12-20-05 Big changes in atmospheric oxygen
FEATURE ARTICLE from Science News, Vol. 157, No. 9, 2-26-00, p. 138, By R. Monastersky
[There is global warming. But the temperature of things in a warm room increases. So, is global warming the result of current human activities? Or is it the result of something that happened 12,000 years ago which has not yet "turned off?" And is global warming accelerating because of the release of enormous amounts of methane as the tundra thaws? Methane is a more effective greenhouse gas than carbon dioxide. Trying to stop global warming by limiting CO2 production could now be futile.]
"A warm spell in the distant past holds soggy clues to the future
The geological evidence for higher sea level
during stage 11
problem of shifting coastlines
swarthy face of the globe
an increased proportion of the heavier isotope
congregating close to the ocean
a skeptic regarding greenhouse warming
Clinging to a cliff in the Bahamas, high above the pounding surf, geologist Paul J. Hearty takes care to avoid any missteps as he surveys the craggy rock ...
Though the threat of a fall keeps Hearty on edge, the real drama of this spot lies exposed in the alternating stripes of stone running along the face of the cliff. Thin red bands of fossilized soil separate massive white sheets of limestone -- strawberry icing spread between layers of coconut cake ...
The thick limestone layers are ancient beach deposits, formed during warm breaks between ice ages. In those cold times, dust from the Sahara laid down thin red sheets. Hearty has come to the cliffs searching for a distinctive limestone band from 400,000 years ago, a warm interlude known as stage 11.
Today, the former beach sits so far above the ocean that a fall from this height could kill. Back then, however, the seas washed over it. For the oceans to have swollen that much, significant portions of Earth's polar ice must have melted, says Hearty, who works as a geologic consultant in Honolulu. If such a melting were to happen today, breakers would crest over much of the property in Miami, New York, and other cities near the sea ...
"The geological evidence for higher sea level during stage 11 is beginning to mount up, so it's got to be given a lot of credibility," says Richard Z. Poore, an oceanographer with the U.S. Geological Survey in Reston, Va. "I think the evidence that sea level was significantly higher than today is pretty good."
The stage 11 question is critical because scientists are starting to view this time as a twin of our present climate. If we want to know what bobs ahead in the future -- if polar ice will melt and sea level will rise catastrophically -- this long-gone period could offer the clearest view.
The last ice age ended over 10,000 years ago, an apparent eternity to a society fueled by drive-through restaurants, microwave popcorn, and E-mail. To a geologist, however, today's warmth is just a blip. For much of the past million years, Earth has shuddered through a series of ice ages, each lasting close to 100,000 years. Punctuating these chills are relatively brief interglacial periods like the current time.
In the 1950s, when oceanographers first discovered signs of this glacial cycle recorded in deep-sea sediments, they named the various epochs going backward, starting with the present interglacial as stage 1. Four separate glacial periods separate modern times from the epoch known as stage 11 ...
The new evidence of elevated sea levels is rewriting that well-worn tale. Along the cliff face on the island of Eleuthera, in the Bahamas, Hearty has found a distinctive herringbone pattern in the limestone. This arrangement of ridges would be familiar to any beachgoer. It's made up of fossilized relicts of the sand ripples that swimmers can feel beneath their feet as they wade out from shore. The pattern means that these rocks, now 20 m above the sea, were once below the low-tide mark, Hearty and his colleagues reported in the April 1999 Geology.
His team has found similar evidence in Bermuda and, more recently, on the Hawaiian island of Oahu, he announced last December at a meeting of the American Geophysical Union (AGU) in San Francisco ...
The problem of shifting coastlines complicates the job of geologists who are trying to unravel exactly how high the seas crested in stage 11. As the land rises or sinks, it skews the evidence of past sea levels.
Some scientists question whether even Bermuda and the Bahamas have provided a true record. "Given what we know about plate tectonics, it's very unlikely that anywhere on the coast has been stable for 400,000 years," says David Q. Bowen of Cardiff University in Wales.
