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16 2.10 Coral Reefs
It may seem odd to be discussing coral reefs in a section about geology, but due to the stony calcium carbonate skeletons secreted by many coral species, coral reefs are as interesting as geological features as they are biological ones. Corals grow best in warm, clear, tropical water, that is close enough to the surface for light to support photosynthesis by the algae living in the coral tissues. Because of this need for light, new coral will often grown on top of the stony skeletons of older corals.
In the 1830s Charles Darwin made some observations about different types of coral reefs, and hypothesized that they represent a progression from one form to the next. The types of reefs he examined were fringing reefs, barrier reefs, and atolls, which are associated with oceanic islands (Figure 2.10.1). Fringing reefs are reefs that are close to or are connected to shore. Barrier reefs are offshore reefs that are separated from the land by an expanse of water, such as a lagoon. Atolls are circular or oval reefs surrounding a lagoon, without any central land mass in the lagoon. Darwin speculated that reefs progressed from fringing, to barrier, to atolls as the land mass subsided. However, he had no explanation for how volcanic islands could sink. Today we know that Darwin was correct, and that islands can sink as oceanic crust subsides as it moves away from a spreading center, or as sea level rises as glaciers melt.
Figure 2.10.1 A fringing reef (left), barrier reef (center), and atoll (right) (Public domain, via Wikimedia Commons).
The progression starts with a fringing reef built against the shores of island (Figure 2.10.2). If sea level doesn’t change or the land doesn’t sink, many reefs will not progress beyond this stage. But if the land does subside, the corals would eventually sink too deep for light penetration and they would die. So as the reef gets deeper, the corals continue to grow upwards, at a rate of about 3-5 m per 1000 years, and eventually a lagoon develops between the reef and the island; the reef is now a barrier reef. If the land continues to subside until it is completely submerged, all that is left is a ring of coral that has been growing upwards around the central lagoon; an atoll.
Figure 2.10.2 Steps in the development of coral reefs (Steven Earle, “Physical Geology”).
Modified from “Physical Geology” by Steven Earle used under a CC-BY 4.0 international license. Download this book for free at http://open.bccampus.ca
definition
With so many variables playing a role in the production of tides, it is understandable that not every place on Earth will experience exactly the same tidal conditions. There are three primary classifications for tides, depending on the number and relative heights of tidal cycles per day.
A diurnal tide consists of only one high tide and one low tide per day (Figure 3.7.1). "Diurnal" refers to a daily occurrence, so a situation where there is only one complete tidal cycle per day is considered a diurnal tide. Diurnal tides are common in the Gulf of Mexico, along the west coast of Alaska, and in parts of Southeast Asia.
Figure 3.7.1 A diurnal tide, with one high and one low tide per day (By NOAA [Public domain], via Wikimedia Commons).
A semidiurnal tide exhibits two high and two low tides each day, with both highs and both lows of toughly equal height (Figure 3.7.2). "Semidiurnal" means "half of a day"; one tidal cycle takes half of a day, therefore there are two complete cycles per day. Semidiurnal tides are common along the east coasts of North America and Australia, the west coast of Africa, and most of Europe.
Figure 3.7.2 A semi-diurnal tide, with two high and two low tides per day, each of roughly equal heights (By NOAA [Public domain], via Wikimedia Commons).
Mixed semidiurnal tides (or mixed tides), have two high tides and two low tides per day, but the heights of each tide differs; the two high tides are of different heights, as are the two low tides (Figure 3.7.3). The differences in height may be the result of amphidromic circulation, the angle of the moon, or any of the other variables discussed in section 3.6. Mixed semidiurnal tides are found along the Pacific coast of North America.
Figure 3.7.3 A mixed semi-diurnal tide, with two high and two low tides per day, each with a different height (By NOAA [Public domain], via Wikimedia Commons).
Figure 3.7.4 shows the distribution of the various tide types throughout the world.
Figure 3.7.4 Global distribution of the different types of tides (By KVDP (Own work) [Public domain], via Wikimedia Commons).
Tidal Currents
The movement of water with the rising and falling tide creates tidal currents. As the tide rises, water flows into an area, creating a flood current. As the tide falls and water flows out an ebb current is created. Slack water, or slack tides occur during the transition between incoming high and outgoing low tides, when there is no net water movement.
