OUTLINE
1. Earth Structure and Plate Tectonics
1.1. Layers of the Earth
1.2. Mantle Convection and Plate Motions
1.3. Ocean Basins and Mid-Ocean Ridges
1.4. Mountain Ranges and Continental Crumple Zones
2. Tracing the Tethys Suture
2.1. Closing of the Tethys Ocean
2.2. India Collided with Asia
2.3. Crumple Zone from Morocco to Vietnam
2.4. Our whole voyage traces the Tethys Suture
3. Ocean Layers
3.1. Buoyancy and density stratification of water
3.2. Warm and well-lit on top
3.3. Cold and pitch-black below the surface
3.4. About 97% of the ocean is Davy Jones’ Locker!
4. Nutrient Cycling
4.1. Plants need light and nutrients
4.2. Light at the surface, nutrients in the deep/dark
4.3. Food chains and webs
4.4. Food energy pyramid
4.5. Role of vertical motion
4.6. Marine Biology from Space
5. Fishing and Overfishing
5.1. Fish populations
5.2. Fishing effort vs yield
5.3. Maximum sustainable yield
5.4. Overfishing
5.5. Bycatch
5.6. What can be done?
1. Earth Structure and Plate Tectonics
1.1 Layers of the Solid Earth
Like other planets, Earth formed by accretion from smaller chunks of rock called planetesimals. As the pieces accreted, heat from collisions eventually caused the material to melt. In the liquid state, Earth’s materials settled into layers according to density. Heavy metals like iron and nickel sank to the bottom and lighter magma floated to the top.
After the molten Earth cooled, a solid mantle made mostly of a silicate mineral called olivine crystallized above the metallic core. The mantle is about 2900 km thick (about 1800 miles). Beneath the mantle, the Earth’s metallic core is about 3500 km thick (about 2150 miles). The outer core is molten and is responsible for Earth’s magnetic field. The inner core is solid metal.
At the surface is a very thin crust of more familiar rock. Under the continents, the crust is made of rocks like granite that are high in aluminum and silicon 20 to 100 km thick. Under the oceans the crust is much thinner – just 5 to 10 km thick. Ocean crust is made of basalt, a black igneous rock high in iron and magnesium.
1.1. Plate Tectonics
Earth is a dynamic living planet with a churning heat engine that continually reshuffles its materials and repaves its surface. The Indian region is a spectacular result of this ongoing metamorphosis. We are lucky to learn about these changes through our sojourns at sea and on land, and in the incredible diversity of landscapes, ecosystems, and cultures in the region.
Thousands of miles below our feet, Earth’s iron core and mineral mantle drive heat to the surface through a slow churn of solid rock over millions of years. This convective conveyor belt gradually pushes the continents and ocean basins around in a process of continental drift also known as plate tectonics.
Earth’s crust and upper mantle are colder and more rigid than the hotter smushy rocks underneath. The outer layer moves around as rigid plates that slide around on the softer stuff like rafts on water. The plate motions are ridiculously slow on human timescales – plates take many millions of years to cross the world.
The system of interlocking rigid plates that move around on Earth’s surface is known as “plate tectonics.” The specialist word “tectonics” has the same root as “architect,” so plate tectonics is the system by which the Earth’s surface is built up of about 20 smaller parts called plates.
The whole tectonic system is driven by thermal convection in the mantle. Hot mantle rock rises from the core and colder mantle rock sinks toward the core. It’s bizarre but all this flowing up and down happens even though the mantle is solid and not liquid. It’s just super slow and the up and down motions happen over millions and millions of years.
When the plumes of hot mantle rock reach the surface, they have to spread out and move sideways. This horizontal motion of the upper mantle is the motive force that pushes and pulls tectonic plates around on Earth’s surface.
Collisions, divergence, and sliding motions of the plates produce all the topography and geographic features on the surface: ocean basins, mountain ranges, deep valleys, faults, folds, volcanos, and so forth.