At the recent meeting, Bowen reported on his studies of ancient beach deposits around southern Britain that now perch between 20 and 43 m above sea level. Bowen has estimated the uplift rate by looking for evidence of sands laid down during the last interglacial. Because this warm period happened relatively recently, about 120,000 years ago, scientists could independently determine its sea level.
Measuring against the deposits, Bowen calculated how quickly these sites have risen. This rate indicates that stage 11 sea levels reached around 15 m above the modern value. Though lower than Hearty's figures, Bowen's calculations still point to a substantial melting of polar ice at the time.
More support comes from sites in northern Alaska that preserve evidence of a 13-m rise in sea level, reports Julie Brigham-Grette of the University of Massachusetts at Amherst. "A lot of us who work on stage 11 have realized that we really have compelling evidence on a global basis, if we put it together," she says.
The face of the globe would have looked far more swarthy during stage 11 if sea levels then surpassed the modern mark by 13 m or more.
Today's porcelain-white island of Greenland would have lost its icy coating and, as vegetation sprouted across the rocky surface, turned a darker shade more true to its name. The melting of this northern ice could account for 6 m of the sea level shift.
To find the rest, one must head south, says Reed P. Scherer, a researcher at Northern Illinois University in Dekalb who studies the climate history of Antarctica. The most probable suspect in this case is West Antarctica -- the half of the continent that reaches toward South America.
Ice on the other side of Antarctica rests high and dry on rock mostly above sea level, making it far more stable than the West Antarctic ice sheet, whose base lies below sea level. Some scientists have theorized that the arrangement in West Antarctica makes that region prone to collapsing when climate warms because the swelling oceans would undermine its base.
The theory itself lacked much support until Scherer started studying gravel that a drill team had collected from beneath the West Antarctic ice sheet. The marine sediments contained the shells of one-celled algae with a distinctive shape.
From records elsewhere, Scherer knew that the algal species found under West Antarctica had appeared only within the past 750,000 years. Scientists once thought that the West Antarctic ice had been stable for more than 6 million years, but Scherer's evidence revealed its erratic nature. The presence of relatively modern algae indicates that much or all of West Antarctica must have melted sometime within the past million years, leaving open ocean in its place ...
When water evaporates from the seas, it preferentially carries away the lighter isotope of oxygen, which can become trapped in snow and glaciers. The seawater left behind shows an increased proportion of the heavier isotope. Tiny marine organisms preserve a record of the shift in their shells, which eventually fall to the seafloor.
If during stage 11, melting ice pumped up ocean levels by 20 m, it should have lowered the ratio of heavy to light oxygen in these shells ... The records from the North Atlantic, though, show no dramatic events at the time ...
Something else may have happened at the time to hide the evidence of melting. If the North Atlantic cooled slightly, the temperature shift would have pushed the oxygen-isotope ratio in the opposite direction, counterbalancing the signal from polar melting.
That's what Poore thinks happened. He has recently collected sediment cores from the Cariaco Basin off Venezuela, where the stage 11 story reads differently from the way it does in the North Atlantic. The shells of single-celled animals that lived in the basin's waters during stage 11 contain significantly more of the lighter isotope than is seen in modern shells.
"The easiest way to explain this is to say that sea level was higher and global ice volume was less than today," says Poore.
As a species, people have tended to congregate close to the ocean, so a repeat of stage 11 would send much of the world scurrying for higher ground. A full quarter of the current U.S. population would find itself underwater if sea levels were to rise by 10 m, calculate Poore and his colleagues.
Scientists disagree on whether coastal residents should worry about the long-term value of their property. The present interglacial has lasted 10,000 years, only about a third of the length of stage 11. So, melting of the polar ice could lie many millennia in the future.
On the other hand, global temperatures are rising rapidly, and greenhouse gases threaten to accelerate the warming in the next few decades.
