Ocean expedition reveals deep Japan Trench's carbon cycle

Expedition 386 crew collected 58 sediment cores from holes dug up to 37.82 meters below the seabed at 15 different places along the 500-kilometer-long trench.
Mrigakshi Dixit
Representational image
Representational image


A comprehensive analysis of a large set of ocean drilling samples revealed the carbon cycle occurring at record-breaking ocean depths of Japan Trench. 

The Japan Trench is a significant geological feature in the western Pacific Ocean. The trench is situated within a subduction zone and is linked to geological activity, encompassing seismic events such as earthquakes and tsunamis. Furthermore, the region is of particular importance in deep-water studies.

Carbon cycle dynamics

Professor Rui Bao of Ocean University in China led the sample study, which showed a considerable amount of "labile dissolved carbon stored in the sediment interstitial water." 

As per the official release, dissolved carbon storage suggests that there's a more substantial breakdown of organic carbon in the hadal trenches compared to other deep-sea locations in the open ocean. 

Using sophisticated radiocarbon technologies, the researchers noted that organic and inorganic carbon in the deep sediment layers was aging and accumulating over time.

"These are exciting results, because these dissolved carbon fractions may have great impacts on the deep carbon cycle as they are buried still deeper into the trench sediments and the subduction zone," said Mengfan Chu, Ph.D. candidate at the university, and lead author of this new study. 

Collection of the samples

The International Ocean Discovery Program (IODP) Expedition 386 crew collected and analyzed 58 sediment cores from holes dug up to 37.82 meters below the seabed at 15 different places along the 500-kilometer-long trench.

"These operational expedition achievements of successful deep-subsurface sampling at water depths between 7,445 and 8,023 m below sea level set two new records in over 50 years of scientific ocean drilling and coring," said Michael Strasser from the University of Innsbruck and co-chief scientist of the IODP Expedition 386.

"We have cored the deepest water site at a water depth of 8,023 meters and recovered the deepest sub-sea level sample from 8,060.74 meters below sea level,” added Strasser. 

Ocean expedition reveals deep Japan Trench's carbon cycle
Post-expedition science party splitting the samples.

Large methane deposits

Furthermore, the sample analysis indicated substantial methane deposits in the strata of the Japan Trench.

This shows that a lot of microbial methane is generated in the hadal trenches, which is thought to be influenced by the frequent large earthquakes in the subduction zone. 

These earthquakes enhance the flow of organic carbon and alter the physical and chemical characteristics of the sediment. As a result, earthquakes play a significant role in regulating the carbon cycle and the activities of microorganisms in the deep biosphere of these extreme environments.

"These discoveries provide strong evidence that the hadal trenches are not 'tranquil' deep-sea environments as previously considered. We're excited to say that more discoveries about the hadal trench carbon cycle are bound to be made in the future," said Rui Bao.

The study results were published in Nature Communications.

Study abstract:

Hadal trenches are unique geological and ecological systems located along subduction zones. Earthquake-triggered turbidites act as efficient transport pathways of organic carbon (OC), yet remineralization and transformation of OC in these systems are not comprehensively understood. Here we measure concentrations and stable- and radiocarbon isotope signatures of dissolved organic and inorganic carbon (DOC, DIC) in the subsurface sediment interstitial water along the Japan Trench axis collected during the IODP Expedition 386. We find accumulation and aging of DOC and DIC in the subsurface sediments, which we interpret as enhanced production of labile dissolved carbon owing to earthquake-triggered turbidites, which supports intensive microbial methanogenesis in the trench sediments. The residual dissolved carbon accumulates in deep subsurface sediments and may continue to fuel the deep biosphere. Tectonic events can therefore enhance carbon accumulation and stimulate carbon transformation in plate convergent trench systems, which may accelerate carbon export into the subduction zones.

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