9 Trace metals and other minor elements

Transcription

1 9 Trace metals and other minor elements

2 Outline 1 Analytical considerations 2 Various patterns of distribution Cadmium Nickel Selenium Manganese Lead Aluminum Germanium 3 Mercury, an interesting special case 4 Speciation (Cu) 5 Iron, another special case 6 Trace elements in sediments

3 Outline 1 Analytical considerations 2 Various patterns of distribution Cadmium Nickel Selenium Manganese Lead Aluminum Germanium 3 Mercury, an interesting special case 4 Speciation (Cu) 5 Iron, another special case 6 Trace elements in sediments

4 Difficulty of the analysis 1 µg kg 1 (a value common for some important elements) in a matrix consisting of µg of a complex mixture of salts and other substances. contamination - reagent, air, glassware, other equipment

5 Results from an intercalibration in no trace-element data could be considered entirely trustworthy until the methods and data had been duplicated in more than one laboratory. As a very powerful test, the data must make oceanographic sense.

6 The first believable data from Boyle and Edmond (1975) Cu is biologically transported and is consistent with its status as an essential constituent element in living organisms. A great deal of time and material effort is (still) required to achieve reliable results for some of the important elements.

7 GEOTRACES (Since 2010) An International Study of the Marine Biogeochemical Cycles of Trace Elements and Their Isotopes

8 Outline 1 Analytical considerations 2 Various patterns of distribution Cadmium Nickel Selenium Manganese Lead Aluminum Germanium 3 Mercury, an interesting special case 4 Speciation (Cu) 5 Iron, another special case 6 Trace elements in sediments

9 Summary of concentration and distribution for the minor elements

10 ... continued

11 Distriubtion types Conservative Li, Rb, Cs - similar to Na and K (Group 1 in the periodic table) Mo as MoO 2 4 (similar to SO 2 4 ) U - forms a stable complex with carbonate ions in seawater; relatively soluble and abundant Nutrient-like incorporated into body materials; transported downwards when organisms or their debris sink; released into the water when the organic matter is metabolically destroyed. Particle-reactive (scavenged) surface depletion

12 Cd vs. P Cd is toxic. Any biochemical function? * active center in some forms of the enzyme carbonic anhydrase * cadmium may substitutes for zinc Cd appears to be taken into marine organisms in proportionality to the total mass of tissue and regenerated at about the same rate as phosphate during decomposition.

13 Cd in the Atlantic and Pacific [Cd] [P] oceanic circulation at times in the past (Figure 9.2)

14 Nickel resembles silicon. Nickel is an essential trace element for many organisms, as it is a component of the active site of hydrogenases and some other enzymes. (Figure 9.3)

15 Two oxidation states of selenium Se(VI) is thermodynamically stable in the presence of oxygen. Se(IV) seems to be taken up by organisms. Se correlates much better with dissolved silica than phosphate. (Figure 9.4)

16 The redox cycling of selenium The preferential uptake of selenite in surface waters is indicated by a larger dissolved-to-particulate arrow.

17 Hydrothermal input of manganese * The high concentrations can only be maintained by continuous injection into the water through hydrothermal vents. * Mn (and Fe) plays important roles in electron transport in photosynthetic systems.

18 Atmospheric input of lead mostly the exhausts from cars using leaded gasoline. phasing out of leaded gasoline

19 Mid-depth minima of aluminum * dissolution of aluminum from clays in dust * scavenging at mid-depths * release at greater depths (from sediments?)

20 Ge resembles Si. Ge/Si = (Group 14)

21 Germanium in organic combination monomethylgermanium and dimethylgermanium Deep water: MMGe (330 pm), DMGe (120 pm), inorganic (100 pm) conservative mixing in esturaies just like the salt

22 Periodic table of the elements

23 Outline 1 Analytical considerations 2 Various patterns of distribution Cadmium Nickel Selenium Manganese Lead Aluminum Germanium 3 Mercury, an interesting special case 4 Speciation (Cu) 5 Iron, another special case 6 Trace elements in sediments

24 Mercury Cinnabar (ore) is red in color, soft, and easily ground into a paste, so perhaps its first use was as the pigment also known as vermilion. HgS + O2 Hg + SO2 Mercury vapor is very toxic, causing neurological damage, kidney damage, and death. argentum vivium in Roman, equivalent to quicksliver High surface tension ( 6 of water) High density ( 13.5 of water) Low vapor pressure use in thermometers and barometers

25 Three oxdation state of mercury Hg 0, Hg +, and Hg 2+ Singly charged (Hg + ): Hg 2 Cl 2, calomel used in medicine and component of electrodes Doubly charged (Hg 2+ ): the stable valence state in the presence of oxygen, HgCl 2 4 in seawater Hg 2+ bacteria monomethylmercury (CH 3 Hg + ), dimethylmercury (CH 3 HgCH 3 ) and Hg 0 in reducing conditions 2+ light Hg Hg 0

26 Monomethylmercury CH 3 HgCl in seawater is relatively stable, accumulated biologically, and toxic to mammals. In the 1950s around Minamata Bay, Japan: acetaldehyde plant mercury bacteria MMHg fish, shellfish >100 people died, and >2200 babies were affected.

