Ocean Chemical Dynamics. EAS 2200 Lecture 21

Transcription

1 Ocean Chemical Dynamics EAS 2200 Lecture 21

2 Summary of Water s H-bonding accounts for high heat capacity, latent heats of fusion and evaporation. important for heat transport, transfer to atmosphere, and thermal buffering Polar nature of molecule accounts for its high dissolving power. Temperature and salinity determine density of seawater

3 Seawater Composition

4 Other Constituents of Seawater dissolved organic carbon (DOC) compounds sugars, proteins, amino acids, nucleic acids suspended particulate materials inorganic compounds fragmented products of rock weathering organic compounds living - phytoplankton, bacteria, larger organisms dead - detrital materials Dissolved gases CO 2, O 2, N 2, noble gases, etc.

5 Seawater Salinity The dissolved salt content of seawater is called salinity. Average salinity of seawater is 3.5% or 35 ( = parts per thousand). Why is the sea salty?

6 Seawater ph ph is an important property of any solution, governing much of its behavior. ph is defined as: Negative logarithm of the hydrogen ion concentration. ph = -log [H + ] Range of possible ph values is 0 to 14. In pure water, ph = -log [10-7 ] = 7. Seawater has a ph of 7.6 to 8.2. Seawater is mildly alkaline. Why?

7 Rinsings of the Earth Antoine Lavoisier, father of modern chemistry, said the oceans are the rinsings of the Earth. In other words, the sea is salty, and alkaline, because it contains the weathering products of the continents, delivered to the oceans by rivers. Antoine Lavoisier ( )

8 Weathering Igneous Rock + Water = Sediments + Seawater 2NaAlSi 3 O 8 +3H 2 O = Al 2 Si 2 O 5 (OH) 4 +4SiO 2 +2Na + +2OH albite (NaSpar) + water = kaolinite + quartz + solution Weathering of rock on continents generates both solids (such as clays and quartz) and an alkaline solution. The solution is then carried to the ocean by rivers. This is why the sea is salty and why it is alkaline.

9 Ion Dissolved Salts in Parts per thousand by weight Percent of Dissolved Solids Seawater Ave. Rivers (parts per million) Cl SO HCO Na Mg Ca K

10 The River Water Paradox Seawater is approximately a sodium chloride solution; river water is approximately a calcium bicarbonate solution. If rivers deliver salts to the sea, why are their compositions so different? Was Lavoisier wrong?

11 Another Paradox At present rates, it would take rivers about 90 million years to deliver all the salt now in the oceans. Yet, the oceans are over 4 billion years old. Furthermore, the oceans are not getting saltier with time. Why? Dissolved salts are also being removed from seawater. Seawater composition is controlled by both the rate at which elements are added and the rate at which they are removed.

12 Processes Controlling Seawater Composition Components that make up seawater are dynamic, not static. There is a continuous input and loss of these components from seawater inputs seawater losses cyclic processes

13 The Steady-State Ocean Evidence that over the last 100 Ma, the composition of seawater has remained ~ constant Therefore, salts must be removed from seawater in the same proportion and at the same rate at which they are being delivered. If the oceans are not getting saltier we may say they are in steady-state. inputs seawater losses cyclic processes

14 Concept of Residence Time Residence Time is the average time an atom of an element will spend in seawater. Residence Time = Amount dissolved in ocean Rate at which it is delivered

15 The Hydrologic Cycle

16 Residence Time of Water Residence time of water in seawater average depth of the ocean 4000 m average rate of evaporation 0.1 m/year Residence time of water 4000/0.1 = 40,000 years Residence time of major ions in seawater residence time of Cl - = 68,000,000 years residence time of Na + = 260,000,000 years Residence time of water is >100X shorter than for ions Salinity controlled by evaporation and precipitation

17 Residence Times of Some Elements Element Amount in Ocean (g) Residence Time (years) Na (sodium) ,000,000 K (potassium) ,000,000 Ca (calcium) ,000,000 Si (silicon) ,000 Mn (manganese) Fe (iron) Al (aluminum)

18 The Cyclic Salts Some elements, most notably anions such as Cl and SO 4 2-, are not released from rock by weathering (because they are not in the rock in the first place). They are present in volcanic gases and have been slowly added to the oceans by volcanic eruptions. They nevertheless cycle through the hydrologic system (oceans atmosphere rivers oceans). Cyclic salts find their way into atmosphere as breaking waves produced small droplets; water evaporates, leaving salt particles.

