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Transcript
“The highest form of human intelligence is the ability to observe without judging” Krishnamurti “The intuitive mind is a sacred gift and the rational mind is a faithful servant. We have created a society that honors the servant and has forgotten the gift” Albert Einstein “The mind is everything, Oceanography Lecture 15 a. Seawater Chemistry: Sources of Sea salts – Constancy rule c. Residence time: Steady-state d. Dissolved gases e. The carbonate system and Carbon cycle what you think you become” Buddha Vertical Temperature & Salinity Salinity vs Depth Temperature vs Depth Salinity (‰) Temperature (°C) 0 5 10 Density Structure of the Oceans: Depth 34 15 20 34.5 35 35.5 36 0 0 2000 3000 4000 5000 1000 Thermocline Halocline 2000 Depth (m) Depth (m) 1000 3000 Halocline 4000 5000 The Ocean can be divided into three structures: Surface layer; Pycnocline; Deep-layer. Pycnocline: Amounts to ~18% of the Oceans volume) Salinity So, What is salinity? • By weight, 96.5% of the Ocean are water, whereas the remainder 3.5% are composed of dissolved substances (salts). • Oceans: 1,370.103 km3. • Total weight of salts: 1,370.106 km3 ! 1.1015 cm3/km3 ! 1.03 g/cm3 ! 0.035 4.94! 1022 g of salts ~50 000 Trillion Tons!!!!! ! Could cover the entire planet with an even layer > 50 m in thickness! Salinity: Composition Major Elements Only 7 elements belong to this class > 100 ppm Sodium Sulfate Magnesium Common form ClNa+ SO4Mg2+ Concentration (ppm) 18,980 10,556 2,649 1,272 Percent by weight 50.08% 30.63% 7.69% 3.69% Calcium Potassium Carbon Ca2+ K+ HCO3- 400 380 140 1.16% 1.10% 0.41% 34.377 99.76% Constituent Chloride Only 12 elements make up 99.9% of the dissolved constituents of seawater! Total Minor Elements Trace Elements Only 5 elements belong to this class 1 ppm > x > 100 ppm Constituent Common form Concentration (ppm) Percent by weight Bromide Br- 65 0.19% Strontium Sr2- 8 0.02% Boron B3+ 4.4 0.01% Silicon Si(OH)4 4.9 0.01% Fluorine F- 1.4 0.004% 83.7 0.234% Total Important trace elements: Concentration < 1 ppm Nitrogen Lithium Iodine Phosphorus Common form N Li I P Concentration (ppb) 280 125 60 30 Zinc Iron Aluminum Manganese Zn Fe Al Mn 10 6 2 2 Constituent Etc… … Salinity distribution Uniformity? Uniformity? Sea water chemistry Percent by weight <0.001 Uniformity of Ocean Water 75% of the total Ocean volume have a • Temperature: 0-5°C • Salinity: 34-35‰ Oceans’ depths are filled with cold water (colder than the ~17.5°C average T of the Oceans’ surface waters). ! Most of this water must have originated in polar latitudes, where it was chilled by losing heat to the frigid air. ! Uniformity of T and salinity of subsurface seawater from Ocean to Ocean suggests that Ocean basins are open systems! Departure: Departure: Ø Utilization of carbon, calcium and magnesium by biological activities (hard skeletons) Ø Enclosed seas: estuaries and other regions can receive substantial inflow of river water which may contain less salts in different proportions Ø Basins, Fjords, etc., where bottom circulation is severely restricted (i.e. Black Sea). Complete utilization of O2 followed by SO4-. Principle of constant proportions • In 1865, the chemist George Forchammer noted that, although the total amount of dissolved solids (salinity) might vary among samples, the ratio of major salts in samples of seawater from many locations was constant! Na/Cl = 0.56; S/Cl = 0.14; Mg/Cl = 0.07 Regardless of changes in salinity (total amount of salts)! The way salinity changes throughout the Oceans depends almost entirely on: - evaporation/precipitation balance, and - extent of mixing between surface and deep waters. Departure: Departure: Ø Extensive areas of warm, shallow waters (i.e. Bahamas), characterized by very active chemical and/or biological precipitation of calcium carbonate leading to changes in Ca2+ and HCO3- to total salinity. Ø Areas of sea-floor where interstitial (pore) waters in sediments react with sediments ! leading to large changes in ionic ratios. Ø Regions of sea-floor spreading and active submarine volcanism where heated seawater circulates through cracks of the oceanic crust. Ionic ratio in hydrothermal solutions are very different from those of normal seawater Sources of Sea Salts? Example: Water on Earth ! Water is a great solvent: a great many substances can dissolve within its matrix. ! Rain and river dissolve continental crustal rocks due to the formation of acids (H2CO3; H2SO4, HCl) and the solvent properties of water. ! H2O + CO2 " H2CO3 (acid) ! SO2 + OH + H2O # # H2SO4 (acid) !These dissolved species then reach the Oceans through surface flow in rivers (rivers provide most of the dissolved elements to the Oceans!). Sources of Sea Salts? !So, does that mean that Oceans: • Have a diluted but similar composition as crustal rocks? Element Si Al Fe Ca Na K Mg Ti Mn P % by weight 28.2 8.2 5.6 4.2 2.4 2.4 2.0 0.6 0.1 