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 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.

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 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)

Surface enrichment from Atmos. deposition

Distributions below the euphotic zone are influenced by scavenging

Cadmium displays a nutrient-type distribution (similar profile to that of PO4

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 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 (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

Lead

Years since 1980

From Emerson & Hedges 2007 (similar to Fig 11-16 in Libes)

Factors affecting the cycling and fate of Metals

Controlled by:

Complexation

Uptake

Advective transport

Remineralization

Scavenging from the water columnScavenging from the water column and ultimate burial in sediments.

Key chemical and biochemical reactions include:

• Bioreduction/oxidation• PhotoReduction/oxidation• Methylation• Ligand binding• Surface adsorption

The trace element continuum

Dissolved Particulatecolloidal

Free Inorganic complexes

Organic complexes

Organic detrital

Inorganic detrital

BiotaColloids

Total Trace element

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 ligands must compete for metals with inorganic ligands such as OH-, Cl-, CO3

2- 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)

Most metals are highly chelated in seawater (i.e. low concentration of unchelated metal)

Emerson and Hedges, 2007

Note much lower conc. & log scale

“Free” “Free”

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]

+

OH-Soluble metal complex (longer residence time in euphotic zone)

Me(OH)n

Metal oxide (insoluble)

Scavenging & Sinking

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

Deg

ree

of o

rder

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).

Detrital particle

Me2+ Me2+

-OO-

-O

-O

Detrital particle

234Th3+ 234Th3+

-OO-

-O

-O

238U

Sinking

Export

Abundant, long lived isotope, rare decays

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

Thorium deficit as an index of scavenging

Scavenging of trace elements from the euphotic zone

Euphotic depth

Scavenging Intensity

Depth

(m)

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

A M

A g

Z n

C d

S ( l o g )

Z n ( s t a )

1 0 0

5 0

5 0 1 0 0

S ( s t a )

P C

S e

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

Assimilation efficiency

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

Thalasiosira pseudonana is a diatom (phytoplankter)

Ass

imil

atio

n ef

fici

ency

(%

)5

0

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 MercuryHg• 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 natureHg2Cl2 (Calomel) Hg in the +I oxidation state

CH3HgCH3 dimethyl mercury

CH3Hg+ monomethyl mercury (found mainly as mono

chloride CH3Hg:Cl complex in seawater)

HgCl42- inorganic chloride complex (the most common

form of Hg2+ in seawater)

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 atmosphere is the major source of Hg to the marine environment.

The ultimate fate 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 Hgo

Relative concentrations in ocean water column:

Hg2+ > Hgo > dimethyl-Hg > monomethyl-Hg

Pacific Ocean

From Laurier et al., 2004

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.

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

Hawaii

Japan

Hg concentrations in picomolar (10-12 M)

AQUATIC CYCLING OF MERCURY

Water

Hg0 (g)Algae

Bacteria CH3Hg+ Hg(II)Bacteria

Hg(II)- particle

MeHg- particle

Hg- colloid

MeHg- colloid

AirHg0 (g) Hg(II)

Sunlight

Sediment CH3Hg+ Hg(II)Bacteria

HgS (s)

Phyto- plankton

Fish

Zoo- plankton

Marine mercury cycle

Libes, Chap 28

=106 mol

Preindustrial fluxes in parentheses

HgCH3+ is produced by methylation (CH3 transfer

to Hg) , a reaction carried out by bacteria, mainly anaerobes.

Monomethylmercury – the key to mercury’s toxicity in animals

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+

Kannan et al 1998. Arch Environ. Contam. Tox. 34: 109

1:1 line

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

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

T-Hg (ng g-1)

10-1 100 101 102 103 104 105 106

CH

3Hg

(ng

g-1)

0.001

0.01

0.1

1

10

100

1000

R2 = 0.40

Rivers

Marine & EstuariesFreshwater Wetlands

Lakes

Regression95% Prediction Interval

Benoit, Gilmour, Heyes, Mason and Miller, 2002

HgSo

Hg:ligand

CH3-Hg:ligandCH3-Hg:ligand

HgSo

Enzymatic methylation via methyl B12

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

Methylation occurs inside cells

Vitamin B12 is the proximate methylating agent

Inorganic Hg speciation determines uptake rate by cells

Hg methylation by Desulfobulbus propionicus

Uptake of a neutral Hg species

Hg2+ (Hg(HS)2)

CH4 + Hg0

CH3Hg+

Sulfate & Iron reducing bacteria

ReductiveProcess merA & merB genes

Hg0

Anoxic SedimentsOxic Water

Hg2+

CH4 + CO2 + Hg2+

CH3Hg+

CO2 + Hg2+

Oxidative demethylationProcesses

CO2 + Hg2+ OxidativeProcess

Courtesy of Tamar Barkay (via Mark Hines)

bacteria

scavenging

Hg2+ HgSo Hg2+CH3Hg+

Hg:Organic Matter

HgHS2-

Kmethylation Kdemethylation

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

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

Potential methylation rate

Con

cen

trat

ion

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 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.

AntagonismsThe 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 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 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?

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 (NH2CONH2) 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 CaCO3 serves as proxy for ocean nutrient concentrations in paleoreconstructions

Arsenic (As)

Nutrient-type distribution. Similar chemistry to P

Ratio of HAsO42- to HPO4

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 (CH3)3AsCH2COOH 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 (HgSo, Hg(HS)2

o, HgHS2

-, HgS2--)

•

• 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

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

Methylation rate constant

Demethylation rate constant

Heyes et al 2006