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

http://www.lab-initio.com/screen_res/nz015.jpg

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 Mn 2+ 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.

Most metals are enriched in organisms as compared with seawater

Exceptions are Na and Mg which are excluded from the intracellular environment.

Enrichment factors (from Libes)

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 CaCO 3 ) 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 CaCO 3 ; Zn in SiO 2 .

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 PO 4 3 )

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 10 6 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 3 He released from the mantle

Libes, Chap 11

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) Lead Years since 1980

Factors affecting the cycling and fate of Metals

Controlled by:  Complexation  Uptake Key chemical and biochemical reactions include: • • • • • Bioreduction/oxidation PhotoReduction/oxidation Methylation Ligand binding Surface adsorption  Advective transport  Remineralization 

Scavenging from the water column

burial in sediments. and ultimate

The trace element continuum

Total Trace element

Dissolved Free Inorganic complexes Organic complexes

colloidal

Colloids Particulate Organic detrital Inorganic detrital Biota Dissolved and particulate are operationally defined!

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.

Ligands

- electron donors molecules capable of forming relatively stable complexes with cations including metals. Ligands may be organic or inorganic

Organic ligand include:

 Siderophores   Phytochelatins Specific Cu and Zn binding ligands in surface ocean  Humic material (amorphous organic matter with metal binding sites)

Organic ligands must compete for metals with inorganic ligands such as OH , Cl , CO 3 2 constants and etc. It is the relative stability concentrations of the ligands which will determine which complexes will dominate

the speciation of a metal.

Most metals are highly chelated in seawater

(i.e. low concentration of unchelated metal)

“Free”

Note much lower conc. & log scale

“Free”

Emerson and Hedges, 2007

Most metal oxides are extremely insoluble. Amorphous iron oxide (Fe(OH) 3 ) s for example has a K sp 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.

+ Me 2+ + L OH [Me 2+ -L] Soluble metal complex (longer residence time in euphotic zone) Me(OH) n Scavenging & Sinking Metal oxide (insoluble)

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

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 ( 234 Th). Me 2+ O Me 2+ O O Detrital particle O -

238 U

Abundant, long lived isotope, rare decays

234 Th 3+ Thorium deficit as an index of scavenging O O O 234 Th 3+ O Detrital particle

Short-lived nuclide.

Abundance depends on supply by decay of 238 U (parent)

Deficit of 234 Th is an index of removal by scavenging. 234 Th serves as a proxy for all other particle reactive elements Sinking Export Scavenging

Scavenging of trace elements from the euphotic zone Depth (m)

Scavenging Intensity Euphotic depth

See Coale and Bruland 1987. L&O 32: 189

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

1 0 0 R

e i n f e l d e r a n d F i s h e r , 1 9 9 1

P C S ( l o g ) S ( s t a ) C d Z n ( s t a ) Z n S e A g A M 5 0 1 0 0 F r a c t i o n i n c y t o p l a s m ( % ) T . P s e u d o n a n a

Thalasiosira pseudonana

is a diatom (phytoplankter) Grazers (copepods) assimilated elements from the

cytoplasm

of prey with high efficiency. Elements that are in hard parts were assimilated with lower efficiency.

Elements in hard parts are more likely to be exported from euphotic zone in fecal pellets and other excreta.

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: Hg 2+ +S 2  HgS (mercury in the Hg(II) form) • Heating of ore causes reduction of the Hg(II) to Hg o (elemental mercury). Hg o 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

! Hg o can evaporate and go into the atmosphere.

Other forms of Hg in nature

Hg 2 Cl 2

(Calomel) Hg in the +I oxidation state

HgCl 4 2-

inorganic chloride complex form of Hg 2+ in seawater) (the most common

CH 3 Hg +

monomethyl mercury chloride

CH 3 Hg:Cl

(found mainly as mono complex in seawater)

CH 3 HgCH 3

dimethyl mercury

HgS, HgSCH 3

(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 (BaSO 4 ) drilling muds (these contain some Hg as HgS.

The ultimate fate The atmosphere is the major source of Hg to the marine environment. of Hg is burial of Hg-containing particles on the sea floor.

Hg in seawater

Concentration range of 1-5 pM in water column Most is inorganic Hg. Small amounts of Monomethyl-Hg, Dimethyl-Hg and Hg o Relative concentrations in ocean water column: Hg 2+ > Hg o > 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

Hg concentrations in picomolar (10 -12 M)

Japan

There is some spatial variability in total Hg concentrations in surface waters of the Pacific Ocean – but concentrations are extremely low everywhere

Hawaii

AQUATIC CYCLING OF MERCURY Air Hg 0 (g) Sunlight Hg(II) Water Hg 0 (g) Algae Bacteria Hg(II) Hg(II) particle Bacteria Hg colloid MeHg colloid CH 3 Hg + MeHg particle Phyto plankton Zoo plankton Fish Bacteria Sediment Hg(II) CH 3 Hg + HgS (s)

Marine mercury cycle

=10 6 mol

Preindustrial fluxes in parentheses Libes, Chap 28

Monomethylmercury

– the key to mercury’s toxicity in animals

HgCH 3 + is produced by

methylation

(CH 3 transfer to Hg) , a reaction carried out by bacteria, mainly anaerobes. HgCH 3 + is concentrated in animal and plant tissue, and is

biomagnified

. Higher trophic levels have higher HgCH 3 + content. Nearly all Hg in fish is HgCH 3 +

