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Unformatted text preview: "Water, water every where, nor any drop to drink..."
The Rime of the Ancient Mariner, by Samuel T. Coleridge, 1798 Approximately 3/4 of Earth's surface is covered by water. Humans cannot survive more than 3 days without water... 97% of all the water on Earth is salty... it is very costly to
Group 6!). make this water potable (fit for drinking- more on this later from 2% of all the water on Earth is glacier ice at the North and South Poles the melted water from which is too far from the people who need it. Fresh water for us to use makes up only 1% of all the water on Earth. Seawater chemistry... chemistry...
The polar nature of the water molecule produces some unexpected chemical properties. One of the most important is water's remarkable ability to dissolve more ability substances than any other natural solvent. The most abundant ions dissolved in seawater are chloride (Cl), sodium (Na), (Cl), sulfate (SO4). The quantity of dissolved inorganic solids in water is its salinity. The salinity. proportion of ions in seawater is not the same as the proportion in concentrated river water, which indicates that ongoing geological and chemical processes affect the ocean's salinity. salinity. Though most solids and gases are soluble in water, the ocean is in chemical water, equilibrium, and neither the proportion nor the amount of most dissolved equilibrium, substances changes significantly through time. Gases dissolve in water in proportions that vary with their physical properties. N is physical the most abundant dissolved gas in seawater; O is the 2nd most abundant. CO2 is abundant. the most soluble gas, and one of many substances that affect the ocean's pH balance. Seawater acts as a buffer to prevent broad swings of pH when acids or bases are introduced. There is no "salt" in seawater; when salt crystals dissolve, components are separated into ions... salt crystals can reform as water evaporates What are solutions and mixtures?
A solution is made of two components, with uniform molecular properties throughout: The solvent, which is usually a liquid, and is the more abundant component. The solute, often a solid or gas, is the less abundant component. A mixture is different from a solution. In a mixture the components retain separate identities, so it is NOT uniform throughout.
"solution" sugar in coffee "mixture" noodles in soup Salt in solution. When a salt such as NaCl is put in water, the positively charged hydrogen end of the polar water molecule is attracted to the negatively charged Cl- ion, negatively and the negatively charged oxygen end is attracted to the positively charged Na+ ion. positively The ions are surrounded by water molecules that are attracted to them and become solute ions in the solvent. Fig. 7-2, p. 187 1 Hydrologic cycle evaporation - change of state of water from a liquid to a gas. On average,
~120 cm is evaporated from the ocean each year. transpiration - evaporation of liquid water from plants and trees into the atmosphere. About 90% of all water that enters the roots transpires into the atmosphere. without moving through the liquid phase (our fog this week! energy = heat of sublimation - the process where solid ice and snow changes into water vapor
fusion + heat of vaporization). condensation - the process where water vapor changes back into a water
droplets. This is when we begin to see clouds. precipitation - water that falls to the Earth (rain, snow, sleet, drizzle, and hail).
~500,000 km3 of water falls each year, mainly over the ocean. runoff - the variety of ways of which water moves over the Earth's surface. infiltration - the movement of water Add heat
into the ground from the surface. groundwater flow - the flow of water (km3/yr).
Fig. 7-1, p. 186 underground in aquifers. The water may return to the surface in springs or eventually seep into the oceans. Remove heat Table 7-4, p. 194 Processes that regulate the major constituents in seawater:
Ions are added to seawater by rivers running off crustal rocks, volcanic activity, groundwater, hydrothermal vents and cold springs, and the decay of once-living organisms. Ions are removed from the ocean by chemical entrapment as water percolates through the mid-ocean ridge systems and seamounts, sea spray, uptake by living organisms, incorporation into sediments, and ultimately by subduction. A Few Ions Account for Most of the Ocean's Salinity Ocean'
If the Earth's ocean evaporated, the entire planet would be covered with a layer of salt 150 feet thick! Fig. 7-4a, p. 190 Fig. 7-3, p. 189 "Earth tea" "Earth tea" Major solid and gas solutes dissolved in the ocean! Table 7-1, p. 188 Table 7-2, p. 189 2 The Ocean Is in Chemical Equilibrium
Is the ocean becoming progressively saltier with age?
No, the ocean is in chemical equilibrium. The proportion and amounts of dissolved solids remain constant. This concept is known as the "steady the state ocean." ocean." Ions are being added to and removed from the ocean at the same rate. rate. Residence time is the average length of time an element spends in the in ocean. Residence time can be calculated by the equation: Residence Time = _Amount of element in the ocean___ _Amount The rate at which the element is added to (or removed from) the ocean Table 7-3, p. 192 Conservative or Nonconservative
Conservative constituents of seawater are those constituents that occur in constant proportions. Conservative elements have proportions. long residence times and are the most abundant dissolved material in the ocean Nonconservative constituents have short residence times, times, and are usually associated with seasonal, biological or short geological cycles Salinity is the total quantity of dissolved inorganic solids in water
conservative constituents - water itself, major salts that maintain a relatively constant ratio or change very slowly (geologic time scales Seawater's Constituents May Be Seawater' Seawater's nonconservative constituents -tied to biological / seasonal cycles (dissolved O produced by plants, Si and Ca needed for plant / animal shells, nitrates and phosphates needed for production of protein and other biochemicals, etc.
