1. Introduction 2. Ocean circulation a) Temperature, salinity, density b) Thermohaline circulation c) Wind-driven surface currents d) Circulation and

Transcription

1 1. Introduction 2. Ocean circulation a) Temperature, salinity, density b) Thermohaline circulation c) Wind-driven surface currents d) Circulation and climate change e) Oceanic water residence times 3. Seawater composition a) Major ions b) Ion residence times c) Ion removal 4. Summary 1

2 The Earth s waters constitute its hydrosphere The oceans dominate: almost 96 % of the total The deep ocean (below m) accounts for 91 % of total Solar radiation heats the earth s surface The absorbed amount (e.g., J cm -2 min -1 ) varies depending on the latitude Begon et al. (2006) 2

3 Surface mixed layer: m, o C Generated by physical mixing of the sea surface by wind waves The higher the temperature, the lower the density Region of rapid change -cline Temperature Thermocline Density pycnocline G. Ravizza, Unpublished data, Equat. Pacific Latitudinal variations Seasonal variations Tropical Temperate Polar Earthguide, UCSD 3

4 Latitudinal variations NOAA ( Originates from continental weathering and erosion, as well as release of matter from the planet s interior Average salinity: 35 psu (roughly analogous to 35 by weight) Variations in salinity are due to differences is the balance between: - precipitation, and - evaporation Ferret, NOAA/PMEL 4

5 T e m p e r a t u r e ( C ) Halocline 8 0 Depth (m) S a l i n i t y G. Ravizza, Unpublished data, Equat. Pacific If you know the temperature and salinity of seawater, the density of seawater can be calculated directly Density of water at the sea surface is typically 1027 kg/m 3. For simplification, physical oceanographers often quote only the last 2 digits of the density, a quantity they call density anomaly or Sigma (σ): σ = density kg/m 3 σ is typically 27 kg/m 3 or 27 g/cm 3 5

6 Data: World Ocean Atlas, In the polar oceans in winter, fresh water freezes out of surface waters, leaving behind saltier, colder, higher density seawater that can sink to the deep ocean (downwelling) In summer, due to ice melt, surface waters are fresher and more stable Kurt Hollocher, Union College 6

7 Because seasonal downwelling of cold polar surface waters is driven both by temperature and salinity, it is called thermohaline circulation Kurt Hollocher, Union College North Atlantic Deep Water (NADW) forms near Greenland, moves south through the deep Atlantic and then into the Indian and Pacific Oceans Upwelling occurs in the Indian, Pacific and Southern Ocean, bringing nutrients to the surface. 7

8 [Winds: see Chapter 3, Fig. 3.3 and relevant text, p ) Ultimately, wind direction, and consequently water motion, are controlled by: - friction between the atmosphere and the underlying sea surface, - the configuration of continental masses and oceanic basins Trade Winds drive surface currents E-to-W along equator Currents along the eastern side of continents (western boundary currents) are deflected toward the poles As they move pole-ward, they are deflected by the Coriolis force Mid-latitude Westerlies drive W-to-E currents Surface currents along the western side of continents (eastern boundary currents) return cold water to tropics The result is circular subtropical gyres that transfer heat from the tropics to the poles and complete the thermohaline circulation 8

9 The direction of net transport of a surface layer current is at 90 o to the direction of the wind Causes upwelling of deeper water to replace displaced water, mainly along eastern boundary currents and the equator Shown: Northern Hemisphere 9

10 Formation of deep waters may be associated with changes in global climate For example, an increase in the rate of downwelling of cold, saline N. Atlantic waters at the start of the last glacial epoch may have resulted in a lowering of atmospheric CO 2 - CO 2 is more soluble in cold waters - Atmospheric CO 2 during last glacial was 200 ppm, vs. preindustrial 280 ppm However, deep water formation depends on the contrast in temperature and density between surface and deep waters - once the glacial epoch was fully developed, the production of NADW was likely to have declined - Causing a reduction in transport of atmospheric CO 2 to the deep ocean - Allowing warmer conditions to return Residence Time (T R ) = Mass / Flux In = Mass / Flux Out Residence time with respect to river flow: T R = total ocean volume / annual river flow = 3400 years - However, most rivers mix only with surface waters, which have T R = 1700 years (with respect to river waters) - Considering rain water and upwelling input to surface waters reduces the T R even more For example, the mean age of North Pacific surface water is 9-15 y Surface waters are also in rapid gaseous equilibrium with the atm - The mean T R of CO 2 in the surface ocean is ~6 years Estimates of T r can be constrained utsing transient tracers (e.g., bomb tritium - 3 H, or bomb carbon - 14 C) 3 H 2 O dating of downwelling polar waters reveals that NADW transport toward the equator is ~10 times faster than the annual rate of riverine input to the ocean (downwelling also occurs in the Southern Ocean) 14 C dates of dissolved CO 2 range from 275 years for Atlantic Ocean to 510 years for Pacific Ocean 10

