Microfossils reveal warm oceans had less oxygen, geologists say

Assistant Professor of Earth Sciences Zunli Lu was among the researchers to release these findings. -  Syracuse University
Assistant Professor of Earth Sciences Zunli Lu was among the researchers to release these findings. – Syracuse University

Researchers in Syracuse University’s College of Arts and Sciences are pairing chemical analyses with micropaleontology—the study of tiny fossilized organisms—to better understand how global marine life was affected by a rapid warming event more than 55 million years ago.

Their findings are the subject of an article in the journal Paleoceanography (John Wiley & Sons, 2014).

“Global warming impacts marine life in complex ways, of which the loss of dissolved oxygen [a condition known as hypoxia] is a growing concern” says Zunli Lu, assistant professor of Earth sciences and a member of Syracuse’s Water Science and Engineering Initiative. “Moreover, it’s difficult to predict future deoxygenation that is induced by carbon emissions, without a good understanding of our geologic past.”

Lu says this type of deoxygenation leads to larger and thicker oxygen minimum zones (OMZs) in the world’s oceans. An OMZ is the layer of water in an ocean where oxygen saturation is at its lowest.

Much of Lu’s work revolves around the Paleocene-Eocene Thermal Maximum (PETM), a well-studied analogue for modern climate warming. Documenting the expansion of OMZs during the PETM is difficult because of the lack of a sensitive, widely applicable indicator of dissolved oxygen.

To address the problem, Lu and his colleagues have begun working with iodate, a type of iodine that exists only in oxygenated waters. By analyzing the iodine-to-calcium ratios in microfossils, they are able to estimate the oxygen levels of ambient seawater, where microorganisms once lived.

Fossil skeletons of a group of protists known as foraminiferas have long been used for paleo-environmental reconstructions. Developing an oxygenation proxy for foraminifera is important to Lu because it could enable him study the extent of OMZs “in 3-D,” since these popcorn-like organisms have been abundant in ancient and modern oceans.

“By comparing our fossil data with oxygen levels simulated in climate models, we think OMZs were much more prevalent 55 million years ago than they are today,” he says, adding that OMZs likely expanded during the PETM. “Deoxygenation, along with warming and acidification, had a dramatic effect on marine life during the PETM, prompting mass extinction on the seafloor.”

Lu thinks analytical facilities that combine climate modeling with micropaleontology will help scientists anticipate trends in ocean deoxygenation. Already, it’s been reported that modern-day OMZs, such as ones in the Eastern Pacific Ocean, are beginning to expand. “They’re natural laboratories for research,” he says, regarding the interactions between oceanic oxygen levels and climate changes.”


The article’s lead author is Xiaoli Zhou, a Ph.D. student of Lu’s in Syracuse’s Earth sciences department. Other coauthors are Ellen Thomas, a senior research scientist in geology and geophysics at Yale University; Ros Rickaby, professor of biogeochemistry at the University of Oxford (U.K.); and Arne Winguth, assistant professor of oceanography at The University of Texas at Arlington.

Housed in Syracuse’s College of Arts and Sciences, the Department of Earth Sciences offers graduate and undergraduate degree opportunities in environmental geology, wetland hydrogeology, crustal evolution, sedimentology, isotope geochemistry, paleobiology, paleolimnology, and global environmental change.

Modern ocean acidification is outpacing ancient upheaval, study suggests

<IMG SRC="/Images/964394569.jpg" WIDTH="350" HEIGHT="237" BORDER="0" ALT="The deep-sea benthic foram Aragonia velascoensis went extinct about 56 million years ago as the oceans rapidly acidified. – Ellen Thomas/Yale University”>
The deep-sea benthic foram Aragonia velascoensis went extinct about 56 million years ago as the oceans rapidly acidified. – Ellen Thomas/Yale University

Some 56 million years ago, a massive pulse of carbon dioxide into the atmosphere sent global temperatures soaring. In the oceans, carbonate sediments dissolved, some organisms went extinct and others evolved.

Scientists have long suspected that ocean acidification played a part in the crisis-similar to today, as manmade CO2 combines with seawater to change its chemistry. Now, for the first time, scientists have quantified the extent of surface acidification from those ancient days, and the news is not good: the oceans are on track to acidify at least as much as they did then, only at a much faster rate.

In a study published in the latest issue of Paleoceanography, the scientists estimate that surface ocean acidity increased by about 100 percent in a few thousand years or more, and stayed that way for the next 70,000 years. In this radically changed environment, some creatures died out while others adapted and evolved. The study is the first to use the chemical composition of fossils to reconstruct surface ocean acidity at the Paleocene-Eocene Thermal Maximum (PETM), a period of intense warming on land and throughout the oceans due to high CO2.

“This could be the closest geological analog to modern ocean acidification,” said study coauthor Bärbel Hönisch, a paleoceanographer at Columbia University’s Lamont-Doherty Earth Observatory. “As massive as it was, it still happened about 10 times more slowly than what we are doing today.”

The oceans have absorbed about a third of the carbon humans have pumped into the air since industrialization, helping to keep temperatures lower than they would be otherwise. But that uptake of carbon has come at a price. Chemical reactions caused by that excess CO2 have made seawater grow more acidic, depleting it of the carbonate ions that corals, mollusks and calcifying plankton need to build their shells and skeletons.

In the last 150 years or so, the pH of the oceans has dropped substantially, from 8.2 to 8.1–equivalent to a 25 percent increase in acidity. By the end of the century, ocean pH is projected to fall another 0.3 pH units, to 7.8. While the researchers found a comparable pH drop during the PETM–0.3 units–the shift happened over a few thousand years.

