Trace Metals and Trace Elements Definition of trace elements Minor elements (< 50  mol kg -1 )

Trace Metals and Trace Elements • Definition of trace elements • Minor elements (< 50 mol kg-1) • Trace elements (< 0.05 mol kg-1) i.e. < 50 nM • The distinction is arbitrary. Assign the Boyd & Ellwood Iron Cycle paper Nature Geosciences

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Trace metal data for oceanic distributions measured prior to mid 1970’s is not reliable. • Contamination artifacts were not recognized. • Data had no “oceanographic consistency”. • Profiles could not be interpreted. • Developments in analytical capabilities by Martin, Bruland and others in the 1970’s finally allowed good data on trace metal distributions to be obtained. • New data show much better profiles which can be explained by other things we know about ocean structure and distribution of other elements. • New data revealed very low trace metal concentrations in most parts of the ocean, and ultimately to the realization that they impact productivity.

Sources and Sinks of metals in the ocean • Sources • Rivers- particulate (clays) mostly but also some dissolved. • Atmosphere- wet and dry deposition. Particularly important in gyres and areas well away from land masses and sources of atmospheric dust e.g. Equatorial Pacific, Southern Ocean near Antarctica, subarctic Pacific • Hydrothermal vents - Major source of metals, but many are immediately precipitated as metal sulfides. Reduced Fe and Mn are emitted from vents and due to relatively slow oxidation kinetics for Mn2+ this metal can be transported significant distances from the vents. Ultimate Sink • Sediments – Precipitation of metals as insoluble oxides or other minerals; adsorption of trace metals to particulates (e.g. clays) – all result in sedimentation and ultimate burial.

Enrichment factors (from Libes) Most metals are enriched in organisms as compared with seawater Exceptions are Na and Mg which are excluded from the intracellular environment.

Metals are actively taken up by biological systems for use as cofactors in enzymes etc. • Biologically active trace metals include: Fe, Zn, V, Cr, Mn, Ni, Co, Cu, Mo • Many other metals and trace elements are influenced by biological activity in some way including Cd, Se, Pb, Hg, Au, Sn, Sb, Ge, and As • Certain metals can be considered nutrients and can become limiting. They can also be toxicants whereby they inhibit biological processes such as primary production. Metal availability and chemical speciation is critical.

In some cases metals or trace elements are taken up inadvertently because of their chemical similarity to other elements. • This happens in living biomass: • Se taken for S • As taken for P • and also in hard parts (opal and CaCO3) • Ge for Si • Ra for Ca • Other elements are simply incorporated into the crystalline matrix of the hard parts i.e. Cd and Sr in CaCO3; Zn in SiO2. • The distributions of these reactive elements is influenced by these reactions!

Metals with nutrient-type distributions (except Mn) Distributions below the euphotic zone are influenced by scavenging Surface enrichment from Atmos. deposition

Cadmium displays a nutrient-type distribution (similar profile to that of PO43-) Millero

Germanium has a chemistry similar to that of Silicon, and as a result, the distribution of Ge in the ocean is similar to that of Si. Note the difference in scales for the concentrations – Si is 106-fold (a million times) higher than Ge! Fig. 3.10 in Millero. Data are from Pacific Ocean

Manganese is added to seawater at hydrothermal vents along with 3He released from the mantle Libes, Chap 11

Lead Lead (Pb) is transported in the atmosphere and deposited on the surface of the ocean, resulting in surface enrichments. It is scavenged at depth. Lead is a serious pollutant, but its concentration has diminished over the last ~25 years From Emerson & Hedges 2007 (similar to Fig 11-16 in Libes) Years since 1980

Factors affecting the cycling and fate of Metals • Controlled by: • Complexation • Uptake • Advective transport • Remineralization • Scavenging from the water column and ultimate burial in sediments. • Key chemical and biochemical reactions include: • Bioreduction/oxidation • PhotoReduction/oxidation • Methylation • Ligand binding • Surface adsorption

Dissolved Particulate Total Trace element colloidal Organic complexes Organic detrital Inorganic detrital Free Inorganic complexes Biota Colloids • Metal speciation is extremely important • governs reactivity, toxicity & nutritional function. • “Free” (uncomplexed) metals are most accessible to organisms. • Complexation (organic or inorganic) generally lowers bioavailability • Ocean waters are extremely “clean” with respect to trace metals, and even very low concentrations of trace metals can be toxic. The trace element continuum Dissolved and particulate are operationally defined!

