Widespread ancient ocean ‘dead zones’ challenged early life

The Notch Peak Formation, located in Lawsons Cove, Utah, preserves ancient biogeochemical events. -  Ben Gill, UC-Riverside and Harvard University
The Notch Peak Formation, located in Lawsons Cove, Utah, preserves ancient biogeochemical events. – Ben Gill, UC-Riverside and Harvard University

The oceans became oxygen-rich as they are today about 600 million years ago, during Earth’s Late Ediacaran Period. Before that, most scientists believed until recently, the ancient oceans were relatively oxygen-poor for the preceding four billion years.

Now biogeochemists at the University of California-Riverside (UCR) have found evidence that the oceans went back to being “anoxic,” or oxygen-poor, around 499 million years ago, soon after the first appearance of animals on the planet.

They remained anoxic for two to four million years.

The researchers suggest that such anoxic conditions may have been commonplace over a much broader interval of time.

“This work is important at many levels, from the steady growth of atmospheric oxygen in the last 600 million years, to the potential impact of oxygen level fluctuations on early evolution and diversification of life,” said Enriqueta Barrera, program director in the National Science Foundation (NSF)’s Division of Earth Sciences, which funded the research.

The researchers argue that such fluctuations in the oceans’ oxygen levels are the most likely explanation for what drove the explosive diversification of life forms and rapid evolutionary turnover that marked the Cambrian Period some 540 to 488 million years ago.

They report in this week’s issue of the journal Nature that the transition from a generally oxygen-rich ocean during the Cambrian to the fully oxygenated ocean we have today was not a simple turn of the switch, as has been widely accepted until now.

“Our research shows that the ocean fluctuated between oxygenation states 499 million years ago,” said paper co-author Timothy Lyons, a UCR biogeochemist and co-author of the paper.

“Such fluctuations played a major, perhaps dominant, role in shaping the early evolution of animals on the planet by driving extinction and clearing the way for new organisms to take their place.”

Oxygen is necessary for animal survival, but not for the many bacteria that thrive in and even demand life without oxygen.

Understanding how the environment changed over the course of Earth’s history can give scientists clues to how life evolved and flourished during the critical, very early stages of animal evolution.

“Life and the environment in which it lives are intimately linked,” said Benjamin Gill, the first author of the paper, a biogeochemist at UCR, and currently a postdoctoral researcher at Harvard University.

When the ocean’s oxygenation states changed rapidly in Earth’s history, some organisms were not able to cope.

Oceanic oxygen affects cycles of other biologically important elements such as iron, phosphorus and nitrogen.

“Disruption of these cycles is another way to drive biological crises,” Gill said. “A switch to an oxygen-poor state of the ocean can cause major extinction of species.”

The researchers are now working to find an explanation for why the oceans became oxygen-poor about 499 million years ago.

“We have the ‘effect,’ but not the ’cause,'” said Gill.

“The oxygen-poor state persisted likely until the enhanced burial of organic matter, originally derived from oxygen-producing photosynthesis, resulted in the accumulation of more oxygen in the atmosphere and ocean

“As a kind of negative feedback, the abundant burial of organic material facilitated by anoxia may have bounced the ocean to a more oxygen-rich state.”

Understanding past events in Earth’s distant history can help refine our view of changes happening on the planet now, said Gill.

“Today, some sections of the world’s oceans are becoming oxygen-poor–the Chesapeake Bay (surrounded by Maryland and Virginia) and the so-called ‘dead zone’ in the Gulf of Mexico are just two examples,” he said.

“We know the Earth went through similar scenarios in the past. Understanding the ancient causes and consequences can provide essential clues to what the future has in store for our oceans.”

The team examined the carbon, sulfur and molybdenum contents of rocks they collected from localities in the United States, Sweden, and Australia.

Combined, these analyses allowed the scientists to infer the amount of oxygen present in the ocean at the time the limestones and shales were deposited.

By looking at successive rock layers, they were able to compile the biogeochemical history of the ocean.

Oxygen’s challenge to early life

Researcher Benjamin Gill near the top of a stratigraphic section at Lawsons Cove, Utah. -  Steve Bates.
Researcher Benjamin Gill near the top of a stratigraphic section at Lawsons Cove, Utah. – Steve Bates.

The conventional view of the history of the Earth is that the oceans became oxygen-rich to approximately the degree they are today in the Late Ediacaran Period (about 600 million years ago) after staying relatively oxygen-poor for the preceding four billion years. But biogeochemists at the University of California, Riverside have found evidence that shows that the ocean went back to being “anoxic” or oxygen-poor around 499 million years ago, soon after the first appearance of animals on the planet, and remained anoxic for 2-4 million years. What’s more, the researchers suggest that such anoxic conditions may have been commonplace over a much broader interval of time, with their data capturing a particularly good example.

The researchers argue that such fluctuation in the ocean’s oxygenation state is the most likely explanation for what drove the rapid evolutionary turnover famously recognized in the fossil record of the Cambrian Period (540 to 488 million years ago).

They report in the Jan. 6 issue of Nature that the transition from a generally oxygen-rich ocean during the Cambrian to the fully oxygenated ocean we have today was not a simple turn of the switch, as has been widely accepted until now.

