How much do we know about the murkiest depths of our deepest oceans? A lot less than we thought, new research shows. Whereas it was previously suspected that the majority of sediment covering the seafloor was devoid of life, drill cores from the South Pacific Gyre, considered the 'deadest' place in our oceans, have provided new evidence of numerous oxygen-breathing microbial communities.

The South Pacific gyre is considered the most nutrient-poor oceanic area on the planet, and one of the least inhabited places on the earth – something of an oceanic desert. Earlier explorations had shown that microbes were present here only to a maximum depth of around eight metres, a finding that has remained consistent for the past 60 years. Until now.

An international team led by Professor Steven D’Hondt at the University of Rhode Island Graduate School of Oceanography collected core samples from seven sites in the South Pacific Gyre. By combining predictive modelling with first-hand analyses of the microbial communities within these cores, the scientists were able to estimate that microbes are present throughout the sediment in up to 44% of the Pacific Ocean, and 37% of all the world’s oceans.

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A sediment core from the South Pacific Gyre is analysed. Photo courtesy of Fumio Inagaki.

What's more, the team demonstrated that microbial life exists in these sediments at a much deeper level than previously thought. “We found that there is no limit to life within this sediment. Oxygen and aerobic microbes hang in there all the way to the igneous basement, to at least 75 metres [246ft] below the seafloor,” D’Hondt notes.

This process occurs over very long time periods. Sediment in the deep oceans accumulates extremely slowly, at most a few meters per million years in the South Pacific Gyre. Having a low sedimentation rate in this case probably helped oxygen penetrate to greater depths, fuelling the deeper microbial communities.

More sophisticated detection techniques were crucial in finding evidence of life here. The team found that microbes were present in extremely low quantities – only about 1,000 cells per cubic centimetre of sediment. These small amounts are probably why previous analyses using less-precise equipment were unable to detect the presence of life at these depths.

These findings are also important for our understanding of how geochemical cycles operate within the Earth. As you may know already, the planet's crust is made of large tectonic plates. Though we don’t feel it, these plates are in constant motion, moving not only past each other, but also under each other in regions known as ‘subduction zones’. The zones work like giant conveyor belts, pulling the edge of one plate – and the sediments, minerals and chemicals within it – under the edge of another plate and into the hot mantle below.

If this sediment is more oxygenated, then this has implications for the chemistry of material being carried into mantle at these subduction zones. “Subduction of these big regions where oxygen penetrates through the sediment and into the igneous basement introduces oxidised minerals to the mantle, which may affect the chemistry of the upper mantle and the long-term evolution of Earth’s surface oxidation,” D’Hondt says.

It's fascinating that the detection of the smallest and simplest of things, in the most remote, barren places on the planet, can tell us so much about the complex internal workings of the world we live in. 

This research forms part of a much larger international project, the Deep Carbon Observatory, which has been operating for ten years to investigate 90% of the carbon locked up in Earth’s deepest areas.

Top header image: David Minder, Flickr