Table 7-3, p. 192 Temperature, salinity, density characteristics of the ocean As you might expect, temperatures are lowest in
the polar regions and highest near the equator. Heavy rainfall in the equatorial regions "freshens" the ocean near the equator, whereas freshens" hot and dry conditions near the tropic lines (Tropic of Capricorn and Tropic of Cancer) result in higher surface salinity in those areas. remember- cold + saline = more dense water rememberFig. 6-14, p. 167 3 Sea-surface temp (Jan 2001) poles solar energy over larger surface area equator more solar energy per area Ocean-Surface Conditions Depend on OceanLatitude, Temperature, and Salinity Fig. 6-15, p. 168 Fig. 6-16, p. 168 Ocean is divided into 3 density zones: Surface zone the upper layer of the ocean, containing the least dense , relatively warm water. The surface zone is only about 2% of total ocean volume. Pycnocline a zone in which density increases with depth, containing about 18% of all ocean water Deep zone contains about 80% of all ocean water. There is little change in density throughout this layer. Ocean density stratification Fig. 6-18a, p. 169 4 The rapid density increase in the pycnocline is mainly due to a decrease in temperature with depth in this area--the thermocline. In some regions, especially in shallow water near rivers, a pycnocline may develop in which the density increase with depth is due to vertical variations in salinity. In this case, the pycnocline is a halocline. Fig. 6-18b, p. 169 Fig. 6-18c, p. 169 Typical temperature profiles at polar, tropical, and middle (temperate) latitudes. Note that polar waters lack a thermocline. Note the similarity in temperature in all deep waters. ...only a surface phenomena!
Fig. 6-12, p. 165 Light and sound are affected by the physical properties of water, with refraction and absorption effects playing important roles Refraction can bend the paths of light and sound through water Sound and light both travel in waves: Refraction is the bending of waves, which occurs when waves travel from one medium to another. The refractive index is a ratio that expresses how much light is refracted from one medium to another. Scattering and absorption weaken light: Scattering occurs when light is bounced between air and water molecules, dust and other objects. Absorption occurs when light's electromagnetic energy is converted light' to heat in the molecules of seawater. The relationship between water depth and the speed of sound the "shadow zone" Sunlight does not travel well in the ocean. Fig. 6-23, p. 174 5 Light in the deep Only a thin film of seawater is illuminated by the sun. Except for light generated by living organisms, most of the ocean lies in complete blackness.
Fig. 6-22a, p. 173 A fish photographed in normal oceanic light--the color blue predominates; and with a strobe light. The flash contains all colors, and the distance from the strobe to the fish and back to the camera is not far enough to absorb all the red light. The fish shows bright warm colors otherwise invisible. Fig. 6-22b, p. 173 Gases in seawater Most gases in the air dissolve readily in seawater at the ocean''s surface. Plants ocean In order of relative abundance, the major gases found in seawater are nitrogen, seawater The proportions of dissolved gases in the ocean are very different from the different oxygen, and carbon dioxide.
proportions of the same gases in the atmosphere because of differences in their differences solubility in water and air. Oxygen is abundant near the surface because of the photosynthetic activity of marine plants. Oxygen concentration decreases below the sunlit layer because of the respiration of marine animals and bacteria, and because of the oxygen consumed by oxygen the decay of tiny dead organisms slowly sinking through the area. area. In contrast, because plants use carbon dioxide during photosynthesis, surface levels photosynthesis, of CO2 are low. Overall, the ocean has ~60x as much CO2 as the atmosphere! Because photosynthesis cannot take place in the dark, CO2 given off by animals and bacteria tends to build up at depths below the sunlit layer. CO2 also increases with depth because its solubility increases as pressure increases and temperature decreases.
Fig. 7-8, p. 195 Average O2 & CO2 versus ocean depth Gases dissolve more readily in COLD water and animals living in the ocean require these dissolved gases to survive. O2 levels may be so low in 2 tropical oceans that animals may be unable to survive! The Ocean's Acid-Base Balance Varies with Ocean' AcidOcean's Acid-Base Dissolved Components and Depth
... what in the world does that mean??
What are acids and bases?
An acid is a substance that releases a hydrogen ion (H+) in solution. A base is a substance that combines with a hydrogen ion in solution. A solution containing a base is called an alkaline solution. Acidity or alkalinity is measured on the pH scale. Fig. 7-9, p. 196 6 The Ocean's Acid-Base Balance Varies Ocean' AcidOcean's Acid-Base with Dissolved Components and Depth Carbon dioxide (CO2) combines readily with seawater to form Carbonic acid can then lose a H+ ion to become a bicarbonate
carbonic acid (H2CO3). Some bicarbonate ions dissociate to form carbonate ions, which combine with calcium ions in seawater to form calcium carbonate (CaCO3), used by some organisms to form hard shells and skeletons. When their builders die, these structures may fall to the seabed as carbonate sediments, eventually to be redissolved. redissolved. As the double arrows indicate, all these reactions may move in either direction.
CO2 + H2O H2CO3 H2CO3 HCO3- + H+ HCO3- + H+ CO32- + 2H+
Fig. 7-10, p. 197 ion (HCO3-), or two H+ ions to become a carbonate ion (CO32-). pH of seawater changes slightly with depth in surface ocean areas of rapid photosynthesis, pH will rise at middle depths and in deep water, more CO2 may be present
than at the surface (cold temps, high pressure, and no photosynthetic plants, organic C produced in surface ocean sinks to bottom and decays, producing CO2) "buffering" prevents broad swings of pH when acids or bases buffering" are introduced into the ocean slightly because CO2 is used by plants 7 ...
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This note was uploaded on 04/29/2008 for the course EAS 104 taught by Professor Brown during the Spring '08 term at Purdue University-West Lafayette.
- Spring '08