1 / 32

Lecture 15 – Iron Ocean Chemistry

Lecture 15 – Iron Ocean Chemistry. Iron Distributions Iron Speciation and Solubility. See: Johnson, Gordon and Coale (1997) What controls dissolved iron concentrations in the world ocean? Marine Chemistry, 57, 137-161. The Biogeochemical Cycle of Iron in Seawater. Biological component.

abba
Download Presentation

Lecture 15 – Iron Ocean Chemistry

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Lecture 15 – Iron Ocean Chemistry Iron Distributions Iron Speciation and Solubility See: Johnson, Gordon and Coale (1997) What controls dissolved iron concentrations in the world ocean? Marine Chemistry, 57, 137-161

  2. The Biogeochemical Cycle of Iron in Seawater Biological component Biochemistry and Physiology Concentrations of and exchange between different Fe species Chemical component Ecosystem structure, Fe and C inputs, exports, and internal cycling Ecological/Physical/Geochemical component

  3. Vertical Profiles of Selected Metals in the North Pacific Ocean (from Butler (1998) Science, 281, 207)

  4. All Fe data as of 1997 (Johnson et al, 1997) coastal What are key features?

  5. Dissolved Fe model (Johnson et al 1997) Vertical flux of POC FC = F100 (Z/100)-b Q = Fe/C Remineralization Flux PFe = Q FC/Z = (b/100-b) QF100 Z-(1+b) = 45 Q F100 Z-1.858 Scavenging sink = RFe = kFe [(Fe) – (Fe)solubility] where Fesolubility = 0.6 nmol kg-1 Fe/t = 45 Q F100 Z-1.858 - kFe [(Fe) – (Fe)soly] + KZ 2Fe/Z2 remineralization scavenging mixing

  6. Fe at EqPac and HOT –different years; different methods

  7. Average Surface Ocean Nitrate HNLC : High Nitrate Low Chlorophyll Due to Fe limitation??

  8. Dust Transport from continents

  9. North Atlantic dust storm from the Sahara

  10. Aerosol optical depth

  11. Fe flux from dust to the ocean (Duce and Tindale, 1991)

  12. Fe dissolved EqPac results (1992) Meridional section across equator at 140°W. Is there an Fe maximum in the EUC??? What is the EUC?? Fe particulate

  13. Cruise Track – EUCFe on the R/V Kilo Moana 2006

  14. Equatorial Undercurrent during Kilo 0625 sө(EUC) = 25.0 to 25.5 ADCP From P. Dutrieux (Hawaii)

  15. Origin of the Equatorial Undercurrent (EUC)

  16. Equatorial Zonal Sections of Al, Fe and Mn nM

  17. Speciation is important!

  18. 1nm 10nm 0.1m 1m 10m 100m soluble species colloids particulates iron oxide mineral What constitutes “bioavailable” iron in seawater? Fe(III)' - Fe(OH)2+ Fe(OH)30 Fe(OH)4- Fe(II)' - Fe2+ FeCO30 organic detritus

  19. Inorganic speciation of iron – hydrolysis reactions Metals are acids For a homogeneous system: FeT = [Fe3+] + [FeOH2+] + [Fe(OH)2+] + [Fe(OH)3º] + [Fe(OH)4-] Hydrolysis Reactions Fe3+ + H2O = FeOH2+ + H+ log *K1 = -2.2 FeOH2+ + H2O = Fe(OH)2+ + H+ log *K2 = -3.4 or log *b2 = -5.6 Fe(OH)2+ + H2O = Fe(OH)3º + H+ log *K3 = -6.8 Fe(OH)3º + H2O = Fe(OH)4- + H+ log *K4 = -9.1

  20. Heterogeneous System – Mass Balance with Iron Solubility Mass balance: FeT = [Fe3+] + [FeOH2+] + [Fe(OH)2+] + [Fe(OH)3º] + [Fe(OH)4-] FeT = [Fe3+] + [Fe3+] *K1/H+ + [Fe3+] *b2/(H+)2 + [Fe3+] *b2 / (H+)3 + [Fe3+] *b4 / (H+)4 FeT = [Fe3+] { 1 + *K1/H + *b2/H2 + *b3/H3 + *b4/H4 } aFe3+ = Fe3+ / FeT Solubility product of iron oxide is written as: Fe(OH)3am = Fe 3+ + 3 OH- 3 OH- + 3 H+ = 3 H2O or Fe(OH)3am + 3H+ = Fe3+ + 3 H2O *KSO = 103.2 = [Fe3+] / (H+)3 Then: FeT = (*KSO (H)3 ) { 1 + *K1/H + *b2/H2 + *b3/H3 + *b4/H4 }

  21. Example: What is the solubility of Fe(OH)3(s) in terms of the uncharged dissolved species, Fe(OH)3º? Add two reactions: Fe(OH)3(s) + 3H+ = Fe3+ + 3 H2O log K = 3.2 Fe3+ + 3H2O = Fe(OH)3º + 3H+ log K = -12.4 ------------------------------------------------------------------- Fe(OH)3 (s) = Fe(OH)3º log K = -9.2 K = Fe(OH)3º / Fe(OH)3(s) ≈ Fe(OH)3º 0 Dissolved Fe = Fe(OH)3º = 10-9.2 M

  22. Solubility diagram for iron oxide Fe in deep seawater = 0.7 x 10-9M = 10-9.15 Solubility minimum at pH 8

  23. Other inorganic species of iron

  24. Organic Ligands (see Rue and Bruland, 1995) Organic Ligand Reactions Fe3+ + Ln- = FeL K = [FeL] / [Fe3+] [Ln-] = 1026.5 for Desferol The ligands (L) can be made by (an iron chelating siderophore) both bacteria and plankton so: FeL = K [Fe3+] [Ln-] Then: FeT = Fe’ + FeL (inorganic species) (organic species) FeT = [Fe3+] { 1 + *K1/H + *b2/H2 + *b3/H3 + *b4/H4 + KFeL [L]} Example assuming a ligand concentration Ln- = 0.44nM = 10-9.35M and pH = 8 FeT = [Fe3+] ( 1 + 105.8 + 1010.3 + 1011.5 + 1010.4 + 1017.5) Then: Fe’ = 0.0039% FeL = 99.996% Rue and Bruland (1995) analyzed natural seawater and found Fe’ = 0.03% FeL = 99.97% In surface SW most Fe is chelated!

  25. Much of dissolved iron is colloidal (from Wu et al (2001) Science, 293, 847)

  26. Concentration of soluble (<0.02mm) ligands in the eastern North Atlantic. (Wu et al (2001) Science 293, 847)

  27. Metallenzymes containing Iron and other transition Metals. From Butler (1998) Science, 281, 207.

  28. Fe redox/photo chemistry

  29. Fe – redox/photo chemistry

  30. Speciation of Major Ions in Seawater

  31. Example Calculation The MgSO4° ion pair MgSO4° = Mg2+ + SO42-log K = -2.4 = 4 x 10-3 K = (MgSO4°) / (Mg2+)(SO42-) MgT = Mg2+ + MgSO4° = 50 x 10-3 M SO4T = 28 x 10-3 M Answer: 11% of MgT is MgSO4 21.5% of SO4T is MgSO4

More Related