1.2. Types of Plate Motion
1.2.1 Ripping
When a mantle plume hits a tectonic plate from beneath and spreads out, the mantle moves in opposite directions in different parts of the plate. This ripping motion pulls the plate apart!
Mid-Ocean Ridges: Where Ocean Plates Rip Apart
The brand-new crust formed along the fissure where ocean plates have pulled apart due to a mantle plume is hot and takes up a lot of room. It sticks up into the overlying ocean water as a mountain range called a “mid-ocean ridge.” The Mid-Atlantic Ridge is a particularly famous example, with the island nation of Iceland rising from its midst.
All ocean crust is formed at mid-ocean ridges. As more and more mantle rocks pushes up underneath the mid-ocean ridge, the plates continue to separate and more basalt magma fills in the crack.
The world’s ocean basins have very long mid-ocean ridges (underwater mountain ranges) running down their middles, like seams on a baseball. The overall length of this world-spanning mountain range is about 40,000 km!
Continental Rift Valleys: Where Continental Plates Rip Apart
1.2.2 Crunching
Tectonic plates collide a lot. Since the Earth remains a constant size, spreading in one place has to be taken up by crunching someplace else.
Subduction Zones: Ocean Plate Sinks Into Mantle
What happens when plates converge is different for oceanic vs continental plates. Ocean plates are thin and dense. Continental plates are thick and buoyant. When a land plate and an ocean plate come together, the ocean plate always loses. It sinks under the over-riding continental plate and disappears back into the mantle. When two ocean plates collide, one rides over the other. The sinking plate dives down into the mantle.
Plates that sink into oblivion get “digested” inside the Earth in what’s called a “subduction zone.” Over millions of years, carbon and water melt or boil off of the subducted plate. Most of it comes back to the surface in volcanic eruptions.
When an ocean plate dives under a continental plate, volcanic mountain ranges form inland of the collision. Famous examples are the Cascade Mountains in North America and the Andes Mountains in South America. Both result from material melting off the subducting Pacific Ocean Plate as it falls under the west coast and begins to melt.
When one ocean plate dives under another ocean plate, an arc of volcanic islands is formed above the subducting plate. For obvious reasons, these features are called “island arcs.” Famous examples are Japan, Indonesia, and the Aleutian Islands of Alaska.
Gigantic Mountains: Land Plates Collide and Neither Can Sink
The biggest and most spectacular mountain ranges on Earth form when continental plates plow into each other. Because both plates are thick and buoyant, neither can sink out of the way. Masses of rock fold and buckle, piling into huge masses of high mountains. Below the surface, there’s typically an even bigger bulge of continental rock than the one above sea level.
Famous examples of continent-continent collisions are the Himalayas, the Alps, and the many ranges of Central Asia.
2. Tracing the Tethys Suture
Over the past 200 million years, the continents of Earth’s Eastern Hemisphere have been assembled through a series of progressive collisions as landmasses moved from south to north. The deep south-to-north conveyor pushed continents together and dragged heavier ocean crust downward into the underlying mantle, leaving the largest landmasses in today’s world.
Siberia and Scandinavia hosted Europe and Central Asia and then Africa and India docked somewhat later. Each of these great gatherings consumed ocean plates, which now dangle thousands of miles below like tattered draperies. Each suture or seam is a crumple zone, where solid rock was smushed together into massive folds and gigantic west-to-east mountain ranges from the Atlantic to the Pacific.
Most of our voyage will be an exploration of this great suture zone where once the Tethys Ocean lapped the shores of Laurasia and Gondwana. From the Atlas Mountains of Morocco, along the Alps, to the roof of the world in Central Asia, to the volcanic ridges of Indonesia, the Tethys Suture divides the Old World into northern and southern domains. The Mediterranean is a tiny remnant of the former glory of the Tethys Ocean.