Ligands - electron donors molecules capable of forming relatively stable complexes with cations including metals. Ligands may be organic or inorganic Organic ligands must compete for metals with inorganic ligands such as OH-, Cl-, CO32- etc. It is the relative stability constants and concentrations of the ligands which will determine which complexes will dominate the speciation of a metal. • Organic ligand include: • Siderophores • Phytochelatins • Specific Cu and Zn binding ligands in surface ocean • Humic material (amorphous organic matter with metal binding sites)

“Free” “Free” Note much lower conc. & log scale Most metals are highly chelated in seawater(i.e. low concentration of unchelated metal) Emerson and Hedges, 2007

Most metal oxides are extremely insoluble. Amorphous iron oxide (Fe(OH)3)s for example has a Ksp of 10-38.8 Ligands are responsible for keeping some trace metals in the euphotic zone. Were metals not complexed in a soluble form, they would precipitate as insoluble oxides (particles) or they would be scavenged from the water column by adsorption/packaging and vertical export. Me2+ + -L [Me2+ -L] Me(OH)n Metal oxide (insoluble) Scavenging & Sinking + OH- Soluble metal complex (longer residence time in euphotic zone)

Different degrees of surface “adsorption” for metals with solid surfaces Degree of order Surfaces could include things like clay particles, sediments, diatom frustules, colloids, chitin, viruses etc. Libes, Chap 11

-O Detrital particle O- -O -O Me2+ Me2+ Scavenging - The stability constants of metals with surfaces of clays, metal oxides, opal and organic coatings are often sufficiently high to allow “adsorption” and scavenging of the trace metal from solution. Scavenging loss rates from the water column to depth can be estimated by looking at the distribution of a particle reactive radionuclide such as Thorium-234 (234Th).

Abundant, long lived isotope, rare decays Thorium deficit as an index of scavenging -O Detrital particle O- -O -O 238U 234Th3+ 234Th3+ Sinking Export Short-lived nuclide. Abundance depends on supply by decay of 238U (parent) Deficit of 234Th is an index of removal by scavenging. 234Th serves as a proxy for all other particle reactive elements Scavenging

Scavenging Intensity Depth (m) Euphotic depth Scavenging of trace elements from the euphotic zone Scavenging intensity is highest where biological particle production is highest. This is true in the vertical and horizontal sense (i.e. its higher in coastal areas where particles are abundant and in high productivity zones). Scavenging in the deep-sea water column (>1000 m) is low and some metals are released from particles at depth See Coale and Bruland 1987. L&O 32: 189

Grazers (copepods) assimilated elements from the cytoplasm of prey with high efficiency. Elements that are in hard parts were assimilated with lower efficiency. Assimilation efficiency (%) 50 Elements in hard parts are more likely to be exported from euphotic zone in fecal pellets and other excreta. Thalasiosira pseudonana is a diatom (phytoplankter)

Role of metals in maintaining variability/diversity in the ocean. Because trace metals have short residence times in surface waters, and their input is episodic (depending on atmospheric sources, upwelling etc.) this results in changeable conditions for organisms that might be starved for, or inhibited by those metals. Such a scenario could explain why blooms of certain algae appear somewhat randomly. It may provide an environment, which on the surface appears very uniform and unchangeable, with enough variability to support diverse group of organisms. More on Fe later

Biogeochemistry of Mercury Hg • Rare in the Earth’s crust, but concentrated in ores. • Most common ore is cinnabar (mercuric sulfide, HgS). • Cinnabar forms as follows: • Hg2+ +S2- HgS (mercury in the Hg(II) form) • Heating of ore causes reduction of the Hg(II) to Hgo (elemental mercury). Hgo occurs naturally too. • Hg is present in coal and is emitted to atmosphere when coal is burned. Pure elemental mercury is liquid at room temperature. Although it as a low vapor pressure, it is somewhat volatile! Hgo can evaporate and go into the atmosphere.

