They estimate that worldwide 19–45 Gigatonnes of organic carbon are locked up in surface marine sediments in this way. Because reactive iron phases are metastable over long time periods, the sediments could be an efficient "rusty sink" for organic carbon.

"Burial in sediments is the only long-term sink of organic carbon on the planet, on geological timescales," Yves Gélinas of Concordia University told environmentalresearchweb. "Yet only a tiny fraction of organic carbon – about 0.3% – produced in the surface waters through photosynthesis eventually reaches the seafloor and is preserved in sediments. The rest is degraded in the water column and at the surface of the sediment."

The most widely accepted explanation for the tiny proportion of organic carbon preserved, says Gélinas, is that it's protected by sorption on clay mineral surfaces in the water column and in sediments.

"Our work shows that iron oxides are also very important, which is totally new," he added. "Why does it matter? Simply because iron oxides are not stable in anoxic [no-oxygen] environments (they form only in oxic settings), while clay minerals are stable whatever the redox conditions of the system."

The expansion of oceanic "dead zones" – regions where oxygen levels are too low to sustain life – could eventually affect this iron complexation mechanism for preserving organic carbon. According to Gélinas, this will "create a positive feedback mechanism fuelling greater oxygen consumption" as more organic matter will be degraded in the water column or at the surface of the sediments, using additional oxygen and so contributing to the expansion of dead zones.

"Iron has become a very popular research topic in chemical oceanography since the discovery that it is a limiting nutrient for primary productivity in large areas of the ocean," said Gélinas. "We show for the first time that iron also plays an important role in organic carbon preservation. We are definitely in the Iron Age of oceanography."

Gélinas and colleagues from Concordia University and McGill University tested sediments from around the world sampled from freshwaters, estuaries, river deltas, continental margins and the deep sea, using an iron reduction method previously used in soils.

"We now better understand the controls on organic matter preservation in sediments through its stabilization by iron complexation," said Gélinas. "[This] means that we can do a better job building models representing carbon preservation and cycling in marine environments. It also means better prediction of the evolution of the organic carbon preservation function in these models as bottom-water dissolved oxygen concentrations keep decreasing."

Now Gélinas says the team plans to get a clearer idea of the types of chemical bonds that link iron and organic matter, and how changes in local redox conditions affect their relative proportions in sinking particulates and sediments.

"We have recently obtained Synchrotron X-Ray beam time at the Brookhaven National Light Source Laboratory (NLSL) in Long Island, US, and acquired a very promising first set of data showing that our working hypothesis – that iron complexation to organic carbon plays a much greater than anticipated role in preserving a large fraction of organic carbon in sediments – is correct," he said. "We have applied for more beam time at the NSLS and at the Canadian Light Source Synchrotron in Saskatchewan to pursue this work."

The team would also like to estimate the importance of the mechanism in the stabilization of soil organic carbon.