The west-east vastness of the Old World was built in long stages, forming globe-spanning ecozones that allowed evolving plants and animals to flourish across broad latitude zones. This in turn encouraged emerging humanity to spread pastoral and farming practices and our associated cultures along the natural grain of Earth’s surface. Boreal forests stretch from Norway across Siberia. Dappled summer sunlight filters through leafy deciduous canopies from the Lime trees of Ireland across to Shinto shrines of Japan. Vast grasslands are home to herds of hoofed mammals, nomadic pastoralists, and horse-riders across 12 time zones from Spain to Manchuria. Sun-scorched sands stretch from the Sahara to the Gobi, with seasonal savannas and teeming tropical forests further south.
India was a late arrival to the Tethys Suture. The Indian Plate swept rapidly up from the deep south, with subduction swallowing the intervening ocean starting around 70 million years ago. By 55 million years ago, compression had formed huge chains of volcanos and mountain belts as the ocean basin closed. India “docked” with the Asian mainland around 10 million years ago but the slow catastrophe in ongoing. The greatest mountain ranges on Earth wrap around the leading edge where India plows deep into central Asia. These ranges wrap around Iran, Afghanistan, Pakistan, Tibet, Nepal, and Southeast Asia. They structure climate, hydrology, vegetation, food supply, and culture over a vast region affecting billions of people.
3. Ocean Layers
Like the Earth itself, ocean water is also layered according to its density. The layered structure of the ocean is critically important for climate, life, food, and people.
Buoyancy is the force that makes dense things sink and less dense things float. Think about trying to swim to the bottom of a pool with a basketball. The reason you can’t is buoyancy. Buoyancy is what holds our ship up at the top of the ocean.
The density of water is mostly determined by temperature and salinity. Cold water is more dense than warm water and salt water is more dense than fresh water. Surface water is warmed by absorbing sunlight. The warm water is less dense than colder water. It literally floats on top of the ocean like a raft.
Water is so good at absorbing sunlight that virtually no light gets deeper than a few hundred feet. It’s perpetually pitch-black dark down there. Because there’s no sunlight to absorb, there’s no source of heat either. Over most of the world oceans, the deep water is much colder than the surface water.
The force of buoyancy acting on the warm surface water makes it virtually impossible for surface water to sink or deep water to float. They can’t just mix unless the surface water gets really cold.
The only place in the world where surface water gets cold enough to sink into the depths is the polar winter. Polar winters are dark for months. The water loses heat to the bitterly cold air and starts to freeze. Sea salt keeps it liquid down to about -2 ºC (this is why people put salt on roads and sidewalks to melt ice). But ice is not salty, even when it’s made from seawater. Sea ice is fresh water, and the salt in the freezing water stays in the liquid. So as sea ice forms the surface water gets saltier and saltier, becoming a bitterly cold brine that sinks like a rock.
In the Arctic in January and the Antarctic in July, streamers of cold dense brine fall to the ocean bottom. Cold water slowly fills the oceans from the bottom up and from the poles toward the Equator. Each year about 1/1000 of the ocean undergoes this freezing/sinking process, so it takes about 1000 years to fill up the ocean this way.
Visualize the world’s oceans as a giant bathtub. On the surface is a thin skin of water that’s warm and well-lit. Air and water are in physical contact in the surface layer, so gases pass freely from the atmosphere to the oceans. But the surface layer is so buoyant that it seals off the water below from ever touching the air.
The surface layer is only about 3% of the ocean. The remaining 97% is utterly dark and very very cold. Deep water is essentially a storage pool for the dense water formed at the surface in the polar night. The temperature of deep water doesn’t vary much from top to bottom or from north to south. It’s perpetually about 3 ºC (37 ºF) everywhere, even at the Equator. Because only 1/1000 of the ocean sinks at the poles each year, the deep water has been sealed off from the atmosphere for 1000 years. It last touched the air during Europe’s high Middle Ages / the Song Dynasty in China / Abbassid Caliphate.
The deep water doesn’t know we’re here yet!
The thermal stratification of the oceans – that is, the physical separation of the well-lit surface from the cold dark deep water beneath – has profound implications for life, climate, the composition of the atmosphere, and the future of human civilization.
4. Nutrient Cycling
4.1 The Miracle of Photosynthesis
Photosynthesis literally means “to make from light” and it’s a freaking miracle!
