Low oxygen in the open ocean: A case study of mild oxygen minimum zone (OMZ) in the Western Pacific

Marine hypoxia, exacerbated by global warming, has led to the expansion of oxygen minimum zone (OMZ) in the ocean. Based on investigations across four sections in the Tropical Western Pacific Ocean, this study examines the characteristics and key controlling factors of OMZ in the region, with a focus on the interactions between organic matter and hypoxia, using particulate organic carbon (POC) as an example. Results indicate that OMZ are located at depths of 550-1380 m, 290-1120 m, 740-1380 m, and 730-1380 m across the four sections, with a dissolved oxygen threshold of 3.2 mg/L. Seawater stratification plays a crucial role in defining OMZ distribution, with OMZ upper boundaries occurring below the halocline and minimum oxygen levels found beneath the thermocline. Ocean circulation facilitates the westward expansion of the intense OMZ from the Eastern Pacific, influencing its extent in the study area. OMZ enhance the transport of POC to the deep sea, with decomposition rates within OMZ significantly lower than in overlying waters, 17.7 %, 3.1 %, 3.7 %, and 13.6 % in sections A, B, C, and D, respectively. These findings provide valuable insights into the fundamental characteristics, drivers, and biogeochemical implications of OMZ in the Western Pacific.