Microfossils reveal warm oceans had less oxygen, according to Syracuse geologists
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Foraminiferas registers iodine ratio to calcium, determining oxygen levels.
Researchers from Syracuse University's College of Arts and Sciences are pairing chemical analyses with micropalaeontology - the study of tiny fossilized organisms - to obtain a better understanding of how global marine life was affected by a rapid warming event over 55 million years ago.

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

Zunli Lu
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Asst. Professor of Earth Sciences Zunli Lu among those researching oxygen saturation.
"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 results in 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 problematic due to the lack of a sensitive, widely applicable indicator of dissolved oxygen.

In order to address the problem, Lu and his colleagues have started working with iodate, a type of iodine that is apparent in oxygenated waters only. By analysing the iodine-to-calcium ratios in microfossils, they are able to estimate the oxygen levels because of the lack of sensitive widely applicable indicator of dissolved oxygen.

Fossil skeletons of a group of protists known as foraminiferas have long been used for palaeo-environmental reconstructions. Developing an oxygenated proxy for foraminifera is important to Lu because it could allow him to 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 in the PETM, prompting mass extinction on the seafloor."

Lu thinks analytical facilities that combine climate modeling with micropalaeontology will aid scientists in anticipating trends in ocean deoxygenation. Already, it's been reported that modern-day OMZs, such as those 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.