0.1 Average percentages by weight of the ten most abundant elements (other than oxygen) in the Earth’s crust The three most abundant elements in the crust (Si, Al, Fe) do not appear at all in the major-minor dissolved elements of the Oceans. Why? ! Solubility and reactivity of different elements. ! Si, Fe, Al (very common) not very soluble. Remain as particles ! Na, Ca, K, are relatively soluble and remain in solution !So, does that mean that Oceans: • Have a diluted but similar composition as crustal rocks? • Are concentrated versions of river water? Sources of Sea Salts? !So, does that mean that Oceans are: • Concentrated versions of rivers? Rain River Seawater Sources of Sea Salts? ! So, what is it? It’s a matter of how fast the “clock ticks”… • As water moves through the hydrological cycle on Continents, it picks up salts (dissolved constituents) ! Weathering. • Salts can then be maintained in solution for a long time (rates!). Sources of Sea Salts? Origin of Chloride: Negligible proportions of chloride in river water comes from weathering. Chlorine comes from volcanism (excess volatile) So does sulfur! But, if input is so small how can the total mass be so high? Simplified Examples: H2O + CO2 " H2CO3 (acid) • CaCO3 (calcite - sed) + H2CO3 (acid) " Ca2+ + 2 HCO3(bicarbonate) • 2NaAlSi3O8 (albite - metam/ign) + 5H2CO3 " Al2Si2O5(OH)4 (kaolinite) + 2Na+ + H2CO3 + 4 SiO2 (partly sol) Ocean Salinity • Paradox: Oceans contain, in high concentrations, elements that are rare in Earth Crust; and, in low concentrations, elements that are abundant in the Earth’s crust. Sources of Sea Salts? Residence time! Total mass of dissolved compounds in the Ocean Rate of supply (or removal) Chloride: 261 1014 t/2.54 108 t/yr – Selective weathering – Residence time 1.03 108 yrs Iron: 103 Million Years! (Corrected ! ) 1.4 109 t/0.22 108 t/yr 64 yrs! Changes in residence time = Changes in reactivity Residence time – reactivity – mixing rate Steady State ! It seems so for several 100’s of My! And how do these elements leave? Ø Biological uptake/Chemical precipitation Ø Precipitation/Adsorption Ø Reverse weathering Ø Hydrothermal fluids Major elements Minor elements Biolimiting elements Steady State ! It seems so for several 100’s of My! And how do these elements leave? • Biological uptake/chemical precipitation: Ca2+ + CO32- " CaCO3 • Precipitation/Adsorption Fe + particles ! Iron-rich Clay particles Sediment diagenesis (Fe, Mn, S, metals) • Reverse Weathering ! 5Al2Si2O5(OH)4 + 2K+ + 2HCO3- + 4SiO2" 2KAl5Si7O20(OH)4 (Illite)+ 7H2O + 2 CO2 ! Aluminosilicate + Na+ + HCO3- + SiO2 " Na0.33Al2.33Si3.67O10(OH)2 (Montmorillonite)+ H2O + CO2 • Hydrothermal fluids (1979) Sink for Mg. But source of Ca, K, Si, Fe… (?) Dissolved gases in water Take a break, Grab a soda… Let’s assume we have a clear bottle of soda… You pop the lid ! gas escapes! (pressure) You shake it ! more gas escapes! (mixing) You warm it up ! even more gas escapes! (temperature) Does it stop? !When final gas content of soda is at equilibrium with temperature and pressure Dissolved gases in water Take a break, Grab a soda… soda… Density structure of the Ocean The total amount of gas that can be dissolved eventually reaches an equilibrium concentration which is proportional to: - Atmospheric concentration of gas in atmosphere - Temperature, - Pressure, - Salinity of water. The amount of gas usually $ as: P$ T% S% Oxygen in the Ocean CO2(gas) + H2O + Energy " H12C6O6 (sugar) + O2 (gas) Photosynthesis possible only in photic zone (0-200 m) Oxygen minimum (utilization) at pycnocline – O2 Minimum Dissolved gases in the Ocean CO2(gas) + H2O + Energy " H12C6O6 (sugar) + O2 (gas) Photosynthesis only in possible in photic zone (0-200 m) Oxygen limitation (utilization) at pycnocline – O2 Minimum Dissolved gases in the Ocean Temperature is the most important controlling factor! The most abundant gases in the atmosphere: - N2 (78%) - O2 (21%) - Argon (0.9%) - CO2 (0.03%) The most abundant gases in the Oceans: - CO2 (94.3%). Much more soluble! - N2 (3.4%) - O2 (2%) - Argon (0.3%) Difference due to the reactivity of CO2 is seawater leading to the various carbonate and bicarbonate equilibria Dissolved gases in the Ocean Argon and the other noble gases (He, Ne, Kr, and Ra) and Nitrogen are essentially unreactive in the Oceans: Conservative gases In contrast, CO2 and O2 concentrations are altered by many biological and chemical processes in the sea. Non-conservative gases Photosynthesis/Respiration: CO2(gas) + H2O + Energy # H12C6O6 (sugar) + O2 (gas) H12C6O6 (sugar) + O2 (gas) # CO2(gas) + H2O + Energy CO2(gas) + H2O " H2CO3 " H+ + HCO3- " H+ + CO32- Dissolved gases/nutrients in the Ocean CO2(gas) + H2O + Nutrients (N,P) + Energy " OM + O2 (gas) Dissolved gases in the Ocean CO2(gas) + H2O + Energy " H12C6O6 (sugar) + O2 (gas) Photosynthesis only possible in photic zone (0-200 m) Oxygen limitation (utilization) at pycnocline – O2 Minimum