19 Sources and Sinks Sources refer to the mechanisms by which elements are added to seawater. Sinks refer to the mechanisms by which elements are removed from seawater.

20 Sources and Sinks of Sea Salts Sources Rivers (and glaciers) Atmosphere gases Dust (delivered by wind) Reaction with oceanic crust Low temperature weathering and high-temperature hydrothermal systems. Dissolution of sediments Sinks Biological precipitation Direct precipitation Evaporites, Mn nodules Reaction with oceanic crust

21 Sinks: Biological Precipitates Much of ocean floor covered by biological precipitates

22 Sinks: Direct Precipitation Manganese Nodules are a good example of direct precipitation Actually a mixture of hydroxides of iron (Fe) and manganese (Mn) and of other metals Adsorption onto clay particles also removes Mn and Fe (forming the red clays of the deep ocean bottom).

23 Sinks: Evaporites form when seawater in a closed basin evaporates until salt precipitates Calcium carbonate first to precipitate Gypsum precipitates after 80% evaporation Halite forms after 90% evaporation This forms valuable minerals deposits (e.g., table salt, road salt, gypsum for plaster and sheetrock.) Ancient (Paleozoic) evaporites underlay Ithaca. 6 million years ago, the Mediterranean became closed off and almost completely

24 Salt Salt is essential as a nutrient and as a chemical reagent And in modern times as a road deicer. Salt can be mined from ancient evaporites or directly extracted from seawater - e.g., San Francisco Bay (below), Mediterranean.

25 Salt in History Salt has been a valuable resource throughout history. Salt was a commodity perhaps the first commodity - in the earliest trading relationships - because it was essential and scarce, it help drive the development of civilization. Prehistoric human settlements in Europe and elsewhere were often located around salt deposits (e.g., Halle in Germany, Salzburg in Austria). Salt industry and trade often controlled by monarchies the monarch had the exclusive right to mine and sell it. Salt from upstate New York was a primary economic reason for building the Erie Canal. Syracuse has the nickname salt city.

26 Salt Mining in New York Salt deposits underlie much of Michigan, Ohio, Pennsylvania, and New York. Major salt mine in Lansing, 6 mi north of Cornell; extends under Cayuga Lake (at a depth of 2300 feet). This salt precipitated in the Silurian (420 Ma ago) when a shallow arm of the ocean dried up.

27 The Messinian Salinity Crisis 5.9 million years ago (late Miocene), the convergence of the African and Eurasian plates closed the Strait of Gibraltar. Consequently, much of the Mediterranean Sea dried up. Mediterranean sea level dropped 3000 m. Evaporites, gypsum and halite deposited widely on Mediterranean Sea floor.

28 Evaporites in Mediterranean Sediments The Deep Sea Drilling Project found vast salt deposits on the Mediterranean seafloor. Salt is sometimes 2000 m thick!

29 Mediterranean Water Today, 1000 cubic miles of water are evaporated yearly from the Mediterranean (fresh water influx only 1/10 of this). At this rate, it would take only 1000 years for the Mediterranean to evaporate! As much as 6% of the ocean s salt precipitated in the Mediterranean. Loss of this salt reduced ocean salinity elsewhere, producing an ecological crisis and climatic crisis. Messinian Salt deposits in Italy.

30 The Messinian crisis ended (630,000 years after it began) as a channel through Gibraltar was eroded. At that time, a huge waterfall, with 1000 times greater flow than Niagara, formed in The Mother of All Waterfalls

31 Hydrothermal Activity: Both Sink and Source Hydrothermal reactions remove some elements, such as Mg and U, from seawater, and add other elements, such as Si and K.