27 Anthropogenic emissions of mercury usage: catalyst, artisanal gold mining, manufacture of batteries, dental amalgam (Hg-alloy), and lamps.

28 Mercury in the atomosphere 1.5 ng m 3 (>90% in Hg 0 ) 5900 t or 29 Mmol. equilibrium concentration in surface seawater: fm (Box 9.1) * Equatorial Pacific in 1984: fm * North Atlantic in 1998: average of 650 fm ( fm) * Arctic Ocean in 2008: 220 fm Hg 0 must have been diffusing into the atmosphere.

29 Evasion flux of mercury from the ocean to the atmosphere [Hg 0 ] is related to the chlorophyll a. Production of Hg 0 by bacteria and photochemistry, both mediated by organic matter Mason et al. (1994) reasoned that the evasion rates in the tropical Pacific, calculated from gas exchange equations, could be scaled to the whole ocean according to the biological productivity in the water. Global primary productivity of 23 Gt C yr 1 10 Mmol Hg 0 yr 1 A significant input to the atmosphere of 29 Mmol of standing stock!

30 Global cycle of mercury The annual fluxes to and from the atmosphere (25 Mmol per year) = the total mass of mercury in the atmosphere residence time of Hg in atmosphere is 1 year.

31 Outline 1 Analytical considerations 2 Various patterns of distribution Cadmium Nickel Selenium Manganese Lead Aluminum Germanium 3 Mercury, an interesting special case 4 Speciation (Cu) 5 Iron, another special case 6 Trace elements in sediments

32 Cu in the central North Pacific Copper (and iron, cobalt, zinc) is strongly complexed with organic matter nmol kg 1 Copper is stripped out of surface water and carried by transport on particles down to depth where it is released. A source by diffusion out of bottom sediments?

33 Growth rates of five species of marine phytoplankton The most sensitive species (3) could not grow at copper ion activities as low as mol kg 1.

34 Activity of Cu in the North Atlantic [Cu] of 1 nmol kg 1 in surface water 90% of Cu is complexed with inorgnaic anions (CO 2 3 or OH ) 10% of free ion Activity of 0.21 (similar to Ca 2+ ) activity of Cu ion mol kg 1 (= 1 nmol kg ), very close to the toxic acitivity in Figure Most of the surface water of the ocean could be marginally or lethally toxic to many of the species of phytoplankton that grow there?

35 Binding between copper ions and the organic ligands More than 99% of the copper is effectively unavailable due to the binding with (unknown) ligands. L1 binds copper extremely strongly {Cu 2+ } of M in surface water {Cu 2+ } at 500 m three order of magnitude larger Organic ligands must be made by phytoplankton in the surface water.

36 Outline 1 Analytical considerations 2 Various patterns of distribution Cadmium Nickel Selenium Manganese Lead Aluminum Germanium 3 Mercury, an interesting special case 4 Speciation (Cu) 5 Iron, another special case 6 Trace elements in sediments

37 Iron in seawater ferrous (Fe II ) and ferric (Fe III ) Fe III is stable in the presence of oxygen. Strong affinity for OH ions; Fe(OH) 2+ 1, Fe(OH) + 2, Fe(OH) 0 3, and Fe(OH) 4

38 Station Papa in the eastern North Pacific solubility of iron in organic-free seawater: nmol kg 1 Most of the iron in seawater is complexed with organic ligands Martin et al. showed that growth of phytoplankton could be enhanced by the addition of iron to samples of surface water. Fe:C in marine organisms bacteria - 1:125,000 diatoms (NH 3 ) - 1:310,000 diatoms (NO 3 ) - 2:310,000 Trichodesmium - 1:3,100

39 Annual surface mixed-layer nitrate (µmol/l) Boyd et al. (2007)

40 Fe addition to the surface waters of Ocean Station PAPA (north-east Pacific subarctic) Martin and Fitzwater (1988) The addition of nmol amounts of dissolved iron resulted in the nearly complete utilization of excess NO 3. Fe deficiency is limiting phytoplankton growth in high-nutrient low-chlorophyll (HNLC) regions. Give me half a tanker of iron and I ll give you an ice age. (John Martin, 1988)

41 Siderophores ( iron carrier ) Bacteria may acquire iron by recognizing and absorbing their own siderophore iron complex; some bacteria have evolved receptors to recognize the siderophore iron complexes of their competitors. Probably most of the iron in surface water is complexed by these specialized organic ligands.

42 Outline 1 Analytical considerations 2 Various patterns of distribution Cadmium Nickel Selenium Manganese Lead Aluminum Germanium 3 Mercury, an interesting special case 4 Speciation (Cu) 5 Iron, another special case 6 Trace elements in sediments

43 Manganese nodules 1 10 cm in diameter, roughly spherical, concentric layers Mn:Fe 1:1 on average ( manganese iron nodules) grow extraordinarily slowly (millimeters per million years) The concentric layering in the nodules suggests that the conditions for growth have changed periodically over time, but there is no evidence about what these changes might consist of.