The strength of a tidal current depends on the volume of water that enters and exits with each tidal cycle (the tidal volume or tidal prism), and the area through which the water flows. A large tidal volume moving through a large area may create only a weak tidal current, as the volume is spread over a wide area. On the other hand, a narrow area may produce a strong tidal current even if the tidal volume is small, as all of the water is forced through a small area. It follows that the strongest tidal currents will result from a large tidal range moving through a narrow area.
Tidal bores occur where rivers meet the ocean. If the incoming tidal current is stronger than the river outflow, the tidal bore appears as a wave, or moving wall of water that moves up the river as the tide comes in (Figure 3.7.5).
Figure 3.7.5 A tidal bore near Silverdale in the United Kingdom (Arnold Price [CC BY-SA 2.0], via Wikimedia Commons).
In many cases these tidal bores may move through a river or inlet for many kilometers, and if they are large enough they can form continually breaking waves that surfers can ride much farther and longer than a traditional ocean wave, such as the Severn Bore in England, shown in the video below.
Many types of organic compounds produced by humans have a toxic effect on the environment. Many synthetic organic chemicals are manufactured or are a byproduct in the production of many industrial, agricultural, and household products. Large quantities of synthetic materials were produced and released into the environment in the period after WWII until environmental regulation and control began to prevail in the 1970s. In that time interval, a great amount of damage was done in many industrialized coastal regions.
Among the worst are chlorinated and halogenated hydrocarbons: DDT, TCE, and PCB’s.
* DDT (dichlorodiphenyltrichloroethane) was heavily used as an insecticide throughout the United States, particularly starting after World War II. DDT is an insecticide that was initially used by the military in WW II to control malaria, typhus, body lice, and bubonic plague. After the war, this inexpensive-to-produce chemical was extensively used with agriculture for insect control (insecticide). In the 1960’s DDT was found to cause adverse affects to wildlife, most notably causing predatory birds to produce thin shells, too thin for offspring to survive. DDT was banned from agricultural uses in 1972 over concerns of the unmitigated toxic effects on human health and many organisms in the natural environment. Unfortunately, DDT and other similar pesticides are still used in poor countries around the world to fight mosquitoes carrying malaria and other diseases.
* PCBs (polychlorinated biphenyls) are industrial products or chemicals, commonly used as insulator fluids in old transformers. PCB contamination is common in old industrial regions, particularly in the eastern United States. PCBs were banned in the U.S. in 1979 because of concerns about unintended impacts on human and environmental health. Like DDT, PBCs bio-accumulate.
* TCE (trichoroethlene) as originally introduced as a general anesthetic until it was linked to severe neurological disorders. After WWII it is was widely used as an industrial solvent, used primarily to degrease engine parts. TCE poisoning began to increase in many areas where the liquid chemical was dumped into sewers and wells, contaminating water supplies for many communities. It was later determined to be carcinogenic was banned in the 1980’s by most developed nations.
Medical Wastes
Biologists are also reporting negative impacts of pharmaceutical compounds, medical wastes, and byproduct, including birth control pills, anti-depressants, and chemotherapy drugs finding their way into coastal waters. Illegal dumping of medical wastes at sea has been a large problem. Many coastal cities have had to deal with illegal dumping of medical wastes (commonly used hypodermic needles). Beaches were closed in many areas in the New York City region during the 1990s because of dangerous quantities of contaminated needles washing up on shore.
Large marine disasters involving petroleum spills have happened many times over the past century. The impact of these events depends on where and how they occur. Some are accidents, others include intentional acts of war, such as the destruction of Kuwait’s oil fields in the First Gulf War. Oil spills can have a wide variety of impacts ranging from minimal (when far from coastal regions) to catastrophic when they impact shore regions. Two of the largest (most expensive) petroleum-related disasters affecting North American coastal waters are discussed below.
The Deepwater Horizon Disaster, 2010
The Deepwater Horizon Disaster started on April 20, 2010 with an explosion and fire of a submersible drilling platform located in the Gulf of Mexico about 40 miles (64 km) offshore from the Louisiana coast. The drilling operation involved tapping an oil reservoir deep in sedimentary deposits on the offshore region beyond the continental shelf. Problems with the drilling operation and failure of equipment to prevent a blowout that resulted in the explosion and fire, and the eventual sinking of the drilling platform two days later. 17 platform workers were killed and another 11 were injured by the explosion and fire. The open well, sheared of at the seabed, proceeded to spew large quantities of crude oil into Gulf waters until it was shut down in mid July when a 75 ton cap was put in place, sealing off the well. The disaster resulted in the largest oil spill in the history of the Petroleum Industry.