Poore takes a conservative approach. "There's nothing to worry about next week. But it's wrong to just think that it's a problem 500, 600, or 700 years in the future. I think it could be problem much sooner than that."
Hearty agrees, saying, "As the greenhouse effect tends to increase the temperature of the Earth over the next 50 to 100 years, we can probably expect that some of the Antarctic ice will melt and destabilize." He notes that small blocks of Antarctic ice have already started to collapse from rising temperatures.
Bowen, who considers himself a skeptic regarding greenhouse warming, says that human activity has no bearing on the issue of polar melting. "My view is straightforward: The longer an interglacial lasts, the higher the sea level is eventually going to be," he says.
"It's like building a big snowman in the garden. If it's a big enough snowman, it will last long after the weather has warmed up. It will keep trickling away slowly," he explains. "For the present interglacial, sea level is going to trickle upwards until the climate switches and we start descending into the next ice age."
Scientists once thought that polar ice would have little trouble lasting until the next big freeze. The current interglacial is nearing its conclusion, according to conventional wisdom. Once the world made it past a few centuries' worth of greenhouse warming, the climate would start getting cold again.
This conclusion rested on the assumption that the current interglacial would persist only about 10,000 years, the length of the last interglacial, which occurred 120,000 years ago. Recent studies, however, have demonstrated that the last interglacial does not make a good model for understanding the present. Instead, stage 11 has emerged as the better example.
Scientists base the comparison on features of Earth's orbit. Every 400,000 years, the shape of the orbit varies from a squashed circle to a more nearly perfect one. This shape alters the amount of summer sunlight hitting the Northern Hemisphere -- the factor believed to push Earth into and out of ice ages.
Modern times resemble stage 11 in that the orbit is nearly circular, which tends to dampen the climatic influences of other orbital factors, says André L. Berger of the Catholic University of Louvain in Belgium, who presented his findings at the recent AGU meeting.
Berger uses Earth's orbital variations to calculate how insolation -- the amount of sunlight hitting the planet -- changes with time. In simulations using a computerized climate model, Berger and his colleagues found that the orbital effects would keep Earth out of an ice age for 30,000 years, producing an exceptionally long interglacial matched only by stage 11.
For the next few thousand years, he says, the sunlight variations will be rather weak. "The insolation is not going to vary that much. It will almost remain constant. That means that the other factors are going to play a very important role. Among those other factors are greenhouse gases."
This quirk in the timing of Earth's orbital wiggles has given greenhouse gases unusual potency in terms of changing climate, he says. Even just a few thousand years ago, the shape of the orbit was different enough that the astronomical forces would have exerted a controlling influence on climate, leaving less room for carbon dioxide and other gases to exert power.
The result is the climatic equivalent of Murphy's law: Humans have started exploiting fossil fuels and altering Earth's atmosphere at precisely the moment when greenhouse gases could do the most damage to climate.
If societies had bloomed and used up all the coal, oil, and natural gas several millennia earlier, greenhouse warming might have come and gone quietly, without any possibility that melting polar ice caps would eventually flood what people have erected."
Brigham-Grette, J. 1999. Marine isotopic stage 11 high sea level, record from northwest Alaska. Meeting of the American Geophysical Union. Dec. 13-17. San Francisco.
Hearty, P.J., et al. 1999. The Kaena Highstand
on Oahu, Hawaii: Further support for partial Antarctic ice collapse during marine isotope stage 11. Meeting of the American Geophysical Union. Dec. 13-17. San Francisco.
Hearty, P.J., et al. 1999. A + 20 m Middle Pleistocene sea-level highstand (Bermuda and the Bahamas) due to partial collapse of Antartic ice. Geology 27(April):375.
McManus, J., et al. 1999. Marine isotope stage 11 (MIS 11) as analog for the current Holocene interglacial. Meeting of the American Geophysical Union. Dec. 13-17. San Francisco.
