Shulou(Shulou.com)11/24 Report--

This article comes from the official account of Wechat: Earth knowledge Bureau (ID:diqiuzhishiju), author: senior Fan, revised manuscript: gu Hanying, Editor: wandering son

In addition to the Pacific Ocean, Atlantic Ocean, Indian Ocean and Arctic Ocean, there is actually a "fifth ocean" in the world-the Southern Ocean. Although there is still controversy in the academic circles, this area that surrounds Antarctica but has no obvious geographical barrier from other oceans does have a lot of unique features.

The climate of the Southern Ocean is unique, and a variety of marine life, mainly krill and whales, form a unique biosphere here. At present, it is greatly affected by climate change, and it is also the most unfamiliar sea area for human beings. One of the purposes of National Geographic's move is to draw attention to the waters around Antarctica.

How many oceans are there in the world? So what is the basis for the division of the Southern Ocean?

Welcome to the Southern Ocean (south of latitude 60 degrees south)

(figure: Wiki@Apcbg) ▼

Several oceans after human beings have a basic understanding of the land and sea distribution of the world, the debate about how many oceans there is has never stopped. From different classification criteria and positions, it can be roughly divided into one, three, four and five sets of different cognition.

On the whole, the earth is surrounded by oceans.

Most of the waters are connected to each other.

So it makes sense to have only one big ocean on earth, ▼.

The oceans of the earth are not completely cut off by land. All oceans are actually the same sea. At the same time, we cannot regard the huge ocean that surrounds the earth's surface as water in a giant tank. Because it is not a static pool of stagnant water, but under the comprehensive influence of temperature, density, salinity, ocean surface climate, wind currents, ocean currents, and so on, it is a never-ending source of life carrying salt ions, nutrients, dissolved gases, heat and suspended matter.

▼ of the sea (Caribbean Sea, South China Sea) that has been circling.

The change of salinity temperature will bring about the change of sea water density, and the dense water masses will sink, forming a downflow zone, bringing oxygen-rich surface water to the depths of the ocean and bringing precious oxygen to deep-sea animals.

The rising low-density water masses will form an upwelling zone, bringing nutrients to the surface of the sea, and nutrients will be used as nutrients to promote the growth of plankton and increase the oxygen content of the surface water. The water in the depths of the ocean may slowly rise in a thousand years, return to the surface, and complete a cycle of life.

Salinity distribution of sea water on the earth's surface

It can be seen that the distribution of surface salt in different sea areas is not uniform.

Among them, the highest salinity is in the "closed waters" such as the Mediterranean Sea.

And the salinity of the waters around Antarctica is indeed quite close.

(figure: Wiki@Plumbago) ▼

Precisely because the bodies of water within the oceans are not static, and there are water bodies and ocean currents connecting each other, the definition of how many oceans are changing all the time.

There is almost no land between 50 degrees south of latitude and the Antarctic Circle, where oceans beyond the Arctic Ocean connect, and there is frequent exchange of water between the oceans. Because of this, the Russian oceanographer Yuri Mikhilovich Shokalski put forward the concept of a world ocean at the beginning of the 20th century.

On the other hand, the distribution of land divides the ocean into relatively independent components. Therefore, the theory of three oceans is formed, which is composed of the Pacific Ocean, the Indian Ocean and the Atlantic Ocean. Because the Arctic Ocean accounts for only 4.2% of the global ocean area, the sea connects with the Atlantic Ocean, and there are a large number of ocean currents shuttling between the two oceans, so it is regarded as part of the Atlantic Ocean in the theory of the three major oceans.

On the Pacific side is the narrow Bering Strait.

Facing the Atlantic Ocean is the vast Norwegian Sea and Greenland Sea.

So some people see it as part of the ▼ of the larger Atlantic Ocean.

More oceanographers regard the Arctic Ocean as a separate ocean. Unlike the Atlantic Ocean, 1/3 of its sea area is covered by the continental shelf and is the broadest ocean on the continental shelf. Because of the existence of sea ice, its temperature and salinity are different from those of the North Atlantic Ocean. It also has its own unique ecosystem.

