The study is the first to outline a global marine oil slick map. The team's research found that human activities are the most important source of oil slicks on the global sea surface, and anthropogenic oil slicks account for about 94% of the total global oil slick area. At the same time, offshore oil slicks are more polluted, with 50% of the oil slick concentrated in the sea 38 kilometers from the shore. In the busy shipping waters, the study also observed for the first time 21 high-density oil slick pollution zones that are highly consistent with the route.
The oil slick of the ocean, like a piece of "psoriasis" scattered on the surface of the ocean, makes a piece of "deep blue" become turbid. Where do they come from and where do they form "hardest hit areas"?
On June 17, a paper published in the international authoritative academic journal Science outlined for the first time a global marine oil slick map.
A team of Chinese and American scientists led by Nanjing University found that human activities are the most important source of oil slicks on the global sea surface, and anthropogenic oil slicks account for about 94% of the total global oil slick area. At the same time, offshore oil slicks are more polluted, with 50% of the oil slick concentrated in the sea 38 kilometers from the shore. In the busy shipping waters, the study also observed for the first time 21 high-density oil slick pollution zones that are highly consistent with the route. The article was also recommended by the Science news team as the highlight of the current issue.
Oil slicks are mainly distributed in coastal areas
"Ocean slicks are a film of mineral oil floating on the surface of the sea." The paper's corresponding author, Liu Yongxue, a professor at Nanjing University's School of Geography and Oceanographic Sciences, divides marine oil slicks into natural sources, from natural hydrocarbons that leak from the seabed oil and gas layer, and from ships, offshore oil and gas platforms, submarine oil and gas pipelines, and land emissions.
"Oil slick pollution seriously threatens marine biodiversity and the ecological environment, and toxic substances such as polycyclic aromatic hydrocarbons in pollutants will accumulate along the food chain, threatening food safety." Liu Yongxue has been concerned about marine ecology for many years, a few years ago, when he and his team established a database of global offshore oil and gas development platforms, they often found that there were many oil slicks near these offshore platforms, and several questions also came to mind, "Where does these oil slicks come from, is it related to human activities, and how much of the global marine slick is caused by man?" ”
From 2019 onwards, Liu Yongxue's team decided to clarify the "home base" for the global marine oil slick distribution. However, the survival cycle of oil slicks is often less than 1 week, and they will slowly spread, disintegrate and differentiate with the help of marine dynamics such as wind and waves.
"The surface oil slick suppresses sea surface capillaries and short gravity waves, reduces the roughness of the sea surface, and in synthetic aperture radar images imaged at appropriate wind speeds, the oil slick backscatter signal is weakened, manifesting as a black dark spot." We obtained and processed more than 560,000 synthetic aperture radar images from 2014 to 2019 from Google Earth Engine's cloud computing platform, indicated the possible oil slick area in the image through marker points, and combined with multi-source data such as sea surface temperature, chlorophyll, bottom shape, wind speed, etc., to remove marine slick analogues, we can obtain information such as the distribution and area of the slick. Liu Yongxue introduced that through this method of "visual marking + automatic segmentation", the team for the first time established a global sea surface oil slick dataset with a resolution of 10 meters, and the detection found that the global marine slick area was more than twice the area of France.
"The oil slick is mainly distributed in the coastal area, and the farther away from the coast, the overall oil slick area decreases. The most densely distributed area of oil slick is within 7 km of the sea, about 50% of the oil slick is in the sea 38 km from the shore, and about 90% of the slick is within 160 km of the shore. Liu Yongxue explained.
The area of the oil slick varies greatly from coast to coast and sea. He introduced that in the Java Sea, the South China Sea, the Gulf of Guinea and its surrounding waters, the sea surface oil slick pollution is the most serious.
In the busy shipping waters, the study also observed for the first time 21 high-density oil slick pollution belts that are highly consistent with the route. Among them, there are 10 oil pollution belts in the Mediterranean Sea and the Black Sea, and 11 oil pollution belts for ships in the Strait of Hormuz, the Bay of Bengal, the Strait of Malacca, the Java Sea and other seas. Outside of these oil slick zones, the study also identified 82,982 strip oil slicks that may have been emitted from ships, which account for at least 19.8% of the total global oil slick area.
There are 435 natural seepage centres on the seafloor worldwide
Previously, the natural leakage of the seabed oil and gas layer has always been considered a major source of oil slick, so where is the natural leakage of oil slick, and is there a law?
