest study, published in March by researchers from Brazil and the United States, makes the case for a much more intensive period of marine incursions during the late Miocene (11.1 million to 8.8 million years ago). It also uses fossil evidence to show that the source of this flooding was the Caribbean, with the water flowing down what is today the Orinoco River Basin in Venezuela.
“Researchers talked about six possible routes to the marine incursions,” lead author Lilian Leandro, a researcher at Unisinos University in the Brazilian state of Rio Grande do Sul, told Mongabay in a video call. “But now we have managed to hammer out that it came from the Caribbean Sea.”
To get to the bottom of the mystery, Leandro and her colleagues compared microfossils from samples from three offshore cores in the Atlantic and Pacific Oceans with six core samples taken from onshore sites in Brazil’s Solimões River Basin. (The Solimões is the stretch of the Amazon River in the western Amazon that runs from the border with Peru to where it meets the Negro River in the city of Manaus, in Amazonas state.)
“The similarity between microfossil assemblages of the Solimões Basin and the Caribbean Sea, and evidence of increased runoff from the Orinoco River drainage system, strongly suggest the Caribbean Sea as the primary source area of the marine incursions, supporting a Venezuelan seaway,” the study says.
Carlos D’Apolito, who studies fossilized spores and pollen at the Federal University of Mato Grosso but was not involved in the recent study, said the mix of freshwater and saltwater environments, nutrients washed down with from the Andes, and the tropical climate all combined to conjure up “a melting pot ready to produce [this] spectacular [Amazonian] biodiversity.”
Leandro’s study also helps flesh out what researchers now understand about the intensity and the timing of this phenomenon. Since 1993, when Hoorn found evidence of fossilized pollen from coastal mangrove trees during her fieldwork deep inland in northwestern Brazil, there has been a general consensus among researchers that the region received repeated marine incursions for at least two periods of time, in the early and middle Miocene.
The possibility of a third and more recent period of incursions, during the late Miocene, was already suggested by researchers like Ana Paula Linhares, from the Museu Paraense Emílio Goeldi, in 2017, and Bruno Espinosa, from the Federal University of Mato Grosso, in 2021. The new study, co-authored by Linhares, not only confirms this hypothesis, but also identifies this third episode as being far more intense than the previous ones.
“During this time interval we saw a greater magnitude of this marine incursion, meaning that it lasted longer than in previous periods,” Leandro said. “We found shark scales from the late Miocene in the Amazon.”
D’Apolito, a co-author on Espinosa’s 2021 study, said the new study confirms their previous research, and adds a much larger amount of data to the scientific record. What makes this third incursion so important, he said in a video call, is that “it means the duration and impact of this event over the Amazon biodiversity formation is greater than we thought.”
All the researchers interviewed by Mongabay agreed that the effects of this period of marine incursions are still visible in the rainforest, in the form of animals and plants that originated in the ocean but have over the course of millions of years adapted to the freshwater environment.
“The river dolphins are part of that history, they are a relic of the connection with the ocean,” Hoorn said. “And there is a lot we don’t know yet.”
Leandro’s paper also raises the possibility of a fourth and even more recent period of marine incursions, during the early Pliocene (4.7 million to 3.8 million years ago), but she said this still needs to be confirmed through further investigation.
A vast wetland with giant animals
There are two plausible explanations for the marine incursions into the Amazon, experts say: a global warming period, which led to the melting of the glaciers and a corresponding rise in the sea level; and the formation of the Andes mountain range, which created a vast area of lowlands in the western Amazon.
“It transformed the Amazon into a large drain, a lower area that became a channel for these marine incursions,” Leandro said.
The precise characteristics of this landscape are still uncertain, but several studies point to a vast wetland, known as the Pebas Mega Wetland, that was permanently flooded with freshwater and that periodically received pulses of seawater.
“This landscape has been shaped over the years, but the data indicate that the western Amazon was generally a lacustrine [lake] environment with occasional saline and brackish records,” Linhares said .
This wetland was home to an impressive array of animals, D’Apolito said: “There are records of a dinosaur-sized crocodile, almost 12 meters long [39 feet], and a turtle the size of a car. It was an incredible swamp system.”