An estimated 200 million gallons (about 5 million barrels) of oil poured into ocean from the unconstrained well. Some of oil stayed on or near the seabed, much of it formed a large plum in layers within the ocean waters, and some migrated to the surface where wind and currents dispersed it. An extensive cleanup effort was undertaken to trap, degrade and disperse, or burn off much of the oil on the surface. Unfortunately, large amounts found its way onshore, impacting beaches and coastal wetlands, and severely impacting wildlife. The bad publicity wreaked economic havoc on coastal communities and businesses involved in fishing and recreation from Texas to Florida. As of 2015, BP (the company that operated the drilling program) agreed to pay $18.7 billion to settle all federal and state claims for the disaster - the biggest pollution penalty in U.S. history. Total settlement costs was in the range of $54 billion.
Settlement of all federal and state claims brings total costs to nearly $54 billion. BP PLC agreed to pay $18.7 billion to settle all federal and state claims arising from the 2010 Deepwater Horizon oil spill, including the biggest pollution penalty in U.S. history.
Because the spill happened in the warm open ocean waters, much of the crude oil from the spill eventually dispersed (evaporated or diluted) or was consumed by microbial activity.
Figure 12.7.1. An oil slick spreads on the Gulf of Mexico from the Deep Horizon disaster.
Exxon Valdez Oil Spill Disaster, 1989
Prior to the Deepwater Horizon disaster, the worst petroleum-related disaster affecting the US coastline was the Exxon Valdez oil spill (Figures 17-24 and 17-25). The oil spill occurred in Prince William Sound in the Gulf of Alaska. The Exxon Valdez, a large oil tanker bound for refineries in Long Beach, California, veered off course and struck a submerged rock “reef” outcrop on March 24, 1989. The disaster was blamed on poor navigation by a drunken ship captain, poorly trained personnel, and faulty and unused navigation equipment.
Estimates by governmental and other sources suggest that at least 10 to 11 million gallons (about 250,000 barrels) of Alaskan crude oil spilled into the coastal waters. The spill, so close to shore, eventually impaction about 1,300 miles (2,000 km) of coastline in the Gulf of Alaska (closer to 9,000 miles [14,500 km] considering all the islands, headlands, and bays along the rugged coastline). Rough seas, and the rugged and remote coastline made clean-up efforts extremely difficult, and the cold-water setting hampered the rapid decay and dispersion of the oil. The spill devastated habitats for salmon, seals, seabirds, and sea otters, and had a catastrophic effect of coastal communities in the region. The cost of the disaster, spread over many years, was in the range of about $7 billion. Hard facts were learned from the disaster. It turns out that some of the beach areas were "cleaned" - basically cooked with 150 F water. These areas were actually harmed more by the cleaning processes used. It was determined that about 35% of the oil evaporated, 8% burned, 5% dispersed by surf , and only about 5% biodegraded; the rest formed slicks that dispersed into the greater ocean currents offshore.
An outcome of the disaster is that all new large petroleum-transport vessels are now being built with double hulls to hopefully prevent future transport-spill disasters.
Impact of Petroleum Pollution on Wildlife
Pollution from petroleum-source products is a major problem in parts of the worlds oceans and coastlines. In addition, tar-ball, waxes, and other petroleum-derivative products can now be found throughout the world’s oceans. Evidence of the pollution is most abundant along developed (urban and industrial) coastlines where accidental spills occur most frequently. The risks of spills occur along all infrastructure systems associated with the petroleum production, refining, transportation, and consumption. Leaky oil from cars and trucks are a major non-point source of water pollution. Large oil spills and oil production disasters are some of the most costly, devastating both wildlife and the economic livelihoods of communities in regions where they occur (Figure 17.26).
Oil spills are particularly bad for homeothermic (warm blooded) organisms with fur of feathers. Saturation with oil causes these animals to loose insulation and they die from hypothermia. Oil slicks poison kill 150-450,000 sea birds killed each year. Organisms living in the intertidal zone most sensitive to oil contamination.
Refined oil and oil-derivative products tend to be non-biodegradable, and are more toxic to wildlife.