Scherer, R.P. 1999. Quaternary collapse of the west Antarctic ice sheet: MIS 11, yes, but was it a unique event? Meeting of the American Geophysical Union. Dec. 13-17. San Francisco.
André L. Berger
Université Catholique de Louvain
Institut d'Astronomie et de Géophysique G.
2 Chemin du Cyclotron
David Q. Bowen
Department of Earth Sciences
P.O. Box 914
Cardiff CF10 3YE
University of Massachusetts
Department of Geosciences
Amherst, MA 01003
Paul J. Hearty
4208 Ai Road
P.O. Box 190
Kalaheo, HI 96741
Jerry F. McManus
Woods Hole Oceanographic Institute
Woods Hole, MA 02543
Richard Z. Poore
U.S. Geological Survey
Mailstop Code 955
Reston, VA 21092
Reed P. Scherer
Northern Illinois University
Department of Geology and Environmental
DeKalb, IL 60115-2854"
REVIEW ARTICLE from Science News Vol. 168, No. 25, Dec. 17, 2005, p. 395; By Sid Perkins
[Big changes in atmospheric oxygen]
``Variations in atmospheric oxygen have affected evolution in big ways
... All animals require oxygen to extract energy from their food and to fuel their activity.
It's no surprise, then, that during geologic periods when atmospheric oxygen concentrations have been high, biological innovation has blazed brightly. At such times, insects grew to gargantuan proportions, reptiles took to the air, and the forerunners of mammals developed a warm-blooded metabolism.
When oxygen concentrations fell precipitously, biodiversity was smothered. Some of the planet's mass extinctions occurred during or after geologically sudden drops in atmospheric oxygen.
Earth's atmosphere had little if any oxygen until about 2.5 billion years ago ... after organisms began using chlorophyll to convert sunlight into useful energy. Because this reaction releases oxygen, the gas slowly became more plentiful in the atmosphere ...
These boosts have been tempered by geological processes that remove oxygen from the air. Widespread episodes of mountain building were inevitably followed by extended periods of erosion and chemical breakdown of the newly exposed rocks, a process that consumes oxygen.
Atmospheric oxygen concentrations have risen and fallen, sometimes gradually and sometimes rapidly, at various times ...
Imagine a world populated by meter-long millipedes, mayflies with the wingspans of today's robins, and dragonflies with wingspans rivaling those of hawks. This isn't science fiction; it's our world at the end of the Carboniferous period, about 300 million years ago. The atmospheric concentration of oxygen then probably was about 35 percent, an all-time high that far exceeds today's 21 percent figure.
The Carboniferous abundance of oxygen enabled insects and other arthropods, which get their oxygen via diffusion through holes in their exoskeleton, to grow to immense proportions, says Robert A. Berner, a geophysicist at Yale University. Models of insect physiology suggest that the higher atmospheric concentrations of oxygen, as well as the increased air pressure that resulted, would have increased the diffusion rate of oxygen into an insect's bloodstream as much as 67 percent.
The evolution of large-bodied arthropods occurred slowly but steadily as atmospheric oxygen gradually increased during the 60-million-year-long Carboniferous period. In laboratory experiments today, insects raised in oxygen-rich conditions can attain beefier proportions in just a few generations.
For example, fruit flies and mealworms raised in chambers for just one generation with twice the normal concentration of oxygen grew 3 percent larger bodies than those reared in standard conditions, says Jon F. Harrison of Arizona State University in Tempe. In other fruit fly-breeding experiments, low oxygen prevented the insects from becoming as large as other flies raised under normal or oxygen-rich conditions.
These findings, as well as evidence from the fossil record, suggest that the atmospheric concentration of oxygen tends to constrain the maximum size that an insect can attain ...
Experiments show that variations in oxygen abundance can also affect the development of reptiles, says John M. Vanden Brooks, a vertebrate paleontologist at Yale University ...
