New method helps understand the global organic carbon cycle

Oceans serve as a sink for carbon. This means that carbon from the atmosphere is stored in the oceans. However, a distinction is made between organic and inorganic carbon. The organic carbon bound in marine sediments is also a source of oxygen. Until now, it has been common practice to determine the mass balance between inorganic and organic carbon – but this method is considered inaccurate. A team from the USA, Great Britain and Germany has now developed a different approach. Their goal was to be able to better determine the rate of carbon over a longer period of time.

To do this, they used data from deep-sea drilling at 81 global sites to determine the history of organic carbon burial during the Neogene (about 23 to 3 million years ago). This approach makes it possible to better determine the variability over such a long period of time.

”Our results support the assumption that rates were high in the early Miocene and Pliocene and low in the middle Miocene,” explains first author Dr. Ziye Li from MARUM – Center for Marine Environmental Sciences at the University of Bremen. “We calculated the mass accumulation rate of organic carbon directly from the organic carbon content of marine sediments. This is possible thanks to standardized measurements combined with well validated age models from sites from the international drilling program IODP and its predecessors DSDP and ODP. Traditionally, estimates have been based on the isotopic composition of carbon, which requires, among other things, a number of assumptions about carbon sources and key fluxes within the carbon cycle,” says Li, who works at MARUM in the Low Latitude Climate Variability group.

“Our new results are very different – they are the opposite of what the isotope calculations are suggesting,” says co-author Benjamin Mills from the University of Leeds, an expert on the established isotope methods. “I was really surprised how wrong our current ideas might be.”

Li and her colleagues assume that carbon sequestration, or rather its absence, is related to temperature-dependent bacterial decomposition of organic matter during the warm period of the middle Miocene. Thus, this feedback mechanism would be expected to play out during other warming intervals in Earth's history, as well as in any future warming of the global ocean.

“As we warm up the ocean, it will make it harder for organic carbon to find its way to be buried in the marine sediment system, and that is what we have found in our study – the lowest rates of carbon sequestration happen when the planet was warm”, said co-author Yige Zhang of Texas A&M University. “So that’s not helping from this perspective, in terms of the issues that we’re facing in the present day.”

However, the team's research suggests that this respiration-like process prevents organic carbon sequestration from reducing carbon dioxide emissions to the atmosphere. When bacteria process the organic carbon, it is returned to its original form as CO2.

First author Ziye Li calls the team's work the beginning of a potentially significant new method for data analysis that can help understand climate change and mitigate its effects.

MARUM produces fundamental scientific knowledge about the role of the ocean and the ocean floor in the total Earth system. The dynamics of the ocean and the ocean floor significantly impact the entire Earth system through the interaction of geological, physical, biological and chemical processes. These influence both the climate and the global carbon cycle, and create unique biological systems. MARUM is committed to fundamental and unbiased research in the interests of society and the marine environment, and in accordance with the Sustainable Development Goals of the United Nations. It publishes its quality-assured scientific data and makes it publicly available. MARUM informs the public about new discoveries in the marine environment and provides practical knowledge through its dialogue with society. MARUM cooperates with commercial and industrial partners in accordance with its goal of protecting the marine environment.