Dissolved Gases other than Carbon Dioxide in Seawater

Transcription

1 Dissolved Gases other than Carbon Dioxide in Seawater OCN 623 Chemical Oceanography 10 February 2015 Reading: Libes, Chapter 6 pp Frank Sansone

2 1. Basic concepts Gas laws Gas solubility and air-sea equilibrium 2. Dissolved oxygen variability in the ocean 3. Unusual gases in the ocean Hydrogen sulfide Methane hydrates Outline

3 Fundamental Gas Laws 1) C G = [G] = gas concentration (mole/l) of gas G in solution P G 2) = partial pressure (atmor kpa) of gas G in a gas mixture ora liquid 3) P = PN + PO + PAr+ PH O... (in gas phase) (Dalton s Law) Total P G = P Total (mole fraction of G in a gas mixture) In the atmosphere (at 1atm TOTAL pressure and 100% relative humidity): P P N O 2 2 = 0.78 atm = 0.21 atm P P CO CH 2 4 = atm = atm

4 4) IDEAL GAS LAW: P Total V= nrt (for a closed system) (V = volume, n = # of moles, R = ideal gas constant, T = ºK) AT STP (0ºC or273 K, 1atm) in the gaseous state: 1 mole = 22.4 L P 5) HENRY S LAW: (under equilibrium conditions) G = K G C G K G = Henry s Law Constant (a function of S and T) 1/K G = β G = Bunsen Coefficient = Amount of gas which can be dissolved in a unit of volume of ml(or moles) GAS Units: water at a given T and S, when LSW atm P G is given (assume 1 atmif not stipulated)

5 6) Setchenow Relationship ( Salting-out Effect ) for air-equilibrated water: lnβ lnβ= b 1 + (b 2 S) where b 1 and b 2 are constants for each gas for a given temp S Lower salinity -- higher solubility 7) Effect Of Salinity and Temperature: lnc G = A 1 + A 2 (100/T) + A 3 ln(t/100) + A 4 (T/100) + S [ B 1 + B 2 (T/100) + B 3 (T/100) 2 ] Lower temp -- higher solubility T = ºK S = salinity (Weiss, 1970) Constants are quoted for a given (e.g., Table 8.3) P Tot

6 Gas Solubility - Empirical Data ( ) 2 ( 100/ T) + A ln( T/100) + A( T/100) + SB + B ( T/100) B ( T/ ) lnc G = A + 1+ A where: T = Absolute temperature ( K) S = Salinity

7 Seawater Gas Concentrations in Equilibrium With the Atmosphere NAEC = normal atmospheric equilibrium concentration

8 8) % Saturation G SW = P P G( seawater) G( atmosphere) 100% = C G NAEC 100% Examples: 100% Saturation: Gas and liquid phases in equilibrium ( PG( seawater ) = PG( atmosphere) ) <100% Saturation: Gas transfer into solution (undersaturated) ( PG( seawater ) < PG( atmosphere) ) >100% Saturation: Gas transfer out of solution (supersaturated) ( PG( seawater ) > PG( atmosphere) )

9 Example: O 2 Deficit in Mediterranean Outflow Water West O 2 H 2 O East Straits of Gibraltar Hot & salty Measure T, S, [O 2 ] Loss of O 2 in Mediterranean bottom waters = NAEC Originalwatermass - [O 2 ] Measured in outflow From Table 6.2 or 8.3, T and S data Note: [O 2 ] is commonly known as dissolved oxygen or DO

10 Conversion factors for pressure: 0.1 bar = 1 decibar In seawater: 1 dbar 1 m depth

11 Vertical Distribution of O 2 Dominant trends: Sub-thermoclineO 2 minimum O 2 depletion at depth: Atlantic < Indian < Pacific High O 2 solubility in cold high-latitude water Low O 2 concsin upwelled waters

12 Horizontal Distribution of O 2 West-coast upwelling systems: Eastern Tropical North Pacific (ETNP) Peru Namibia

13 Eastern Tropical North Pacific (ETNP) Sansone et al. 2001

14 Hydrogen Sulfide a gas under certain conditions S 2- + H + HS - HS - + H + H 2 S Ion Dissolved gas Under acidic conditions: neutral species that obeys gas laws

15 Gas Hydrates (Clathrates) Hydrate: Crystalline structure of water molecules in which hydrocarbon or CO 2 gas molecules are physically trapped (not bonded) At appropriate T and P (abovethe freezing point of water) many gas hydrates are possible (up to iso-butane, C 5 ) Of special interest: Methane hydrate

16 Methane Hydrate Stability-Field Diagram Dissolved Open-ocean temperature profile (HOT 100)

17 Largest reservoirs: East coast of North America Arctic Ocean East coast of Australia

18 Shelf / Slope Methane Seeps U.S. Atlantic Coast A. Skarke and C. Ruppel, Nature Geoscience (2014)

19 (<180 m)

20 NOAA Okeanos Explorer Program Methane bubbles from exposed methane hydrate

21 Importance of Methane Hydrates Energy source: Blake Plateau (off SE U.S.) alone contains enough methane to meet US natural gas needs for 100 years Methane is a potent greenhouse gas Climatic change or tectonic events may lead to catastrophic methane release via positive feedback: Increased global temp causes hydrate decomposition Release of methane to atm increases global warming

22 Homework due: Tuesday, Feb 17, ) Calculate (as opposed to look up in a table) the concentration of nitrogen in surface seawater (S = 35, T = 25 C) when in equilibrium with the atmosphere. Express your results in units of moles/l, moles/kg, and liters(stp)/l. Show your calculations!! 2) The solubility of methane in 25 C seawater with a salinity of 40 is 1.34 mmol/kg. The solubility in 25 C water with a salinity of 0 is 1.58 mmol/kg. Calculate the solubility of methane in 25 C seawater with a salinity of: a) 15 b) 30