Ancient Marine Sediment Used as Yardstick for Present, Future Climate Models

Christopher K. Junium (Credit: Syracuse University)

Scientists at Syracuse University are exploring the geologic past to generate future projections about climate change.

Christopher K. Junium, assistant professor of Earth sciences in the College of Arts and Sciences (A&S), is the study’s lead author that uses the nitrogen isotopic composition of sediments to understand variations in marine conditions during the Paleocene-Eocene Thermal Maximum (PETM)—a short-term period of fast global warming approximately 56 million years ago.

Junium’s team—which includes Benjamin T. Uveges G’17, a Ph.D. candidate in A&S, and Alexander J. Dickson, a lecturer in geochemistry at Royal Holloway at the University of London—has published an article on the subject in Nature Communications (Springer Nature, 2018).

Their study concentrates on the ancient Tethys Ocean (site of the present-day Mediterranean Sea) and provides a yardstick for present and future ocean and climate models.

The nitrogen isotope record demonstrates that oxygen-free [anoxic] conditions initiated rapidly at the onset of the PETM, changing the way important nutrients, such as nitrogen, were recycled. The magnitude of this nitrogen isotopic shift is similar to those observed during rapid warming intervals in the Mesozoic Era [252 million to 66 million years ago], when broad areas of the Tethys and Atlantic oceans became depleted in oxygen, below the surface.

Christopher K. Junium, Assistant Professor, Earth Sciences, College of Arts and Sciences

Such depletion, called deoxygenation, caused Oceanic Anoxic Events (OAEs) in the Eastern Tethys during the Mesozoic Era. Researchers believe OAEs concurred with fast changes in the ancient Earth’s ocean and climate circulation—changes manifest by an influx of carbon dioxide from periods of strong volcanism.

“While the exact cause of the PETM is an area of active debate, we are certain that potent greenhouse gases, including carbon dioxide and methane, contributed to overall warming,” Junium says.

The fate of the Tethys Ocean and areas surrounding it during the PETM has been the focus of much conjecture by paleoclimatologists, particularly Dickson, who has written expansively about it. He and Junium are convinced that an array of factors—including intense rainfall, ocean acidification, and weathering on land, and an influx of nutrients (for example, phosphorous, nitrogen, and sulfur) from river discharge—set the stage for deoxygenation. Similar to what is taking place currently.

Coastal marine systems may be more vulnerable to OAE-like conditions than previously thought. This is particularly so in enclosed basins, such as the Baltic Sea, or near large river systems, including the Mississippi, which are seeing major influences from anthropogenic activity. ... The expansion of anoxic waters, particularly during summer months, impacts marine communities, as well as those relying on coastal areas for food sources, commercial fishing or recreation.

Christopher K. Junium, Assistant Professor, Earth Sciences, College of Arts and Sciences

Based on data from the ancient Kheu River system in southern Russia, Junium and his colleagues have established that the nitrogen cycle of the Eastern Tethys experienced a “major reorganization” during the PETM. “Pertubations to the nitrogen cycle can have widespread consequences,” says Junium, referring to the process in which nitrogen transform from one form to another, while circulating all over the atmosphere, the marine and terrestrial ecosystems. “Nitrogen is critical for life on Earth.”

The team’s research explores further. Differences in nitrogen isotope data from the Kheu indicate episodes wherein anoxic conditions relaxed, causing oxygen to mix into the water column.

“The transition between oxygen-free and low-oxygen conditions in the Tethys Ocean during the PETM may have created conditions that favored increased production of nitrous oxide, a potent greenhouse gas made by microbes at very low oxygen concentrations,” Junium says. “Studying conditions that fostered nitrous oxide production [during the PETM] enables us to calibrate current and future Earth system models. There is more to warming than just increased concentrations of carbon dioxide.”

Nitrous oxide provides a remarkable, albeit speculative twist to the team’s research as it is not possible to measure the gas directly in ancient rock. “I think we can make a case for finding out whether or not conditions during the PETM favored increased production,” Junium says.

Dickson agrees, adding that the mere proposal of nitrous oxide contributing to global warming during the PETM is “fascinating.”

Events such as the PETM are some of the best geological analogues we have for a warmer world. And yet, for years, a satisfactory explanation of how the climatic drivers of these ancient events interacted to produce the level of observed warming has eluded climate modelers,” Dickson says. “The suggestion of a nitrous oxide feedback on climate warming adds a new layer of intrigue to this discussion and highlights the role a changing nitrogen cycle might have on our future Earth.”

Junium believes his team is on the right path. As carbon dioxide concentrations hazardously approach 400 parts per million (levels not witnessed in three million years), they are mindful that warming will continue to increase. The societal and ecological implications could be enormous.

Navigating such ground, Junium says, requires improved model-based forecasts for global warming.

Indeed, there are gaps in our understanding between the model worlds and the fossil worlds. The past enables us to test and hone models on which future projections are based. It also helps us determine what processes are missing from our current Earth system models. These things combined help us understand and prepare for what is on the horizon.

Christopher K. Junium, Assistant Professor, Earth Sciences, College of Arts and Sciences

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