Other forms of Hg in nature Hg2Cl2 (Calomel) Hg in the +I oxidation state HgCl42- inorganic chloride complex (the most common form of Hg2+ in seawater) CH3Hg+ monomethyl mercury (found mainly as mono chloride CH3Hg:Clcomplex in seawater) CH3HgCH3 dimethyl mercury HgS, HgSCH3(mercury forms strong complexes with sulfhydryl compounds, including thiols. Thiols are also known as mercaptans (meaning literally, mercury capturing).

Mobilization of Hg • Mining activities • Fossil fuel combustion (coal contains 0.5 ppm) • Industrial uses of Hg – subsequent incineration or transport results in mobilization of the Hg. • Use of barite (BaSO4) drilling muds (these contain some Hg as HgS. The ultimate fate of Hg is burial of Hg-containing particles on the sea floor. The atmosphere is the major source of Hg to the marine environment.

Hg in seawater Concentration range of 1-5 pM in water column Most is inorganic Hg. Small amounts of Monomethyl-Hg, Dimethyl-Hg and Hgo Relative concentrations in ocean water column: Hg2+ > Hgo > dimethyl-Hg > monomethyl-Hg

pM = 10-12 M Total Hg shows complex profiles with depth due to differing rates of scavenging and release from particles. All profiles show low concentrations. Pacific Ocean From Laurier et al., 2004

There is some spatial variability in total Hg concentrations in surface waters of the Pacific Ocean – but concentrations are extremely low everywhere Hg concentrations in picomolar (10-12 M) Japan Hawaii

Air Sunlight Hg0 (g) Hg(II) Algae Hg0 (g) Bacteria Phyto- plankton Hg- colloid MeHg- colloid Zoo- plankton Hg(II)- particle MeHg- particle Fish Bacteria Sediment Hg(II) CH3Hg+ HgS (s) AQUATIC CYCLING OF MERCURY Bacteria Hg(II) CH3Hg+ Water

Marine mercury cycle Libes, Chap 28 Preindustrial fluxes in parentheses =106 mol

Monomethylmercury – the key to mercury’s toxicity in animals HgCH3+ is produced by methylation (CH3 transfer to Hg) , a reaction carried out by bacteria, mainly anaerobes. HgCH3+ is concentrated in animal and plant tissue, and is biomagnified. Higher trophic levels have higher HgCH3+ content. Nearly all Hg in fish is HgCH3+

1:1 line Methyl mercury was directly related to total mercury in fish from South Florida estuaries Kannan et al 1998. Arch Environ. Contam. Tox. 34: 109

Factors affecting methylmercury production and destruction • Inorganic mercury loading • Reduction-oxidation conditions in sediments (anoxic conditions most favorable) • Chemical speciation (bioavailability) • Organic carbon availability (for bacteria) • Demethylation (bacterial and photochemical) • Temperature • Sulfate concentrations (freshwater systems)

Methyl mercury concentrations are related to total mercury Loading Benoit, Gilmour, Heyes, Mason and Miller, 2002

Hg methylation is carried out mainly by anaerobic sulfate reducing and related Fe(III) reducing bacteria HgSo HgSo Hg:ligand Enzymatic methylation via methyl B12 CH3-Hg:ligand CH3-Hg:ligand • Methylation occurs inside cells • Vitamin B12 is the proximate methylating agent • Inorganic Hg speciation determines uptake rate by cells Uptake of a neutral Hg species Hg methylation by Desulfobulbus propionicus

Hg0 Hg2+ CH4 + Hg0 Reductive Process merA & merB genes CO2 + Hg2+ CH3Hg+ Oxidative Process scavenging Oxic Water Anoxic Sediments CO2 + Hg2+ Sulfate & Iron reducing bacteria bacteria Oxidative demethylation Processes CH3Hg+ Hg2+ (Hg(HS)2) CH4 + CO2 + Hg2+ Courtesy of Tamar Barkay (via Mark Hines)