Green plants exposed to sunlight turn dead air into living goo, inorganic carbon into protoplasm, non-life into life. It’s the foundational quantum-mechanical miracle that underlies nearly all of life.
Aside from the hydrogen and helium that formed in the Big Bang at the beginning of time, carbon and oxygen are the most abundant substances in the universe. Oxygen is extremely reactive so pretty much all the carbon that gets exposed to air or water is bound up as carbon dioxide (CO2). As you’ve probably heard, CO2 is released when we burn carbon as fuel and causes global warming. But for the purposes of this reading, you can think of CO2 as plant food.
What’s really amazing is that there’s no food energy to be had from CO2. Unlike us, plants don’t get their energy from “CO2/plant food,” but rather they get their energy directly from sunlight. They take in inert, low-energy, dead, inorganic CO2 from thin air and use the light of the Sun to give it the breath of life.
The other thing to know about plants is that they are themselves food! Plant material stores the energy of the Sun as complex, high-energy organic molecules that can be digested by other life forms to provide life-giving power.
Some people say I am a high-energy teacher. I get my energy from breakfast. My food is made of plants and the flesh of animals that eat plants. So you can think of me as Solar-Scott. I use oxygen from the air to bust up the energy-rich plant-based molecules in my food and then I convert their energy into wild gyrations in the front of class.
Nearly all of the food energy that plants capture from sunlight is used up by microscopic organisms through a process we call decomposition. Remember that Mufasa told Simba that “when we die, our bodies decompose and go back to feed the grass.” That “decomposing” part is where almost all the solar energy goes eventually. But along the way, the energy of photosynthesis powers diverse ecosystems in the oceans that include phytoplankton, zooplankton, fish, marine mammals, and even people.
4.2 Primary Production Requires Both Light and Nutrients
The miracle of photosynthesis requires other ingredients besides CO2. The pigment that makes plants green is called chlorophyll, and there’s a special protein that plants use to do their alchemy. These special ingredients are made from CO2, but also from nitrogen and phosphorous and a bunch of other trace elements that we can think of as nutrients.
So plants need both light and nutrients to turn air into plants and feed everybody else. Because this step is the very base of the global food chain, we call it “primary production.” Everything else alive requires food, so they are secondary. Primary producers are plants.
In the oceans, it’s surprisingly hard to find places that have an abundance of both light and nutrients. That’s because of the strongly layered vertical structure of seawater discussed in the previous section. Basically, all the light in the ocean is right at the very top but almost all the nutrients are down below in the crushing black abyss of Davy Jones’ Locker!
Let’s look at how the nutrients and the light get separated and how they can be brought back together to make marine ecosystems flourish.
4.3 Food Chains and Webs
Whenever there’s both light and nutrients at the ocean surface, microscopic algae and other single-celled plants bloom like crazy. These phytoplankton get eaten (“grazed”) by microscopic animals called zooplankton. Small zooplankton get eaten by bigger zooplankton and some of these get eaten by tiny fish that are eaten by bigger fish. As this “food chain” continues, some of the carbon and energy winds up in creatures that are big enough to see. And all the fishes poop.
We can think of marine ecosystems as webs that pass energy and inorganic nutrients from phytoplankton and zooplankton to organic debris – dead stuff and poop -- and back to nutrients. Carbon, nitrogen, and phosphorous pass from box to box in this web in the same proportions, and all the flows are powered by sunlight entering the top.
Dead plankton and fish poop tend to sink. As they slowly drift downward, bacteria and other microbes eat them and turn them back into dissolved CO2 in the water. If this happens before the dead stuff sinks into the cold darkness below, then photosynthesis can recycle the dissolved CO2 into a new generation of plants.
But some of the dead stuff and plankton poop sinks below the warm well-lit water and down into the crushing black abyss of Davy Jones’ Locker. When that happens, the nutrients (especially carbon, nitrogen, and phosphorous a.k.a. C, N, and P) that were captured at the top are lost to the world of light. In fact you can think of Davy Jones’ Locker as a big refrigerator where nearly all the nutrients in the ocean are stored.
