Ocean oxygen levels control seafloor carbon burial

by Lauren Milideo
Thursday, November 12, 2015

Residual organic carbon from dead marine phytoplankton and other oceanic life can follow two pathways: It can be deeply buried in seafloor sediments or it can be oxidized, either in the water column or in shallow sediment layers. The balance between these two processes and the extent of organic carbon burial over time are crucial in the global carbon cycle, but a complete understanding of the factors and rates controlling organic carbon burial versus oxidation in the oceans has been lacking. In a recent study in Geology, scientists improved on an approach to quantify organic carbon oxidation and provided a glimpse of marine organic carbon burial in the Precambrian, when the oceans were anoxic and Earth’s atmosphere was in the early stages of oxygenation.

Since the late 1980s, organic carbon’s tendency to be oxidized in marine environments has been described by an equation known as the Middelburg power law, which describes carbon’s reactivity in seawater as a function of its age — the older the carbon is, the less reactive it is and the more likely it is to be buried. Carbon burial efficiencies calculated with this equation have not always matched field observations, however, and scientists have suggested that factors such as temperature, water chemistry, sedimentation rate and the amount of dissolved oxygen in the water column (whether it’s oxic or anoxic) may affect rates of oxidation versus burial.

Sergei Katsev, a geochemist at the University of Minnesota at Duluth, and Sean Crowe, a geomicrobiologist at the University of British Columbia in Vancouver, were particularly interested in how the oxygenation state of water impacts organic carbon. Middelburg’s law does not factor in oxic versus anoxic conditions, Katsev says, but due to oxygen’s role in organic carbon decay, whether water is well-oxygenated or not may matter.

Previous research on Lake Superior by Katsev’s graduate student, Jiying Li, had found that rates of organic carbon oxidation and burial there were consistent with the Middelburg law. But that was only one example, and it was freshwater, not seawater (upon which the law is based). To further determine whether the Middelburg law is applicable across diverse environments, including both freshwater and seawater, the researchers decided to look at additional freshwater lakes, which “have very different chemistry from seawater,” Katsev notes.

Tapping the published literature and their own work on large lakes, Katsev and Crowe assembled water-column and sediment-core data from oxic and anoxic lakes on several continents to study organic carbon reactivity over time. For each of the lakes studied, they calculated organic carbon reactivities with age in the water column and sediment cores and found trends similar to those seen in the marine data on which Middelburg’s law was originally based. The trends did vary slightly, however, for oxic versus anoxic lakes, showing that more carbon is buried in anoxic environments.

“We see pretty much the same [relationship] as in marine environments, which tells us that the same law” applies in a variety of environments, Katsev says. “And by extension, we can hypothesize that the same law can be applied to times in Earth’s history when the ocean chemistry was very different from today.”

To that end, Katsev and Crowe calculated organic carbon reactivity and burial efficiencies considering the anoxic conditions that prevailed in the oceans billions of years ago. Compared to modern, oxygen-rich oceans, they found that the fraction of organic carbon buried on the abyssal plains of Earth’s Precambrian oceans was likely between six and 40 times higher. As oxygen then began accumulating in the atmosphere, and subsequently in the deep ocean during the Late Neoproterozoic, organic carbon burial would have dropped as more carbon was instead oxidized. With more oxygen being consumed in the ocean, less would have been free to accumulate in the atmosphere. “Over hundreds of millions of years,” Katsev says, this give-and-take between carbon and oxygen “is what controls the oxygen content in the atmosphere.”

“Their conclusions about ancient Earth are very intriguing,” says David Burdige, a geochemist at Old Dominion University in Virginia who was not involved with the research. Regarding the study’s emphasis on oxic and anoxic conditions in determining carbon burial efficiencies, he says, “the question then becomes causality.”

Indeed, Katsev says, the next step is to pursue the underlying causes behind their results. This work has described organic carbon oxidation quantitatively over time, he says, but “we don’t understand what controls the rates” of carbon oxidation.

A deeper understanding of carbon oxidation may also yield insight into future conditions. Given decreases in oxygen levels in such locations as the Baltic Sea, North Sea and Chesapeake Bay, Katsev says, a better understanding of “how the chemistry [of these systems] will respond will allow us to make better predictions of nutrient inputs, of the oxygen levels in those environments, and ultimately of the health of the ecosystems.”