If the oxygen concentration was kept at 16 percent, embryonic development [of alligators] was delayed by almost a week. In experiments where the concentration was raised to 27 percent, development of the embryonic alligators was accelerated by almost a week ...
Although the first reptiles evolved around 350 million years ago, and their descendants thus experienced oxygen concentrations above 27 percent, the first creatures resembling crocodiles appeared about 220 million years ago, when oxygen concentrations had dropped to about 16 percent ...
The experiments on reptiles indicate that a precipitous drop in atmospheric oxygen could severely test animals that required high concentrations of oxygen to fuel an active lifestyle. Evidence from the fossil record bolsters that notion.
Most of Earth's significant die-offs have occurred when atmospheric oxygen was rapidly declining or very low, says Peter D. Ward, a paleontologist at the University of Washington in Seattle. A 10-million-year nosedive in atmospheric-oxygen concentration straddles the largest mass extinction in the fossil record (SN: 2/1/97, p. 74). About 255 million years ago, near the end of the Permian period, the oxygen concentration stood at 30 percent. But it plunged to around 13 percent over just 10 million years.
"Something had wiped out the plants," says Berner. Ecosystems once rife with large, woody, coal-forming trees gave way to systems dominated by small, herbaceous plants such as ferns. That change dramatically decreased the long-term rate of carbon burial in sediments. In general, for every carbon atom that's sequestered in sediments, an oxygen atom returns to the air, Berner notes.
Die-offs that occurred at the end of the Permian and the beginning of the Triassic period claimed as many as 95 percent of the species living in the world's oceans and about 70 percent of those on land.
At the end of the drop in atmospheric oxygen, its availability at sea level would have been similar to that today at an altitude of 4.6 kilometers (15,000 feet), says Ward. Today, a few yak, gazelle, and mountain sheep make up the wildlife at that altitude, but there's far less diversity than at lower altitudes.
Just after the Permian-Triassic extinctions, most land animals lived only at low altitudes. That finding hints that the animals experienced oxygen stress, Ward noted at the Calgary meeting ...
As the atmospheric concentration of oxygen rebounded after the end of the Permian period, so did life on Earth. The rise in oxygen abundance throughout the Triassic period, from about 13 percent up to 16 percent about 200 million years ago, permitted the evolution of several groups of organisms that had higher oxygen demands than creatures of the Permian did, says Lawrence H. Tanner, a paleobiologist at LeMoyne College in Syracuse, N.Y. The long-term increase in oxygen availability during the Triassic seems to have enabled biological innovation to flourish, Tanner notes.
For instance, the groups of relatively small reptiles that had survived the Permian extinctions evolved to become larger and more agile. Just 15 million years or so after the mass die-offs, the first dinosaurs appeared. The body plans of the creatures in this vanguard included hip and ankle adaptations that permitted an upright, bipedal posture and increased mobility. The higher concentrations of oxygen enabled such agile creatures to fuel their metabolisms, says Tanner.
The Triassic period saw the rise of the first flying reptiles, the rhamphorhynchoids. Analyses of fossils suggest that these creatures didn't just glide, but also flapped their wings. Such a lifestyle would have required large quantities of food and abundant oxygen that could extract energy from them.
Ancestors of the first mammals also appeared in the Triassic. Besides having skeletal adaptations that suggested an active lifestyle, these creatures probably were the first to be warm-blooded, another oxygen-demanding metabolic feature.
Analyses of DNA from creatures living today suggest that placental mammals evolved between 100 million and 65 million years ago, a period when oxygen concentrations held fairly steady at 16 percent or so, higher than they had been at any time since oxygen abundance bottomed out about 200 million years ago.
A mammal's highly developed brain, as well as its warm body, requires copious fuel. About one-third of a mammal's energy supply goes to its brain, says Paul G. Falkowski, a biogeochemist at Rutgers University in New Brunswick, N.J. Pound for pound, mammals typically need three times as much oxygen as reptiles do, says Falkowski.