“We are dumping carbon in the atmosphere and ocean at a much higher rate today-within centuries,” said study coauthor Richard Zeebe, a paleoceanographer at the University of Hawaii. “If we continue on the emissions path we are on right now, acidification of the surface ocean will be way more dramatic than during the PETM.”

Ocean acidification in the modern ocean may already be affecting some marine life, as shown by the partly dissolved shell of this planktic snail, or pteropod, caught off the Pacific Northwest.

The study confirms that the acidified conditions lasted for 70,000 years or more, consistent with previous model-based estimates.

“It didn’t bounce back right away,” said Timothy Bralower, a researcher at Penn State who was not involved in the study. “It took tens of thousands of years to recover.”

From seafloor sediments drilled off Japan, the researchers analyzed the shells of plankton that lived at the surface of the ocean during the PETM. Two different methods for measuring ocean chemistry at the time-the ratio of boron isotopes in their shells, and the amount of boron –arrived at similar estimates of acidification. “It’s really showing us clear evidence of a change in pH for the first time,” said Bralower.

What caused the burst of carbon at the PETM is still unclear. One popular explanation is that an overall warming trend may have sent a pulse of methane from the seafloor into the air, setting off events that released more earth-warming gases into the air and oceans. Up to half of the tiny animals that live in mud on the seafloor-benthic foraminifera-died out during the PETM, possibly along with life further up the food chain.

Other species thrived in this changed environment and new ones evolved. In the oceans, dinoflagellates extended their range from the tropics to the Arctic, while on land, hoofed animals and primates appeared for the first time. Eventually, the oceans and atmosphere recovered as elements from eroded rocks washed into the sea and neutralized the acid.

Today, signs are already emerging that some marine life may be in trouble. In a recent study led by Nina Bednarsek at the U.S. National Oceanic and Atmospheric Administration, more than half of the tiny planktic snails, or pteropods, that she and her team studied off the coast of Washington, Oregon and California showed badly dissolved shells. Ocean acidification has been linked to the widespread death of baby oysters off Washington and Oregon since 2005, and may also pose a threat to coral reefs, which are under additional pressure from pollution and warming ocean temperatures.

“Seawater carbonate chemistry is complex but the mechanism underlying ocean acidification is very simple,” said study lead author Donald Penman, a graduate student at University of California at Santa Cruz. “We can make accurate predictions about how carbonate chemistry will respond to increasing carbon dioxide levels. The real unknown is how individual organisms will respond and how that cascades through ecosystems.”

Dating an ancient episode of severe global warming

The image shows a location in Longyearbyen, Spisbergen, where the researchers carried out field work. -  Ian Harding
The image shows a location in Longyearbyen, Spisbergen, where the researchers carried out field work. – Ian Harding

Using sophisticated methods of dating rocks, a team including University of Southampton researchers based at the National Oceanography Centre, Southampton, have pinned down the timing of the start of an episode of an ancient global warming known as the Paleocene-Eocene thermal maximum (PETM), with implications for the triggering mechanism.

The early part of the Cenozoic era, which started around 65.5 million years ago witnessed a series of transient global warming events called hyperthermals. The most severe of these was the PETM at the Paleocene-Eocene boundary, around 56 million years ago. Over a period of around 20,000 years, a mere blink of the eye in geological terms, ocean temperatures rose globally by approximately 5°C. There is evidence that the concentration of atmospheric carbon oxide increased, but the phenomena that triggered the event remain controversial.

One possibility is that these hyperthermals were driven by cyclic variations in the eccentricity of the Earth’s orbit around the sun. At the cycle peaks, increased temperatures could have caused methane hydrate deposits in the deep sea to release large amounts of methane. Some of this potent greenhouse gas would have entered the atmosphere resulting in further intensification of the climatic warming, which would have continued as the methane was fairly rapidly converted into carbon dioxide in the atmosphere.

Alternatively, it may have been geological processes, unrelated to variation in the Earth’s orbit, which could have been the culprit for the warming associated with the PETM. In this scenario, magmatism would have caused the baking of marine organic sediments, leading to the massive release of methane and/or carbon dioxide, possibly through hydrothermal vents, thus initiating the global warming which led to the methane release.

“Determining exactly what triggered the PETM requires very accurate dating of the event itself, to determine whether it occurred during a known maximum in the Earth’s orbital eccentricity” explains Adam Charles, a University of Southampton PhD student supervised by Dr Ian Harding, and first author of the newly published report.

To getter a better grip on the numerical age of the Paleocene-Eocene boundary, the researchers measured radio-isotopes of uranium and lead in the mineral zircon, found as crystals in two volcanic ash horizons deposited during the PETM. These rocks were collected from two locations in Spitsbergen, the largest island of the Svalbard Archipelago in the Arctic.

Based on their data, the researchers dated the Paleocene-Eocene boundary at between 55.728 and 55.964 million years ago, which they believe to be the most accurate estimate to date. Their analyses indicated that the onset of the PETM, unlike those of other Eocene hyperthermals, did not occur at the peak of a 400 thousand year cycle in the Earth’s orbital eccentricity. Instead, it occurred on the falling limb of a cycle when warming by the sun would not have been at a maximum.

“Compared to other early Eocene hyperthermals, it appears that the PETM was triggered by a different mechanism, and thus may have involved volcanism. However, a thorough test of this hypothesis will require further detailed dating studies,” Adam concluded.