Only under special circumstances can the nutrients reach the light at the surface and fuel primary production. Never forget that Davy Jones Locker is cold and dense. That’s why the water can’t rise and bring nutrients into the light. It’s 97% of the ocean and most of the nutrients but it’s too dark down there for photosynthesis.
4.4 The Food Energy Pyramid
Consider a tuna fish sandwich or a small piece of sashimi that contains 100 grams (less than ¼ pound) of fish. This meal is the top of an enormous pyramid of energy that flows from photosynthesis through a bunch of intermediate layers.
Tuna are top predators. They can weigh up to a ton and swim as fast as a motorcycle drives in Mumbai. For each 1 kg (1000 grams) of tuna, about 10 times (10 kg) of mid-sized fish. Growing those 10 kg of mid-sized fish to feed the tuna takes 100 kg of small fish. Growing those 100 kg of little fish requires 1000 kg of zooplankton. And growing the 1000 kg of zooplankton requires them to graze 10,000 kg of phytoplankton. This means that something like 90% of the energy, carbon, and nutrients are lost at each step up the pyramid to fuel the life (metabolism) of each intermediate stage.
Two big lessons: (1) food is precious; and (2) primary production is a priceless gift!
4.5 Vertical Motion and Biological Productivity
In the oceans, biological productivity depends almost entirely on vertical motion.
Anything that stirs the ocean to bring nutrient-rich water up from the deep dark fridge into the light sets in motion the incredible alchemy of phytoplankton blooms that then feed food webs and caloric pyramids.
Ocean upwelling zones are the marine equivalents of rainforests on land, the most productive and diverse and rich ecosystems in the Blue World. By contrast anything that suppresses vertical motion or (worse) drives surface water downward keeps the light and nutrients separate. These are the marine equivalents of deserts on land – mostly clear water without much diversity or multilevel food pyramids.
For reasons we’ll explore in the next leg of our voyage, upwelling is concentrated along coasts, near the mouths of big rivers, and in the polar regions. These are the great fisheries of the world and they provide protein for billions of people. Another zone of highly productive seawater is located right along the Equator, but this has been less exploited by the fishing industry.
By contrast, the middles of the big ocean basins are the deserts of the seas. These are huge areas called gyres where water circulates slowly around the edges and flows toward the center and down into the depths. The five big gyres are in the North Atlantic, South Atlantic, North Pacific, South Pacific, and Indian Ocean.
We’re sailing across the Indian Ocean Gyre as I write these words. Look out the window or over the rail. The water is clear and blue, not brown or green with algae. There are very few fish, so very few birds and no fishing boats. The gyres are enormous but biologically thin.
By contrast the cold water of the polar regions is much less buoyant than the warm subtropical waters of the first half of our voyage. It’s easier for the winds to mix cold surface water down and bring nutrient-rich water up to the surface where the light is. These cold waters around the northern continents and Antarctica are the most productive fisheries
in the world. They’re home to tremendous biodiversity and also marine mammals and birds. Fishing fleets mercilessly trawl those waters.
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The maps above show the distribution of marine biology as seen from cameras mounted on orbiting satellites. The cameras are basically measuring how much photosynthesis is happening in the surface ocean. See if you can spot each of the five great gyres where life is sparse. It’s easy to see where cold water mixes nutrients up from Davy Jones’ Locker. Can you spot any other places that are highly productive? Why might that be?
5. Fishing and Overfishing
5.1 Fish populations
Wherever there’s enough for fish to eat (that is, that they are supported by a food web driven by nutrients and sunlight), fish reproduce. So the more fish there are, the more baby fish there will be. Populations of predatory animals like fish tend to grow exponentially until they start to run out of stuff to eat (smaller prey and the plants and nutrients that feed the prey).
You can think of fisheries like agriculture, which is of course very much the way commercial fishing is managed.
