Chemical Oceanography - Test 2
Distribution of Oxygen in Ocean and the processes driving it
•Near equilibration with the atmospheric O2 in the surface mixed layer. •Air injection (bubble) in surface water. •Biological production (photosynthesis). •Biological use of O2 in respiration and oxidation of plant materials. •The sinking of cold and O2 rich surface water to deeper depth.
Deviation from equilibrium concentrations (undersaturated vs. supersaturated)
- At equilibrium (saturated) 100% - Undersaturated <100% - Supersaturated > 100% Mechanisms leading to deviations from NAEC (normal atmospheric equilibrium concentration) - Bubble injection - Postequilibrium temperature change - Subsurface source (upwelling, respiration) = undersaturation - Biogeochemical process (upwelling water has low oxygen due to anoxic mud and respiration)
CO2 Releases
- Net annual CO2 release occurs at upwelling regions (equatorial zones) which brings CO2 enriched water (cold) back to sea surface (warm) - warming decreases the solubility of CaCO3 and generates CO2 by reaction Ca2+ + 2HCO3- -> CaCO3 + CO2(increase) + H2O - Warming favors calcification - Warming also decreases solubility of CO2 and forces it from the water
Influence on solubility-T
- Partial pressure of the gas. - Size of the molecule. (greater size of molecule then solubility increases) - Temperature, Kh increases as temperature decreases - Salinity, Kh decreases as S increases, salt-out - Total pressure, P, (hydraulic pressure) - at deeper depth pressure increases (a compressed balloon at a deeper depth pressure inside balloon has a higher gas concentration)
CO2 fluxes across the air-sea interface
- Surface water pCO2 varies geographically between 150 - 550 uatm - Current atmospheric xCO2 is about 404 ppm, or 390 uatm after incorporating water vapor (won't be tested though) - So surface water ranges from being undersaturated to being supersaturated with respect to the air - The disequilibria drive net fluxes of CO2 across the air sea interfaces - CO2 flux is determined by wind speed, higher latitude is windier so air exchange would be faster - It takes a couple of years to equilibriate the surface layer of the atmosphere
What causes variations in surface ocean pCo2?
- Temperature (cold water dissolves more CO2) - Salinity - DIC - Alkalinity - Biological activities The latter four factors affect the carbonate system equilibrium - About 6 months is needed to equilibriates a 40-m thick surface layer with the atmosphere - CO2 equilibriates much slower than other gases, due to the presence of carbonate system in seawater
CO2 drawdowns
- regions supporting a net uptake of Co2 are NADW, AAIW, and NPIW (polar oceans) formation sites - Gaseous equilibrium supported by movements of warm waters poleward - Drawdown supported by: - Enhanced solubility due to cooling - High wind speeds (F = G*deltapCO2) - High biological production due to high nutrients (biological process), but there's no evidence of enhanced biological uptake over the years with an increase in CO2
Preservation of calcite in sediments
1. Carbonate saturation horizon: omega = 1, a predicted value of saturation 2. Lysocline: depth at which dissolution rates are highest in sediments 3. Calcite compensation depth (CCD): where calcite <5% rain rate = dissolution rate or no preservation of calcite
Carbonate speciation - Texas shelf seawater, recent and future
1. Nevertheless, regular seawater won't turn to acidic in the foreseeable future by any account 2. Chemical balance dictates that bicarbonate and carbonic acid will increase at the expense of carbonate decline pH measred in 2012 was 8.08 predicted pH in 2100 will be 7.80 (not accounting for temperature change)
Controls on solubility (summary of trends in solubility)
1. Type of gas: Kh goes up as molecular weight goes up 2. Temperature: Solubility goes up as T goes down (Major effect) 3. Salinity: Solubility goes up as S goes down (minor effect) 4. Hydraulic pressure, partial pressure (Kh has the unit mol/L/atm), pressure increase so solubility increase
Oxygen consumption and redfield ratio
138:106:16:1 (O2:CO2:N:P) (CH20)106(NH3)16(H3PO4) + 138O2 -> 106CO2 + 16HNO3 + H3PO4 +H2O Oxygen has to be consumed/decreased for nutrients and CO2 to increase
Acids and Bases
Acids = chemical species that can donate protons Bases = chemical species that can accept protons
Oceanic solubility pump
Atmospheric CO2 uptake when the surface water cools as the current goes northward. When this water sinks to form new deep water, atmospheric CO2 is transferred to deep ocean. Keep in mind CO2 is more soluble in cold water than in warm water, like other gases. The solubility pump is the main uptake and most important process for the uptake of CO2 If seawater pCO2 is lower than it will suck up CO2
Ocean Co2 system basics - What causes ocean acidification? What is the consequence?