11 Downwelled volume >> riverine inflow, so TR << 3400 years Resident times (TR) for major ions are much longer than TR for water in the oceans, so they are uniformly distributed (well mixed) Law of constant proportions: they maintain the same ratio to each other in most waters, even if salinity changes 11

12 Because these elements are conservative, we can calculate total salinity from the concentration of a single ion -- typically chloride is used: Salinity = 1.81 Chloride ( ), where = g/kg of water = 35 in average seawater Mass balance of major elements in seawater (i.e., steady state): - Major element composition has remained constant for long periods of time -This requires that there be processes that remove ions from the oceans to balance new riverine inputs - Residence times for major elements (Table 9.1) vary from 120 Ma for Cl to 1.1 Ma for Ca -- a measure of how reactive an element is - The shorter T R for Ca reflects biological removal as calcium carbonate, and deposition in sediments; there is no similar removal process for Cl 12

13 Cyclic sea salts - Wind blown sea-spray forms aerosols containing seawater ions (Chap. 3) - A significant portion of river-transported Cl derives from these aerosols, returning them to the sea - This process removes ions in proportion to their concentration in seawater Ion exchange on river-borne clays entering the ocean - Most of the cation exchange sites on clays are occupied by Ca 2+ - Upon exposure to seawater, Ca 2+ is released and is replaced by other seawater cations, especially Na +, K +, and Mg 2+ - Most deep sea clays have higher Na +, K +, and Mg 2+ concentrations than riverine clays -- causes a net loss of these ions - Reverse Weathering, an old idea recently finding renewed interest Burial of dissolved ions (particularly Na + and Cl - ) in sediment pore waters Deposition of biogenic CaCO 3 controls Ca 2+ removal from seawater Biogenic SO 4 2- removal resulting from sulfate reduction and formation of pyrite (FeS 2 ), a secondary, authigenic mineral Evaporite minerals (salt flats, sabkhas) - Periodically in the geologic past, vast deposits of evaporite minerals formed when seawater evaporated from shallow, enclosed basins - Although limited in areal extent, this process has been important for Na +, Cl + and SO 4 2- removal from seawater during such periods Removal at hydrothermal vents - Particularly important for Mg 2+ (Mg-silicate formation), but also for SO

14 Summary of removal processes for the six major ions: - Most Na + and Cl - are removed in pore water burial, sea spray, and evaporites - Mg 2+ is largely removed in hydrothermal exchange - Ca 2+ and SO 4 2- are removed by deposition in biogenic sediments - K + is removed by exchange with clay minerals and reverse weathering Eventually, over long periods of time, ocean sediments are subducted into the Earth s mantle - Non-volatile components are melted under pressure and converted into primary silicate minerals - Volatiles are released as volcanic gases (H 2 O, CO 2, Cl 2, SO 4, etc.) Temperature and salinity interact to dictate water column structure through their effect on water density, which varies by latitude and season Thermohaline circulation is driven by formation of deep waters in the polar Atlantic (mainly north), and their transport from the Atlantic to the Pacific (through the Indian ocean) where they are upwelled, and returned to the Atlantic through a system of surface currents, mainly the subtropical gyres Thermohaline circulation is an important mechanism for redistributing heat energy on the Earth s surface Residence times (T R ) of elements in the ocean reflect how much time an element spends in the ocean before removal -- ranges from <100 yr to >100 x 10 6 yr T R indicates relative reactivity of elements (i.e., efficacy of removal processes) 14