In terms of ocean depth, the Arctic Ocean is the shallowest, even far less than the Southern Ocean.

(figure: photo from Pixabay) ▼

In the 1950s, relevant scholars began to realize that there are some continuous ridges in the middle of the ocean, which are 1-3 km higher than the bottom of the ocean basin on both sides, with a water depth of 2-3 km at the top.

In the early 1960s, the "seafloor spreading theory" was proposed, which believed that the mid-ocean ridge was the place where the new crust was born. The existence of ocean ridges has become one of the criteria for judging whether the ocean can be regarded as an ocean alone.

If the existence of sea water is ignored, the topography of Haiti is in fact no different from that of land.

There are also plains, hills, basins and mountains

And the mid-ocean ridge is actually the largest continuous mountain range.

(slide below, figure: Wiki) ▼

This standard supports the view of the four oceans, namely, the division of the Pacific Ocean, the Atlantic Ocean, the Indian Ocean and the Arctic Ocean. At the same time, the division of the four oceans is also the most in line with people's previous understanding of ocean division, is the most mainstream point of view.

The division that people have been most familiar with for a long time.

The waters around Antarctica are divided into the Pacific, Atlantic and Indian oceans.

(figure: NOAA) ▼

It is not a recent new idea to find that the Southern Ocean regards the Southern Ocean as the fifth ocean. early explorers came to the Southern Ocean and could feel its special features intuitively.

This sea area originated 30 million years ago. At that time, Antarctica finally split from South America, forming a vast sea around the South Pole.

The most independent continent in the world

(figure: Wiki@Dave Pape) ▼

The westerly belt between latitude 35-65 degrees north and south is highly coincident with this open sea area. Strong westerly winds blow the surface waters of the Southern Ocean to form currents flowing from west to east, which is called westerly drift. The fact that the sea can circle the earth is obviously a prerequisite for the formation of westerly drift.

On the other hand, the melting and seasonal variation of Antarctic glaciers cause changes in temperature and salinity in the sea area near the Antarctic continent, which leads to the density difference between the Antarctic coastal area and the outer sea, forming a geostrophic flow from west to east. This is also an important reason for the formation of the Antarctic circumpolar current.

Drifting around the South Pole, ▼ after circle.

Antarctic currents are estimated to be about 34 million years old, and fast-moving cold water makes the ecology of the Southern Ocean so unique that it provides a unique habitat for thousands of species. So that in such a cold place, a unique and diverse ecosystem has been formed.

Oxygen content in seawater is a key resource of marine ecology.

The amount of dissolved oxygen in cold water is higher than that in warm water.

This is an important condition for the prosperity of marine ecology in the north and south poles.

(figure: Wiki@Plumbago) ▼

In the 1770s, the explorer Captain Cook first discovered the circumpolar circulation in Antarctica. The harsh climate in the polar regions has slowed the progress of Antarctic scientific research, but subsequent explorers are impressed by the unique ecology and environment here. Tend to think of it as a relatively independent ocean.

After the first international conference held by the International Hydrographic Organization (IHO) on July 24, 1919, the scope of the Southern Ocean was defined as all the sea areas south of Africa, South America and Oceania.

But there is no clear continent between the Southern Ocean and other oceans. The Southern Ocean does not have its own independent basin, which consists of the Pacific Ocean, the Indian Ocean and the southern Atlantic Ocean. This leads to a great deal of controversy over how to define the boundary of the Southern Ocean. So the scope of the Southern Ocean continued to shrink, and between 1952 and 2002, IHO did not even give a unified range of the Southern Ocean.

The boundary of the Southern Ocean is shrinking southward.

The last thing that is left is probably your own "basic disk".

(slide the following image, see: Wiki) ▼

In recent years, with the progress of science and technology, human understanding of the Antarctic continent and the Southern Ocean has improved, but the conclusion is not optimistic. The growth rate of local temperature in Antarctica and the Southern Ocean is much higher than the global average. From the perspective of ocean currents and biodiversity, the Southern Ocean is beginning to be recognized by more and more countries and organizations.