Studies show there were at least three periods marked by marine incursions, all of them coming from the Caribbean Sea. They took place during the Miocene epoch, from 23 million to 8.8 million years ago, bringing saltwater to the western Amazon, which at the time was a vast swamp called the Pebas Mega Wetland. Image by Lilian Leandro.
The same rise of the Andes that allowed seawater into the continent eventually led to the end of the marine incursions, according to the study; the surging of the last part of the mountain range around 1.8 million years ago, in today’s Mérida region of Venezuela, closed off the coastal channel that had allowed the seawater to surge inland. And the accumulation of Andean sediment created new ground in the wetland basin, raising the altitude of much of the western Amazon and shrinking the giant lake of the Pliocene to the Amazon River as we know it today, according to the findings. A period of global cooling, which lowered the sea level further, also contributed to ending the linkage between the ocean and the forest, the researchers say.
The Amazon’s history, however, still holds many mysteries.
“The greatest difficulty in studying the Amazon is to obtain sediments,” Leandro said. “Many of these sediments we can only get on the riverbanks during the dry season. But even these outcrops don’t reach very old ages.”
Most of the time, the only way out is to analyze samples drilled for commercial purposes, especially mining, as Leandro and her colleagues had to rely on for their study.
To address this gap, an international group of scientists will begin the largest sampling project ever in the Amazon, drilling rock cores from three Brazilian sites: in the Acre, Solimões and Marajó basins. Hoorn, who is participating in the project, said the start of drilling was delayed because of the pandemic, but should begin by the end of this year.
“The aim is to drill some key locations to get new sediment samples capable of offering much more precise information about the past, so we can see how the forest responded to climate change, for instance,” she said.
Amazon under double threat
Scientists say that understanding what happened in the Amazon in the distant past can give us a clue to what may happen in the near future, especially when it comes to climate change. The Miocene saw average temperatures around 14° Celsius (25.2° Fahrenheit) above pre-industrial levels — much higher than the 2°C (3.6°F) increase that the international community is trying to avoid with the Paris Agreement.
The difference is that the current rate of global warming is taking place much faster than during the Miocene, D’Apolito said.
“These are very different time scales. In our studies, we analyzed changes that occurred over hundreds of thousands of years. What we are seeing now is a process of decades,” he said.
The result, he added, is a higher risk of mass extinction. “In hundreds and thousands or millions of years, species have time to adapt,” he said. “In a few tens of years, they do not.”
In the Amazon, this process is being intensified by record rates of fire and deforestation.
“The unbridled actions of the human species harm other species, mainly through the destruction of their natural habitats and further undermining the coping with environmental variations that already exist,” Linhares said.
“By cutting forest,” Hoorn said, “you change the landscape in a much more problematic way than with climate change. It changes the surface, it creates erosional processes that make it impossible for the forest to go back.”
Leandro, L. M., Linhares, A. P., De Lira Mota, M. A., Fauth, G., Santos, A., Villegas-Martín, J., … Ramos, M. I. (2022). Multi-proxy evidence of Caribbean-sourced marine incursions in the Neogene of Western Amazonia, Brazil. Geology, 50(4), 465-469. doi:10.1130/g49544.1
Hoorn, C. (1993). Marine incursions and the influence of Andean tectonics on the Miocene depositional history of northwestern Amazonia: Results of a palynostratigraphic study. Palaeogeography, Palaeoclimatology, Palaeoecology, 105(3-4), 267-309. doi:10.1016/0031-0182(93)90087-y
Linhares, A. P., Gaia, V. D., & Ramos, M. I. (2017). The significance of marine microfossils for paleoenvironmental reconstruction of the Solimões Formation (Miocene), western Amazonia, Brazil. Journal of South American Earth Sciences, 79, 57-66. doi:10.1016/j.jsames.2017.07.007
Espinosa, B. S., D’Apolito, C., & Da Silva-Caminha, S. A. (2021). Marine influence in western Amazonia during the late Miocene. Global and Planetary Change, 205, 103600. doi:10.1016/j.gloplacha.2021.103600
Banner image: A frog in Peruvian Amazon. Scientists say there is still a strong marine influence over the forest that created a “melting pot ready to produce spectacular biodiversity”. Image by Rhett A. Butler.
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