Methyl mercury was directly related to total mercury in fish from South Florida estuaries

1:1 line

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

1000 R 2 = 0.40

100 10 1 0.1

0.01

0.001

10 -1 10 0 10 1 Freshwater Wetlands Marine & Estuaries Lakes Rivers Regression 95% Prediction Interval 10 2 10 3 T-Hg (ng g -1 ) 10 4 10 5 10 6 Benoit, Gilmour, Heyes, Mason and Miller, 2002

Hg methylation is carried out mainly by anaerobic sulfate reducing and related Fe(III) reducing bacteria

HgS

Uptake of a neutral Hg species

o CH 3 -Hg:ligand HgS o Hg:ligand

Enzymatic methylation via methyl B12

CH 3 -Hg:ligand  Methylation occurs inside cells  Vitamin B12 is the proximate methylating agent  Inorganic Hg speciation determines uptake rate by cells

Hg methylation by Desulfobulbus propionicus

Hg 0 Hg 2+ Oxic Water Anoxic Sediments CO 2 + Hg 2+

bacteria Oxidative demethylation Processes

CH 4 + Hg 0

Reductive Process merA & merB genes

CH 3 Hg +

Oxidative Process

CO 2 + Hg 2+ scavenging

CH 3

Sulfate & Iron reducing bacteria

Hg +

Hg 2+ (Hg(HS) 2 ) CH 4 + CO 2 + Hg 2+

Courtesy of Tamar Barkay (via Mark Hines)

Hg: Organic Matter Hg 2+ HgS o

K

methylation CH 3 Hg +

K de

methylation Hg 2+ HgHS 2 The balance between Hg methylation and demethylation determines whether methyl mercury builds up.

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

End

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 SiO 2 (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 Mn 2+ can alleviate the effects of high free Cu 2+ 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 of Silicic 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 Mn 2+ or insoluble oxidized Mn(IV) (MnO 2 )  Oxidation kinetics of the Mn 2+ is relatively slow - therefore it can persist metastably for considerable time.  Mn 2+ 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?

Cobalt (Co)

 Present in cyanocobalamin (vitamin B12), a methyl carrier in biochemistry.  Present at only 4-50 pM in North Pacific. Could be biolimiting.  A required growth factor for some species. Uptake may be enhanced by organic complexation (as with Fe).  Recent evidence for a cobalt binding ligand in seawater, similar to that of Cu and Zn ligands.  Prymnesiophytes have a higher Co requirement than diatoms. Required for production of methylated compounds?

Nickel (Ni)

 Nutrient-type distribution.  2-12 nM total dissolved concentrations.  Possible role of Ni in urea (NH 2 CONH 2 ) assimilation. Ni is present in urease.  If all Ni is available in ocean then not likely limiting. However, if some is complexed, it could be limiting. Could be important where regeneration is active since urea is excreted more under those circumstances.

Cadmium

(Cd) 

Potentially toxic in coastal areas due to anthropogenic sources.



Nutrient profile in open ocean - like Phosphate.



Cd incorporation into CaCO paleoreconstructions

3

serves as proxy for ocean nutrient concentrations in

Arsenic (As)

 Nutrient-type distribution. Similar chemistry to P  Ratio of HAsO 4 2 to HPO 4 2 is >1 in oligotrophic surface waters, therefore As may be toxic to phytoplankton by replacing P.  This may be why some organisms methylate As.  One form of methylated As is

arsenobetaine

(CH 3 ) 3 AsCH 2 COOH which is found in a variety of organisms – especially lobster!

Deep water has very little Fe, therefore upwelling supplies little. The exception to this is right along the equator (Coale et al, 1996) where the equatorial undercurrent, which originates near Papua New Guinea, contains relatively high (0.3-0.4 nM) Fe and which upwells the major fraction of Fe into the surface waters. Despite this relatively large flux (as compared with other locations), it is insufficent to remove all the nitrate and phosphate also brought up with the water. Only 20% of these macro nutrients can be utilized at this location given an assumed C:Fe ratio of 167,000:1.

(This ratio differs from that given by Bruland et al (1991)- because they chose this higher ratio to better approximate oceanic phytoplankton C:Fe quotas - much uncertainty here!!).

Mercury - Sulfide Chemical Speciation

• Mercury Forms Polysulfide Species (HgS o , Hg(HS) 2 o , HgHS 2 , HgS 2 - ) • • The Solubility of HgS Increases as Sulfide Levels Increase Benoit, J.M., C.C. Gilmour, R. P. Mason and A. Heyes (1999). Sulfide controls on mercury speciation and bioavailibility to methylating bacteria in sediment pore waters.

Environmental Science and Technology

,

33

: 951-957.

Patuxent Estuary

From Heyes et al. Marine Chemistry 102 (2006) 134 –147

From Heyes et al. Marine Chemistry 102 (2006) 134 –147

Methylation rate constant Demethylation rate constant

From Heyes et al. Marine Chemistry 102 (2006) 134 –147

Heyes et al 2006