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Vestiges of the sea in the Amazon interior

Fossils of marine microorganisms indicate that Caribbean waters inundated the Solimões basin at least 11 times over the last 23 million years

A freshwater snail shell

Maria Inês F. Ramos / MPEG

Don’t be surprised if you encounter plants and animals in the Amazon rainforest that have similarities with marine organisms. Even though more than a thousand kilometers distant from the ocean, fossils of shells and sea centipedes, and teeth from sharks, stingrays, and other marine animals have been found in sediments several meters deep during surveys carried out in Brazil, Peru, and Colombia.

They arrived as far as the interior of what is now dense jungle because the Caribbean Sea flooded the Amazon during the geological period known as the Miocene, from 23 million to 5 million years (Ma) ago. These conclusions emerge from analyses of sediment fossils from the banks of the Solimões, Juruá, and Javari rivers that have been conducted since 1998 by a group from the Emílio Goeldi Museum of Pará (MPEG), coordinated by biologist Maria Inês Ramos, with teams from other institutions and support from the Brazilian National Council for Scientific and Technological Development (CNPq).

“Floods caused by seawater contributed to the high biodiversity of the Amazon by bringing in marine animals and separating populations that then originated new species, which adapted to the new environments,” says geologist Lilian Maia Leandro, of Usinos University in Rio Grande do Sul State, lead author of the article published in January 2022 in the journal Geology. A separate article describing the results came out in December in the Journal of South American Earth Sciences. One example of this adaptation is the pink river dolphin, which seems to have evolved in the sea — its ancestors would have been dolphins — and today lives in the rivers of the Amazon.

Investigating sediments extracted from drilling conducted during the 1970s by the Mineral Resources Research Company (CPRM) in the region under study, the MPEG team found an abundance of bivalve crustaceans called ostracods, currently found in coastal environments, and other marine organisms, such as foraminifera. When identified under a microscope, they showed more similarities with samples extracted from a core drilled in the Caribbean than with two other samples from the Pacific and Atlantic oceans.

Another indication of marine influence in the Solimões basin is the remnants of marine microalgae, also known as dinoflagellates, which live in plankton and are common in the Caribbean Sea. With an estimated age of between 23 Ma and 3.8 Ma, the microfossils indicate periods of higher salinity, resulting from the occupation of sea water in the Amazon.

“We identified a pulsing pattern to the floods, which occurred approximately every 2 Ma or 3 Ma. The most pronounced inundation we detected was between 11.1 Ma and 8.8 Ma,” says Ramos. Sea water would have increased the salinity of a gigantic group of lakes, the Pebas system, which then extended from Colombia and Peru to the central portion of the Brazilian Amazon.

Maria Inês F. Ramos / MPEGForaminifera microfossil of the genus Ammonia and microcrustacean carapace of the genus Cyprideis, found on riverbanks or in boreholes in the western Brazilian AmazonMaria Inês F. Ramos / MPEG

“The sea probably entered from the Orinoco river valley, to the north,” suggests Leandro. “The Andes already blocked the waters to the west, in Peru, but the Mérida Mountain range in Venezuela, which could also have barred the sea, had not yet formed.” She believes the Caribbean Sea must have inundated South America at a time when the continent was flatter and unable to block the waters when the climate heated, glaciers melted, and the sea level rose.

The MPEG team’s hypotheses about the inundations by the Caribbean Sea were reinforced by numerical models developed by geophysicist Victor Sacek, from the University of São Paulo (USP), which considered the internal dynamics of planet Earth and climate and sea level changes. According to his conclusion, described in an article published in March 2023 in the journal Earth and Planetary Science Letters, water from the ocean to the north would have invaded the region intermittently for at least 35 Ma.

The models show that the uplift of the Andes would have changed the region’s landscape, altering the flow of rivers and rainfall patterns. “The weight of the mountain range made the crust of South America bend, as if it were a trampoline, and form a depression near the Andes,” explains Sacek. According to the model, this natural pool would have been flooded with Caribbean waters.

“The convection processes of the Earth’s mantle contributed to curve the Earth’s crust downwards, creating depressions that extended into the center of the Amazon,” he adds. “The models show that these depressions were initially filled with marine water and favored the formation of lacustrine environments.”

Over time, sediments from the Andes filled in the lakes, culminating in the formation of the Amazon River basin approximately 10 Ma ago. In Sacek’s numerical models, marine incursions into the Caribbean Sea in the Solimões basin would have ended before the formation of the Amazon River. The conclusions, however, do not eliminate the possibility of more recent incursions, between 4.7 Ma and 3.8 Ma ago, as suggested by the MPEG team.

“Some of the differences between Sacek’s and Leandro’s results may be due to the scarcity of data to test the scenarios generated by numerical models and doubts regarding the living environments of the fossils,” comments USP geologist André Sawakuchi, who did not participate in the studies. “The topic is controversial and there are a lot of uncertainties.” Sawakuchi believes records of environmental changes in the Amazon are still very sparse and it would be risky to speak of climate variation millions of years ago, when there are only relatively reliable data for the last 250,000 years.

Sawakuchi coordinates the Trans-Amazon Drilling Project (TADP), supported by FAPESP, with the participation of researchers from 12 countries, which has the goal of drilling in the Amazon from the Andes to the shores of the Atlantic. In addition to more precise data to feed the mathematical models, he hopes to verify whether the sea invaded the forest even earlier than indicated by Sacek’s calculations.

TransAmazon Drilling Project: Origin and evolution of the forests, climate, and hydrology of the South American tropics (nº 18/23899-2); Grant Mechanism Thematic Project ‒ Global Climate Change Research Program; Principal Investigator André Oliveira Sawakuchi (USP); Investment R$9,659,178.81.

Scientific articles
LEANDRO, L. M. et al. Multi-proxy evidence of Caribbean-sourced marine incursions in the Neogene of Western Amazonia, Brazil. Geology. vol. 50, no. 4, pp. 465–469. jan. 5, 2022.
SANTOS, K. S. dos & RAMOS, M. I. F. Taphonomic analysis on Neogene ostracods from Solimões formation, Borehole 1AS-5-AM, Brazil: A tool to the paleoenvironmental reconstitution. Journal of South American Earth Sciences. vol. 122, 104172. dec. 2022.
SACEK, V. et al. The Amazon paleoenvironment resulted from geodynamic, climate, and sea-level interactions. Earth and Planetary Science Letters. vol. 605, 118033. mar. 1, 2023.