The human era
Plastic material accumulated on ocean floors may define the Anthropocene, a new period in the Earth’s history
In late April 2016, an international group of geologists, archeologists, geochemists, oceanographers and paleontologists took part in a meeting in Oslo, Norway. The initial objective of the meeting, which brought together researchers from such different fields, was to work out a proposal, to be presented in August 2016 in South Africa. The proposal would mark the beginning of the process of officially recognizing that the Earth is experiencing a new geological epoch, the Anthropocene—the age of man.
After two days of discussion, however, the group decided to postpone presentation of the proposal to formalize the Anthropocene until 2018. In the meantime, more evidence must be gathered to show that the environmental changes caused by human activity are so extreme that they have already produced indelible marks on the planet’s geological record. “We want to present a proposal robust enough that the international scientific community will have no doubt about the formalization of the Anthropocene,” says oceanographer Juliana Ivar do Sul, a researcher at the Federal University of Rio Grande (FURG), in the state of Rio Grande do Sul, who participated in the meeting.
According to the group in Norway, from the 1950s to the present, human activity has caused changes in the Earth’s geological processes, modifying the rate of rock erosion and accumulation of sediments from continental surfaces at the bottom of the oceans—much more extreme than what occurs naturally. One striking characteristic of this new stage in the Earth’s history is the ever more abundant presence of an artificial sediment, formed of mud and sand mixed with grains of synthetic material—plastic in particular—coming from trash produced by humans.
“Proposing a new geological epoch is a very complex undertaking,” says Ivar do Sul. “We need more diverse pieces of scientific evidence, and the effect of plastic on geological processes is only one of them,” the researcher says. Her specialty is investigating the effects of pollution on the oceans caused by plastic, and she is a member of the Working Group on the Anthropocene headed by paleontologist Jan Zalasiewicz of the University of Leicester in the United Kingdom, and geologist Colin Waters of the British Geological Survey. The group was created in 2009 by the International Union of Geological Sciences (IUGS), which produces the International Chronostratigraphic Chart.
This chart organizes the layers of rocks that form the continents and the bottom of the oceans according to the chronological order in which they emerged, with the oldest layers shown at the bottom of the chart. The conventions defined in this chart enable geologists to compare sediments and rocks from different locations and determine their relative ages when there is no direct dating, and in this way to reconstruct the Earth’s history.
According to the chart, the current epoch is the Holocene, which began 11,700 years ago. The beginning of the Holocene was officially defined only in 2008, when a working group reviewed the scientific evidence that the layers of rock, sediment and ice about 11,700 years old showed marks left by climate change events that occurred at the end of the last glacial period.
The idea that the Holocene ended as a result of climate change caused by modern civilization, giving rise to the Anthropocene, gained notoriety at the beginning of the past decade through articles and lectures presented by Dutch chemist Paul Crutzen, a winner of the 1995 Nobel Prize in Chemistry for his work on the formation of the hole in the ozone layer of the Earth’s atmosphere. Crutzen’s ideas inspired Zalasiewicz to propose that the IUGS form a working group to discuss the issue and attempt to define the beginning of the Anthropocene and its characteristics.
Although the group’s conclusions are not expected to be summarized and presented until 2018, the principal points of evidence he introduced have been circulated and discussed for some time. The most recent work in support of the Anthropocene is a review paper written by Waters, Zalasiewicz and 22 other colleagues, published in Science in January 2016. In the paper, the researchers maintain that human activity has already changed the planet enough to produce—on a worldwide scale—sediment and ice with features distinct from those formed in the rest of the Holocene.
According to the review, the layers of recently deposited ice and sediment contain fragments of artificial materials produced in abundance over the past 50 years: concrete, pure aluminum and plastic, as well as traces of pesticides and other synthetic chemical compounds. Even in remote parts of the planet, such as Greenland, the sediments that have accumulated between 1950 and the present reveal concentrations of carbon from burning fossil fuels, and phosphorus and nitrogen, used as fertilizer in agriculture—all of which are much higher than in the past 11,700 years.
Waters, Zalasiewicz and their colleagues also estimate that the impact of present-day human activity could remain imprinted on the Earth’s record for tens of millions of years. Mining, global climate change and an increase in the rate of extinction of plant and animal species are also expected to leave their marks on the rocks. “The article caused a lot of controversy,” Ivar do Sul comments. “Many researchers disagree that the Holocene has ended, and this discussion will likely go on for several years.”
Among critics of the proposal is geologist Stanley Finney of California State University at Long Beach, head of the executive board of the IUGS, which produces the chronostratigraphic chart. Finney and Lucy Edwards of the U.S. Geological Survey disagreed with the idea of establishing the Anthropocene, in an opinion paper published in the March/April 2016 issue of the Geological Society of America’s news magazine GSA Today. In the paper, Finney and Edwards point out that many of the layers deposited in the past 70 years in the deepest sections of the ocean are no more than one millimeter (mm) thick. They also say that most of the evidence presented by the Anthropocene supporters is based on projections of the potential record in rocks in the distant future. The inclusion of the Anthropocene in the chronostratigraphic chart is likely motivated more by politics (proclaiming man’s environmental impact) than by science.