32 Distribution of Elements in the Ocean Elements can be divided based on their distribution into 3 groups: Conservative Biologically Controlled Surface Reaction Controlled

33 Conservative and Nonconservative Conservative properties are those that are only altered at the sea surface. Once the water leaves the surface, these properties are conserved. Salinity Temperature Conservative ions inert gas concentration Nonconservative properties of seawater are those that can be altered anywhere in the water column by: biological processes geochemical processes

34 Conservative Elements Includes most of major ions in seawater Na + (sodium), Ca 2+ (calcium), Mg 2+ (magnesium), K + (potassium), Cl - (chlorine), SO 4 2- (sulfate), but not bicarbonate. Includes minor ions as well such as Rb + and F Always present in constant proportions to one and other and to salinity. Concentrations change only as a result of dilution or concentration

35 What can concentrate or dilute dissolved ions? Concentration by evaporation dilution by rain Concentration by freezing dilution by ice melting Dilution near coasts by river water

36 Biologically Controlled Components Elements and ions taken up by phytoplankton in surface waters. Released back into water by decomposition at depth. Includes nutrients (e.g., nitrate, phosphate), micronutrients (e.g., silicate, iron, zinc), and elements accidentally taken up, e.g., Ge). Consequently, deep water is enriched in nutrients compared to surface water. Oxygen and CO 2 (mostly in the form of bicarbonate) controlled by photosynthesis and respiration.

37 Nutrients Nutrients - elements or compounds required by phytoplankton to grow and reproduce nitrogen: NO 3 - (nitrate), NH 4 + (ammonium) phosphorus:po 4 3- (phosphate) silicon: SiO 4 4- (silicate) trace metals:fe, Zn, Mo, Cu, Co, etc. Needed in small quantities, some times referred to as micronutrients.

38 Nutrient Distributions Nitrate NO 3 - (µm) Phosphate PO 4 3- (µm) Depth (m) 2000 Depth (m) Atlantic Pacific Atlantic Pacific

39 Biological Utilization Si utilized by diatoms to build their tests (shells); consequently, it is depleted in surface water. Ge (Germanium) chemically mimics Si and is inadvertently taken up by diatoms. Pacific deep water is richer in Si than Atlantic deep water because it is older.

40 Surface Reaction Controlled Elements with low solubilities that are quickly adsorbed onto particle surfaces and removed from solution. Such elements delivered to oceans in wind-blown dust, hence are enriched in surface waters. Elements in this category, such as Al and Pb, have very short residence times. Many elements are controlled by a combination of biological utilization and surface reaction.

41 The Carbonate System

42 Importance of carbonate system reactions Buffers the ph of seawater to around 8. Addition of H + will drive reactions to right: H + + HCO 3 - H 2 CO 3 H + + CO 3 2- HCO 3 - Buffers atmospheric CO 2 Atmospheric CO 2 dissolved in seawater is converted to bicarbonate ion. CO 2 + H 2 O H + + HCO 3 - Solubility of CO 2 in ocean water is greatly enhanced as a consequence.

43 Global Carbon Cycle

44 Systematics of CO 2 in Oceans CO 2 gas (1.0) atmosphere (0.1) CO 2 dissolved + H 2 O H 2 CO 3 (0.3) ocean respiration (0.01) organic C photosynthesis biogenic precipitation HCO 3 - CO H + + H + (50) (6) organic C carbonate sediments mineral precipitation (10000) (30000) sediments

45 Seawater chemistry and marine sediments Carbonate sediments (e.g., foraminiferal ooze) are restricted to relatively shallow parts of the ocean. Why?

46 Calcium Calcite (calcium carbonate) solubility increases with depth because of: Decreasing ph because of increasing total dissolved CO 2 Increasing pressure Lysocline Depth where calcite dissolution increases rapidly Carbonate Compensation Depth Depth where rate of dissolution just

47 CCD: The Marine Snowline

48 Distribution of Carbonate Sediment CCD controls distribution of carbonate oozes: they occur only where depths are less than 4500 m.

49 Dissolution of silica shells SiO 2 tests of diatoms and radiolaria are most soluble in surface water (where dissolved silicate is lowest). Diatom and radiolarian oozes found only beneath high productivity areas where dissolved silicate and production rate of tests is highest.

50 Distribution of siliceous Siliceous sediment found beneath equatorial and southern ocean high productivity belts. Rare in Atlantic because water is young and poor in dissolved

51 Bottom Line: Seawater chemistry influences the distribution of life in the oceans and the distribution of sediments on the sea floor. Seawater chemistry is part of a much larger chemical cycle. This cycle is linked to other geologic processes, including plate tectonics. These processes control, and sometimes change, Earth s climate.