Commercial fisheries are a crop like livestock. If we don’t harvest them they will become as numerous as allowed by their food supply. On land this would be like herds of bison or other wild mammals. If there are too many, the resource (plankton for fish or grass for bison) can become overgrazed. If we harvest too many fish, then there will be less to reproduce and the catch will decline over time.
5.2 Economics of Fishing
Environmental economists analyze the relationship between “how much fish you catch” (they call this the yield) vs “how hard you try to catch fish”(they call this the effort). In the figure below the yield (in units of currency like Euros) is shown in blue. The effort is shown on the x-axis (how many days are spent fishing) and the costs of that effort (again in Euros) is shown in red.
Where the blue curve is above the red curve, the fishery is profitable: more money is earned from selling fish than is spent by catching them. Conversely when the red line exceeds the blue curve the fishery is unprofitable.
Obviously more fishing effort is associated with more costs for boats, fuel, equipment, labor, etc. The harder we fish, the more it costs. A little bit of effort produces more income (fish) than no effort, and for awhile more and more effort produces more and more yield. But at some point, more and more fishing (effort) actually produces less and less fish (yield). This is shown by the way the blue curve bends over and decreases. The reason for this is that fish are being harvested form the ocean faster than they can reproduce. Costs go up but yield decreases beyond the peak yield which is labeled Maximum Sustainable Yield (MSY) on the graph.
Fishing fleets are motivated to expend more effort as long as they are making a profit. The overall profit of the entire fishery is greatest at the point labeled Maximum economic yield (MEY). This occurs when the difference between the total yield (area under the blue line) and total expenses (area under the red line) is greatest. Fishing harder produces declining profits. A single rational fishing company would fish just this hard to make the most money.
Most commercial fisheries contain many competing companies. If one fisher limits their effort to harvest just the maximum economic yield, they may suspect that others will continue to scoop up the remaining fish (and profits) by fishing harder. This competitive pressure motivates companies to continue to fish far beyond the maximum economic yield and even beyond the maximum sustainable yield.
In fact without some kind of regulation among companies (perhaps by a government), competitive commercial fisheries are usually fished at a level of effort labeled “yield without regulation.” This is the point where it literally doesn’t pay to buy another liter of fuel to catch another fish, because the fishery has been depleted to the point where it’s very hard to find more fish.
The object of fishing regulations is typically to restrict fishing effort to the level that can supply the maximum sustainable yield or a little less. Paradoxically this can cut into the profit of individual companies while actually protecting the long-term economic interests of all fishers.
Two problems that can lead to serious overfishing are excessive bycatch and the presence of global fleets in commercial fisheries.
Bycatch is the unintentional killing of other species when equipment such as trawling nets are used indiscriminately. Most bycatch (dead fish and other animals) is simply discarded as waste. In many cases, bycatch exceeds the harvest of target fish species! Considering the food pyramid shown above, this can result in extremely inefficient use of the marine resource.
The globalization of mechanized fishing fleets has seriously disturbed many local and regional commercial fisheries. In the past decade, several Indian Ocean nations have generated extra revenue by opening their territorial waters to outside fleets. Most of the new fleets are European. Following this development, the yield of tuna in the Indian Ocean increased exponentially. Tuna production increased 1000% in about a decade and then leveled off as fish populations declined.
Overfishing is not just bad business and isn’t just bad for Indian Ocean commercial interests. Overfishing on a massive scale has severely depleted the population of virtually every top predator in the oceans. This is equivalent to the killing of most apex predators on land, and has profound effects on the biodiversity of the entire global ocean.
5.6 What can be done?
Two important steps to protect marine ecosystems and commercial fisheries are reducing bycatch and improving regional governance of marine resources.
Bycatch is accidental and unprofitable. Better equipment design and practical changes in fishing methods can substantially reduce bycatch and inadvertent impoverishment of marine ecosystems.
Better regulation to limit commercial fisheries to the maximum sustainable yield requires regional cooperation among nations and nongovernmental organizations (NGOs). Some of this is the province of diplomacy at the international level, and some of it can be accomplished through partnerships among industry, government, and NGOs. These steps can help everyone benefit from a more sustainable marine environment.
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