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Chemical Oceanography

Chemical Oceanography. Lecture 3: 5/30/2014. Salinity. Definition: weight of inorganic salts in one kg of seawater There are many ions and salts in seawater, but they are never the dominant mass. Inputs. Outputs. Weathering: the physical & chemical processes that break down rock.

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Chemical Oceanography

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  1. Chemical Oceanography Lecture 3: 5/30/2014

  2. Salinity • Definition: weight of inorganic salts in one kg of seawater • There are many ions and salts in seawater, but they are never the dominant mass

  3. Inputs Outputs

  4. Weathering: the physical & chemical processes that break down rock

  5. A simplified biogeochemical cycle

  6. Steady State and Equilibrium • Draw on board

  7. Acidity • pH = -log[H+] • Dissociated water molecule H2O = H+ + OH- In 1L of water (55.6 moles) 10-7 moles dissociated; therefore, 10-7 moles/L of both H+ and OH-(i.e. pH = 7, pOH = 7) • pH < 7 = acidic, pH > 7 alkaline

  8. Seawater Buffering, Alkalinity • Alkalinity = measure of the amount of ions present that can react with, or neutralize, H+ • Higher alkalinity of a solution  more difficult to produce a pH change by adding acid • Alkalinity measures acid buffering capacity • Simple measure of Alkalinity (A) A = [HCO3-] + 2[CO3-] + [OH]- - [H+] Assumes bicarbonate, carbonate, hydroxyl ions dominate seawater alkalinity

  9. Seawater Buffering, Alkalinity • More substances can react with [H+] From Pilson 1998

  10. Seawater Carbonate Buffer System Two important carbon reactions pertain to primary production: CO2 + H2O  CH2O + O2 (consumes acid) Ca+2 + HCO3-  CaCO3 + H+ (produces acid) CO2 (g)  H2CO3 (aq)  HCO3-  CO3-2  Corg CaCO3 Air Sea – photic zone Sea – aphotic zone ‘export’ Ecology influences the net effect of biology on the air-sea transfer!

  11. H2CO3 – a diprotic weak acid Thermodynamic Constants KH = pCO2/{H2CO3} K1 = {H+}{HCO3-}/{H2CO3} K2 = {H+}{CO3-2}/{HCO3-} ‘Apparent’ Constants K1’ = K1H2CO3/HCO3- = {H+}[HCO3-]/[H2CO3]  10-6.0(@25oC, I=0.7) K2’ = K2 HCO3-/CO3-2 = {H+}[CO3-2]/[HCO3-]  10-9.1(@25oC, I=0.7) • How can system be defined uniquely? • pCO2 (open system) • pH (≡ -log aH+) • SCO2 (mass balance) • Alkalinity (acid-neutralizing capacity) 3 Equations but, 5 unknowns!

  12. mass balance constraint CO2= [H2CO3] + [HCO3-] + [CO3-2] ~1% ~90% ~9% i.e. DIC Respiration CH2O + O2  CO2 + H2O Dissolution CaCO3 + H+  Ca+2 + HCO3- CO2

  13. Total Dissolved Inorganic Carbon DIC, i.e. SCO2 (mmol/kg)

  14. Total Alkalinity (mmol/kg)

  15. Discospaera sp., another coccolithophorid Emiliania huxleyi, a coccolithorophorid planktonicforaminifera These organisms all make skeletal material from calcium carbonate – calcite in some cases, aragonite in others Both CaCO3 sponge spicules bryozoa stalks pteropods

  16. Centric diatoms – an alga Both make a skeleton based on the element Si – ‘biogenic silica’ or SiO2 Radiolarian – a protozoan

  17. Solubility of Calcite versus Aragonite CaCO3 (s) Ca+2(aq) + CO3-2 (aq) Ksp* = [Ca+2]saturated + [CO3-2]saturated Ksp* calcite (e.g., foraminifera, coccolithophorids): 3.3 x 10-9 aragonite (e.g., coral, pteropods): 4.6 x 10-9 Biogenic Silica (e.g. diatoms, radiolarian): 2.0 x 10-3 Q: What is more soluble – CaCO3 or SiO2? Q: Which form of calcium carbonate is more soluble?