“Our research shows the ocean fluctuated between oxygenation states 499 million years ago,” said co-author Timothy Lyons, a professor of biogeochemistry, whose lab led the research, “and such fluctuations played a major, perhaps dominant, role in shaping the early evolution of animals on the planet by driving extinction and clearing the way for new organisms to take their place.”

Oxygen is a staple for animal survival, but not for the many bacteria that thrive in and even demand life without oxygen.

Understanding how the environment changed over the course of Earth’s history can clue scientists to how exactly life evolved and flourished during the critical, very early stages of animal evolution.

“Life and the environment in which it lives are intimately linked,” said Benjamin Gill, the first author of the research paper, who worked in Lyons’s lab as a graduate student. Gill explained that when the ocean’s oxygenation states changed rapidly in Earth’s history, some organisms were not able to cope. Further oceanic oxygen affects cycles of other biologically important elements such as iron, phosphorus and nitrogen.

“Disruption of these cycles is another way to drive biological crises,” he said. “Thus both directly and indirectly a switch to an oxygen-poor state of the ocean can cause major extinction of species.”

The researchers are now working on finding an explanation for why the oceans became oxygen-poor about 499 million years ago.

“What we have found so far is evidence that it happened,” Gill said. “We have the ‘effect,’ but not the ’cause.’ The oxygen-poor state persisted for 2-4 million years, likely until the enhanced burial of organic matter, originally derived from oxygen-producing photosynthesis, resulted in the accumulation of more oxygen in the atmosphere and ocean. As a kind of negative feedback, the abundant burial of organic material facilitated by anoxia may have bounced the ocean to a more oxygen-rich state.”

Gill stressed that understanding past events in Earth’s distant history can help refine our view of changes happening on the planet presently.

“Today, some sections of the world’s oceans are becoming oxygen-poor – the Chesapeake Bay and the so-called ‘dead zone’ in the Gulf of Mexico are just two examples,” he said. “We know the Earth went through similar scenarios in the past. Understanding the ancient causes and consequences can provide essential clues to what the future has in store for our ocean.”

In the study, Lyons, Gill and their team examined the carbon, sulfur and molybdenum contents of rocks they collected from localities in the United States, Sweden, and Australia. Combined, these analyses allowed the team to infer the amount of oxygen present in the ocean at the time the limestones and shales were deposited. By looking at successive rock layers, they were able to compile the biogeochemical history of the ocean.

Lyons and Gill were joined in the research by Seth A. Young of Indiana University, Bloomington; Lee R. Kump of Penn State University; Andrew H. Knoll of Harvard University; and Matthew R. Saltzman of Ohio State University. Currently, Gill is a postdoctoral researcher at Harvard University.

Clarifying the Black Sea region

The Black Sea is the largest anoxic basin in the world. Research opportunities here are growing, due especially to the presence in the region of newly independent states now faced with population pressure and a variety of environmental issues. New GSA Special Paper 473 presents the multidisciplinary work of scientists from 12 countries addressing a range of topics, including climatic and hydrologic modeling, paleogeographic reconstruction of Late Quaternary landscapes, palynology and paleoclimatology, and geoarchaeological studies. -  Geological Society of America
The Black Sea is the largest anoxic basin in the world. Research opportunities here are growing, due especially to the presence in the region of newly independent states now faced with population pressure and a variety of environmental issues. New GSA Special Paper 473 presents the multidisciplinary work of scientists from 12 countries addressing a range of topics, including climatic and hydrologic modeling, paleogeographic reconstruction of Late Quaternary landscapes, palynology and paleoclimatology, and geoarchaeological studies. – Geological Society of America

The Black Sea is the largest anoxic basin in the world. Research opportunities here are growing, due especially to the presence in the region of newly independent states now faced with population pressure and a variety of environmental issues. This new GSA Special Paper presents the multidisciplinary work of scientists from twelve countries addressing a range of topics, including climatic and hydrologic modeling, paleogeographic reconstruction of Late Quaternary landscapes, palynology and paleoclimatology, and geoarchaeological studies.

More reasons for the surge in research opportunities and interest include (1) the Great Flood hypotheses that tie the Biblical Flood to the Black Sea; (2) the presence of huge methane reserves within gas hydrates beneath the seafloor that may be exploitable as new nontraditional energy sources; (3) the growing tangle of underwater infrastructure (e.g., gas pipelines and communication cables) laid across the Black Sea floor that is increasingly subject to geohazards from landslides, tectonics, and other dynamic forces; and (4) the presence of vast amounts of raw materials (e.g., sapropels) that have economic applications in agriculture.

The process of putting together a volume of this magnitude took time and several international meetings. Senior volume editor Ilya V. Buynevich of Woods Hole Oceanographic Institution writes, “No interdisciplinary publication is ever achieved without help from a wide range of contributors whose part in the process deserves a public statement of deep appreciation.”

East-west collaboration is growing through the research programs of individual scientists as well as in international multidisciplinary projects, such as International Geological Correlation Programme (IGCP) 521, “The Black Sea-Mediterranean Corridor during the last 30 k.y.: Sea-level change and human adaptation,” and International Union for Quaternary Research (INQUA) 501, “The Caspian-Black Sea-Mediterranean Corridor during the last 30 k.y.: Sea-level change and human adaptive strategies” Today, these projects involve the work of ~400 scientists, not only from the Black Sea region, but from around the world