Hg:Organic Matter Kmethylation Kdemethylation CH3Hg+ HgSo Hg2+ Hg2+ The balance between Hg methylation and demethylation determines whether methyl mercury builds up. HgHS2-

Concentration Potential methylation rate The concentration of methylmercury is directly related to the potential Hg methylation rate in sediments from the Patuxent River. From Heyes et al., 2006

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Dissolved Cd concentrations are related to those of phosphate in waters below the euphotic zone. Different symbols represent different areas of the ocean. This is the same data as in Fig. 3.7 in Millero Fig 9.2 in Pilson

Zn also displays nutrient-type distribution – but with deeper regeneration pattern – similar to that of SiO2 (opal from diatoms, radiolarians etc) Millero

Synergism (simultaneous limitation by Zn, Mn and Fe is more severe than limitation by any one of these. Antagonisms The uptake of one metal may be inhibited by the presence of another (antagonism) due to competitive uptake. Competition is likely to occur at cell surface and intracellularly since chelating functionalities are never completely specific. Metals compete for binding sites. Cu may outcompete Mg which is coordinated in chlorophyll a. On the other hand, Elevated free Mn2+ can alleviate the effects of high free Cu2+ concentrations. So, it is the ratio of free metal concentrations which is critical!

History • Metal distributions and cycling in the oceans have long been of interest to geochemists and chemical oceanographers. • Early researchers suspected that certain metals might be required by phytoplankton for growth. • These early studies provided some interesting data, but were not entirely conclusive. • Other early studies (Barber and Ryther) suggested that metals in newly upwelled water might be toxic because of a lack of chelators in that “new” water

Need something on other metals Cu Zn Cd etc

Information on the oceanographic distribution and cycling of specific metals

Aluminum (Al) - Generally, low seawater concentration (40 nmol/kg in surface) even though this is one of most abundant elements on earth. • Atmospheric input (via clays etc.) in mid-latitudes therefore high concentrations at surface (low scavenging). At high latitudes, lower atmospheric input and higher scavenging give lower surface water values. • Mid depth scavenging. • Increases at depth due to sediment source.

Zinc (Zn) (bioactive - required for certain enzymes) • Total concentration is about 0.1 nM in surface waters and up to 8 nM at depth. • The profile for Zn is similar to that ofSilicic acid (silicate). • A complexing ligand for Zn is present in surface waters at a concentration of 1.2 nM (ie higher than Zn). This ligand may be responsible for complexing >98.7 % of the Zn. The ligand is uniformly distributed in upper 400 m therefore must be stable. It is presently unknown. Because of complexation, the concentration of inorganic Zn is only about 2 pM while the free, uncomplexed ion is only about 1 pM. At depth the free concentration increases up to 1400 fold! • Oceanic phytoplankton and cyanobacteria can tolerate very low levels of Zn, which is typical of their growth environment. Contrast this with neritic and coastal species which require higher levels of Zn.

Manganese (Mn) • Exists as soluble reduced Mn2+ or insoluble oxidized Mn(IV) (MnO2) • Oxidation kinetics of the Mn2+ is relatively slow - therefore it can persist metastably for considerable time. • Mn2+ forms weak complexes with inorganic ligands and exists mainly as the free ion. There is no evidence for organic complexation. • Surface enrichment due to atmospheric source. Not at all locations, however. • Mid-depth scavenging, therefore upwelled waters are low in Mn - might affect primary productivity. • Photoreduction of Mn(IV) can result in production of Mn(II) (Sunda and others). Diel cycle of Mn(II) is observed. • Mn oxides may serve as abiotic catalysts for oxidation of humic substances - this generates low molecular weight material which is metabolizable by bacteria (Kieber and Sunda).

Lead (Pb) • Strong anthropogenic influence from smelting and fossil fuels. • Higher near continents. • Aeolian inputs. • Scavenged at depth?

Trace Metals and Trace Elements Definition of trace elements Minor elements (< 50  mol kg -1 )