Ocean Drilling Reveals the Secrets of Carbon Cycling in the Japan Trench

The Japan Trench is one of the deepest and least-explored places on Earth, where the Pacific Plate dives under the Eurasian Plate in a process called subduction.

This region is also prone to powerful earthquakes, such as the 2011 Tohoku-oki earthquake that triggered a devastating tsunami.

But what happens to the carbon that is buried in the sediments of the trench, and how does it affect the global carbon cycle?

A recent scientific expedition has shed some light on these questions, by drilling into the trench and analyzing its carbon content.

How ocean drilling reveals carbon cycling in the trench

(Photo : Handout/Getty Images)

The expedition, called IODP Expedition 386 "Japan Trench Paleoseismology," was conducted by an international team of researchers from 2013 to 2015.

The team used a research vessel called R/V Kaimei, equipped with a drilling system that can reach depths of over 8,000 meters below sea level.

The team collected 58 sediment cores from 15 sites along the 500-km-long trench axis, setting two new records in over 50 years of scientific ocean drilling.

The team analyzed the carbon content and isotopic composition of the sediment cores, as well as the interstitial water that fills the pores between the sediment grains.

They found that the trench sediments contain a large amount of dissolved carbon, both organic and inorganic, that is stored in the interstitial water.

The dissolved carbon is derived from the organic matter that is transported by gravity flows from the continental shelf to the trench, as well as from the inorganic carbon that is released by subduction-related processes.

The team also found that the dissolved carbon undergoes active microbial-mediated cycling in the deep subsurface sediments.

The microbes use different metabolic pathways to decompose or transform the organic and inorganic carbon, producing various carbon compounds such as methane, acetate, and bicarbonate.

The microbial activity also affects the isotopic signature of the dissolved carbon, making it older or younger than its source.

Also Read: Study Looks to Dive Deeper at the Oceanic Carbon Cycling

How carbon cycling in the trench affects the global carbon cycle

The findings of the expedition have important implications for understanding the role of hadal trenches, which are the deepest parts of the ocean (>6 km depth), in the global carbon cycle.

Hadal trenches are often overlooked or neglected in global carbon budget estimates, due to their remoteness and difficulty of access.

However, they may play a significant role in sequestering or releasing carbon from or to the atmosphere.

The team estimated that hadal trenches account for about 0.5% of the global oceanic dissolved organic carbon pool and about 0.1% of the global oceanic dissolved inorganic carbon pool.

They also estimated that hadal trenches contribute about 0.2% of the global oceanic methane flux and about 0.02% of the global oceanic bicarbonate flux.

These fluxes may vary depending on the frequency and intensity of earthquakes and gravity flows that affect the transport and transformation of carbon in the trench.

The team also suggested that some of the dissolved carbon fractions may have long-term impacts on the deep carbon cycle, as they are buried deeper into the subduction zone and eventually recycled into Earth's mantle or crust. These processes may affect Earth's long-term climate and geodynamics.

Related article: Researchers Discovers Importance of Carbon Cycle in Predicting Climate Change