CO2 + H2O -> H2CO3 H2Co3 + Co32- -> 2HCO3- CO2, regardless of its source, causes acidification of water body. - Consequence is reduced carbonate saturation state and reduced pH - omega = (Ca2+)(CO32-)/Ksp Ksp-aragonite = 1.5 x Ksp-calcite - A reduction in pH and a reduction in omega means a reduction in carbonate ion
Alkalinity (Concentrations)
More than 80% of TA is HCO3, then CO3 Alk = 2248 umol/kg, DIC = 2017 umol/kg, salinity 35 at 25C CO2 or H2CO3 = 15.7 umol/kg HCO3 = 1827.5 umol/kg CO3 = 173.8 umol/kg x2 because of charge B(OH)4 = 70 umol/kg
Draw colormap (y-axis: TA and x-axis: DIC) pH vs pCO2
Carbonate formation Carbonate dissolution CO2 invasion CO2 degas Aerobic respiration Photosynthesis
Production and dissolution of carbonate and the delta values for the ocean
Carbonate formation = deltaAlk/deltaTCO2 = 2 Carbonate dissolution = deltaAlk/deltaTCO2 = 1 Delta values for the ocean: deltaAlk = 6-7%, deltaTCO2 = 11-12%. About 30-40% of the TCO2 change is due to calcium carbonate dissolution, 60-70% is due to organic carbon degradation.
OA - carbonate saturation state
Carbonate saturation state omega = (Ca)(Co3)/Ksp Ksp is the solubility constant Ksp = (Ca)eq (CO3)eq omega>1 supersaturation - most of the surface ocean omega <1 deep ocean, certain parts of marginal seas The saturation state Omega (Ω) describes the level of calcium carbonate saturation in seawater. If the saturation state for aragonite is less than 1 (Ω<1), conditions are corrosive (undersaturated) for aragonite-based shells and skeletons. If the saturation state is above 1 (Ω>1), waters are supersaturated with respect to calcium carbonate and conditions are favourable for shell formation aragonite is more soluble than calcite, thus it is a better indicator of OA - Seawater calcium ~ 10.3 mmol/kg, relatively uchanged (long residence time of calcium in ocean) - OA decreases omega by decreasing carbonate
Bjerrum plot (CO3, HCO3, H2Co3)
Decrease in carbonate with OA, then HCO3 increases, and H2CO3 increases CO2 is only 0.5% of total DIC
Henry's Law
For a diluted solution, the ideal gas vapor pressure of a volatile solute is proportional to its concentration in the solution (Kh has a unit of mol/kg/atm or mol/L/atm). Ci = Khpi Ci is the solubility and Kh is the solubility constant (Henry's law constant) Thus Kh is also the solubility of the gas when pi equals 1 atm Ci = concentration pi = partial pressure The chemical potential of gas and water have to be the same for gas to reach equilibrium At a fixed temperature, the ratio between partial pressure and concentration is a constant = Henry's Law If you change temperature then the equilibrium will change
Pressure effect--bubble
Forming gas bubbles may greatly increase the solubility due to higher total pressure
Seawater pH
In seawater CO2, a slightly different pH definition is used. That is to use total proton concentration.
O2 concentration as a function of salinity and temperature at equilibrium conditions (colormap)
Freshwater in bottom corner of colormap, GOM in blue color, arctic ocean in yellow color
Total alkalinity in seawater (numbers)
HCO3- = 1827.5, CO3- = 173.8, B(OH)4- = 70 Typically more than 80% of total alkalinity is HCO3-
Increase in CO2 (partial pressure)
If you increase CO2 in air then the ocean will uptake CO2, as it keeps increasing the ocean takes up less because the partial pressure changes
Strength of an acid or base
Measured by its tendency to donate or accept protons. A weak acid has a weak proton-donating tendency, a weak base has a weak proton accepting tendency. This tendency is measured by its dissociation constant, K. Higher K value indicate more complete dissociation
Major components of atmosphere (gases, ppm by volume)
Nitrogen (N2) = 780,840 (78%) Oxygen (O2) = 209,460 (21%) Argon (Ar) = 9,340 (1%) Carbon dioxide = 384 in 2007, 419 in 2023 All gases together equals one million O2 and N2 have long residence time and are spread across the globe
Surface ocean important for CO2 uptake
Only the surface ocean is in contact with the atmosphere and helps to control atmospheric pCO2 (CO2 partial pressure) - 1/4 of human produced CO2 has been taken up by the ocean. Atmospheric CO2 is relatively uniform over the globe, variability in air-sea pCO2 gradient deltapCO2 is mostly driven by variations in surface ocean pCO2. deltapCO2 = pCO2(oc) - pCO2 (atm)
Postequilibrium temperature changes
Solubility of gases is a function of temperature (the conservative mixing line does not always align with actual measurements because of different bodies of water with different temperatures) Mixing of two water masses creates supersaturation
Alkalinity profiles in the ocean
TA: Atlantic and Pacific have different surface TA values because of different salinity effects NTA (Net TA): TA/salinity x 35 (get rid of salinity effect) Carbonate is produced at the surface and then graudally goes into dissolution so NTA increases Pacific has higher NTA than Atlantic (more dissolution)