“In order to define a new epoch, the deposited material needs to be expressed in the sediment column in many locations on the planet and in different environments,” explains Professor Michel Mahiques, a geologist at the Oceanographic Institute at the University of São Paulo (IO-USP). “We don’t know to what extent the Anthropocene satisfies the IUGS in this assumption, since the epoch may already be expressed in some environments, such as the coastal regions, and hardly at all in others, such as the bottom of ocean basins.”
Ivar do Sul points out that there is also no consensus among those who support official recognition for the Anthropocene. Zalasiewicz’s group, for example, advocates a specific day to be designated as the beginning of this new epoch: July 16, 1945, the day on which the first atomic bomb was detonated in Alamogordo, New Mexico. That date marks the beginning of atmospheric pollution by radioactive isotopes released in thermonuclear testing that have likely already had time to become incorporated into the ice and sediment across the entire surface of the planet, leaving a clear signature for future geologists. But other researchers suggest earlier dates, such as the beginning of the Industrial Revolution, around 1800, to encompass all the transformations that man has caused in the Earth’s environment.
Microplastics into the sea
Zalasiewicz and Waters invited Ivar do Sul to participate in the Working Group on the Anthropocene after reading a review that she and oceanographer Mônica Costa of the Federal University of Pernambuco published in 2014 in Environmental Pollution, on the accumulation of microplastics in the oceans. Microplastics are fragments measuring less than five mm that are generally invisible to the naked eye when they float on the oceans or are mixed in with mud or sand. “They wanted to know if they could use microplastics as a geological marker for the Anthropocene,” says the researcher, who has already collected the material on the ocean surface around all the large Brazilian oceanic islands, such as Fernando de Noronha and Trindade. Along with 16 other members of the group, she published a review paper in the journal Anthropocene in January 2016 that summarizes everything we know about the path that plastics have taken across the planet. In the paper, the researchers emphasize that this type of material has high potential for preservation in marine sediments.
The microplastics found in the ocean come from a variety of sources. Pellets, which are lentil-sized spheres, are used as raw material for making large plastic objects. Other particles come from large pieces that have disintegrated in the environment. The most abundant microplastics, however, are the fibers measuring 2 to 3 mm long by 0.1 mm thick that are used in cigarette filters or that detach from synthetic fabrics during washing. Between 1950 and the present day, world production of plastic has grown from 5 million metric tons to 300 metric tons per year. It is estimated that the total amount of plastic produced so far (around 2 billion metric tons) is enough to wrap the planet in plastic film several times over.
Plastic materials discarded into landfills make their way into oceans and coastal regions. A study led by biologist Alexander Turra of IO-USP showed years ago that 10 times more particles of microplastic are buried in the sand of a beach than appear on its surface. “Before our study, we underestimated the amount of plastic in sand,” Turra says. Since plastic tends to float, researchers assumed that microplastics would always remain on top of the sand. Turra and his colleagues, however, found them buried up to two meters deep on four beaches along the coast of São Paulo State (see Pesquisa FAPESP Issue nº 219). Since then, the team has confirmed the phenomenon on 13 more beaches. From the distribution of the particles, Turra suspects that microplastics become buried by the force of occasional maritime storms. Another portion of the plastic produced and discarded is floating in the oceans. And it makes its way to yet another destination: the bottom of the ocean.
Although they initially float, plastic pieces—large or small—that remain in saltwater for a long time become colonized by microorganisms and then sink. They may also be swallowed by larger organisms, from microscopic zooplankton to fish, and submerge with their feces or carcasses. Expeditions have found plastics at different depths in underwater terrain. Robots have photographed bottles, bags and fishing nets in underwater canyons around Europe, and in 2015 researchers found microplastics more than 5 kilometers deep on the sediment of the Kuril-Kamchatka Trench in the Pacific Ocean. Samples of marine sediments indicate that there are plastic fibers throughout the ocean floor.
Zalasiewicz is a specialist in 500-million-year-old microfossils, including graptolites, which were made of organic molecules with a structure similar to that of plastics. If these microorganisms left fossilized records, Zalasiewicz suspects that the plastic deposited at the bottom of the ocean, especially that found in the sediment of underwater canyons near the edges of the continental shelves, also has a high chance of being preserved for millions of years and, one day, may possibly intrigue future paleontologists who find fossilized plastic bottles, CDs and cigarette butts.
ZALASIEWICZ, J. et al. The geological cycle of plastics and their use as a stratigraphic indicator of the Anthropocene. Anthropocene. January 18, 2016.
TURRA, A. et al. Three-dimensional distribution of plastic pellets in sandy beaches: Shifting paradigms. Scientific Reports. March 27, 2014.
IVAR DO SUL, J. A. and COSTA, M. F. The present and future of microplastic pollution in the marine environment. Environmental Pollution. February 2014.