Because the embryos of placental mammals receive their oxygen via the mother's bloodstream, only in an oxygen-rich atmosphere could a mother successfully nourish a growing embryo, Falkowski adds.
Starting about 50 million years ago, the atmospheric concentration of oxygen began to increase sharply. Over the next 5 million years, it rose to 21 percent. At the same time, the average global temperature skyrocketed, jumping about 7°C.
Both flora and fauna flourished under those conditions. DNA analyses suggest that the four major lineages of modern bats, flying mammals whose activity requires large quantities of oxygen, arose during this period (SN: 5/14/05, p. 314: http://www.sciencenews.org/articles/20050514/bob9.asp).
The rise in the concentration of oxygen continued, but at a slower pace, for several million years, says Falkowski. The ancestors of artiodactyls, the group that includes gazelles, cattle, and most of today's other large herbivores, appeared during this interval.
About 25 million years ago, oxygen concentrations topped out at about 23 percent, around 2 percent higher than today's value. By then, many mammals had become huge. Adults of some ground sloth species approached 5 meters in length. Indricotheres, which are relatives of today's rhinos, stood 4.5 m tall and weighed about 15 metric tons, making them the largest land mammals ever.
After peaking, the atmospheric concentration of oxygen began its slide toward modern levels, and many of the world's megamammals died out.
Gargantuan proportions are possible only in a high-oxygen world, says Falkowski. Because large animals don't have as many capillaries per kilogram of muscle as small animals do, the bigger animals require higher oxygen concentrations to maintain a high metabolic rate, he notes.
Some mass extinctions, such as the disappearance of the dinosaurs 65 million years ago, have been pinned on the impact of an extraterrestrial object. But according to the recent data on atmospheric oxygen concentrations, many other die-offs may have been due to something as simple as a change in the air ...
Berner, R.A. 2005. Carbon, sulfur, and oxygen across the Permian-Triassic boundary. Geological Society of America Earth System Processes 2 meeting. Aug. 8-11. Calgary, Alberta. Abstract available at http://gsa.confex.com/gsa/2005ESP/finalprogram/abstract_88638.htm.
Falkowski, P.G. . . . R.A. Berner, et al. 2005. The rise of oxygen over the past 205 million years and the evolution of large placental mammals. Science 309(Sept. 30):2202-2204. Abstract available at http://www.sciencemag.org/cgi/content/abstract/309/5744/2202.
Harrison, J.F., et al. 2005. Does atmospheric oxygen level limit maximal insect size? Geological Society of America Earth System Processes 2 meeting. Aug. 8-11. Calgary, Alberta. Abstract available at http://gsa.confex.com/gsa/2005ESP/finalprogram/abstract_87840.htm.
Tanner, L.H., S. Lucas, and K. Zeigler. 2005. Tetrapod evolution and atmospheric oxygen in the Late Triassic. Society of Vertebrate Paleontology meeting. Oct. 19-22. Mesa, Ariz.
Vanden Brooks, J.M. 2005. Phanerozoic oxygen levels and their effects on modern vertebrate development. Geological Society of America Earth System Processes 2 meeting. Aug. 8-11. Calgary, Alberta. Abstract available at http://gsa.confex.com/gsa/2005ESP/finalprogram/abstract_88421.htm.
Ward, P.D. 2005. Biological implications of varying oxygen levels during the Late Paleozoic through Early Mesozoic. Geological Society of America Earth System Processes 2 meeting. Aug. 8-11. Calgary, Alberta. Abstract available at http://gsa.confex.com/gsa/2005ESP/finalprogram/abstract_88228.htm.
Monastersky, R. 1997. Life's closest call. Science News 151(Feb. 1):74-76.
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Science News, Vol. 168, No. 25, Dec. 17, 2005, p. 395.
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