The team found that there are 435 natural seepage centers on the seabed around the world, located within 400 kilometers of the coastline, with water depths ranging from 4 meters to 5526 meters. Among them, the Gulf of Mexico has the largest number of natural leakage centers, reaching 176, accounting for 40% of the total. There are 62 in the Gulf of Guinea, 43 in the Caspian Sea and 31 on the coasts of Ecuador and Peru.
"A study in the 1970s showed that there were 190 offshore seepage points in the global natural seepage map. Of these 190 sites, we found that 48 were not identified in this study. Considering that these unidentified satellite observations also had a minimum frequency of 138, we judged that this was either a low natural leakage frequency or no leakage. Liu Yongxue said.
The study also found that oil spills and emissions occur frequently near offshore oil and gas infrastructure around the world. "There are 137 newly discovered oil slick concentration centers related to oil and gas platforms and pipelines, the vast majority of which have not been previously discovered." Liu Yongxue introduced that the 111 marine oil slick gathering centers are closely related to oil and gas production platforms. The North Sea and the Norwegian Sea in the eastern Atlantic Contributed 34, followed by the Gulf of Guinea with 33 centres and the South China Sea and Java Sea with 15.
In addition, there are 26 oil slick concentration centers that are close to the submarine oil and gas pipeline, with an average distance of less than 1 kilometer, which researchers classify as pipeline oil slick centers.
"It is important to note that in the north-central Gulf of Mexico, 18 submarine oil and gas pipeline spill centers are not far from known natural seepage points, which requires further investigation into whether they are caused by pipeline leakage or natural leakage. Overall, all pipeline leakage centers account for only 0.5% of the global total. Liu Yongxue introduced.
More than 90% of marine oil slicks come from human activities
Although the number of natural leakage centers on the seabed is more than in the past, the large amount of oil slick around the offshore sea is more natural or man-made?
Identifying oil spills from remote sensing images is key. Liu Yongxue introduced that if there is leakage in the oil and gas layer on the seabed, it will continuously release oil at a fixed location, which shows unique characteristics in the detection results of time series remote sensing images.
However, the oil pollution emissions of offshore oil and gas development platforms and underwater pipelines also have the characteristics of fixed and continuous emissions, how to distinguish between natural leakage points and anthropogenic sources of oil slicks?
"We collected the existing subsea oil and gas pipeline data, and the research team has also established a database of global offshore oil and gas development platforms, which indicates the distribution and number of these oil and gas platforms." Subsequently, according to the spatial aggregation characteristics of the oil slick in the timing image, all fixed continuous emission sources are identified, and the points of the oil and gas development platform and the submarine oil and gas pipeline are excluded, and the remaining natural leakage points on the seabed are eliminated. Liu Yongxue said that after the distinction between the two is good, and then add the data from the ship and the land oil discharge point, you can count the natural and man-made oil discharge points separately.
After statistics, the researchers found that the proportion of natural leakage, oil and gas pipelines, oil and gas platforms, ships and land-based discharge areas was 6.2%, 0.5%, 1.6% and 91.7%, respectively, which means that natural and anthropogenic oil slicks were 6.2% and 93.8%, respectively.
"According to the us National Research Council's previous calculation of the global oil slick from 1990 to 1999, the proportion of natural and anthropogenic sources was 46.15% and 53.85%, respectively, and the proportion of anthropogenic source oil slick we found this time has nearly doubled." Liu Yongxue said that where these artificial sources of oil slicks come from can be glimpsed from another set of data tubes, and according to statistics, ships emit at least three times the area of oil slicks as natural oil slicks. Researchers attribute this to the expansion of global maritime transport.
"From 2000 to 2019, global ocean capacity increased 2.5 times. In addition, oil pollution is particularly pronounced in coastal areas of emerging economies, such as Vietnam, Indonesia, Malaysia and Brazil, which have contributed about 25% to global economic growth over the past 20 years. Liu Yongxue said that this is the potential reason for the increase in the proportion of anthropogenic source oil slicks.
"In fact, polycyclic aromatic hydrocarbons at concentrations of 1 to 10 mg/L can impair the heart development of juveniles, and even a small oil spill may have a direct impact on marine life." Liu Yongxue believes that ecological governance after polluting the ocean is often more than worth the loss, and it takes more manpower and material resources, and the protection of the ocean should be prevented.
"Now that we know where the pollution is serious, we should take the initiative to take measures to leave a blue ocean for future generations." Liu Yongxue appealed.
Reporter Jin Feng
Source: Science and Technology Daily