Seamounts and the oxygen minimum zone (OMZ) are two typical habitats that often co-exist in the deep ocean. However, it remains unexplored and unverified whether the "seamount effect" alters OMZ structure based on marine stratification, thus impacting deep-sea hypoxic environment and carbon sink process.

In a study published in Progress in Oceanography, a research team led by Prof. SONG Jinming from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) clarified the differences in the "seamount effect" between two cruises on M4 seamount in the Western Pacific, and found that seamount promoted the expansion of the OMZ to a certain extent.

During the two cruises carried by RV KEXUE, researchers found obvious uplifts of isotherms and isohalines near the summit of M4 seamount, accompanied by the upwelling of nutrients. The average concentration of particulate organic carbon (POC) near the summit in Cruise 1 was 1.99 times that in Cruise 2, indicating that the "seamount effect" existed in both cruises, and it was stronger in Cruise 1.

Using 3.2 mg/L as the threshold, researchers pointed out the OMZ ranges and intensities in Cruise 1 and Cruise 2 were similar, and the OMZ in this region is a typical Mild OMZ. They then proposed two possible mechanisms by which seamounts affect the OMZ. 

One is the hydrodynamic effect of topographic obstruction: The upwelling triggered by the seamount lifts the isolines of low-concentration dissolved oxygen (DO), causing the upper boundary of the OMZ to shift upward and its vertical range to expand. 

The other is the biogeochemical effect of organic matter degradation: Corals, benthic organisms, and the decomposition of organic matter enriched at the seamount base continuously consume DO, creating a local hypoxic area, thus enhancing the range and intensity of the lower boundary of the OMZ.

"Our findings provide reliable reference and guidance for improving the understanding of seamounts and the OMZ," said Prof. MA Jun, the first author of the study.

Schematic diagram of the potential impact mechanism of seamount on OMZ expansion. (Image by IOCAS)