Total Co2 profiles in ocean
TCO2: Atlantic has higher surface TCO2 than Pacific NTCO2: both Atlantic and Pacific have same surface NTCO2 Pacific has higher TCO2 than Atlantic
Ocean acidification
The cause is the continuous increase in atmospheric CO2 - Preindustrial 280 ppm to current 420 ppm
What affects the distribution of CO2 parameters
The distributions are largely determined by water mass movement/age, organic matter decomposition and CaCO3 dissolution For O2 = -1, deltaAlk = 18/138 = -0.12 deltaTCO2 = 106/138 = 0.768 deltaNO3 = 16/138 = 0.116 deltaP = 1/138 = 0.007
Alkalinity
The excess base of the system. Or the amount of proton needed to titration a system to CO2 equivalency point; a pH of 4.5 is when all inorganic carbon species is converted to Co2
Alkalinity
The excess base of the system. Or the amount of proton needed to titration a system to Co2 equivalency point (pH = 4.5; that is when all inorganic carbon species is converted to CO2) T-Alk = C-Alk + B-Alk + P-Alk + Si-Alk + Org-Alk... = HCO3 + 2(Co3) + (OH) - H - HSO4 + (B(OH)4) + 2(PO4) + (HPO4) - (H3PO4) + (SiO(OH)3) + Org-Alk TA = the difference between total concentration of cations and anions and the sum of HCO3, CO3, and B(OH)4
pH definition
The proton level of an aqueous system. It reflects the thermodynamic states of all various acid-base systems present in the water, particularly the geochemically important carbon dioxide system and is therefore indicative of the processes involved in biological production and respiration
Partial pressure and Dalton's Law
The total pressure, Pt) of a mixture of gases in a fixed volume, V, is equal to the sum of the partial pressures of the components of the mixture. For the atmosphere: Pt = p(N2) + p(O2) + p(Ar) + p(H2O)..... = 0.781 + 0.209 + 0.009 + .... = 1.000 Add major and minor components and you will get 1atmosphere of pressure
Distribution of O2, CO2, N, P, and Si (Why are there such correlations?)
They are all tied together due to photosynthesis and primary production at the surface ocean driving high O2, and an uptake of nutrients (NO3, PO4, SiO2) and CO2 and TA, then respiration occurs as you go deeper in the ocean and organic matter gets broken down, nutrients released, CO2 respired, carbonate goes into dissolution (silicate gets released slower than other nutrients because it forms a hard surface and is not as readily broken down compared to e.g. nitrate) More nutrients in the Pacific because of upwelling Lower oxygen minimum zone in Pacific because more organic matter to respire
Oxygen minimum zone (map showing mean DO levels at 300 m below surface)
When nutrients and CO2 are high there will be low oxygen, upwelled water OMZ are expanding vertically because of climate change in the Tropical oceans (west coast of North and south america in the Pacific; eastern tropical Atlantic or west coast of africa and around India)
DIC vs pCO2
dissolved inorganic carbon = dissolved CO2, HCO3, and CO3 pCO2 refers to the partial pressure of CO2 and measures the contribution of Co2 to total gas pressure
Vertical Distribution Of O2
high at surface (3% supersaturated due to photosynthesis), decreases steeply to the oxygen minimum zone (800-1000 m depth) due to respiration, then increases with depth due to deep water formation (subsurface maximum of O2 near bottom of mixing layer depth caused by photosynthesis) Oxygen higher in Atlantic due to younger water, older respired water in the Pacific
General equations from pH and pCO2
if equilibrium at CO2 between air and sea then henry's Law drives changes in pH, if there is a change in air CO2 then the equilibrium changes
Surface water oxygen
measurements of oxygen in surface waters during GEOSECS almost always showed a small degree (~3%) of supersaturation (more oxygen than at equilibrium). This is partially due to the production of oxygen during photosynthesis, and also due to thermal cycling and to air injection increasing oxygen saturation.
Apparent oxygen utilization (AOU) or DO Graph
negative AOU at surface = supersaturation (measured oxygen greater than surface oxygen) Subsurface maximum of DO due to photosynthesis (little bump at top of graph AOU =a measure of how much oxygen has been taken up by sea life through respiration Higher AOU means higher DIC and pCO2
Global map of annual mean distribution of the sea-air difference in partial pressure of CO2 (deltapCO2 = pCO2 (oc) - pCO2 (atm))
pCO2 gradient controls the direction of CO2 flow pCO2 gradient driven by surface ocean, well mixed system, typically can take one measurement from Hawaii and calculate the gradient for the globe becasue it is uniform globally - Positive sea-to-air gradient at the equator (CO2 source) - Negative gradient would be CO2 absorption at higher latitudes of ocean
pH and pCO2 profiles in the ocean (Atlantic and Pacific)
pH and oxygen is higher in Atlantic and lower in Pacific pCO2 is lower in Atlantic and higher in Pacific Upwelling, productivity at surface and then respiration from productivity