After all, the hole in the ozone layer is already quite large

(figure: NASA) ▼

Abundance and crisis in the Southern Ocean

Krill is the foundation of the Southern Ocean ecosystem. There are 85 species of krill in the world, of which seven to eight live in the Southern Ocean, and the krill usually refers to the Antarctic krill. Although there are big characters in their names, they are actually only about six centimeters long and weigh only two grams.

Whenever summer comes in the Southern Ocean, krill quickly enter the mature stage, and they are distributed in regional groups to avoid small predators. Small as fish, squid, seabirds. As big as seals and baleen whales use krill as an important source of food. It is the huge biomass of krill that supports the Antarctic ecosystem.

General distribution of krill around Antarctica

(photo: https://link.springer.com/ chapter / 10.1007. 978-3-319-29279-3. 2) ▼

The relationship between krill and baleen whales is particularly close, and their interaction is the basis of the Southern Ocean ecosystem. Krill feed on plankton on the ocean surface, but the distribution of krill is not just on the ocean surface. All kinds of baleen whales will dive into the distribution of krill, eat krill and float to defecate. Whale droppings are rich in iron and nitrogen and are nourished by plankton and small fish and shrimps. To complete the reincarnation of nitrogen and iron in the Southern Ocean.

Krill is small, but the number is extremely large.

As a key bridge between plankton and other animals in the Southern Ocean

His eating, excretion and breathing can all affect the marine chemical environment.

(figure: Wiki- https://www.nature.com/ articles / s41467-019-12668-7) ▼

Baleen whales chase krill, while humans chase baleen whales. In the 1930s, the whaling industry reached its heyday, leading to a significant decline in baleen whales in just a few decades. The short-term mass reproduction of krill led to the decrease of plankton, which led to the ecological imbalance and the decline of krill population. Whales are at the top of the food chain and have such a profound impact on the ecological balance. If there is a significant reduction in krill, the Southern Ocean ecosystem will be destroyed.

With the climate change, the number of krill has been affected in recent years, and the observed data have decreased significantly. The estimation of the total amount of krill in the Southern Ocean is also getting lower and lower. From a super-optimistic forecast of 10 billion tons, it fell to 10-2 billion tons in the 1960s, and the recent forecast fell further to about 500 million tons.

A more crucial problem is that humans do not know enough about the Southern Ocean, even how deep krill can live and how long they can live. At present, krill is lack of use value, it is easy to be squeezed out in fishing, and it will rot quickly if it is not treated after fishing. After treatment, it tastes like rice and lacks the flavor of fresh shrimp. How to remove excessive fluorine in krill is also a problem.

So at present, krill is mainly used to make salmon feed, or made into health food krill oil. If there is no restriction on the abuse of this fishery resource. It is likely to repeat the disaster that wiped out many kinds of cetaceans.

Can only feed the salmon, the status is not as good as the shrimp rice skin.

It's a waste of ecological resources.

(figure: Wiki@Judgefloro) ▼

The mysterious Southern Ocean, although the development potential is huge, but the ecology is very fragile. This is probably why this cold sea attracts explorers and scholars to study it repeatedly. The existence of the Southern Ocean is also a reminder that our understanding of the earth has not gone far. It still takes a long time for human beings to think about how to define their own environment and how to define their position in the environment.

References:

1. Https://www.thoughtco.com/the-new-fifth-ocean-1435095

2. Https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0011683

3. Sun Song, Liu Yongqin. Antarctic krill and Southern Ocean ecosystem [J]. Nature Magazine, 2009, 30-32-46.

4. Sun Song, Yang Guang. Antarctic krill, the cornerstone of Antarctic biosphere [J]. Man and the Biosphere, 2017

5. Yang Linlin. Antarctic krill can produce marine natural fertilizer [J]. Fishery Information and Strategy, 2017 Pluto 68.

* the content of this article is provided to the author and does not represent the position of the Earth knowledge Bureau

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