  18. Dissolution of biogenic particles • Solubility also is a function of temperature and pressure • In the deep ocean, CaCO3 becomes very soluble • Carbonate Compensation Depth (CCD) • Below CCD calcium carbonate is under-saturated (like SiO2) • Decrease in pH also can increase calcium carbonate solubility • CCD is a dynamic depth (NOT fixed)

  19. Nutrients • In oceanography, “nutrient” refers to important and commonly measured element needed for growth of plants • Includes the major nutrients (i.e. macronutrients): • Phosphorus • Nitrogen • Silicon

  20. Phosphorus Cycle: global Ruttenberg, 2001 (Encyclopedia of Ocean Sciences)

  21. Phosphorus • Forms of occurrence in seawater • Inorganic phosphate (i.e. orthophosphate) • No major redox state differences • Nearly all dissolved phosphorus present in deep sea • Organic phosphorus • Phospho- … -lipids, -proteins, -carbohydrates • Nucleic acids & nucleotides • Phosphonic acid derivatives • Polyphosphates • Wide variety of straight-chain, branched and cyclic polymeric forms • Sorption affects bioavailability • Fe oxy-hydroxides, Carbonate-mineral sorption • Redox sensitivity • Low Dissolved oxygen induces phosphate release from sediments (VERY IMPORTANT IN Gulf of Mexico and adjacent estuaries)

  22. Distribution of Dissolved organic phosphorus (DOP)and Soluble Reactive Phosphorus (SRP)

  23. Nitrogen in the marine environment Gruber (Ch 1) in Nitrogen in the Marine Environment 2nd Ed (2008)

  24. Nitrogen acquisition • Chemical forms of nitrogen and their major characteristics Oxidized Reduced

  25. Major Chemical forms/transformations Gruber (Ch 1) in Nitrogen in the Marine Environment 2nd Ed (2008)

  26. Gruber (Ch 1) in Nitrogen in the Marine Environment 2nd Ed (2008)

  27. Global Mean Profiles Gruber (Ch 1) in Nitrogen in the Marine Environment 2nd Ed (2008)

  28. BIAS ALERT! Behold … the world’s most awesome element

  29. Silicon • Second most abundant element in earth’s crust • 25.5% of crust by weight (Oxygen is 49%) • Si-O chemical bond one of most abundant • In seawater Si is relatively scarce ~0.0003 atom% • In diatoms (a phytoplankton group beloved by your instructor) = 5.0 atom % • Some vertebrates = 0.001 atom%

  30. Current view of the marine Si cycle • Tréguer and De La Rocha Annu. Rev. Mar. Sci. 2013 • NOTE: • No major gas phase • No major organic Si pool • UNITS: Tmols Si year-1

  31. Dissolved silicate • At seawater pH • >97% Si(OH)4 (orthosilicic acid) • Dominant form transported by diatom (Del Amo and Brzezinski 1999, Journal of Phycology) • pH 8.7-8.9 • 14-23% ionic (Si(OH)3- • May be transported across the membrane but typically much lower rates (Reidel et al. 1984 Journal of Phycology)

  32. Ocean Chemical Tracers • Tracer conservation equations establish the relationship between the time rate of change of tracer concentration at a given point and the processes that can change that concentration (Sarmiento and Gruber 2006) • Processes include: • Physical transport (advection, mixing) • Sources and sinks (biological and chemical transformation) • Examples: chemical ocean tracers • AOU = apparent oxygen utilization • Chlorofluorocarbons (CFC) • Carbon 14

  33. AOU • Apparent Oxygen Utilization • AOU = [O2]saturated – [O2]measured • Difference between measured oxygen and what equilibrium saturation (as a function of the physical/chemical characteristics) • From biological activity • Oxygen increased by primary production • Oxygen used by respiration

  34. Apparent Oxygen Utilization AOU = [O2]saturated – [O2]measured Which locations have the highest AOU at depth? Lowest? Why?

  35. AOU and Preformed Nutrients • Preformed nutrients: those initially present at the time of downwelling = total nutrient – regenerated nutrient - Calculated using AOU • Characteristic of waters originating from different regions • Hence use as tracer ‘Preformed’ Nutrient AOU Phosphate

  36. Preformed P (top) & Preformed N (bottom) From Sarmiento & Gruber 2006 From Broecker et al. 1985

  37. CFC

  38. Manmade compounds (where are highest values?) • High radiative forcing (relative to CO2) • 12,400x higher for CFC-11 • 15,800x higher for CFC-12 • Useful as ocean tracers (i.e. only manmade source is from atmosphere)

  39. Natural vs Anthropogenic 14C Production Industrial Revolution Burning 14C-dead Coal! “Suess Effect” Tree Ring Records Nuclear Weapons Testing! Test Ban Treaty – 1963! 14C now decreasing Coral Records -

  40. surface waters (-50‰) contain more 14C than deep waters • deep waters in the Atlantic contain more 14C than those in • the Pacific while those in the Indian Ocean and Antarctic • have intermediate values.

  41. Radiocarbon age – do trends look familiar?

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