After 250 days and a voyage of 29,821 nautical miles, on August 12, the "Xiangyanghong 10" ship, which went to the Indian Ocean to carry out the scientific expedition mission of China's Ocean Expedition 49, returned to China and docked at Zhoushan.

What is the significance of this voyage in fulfilling the world's first international seabed polymetallic sulfide exploration contract? How are the deep-sea training of domestic equipment such as the "Qianlong 2" autonomous underwater robot (AUV)? What is the "physical examination" of the Indian Ocean? Tao Chunhui, chief scientist of Ocean Expedition 49, was interviewed by Science and Technology Daily and explained one by one.

The prospect of sulfide resources in the exploration contract area of the Southwest Indian Ocean is promising

Polymetallic sulfides are the product of seabed hydrothermal activity. Because they are rich in metal elements such as copper, zinc, lead, gold and silver, they have become an important potential mineral resource. In 2011, China and the International Seabed Authority signed the "Southwest Indian Ocean Polymetallic Sulphide Resources Exploration Contract". This is also the world's first international seabed polymetallic sulfide exploration contract.

At present, China has reached the critical stage of implementing resource exploration and environmental assessment. The main tasks of the first four legs of Ocean 49 are to complete the exploration of polymetallic sulfide resources and environmental baseline surveys in the Southwest Indian Ocean contract area, and basically complete the hydrothermal and mineralized anomaly surveys.

"This voyage carried out comprehensive towing anomaly surveys and geological sampling in 28 of the 100 contract blocks of 10,000 square kilometers, making the overall progress of the hydrothermal and mineralized anomaly surveys in the contract area reach 95%. We also carried out verification of mineralized anomalies discovered in the early stage in 16 blocks." Tao Chunhui revealed that a total of 3 mineralized areas, 2 mineralized anomaly areas and 9 anomaly areas were discovered in this voyage, laying the foundation for the planned completion of resource assessment work before the abandonment of 50% of the area in 2019. At the same time, the distribution range of mineralized areas such as Longqi, Yuhuang and Broken Bridge has also been expanded.

Scientists used to believe that the southwest Indian Ocean belongs to an ultra-slow mid-ocean ridge, which is not easy to develop hydrothermal activities. However, in 2007, Tao Chunhui's team discovered active submarine hydrothermal vents in the southwest Indian Ocean, which was the first time in the world that an active hydrothermal area was found on an ultra-slow spreading mid-ocean ridge.

"The survey of this voyage shows that the frequency of hydrothermal activity in the contract area is high, indicating that the distribution of hydrothermal activity on the mid-ocean ridge is controlled by local magma supply. Therefore, sulfides can also be developed and formed in large quantities on ultra-slow spreading mid-ocean ridges. This deepens our understanding of the distribution control mechanism, mineralization law and resource evaluation of polymetallic sulfides in the contract area, and further reveals the prospects of sulfide deposits on ultra-slow spreading ridges." Tao Chunhui said.

According to the exploration contract, China will further give up 25% in 2021, and the final area will not exceed 2,500 square kilometers. "Although China enjoys the priority right of exploitation, it must first fulfill its obligations such as environmental baseline survey, biodiversity research and training, and eventually return 3/4 of the exploration contract area to the International Seabed Authority." Tao Chunhui said.

Deep-sea equipment training, China's sulfide exploration methods and technologies are at the forefront of the world

In terms of equipment, the biggest highlight is the record-breaking "Qianlong II" AUV. "Qianlong II" has carried out 9 dives, a total of 257 hours of underwater work, and a total voyage of 654 kilometers.

"Qianlong II" has developed two new skills this time: mother ship-free operation and modular operation of multiple detection sensors, which significantly improves the operation efficiency. Through three consecutive years of deep-sea tests and voyage applications, "Qianlong II" has carried out 50 dives of deep-sea safe operations, accumulated on-site engineering application experience, and formed business operation capabilities. The cumulative underwater working time is 763 hours and the voyage is 2,204 kilometers. It constantly breaks the record of China's AUV operations at deep seabeds. There are only a handful of submersibles in the world that can achieve such capabilities.

Through this scientific expedition, China's comprehensive geophysical detection methods such as near-bottom magnetic, electrical and microseismic have been greatly improved. Tao Chunhui introduced that with the iterative development of technology, China's exploration methods have developed from towing to high-precision underwater unmanned and manned submersibles, and from grab surface sampling to transient electromagnetic method, near-bottom magnetic method and shallow drilling for detecting the three-dimensional distribution of ore bodies.

Through more than ten years of accumulation, China has established a mature "near-bottom-bottom-shallow" sulfide prospecting method and technology system, forming the ability to efficiently detect seabed sulfides, and the overall performance indicators and application effects are at the forefront of the world.

Indian Ocean survey achieves multiple firsts

This voyage also conducted a "physical examination" to assess the health of the ocean: the first survey of microplastics in the Indian Ocean was conducted, covering the southwest Indian Ocean, the northwest Indian Ocean and shipping routes; the first survey of hypoxia and ocean acidification in the Indian Ocean was conducted, which preliminarily showed that the water in the surveyed sea area was in an oxidized state, the dissolved oxygen in the surface water was in a saturated to supersaturated state, and the pH value and total alkalinity analysis of the water body showed that the water in the surveyed sea area was weakly alkaline.

In addition, the expedition also obtained new insights into the long-term water profile structure of the 90 Ridge environmental survey for the first time; it deployed meteorological and wave buoys in the Indian Ocean for the first time; it achieved new results in the correlation between microseismic and plume changes, and improved the understanding of the hydrothermal circulation system of the ultra-slow spreading ridge.

It is particularly worth mentioning that around the scientific goal of the distribution law and formation mechanism of the hydrothermal circulation system of the Carlsberg Ridge, the expedition team discovered a new high-temperature vent group in the Tianxiu hydrothermal area, expanding the distribution range of hydrothermal activities; 3 new hydrothermal systems were discovered, bringing the number of hydrothermal systems discovered in China in the area to 9; 2 sets of comprehensive anchor systems were deployed for the first time to carry out multidisciplinary long-term observations, which is expected to enhance the level of understanding of the impact of hydrothermal activities on marine material flux and the environment. (Reporter Chen Lei of this newspaper)

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