New research shows that over Earth’s history, certain minerals have played a role in preserving organic carbon in marine sediment – a finding that could be important for improving Earth’s blue carbon stocks in the future.

Climate Change and Environment

Blue carbon – carbon stored in coastal and marine ecosystems, including seabed sediments – has been recognised as having an important role in limiting the impact of climate change. However, it is not well understood how organic carbon, derived from once-living organisms, avoids being broken down by microbes to become preserved in sediment over millions of years. “The fact that any organic carbon is preserved is profoundly puzzling,” notes MINORG project coordinator Caroline Peacock, professor of Biogeochemistry at the School of Earth and Environment, University of Leeds, United Kingdom. “So our project sought to discover what causes burial of carbon in sediment and how important minerals are for that burial process.” “This is important because burial of carbon in sediments helps regulate long-term climate on Earth,” she explains. “Over long timescales, the burial of organic carbon in sediments also accumulates oxygen in the atmosphere.” But even over shorter timescales, “every bit of carbon that is buried in sediment is locked away from the atmosphere to some degree.”

Experiments to determine mineral association

Minerals similar to those found in sea sediments were synthesised in the laboratory, to look at the mechanisms by which different types of carbon found in the marine environment associate with minerals. “We looked at exactly how a specific molecule of carbon becomes attached to a mineral surface,” Peacock explains. Minerals in sea sediment – in particular those made of iron and manganese – were shown to lock up organic carbon and protect it from degradation. Changes in temperature, salinity, pH and other parameters were also investigated in the laboratory and quantified. Peacock says: “We wanted to understand the stability of those attachment mechanisms, such as whether they survive being buried in sediment or survive different chemical or biological changes that occur during burial.” “Our main conclusion was that the most important type of carbon for preservation and burial is carboxyl-rich carbon,” she notes. This is derived from the breakdown of marine phytoplankton. “Carboxyl carbon is strongly associated with minerals because forces of attraction between the carbon and minerals are really high and remain throughout biological changes, likely long-term over Earth’s history.”

Biogeochemical model simulating the ocean floor

The project, which was funded by the European Research Council, devised a predictive model from scratch. “We tied together all of these recorded processes into a biogeochemical model – a kind of simulation of the ocean floor – which we could use to predict carbon cycling between sediments and seawater,” Peacock remarks. “Our model shows that around 60 % of all carbon that is buried is carbon associated with minerals such as iron and manganese minerals, which, in the modern ocean, can change from place to place let alone over long timescales.” “If iron availability controls carbon burial, then carbon burial will be varied because we know iron availability has dramatically varied over Earth’s history,” she explains, pointing out “that has big implications for climate and oxygenation and even biological evolution.”

Geopolymerisation

The project also found that, under certain circumstances, carbon in sediment is transformed into a highly unreactive type of carbon. “We call it geopolymerisation, where relatively simple forms of carbon become polymerised – they link together to form much larger molecules that are very stable,” Peacock says. “We did modelling that showed that without this burial of geopolymerised carbon, Earth’s surface temperatures would likely have been very different, and oxygenation of the planet would also have been different over many, many millions of years. That was pretty big,” she adds. Whilst the project focus was on how these shaped the planet over history, Peacock notes that there is interest in manipulating such processes to improve blue carbon stocks in future.

Keywords

MINORG, blue carbon, seabed sediment, sediment, climate, iron, manganese, carboxyl carbon, biogeochemical model, ocean floor, carbon cycling, evolution, geopolymerisation