Transcript No Slide Title

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)
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 106fold (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
Key chemical and biochemical
reactions include:
• Bioreduction/oxidation
• PhotoReduction/oxidation
• Methylation
• Ligand binding
• Surface adsorption
 Advective transport
 Remineralization
 Scavenging from the water column and ultimate
burial in sediments.
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-, 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.
“Free”
Most metals are
highly chelated
in seawater (i.e.
Note much
lower conc. &
log scale
“Free”
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
+
OH-
Me(OH)n
Scavenging
& Sinking
Metal oxide
(insoluble)
[Me2+ -L]
Soluble metal complex
(longer residence time in
euphotic zone)
Surfaces could
include things like
clay particles,
sediments, diatom
frustules, colloids,
chitin, viruses etc.
Libes, Chap 11
Degree of order
Different degrees of
surface “adsorption” for
metals with solid
surfaces
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).
-O
-O
Me2+
Me2+
Detrital particle
-O
O-
Abundant, long lived
isotope, rare decays
Thorium deficit as an index of scavenging
-O
-O
238U
234Th3+
234Th3+
Short-lived
nuclide.
Detrital particle
-O
Sinking
Export
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
O-
Scavenging of trace elements
from the euphotic zone
Scavenging Intensity
Depth
Euphotic depth
(m)
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
Re i n f e l d e r a n d Fi s h e r , 1 9 9 1
Assimilation efficiency (%)
100
Se
P
S( st a)
S( l o g )
50
n efficiency
50
C
Zn ( s t a)
Cd
Zn
Ag
Grazers (copepods)
assimilated
elements from the
cytoplasm of prey
with high
efficiency.
Elements that are
in hard parts were
assimilated with
lower efficiency.
AM
Elements in hard
50
100
Fract ion in cyt oplasm ( %) T. Pseudonana parts are more likely
Thalasiosira pseudonana is a diatom (phytoplankter)
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:
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:Cl complex 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
The atmosphere is
Hg-containing
the major source of
particles on the
Hg to the marine
sea floor.
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
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
Sunlight
Hg0 (g)
Hg0 (g)
Hg(II)
Algae
Bacteria
Hg(II)
Bacteria
CH3Hg+
Hg- MeHgcolloid colloid
Water
Sediment
Hg(II)particle
Hg(II)
MeHgparticle
Bacteria
HgS (s)
CH3Hg+
Phytoplankton
Zooplankton
Fish
Marine mercury cycle
=106 mol
Preindustrial fluxes
in parentheses
Libes, Chap 28
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+
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
2
R = 0.40
-1
CH3Hg (ng g )
100
10
1
Freshwater Wetlands
Marine & Estuaries
Lakes
Rivers
Regression
95% Prediction Interval
0.1
0.01
0.001
10-1
100
101
102
103
104
105
106
-1
T-Hg (ng g )
Benoit, Gilmour, Heyes, Mason and Miller, 2002
Hg methylation is carried out mainly by
anaerobic sulfate reducing and related Fe(III)
reducing bacteria
HgSo
Uptake of a
neutral Hg
species
 Vitamin B12 is
the proximate
methylating
agent
HgSo
Hg:ligand
Enzymatic
methylation via
methyl B12
CH3-Hg:ligand
Methylation
occurs inside
cells
CH3-Hg:ligand
Hg methylation by Desulfobulbus propionicus
 Inorganic Hg
speciation
determines
uptake rate by
cells
Hg0
Hg2+
CH4 + Hg0
Reductive
merA & merB genes
Process
CH3Hg+
Oxic Water
Anoxic Sediments
Oxidative
Process
CO2 + Hg2+
scavenging
CO2 + Hg2+
bacteria
Oxidative demethylation
Processes
CH4 + CO2 + Hg2+
Sulfate & Iron reducing bacteria
CH3Hg+
Hg2+ (Hg(HS)2)
Courtesy of Tamar Barkay (via Mark Hines)
Hg:Organic Matter
Kmethylation
HgSo
Hg2+
HgHS2
-
Kdemethylation
CH3Hg+
Hg2+
The balance between Hg methylation
and demethylation determines
whether methyl mercury builds up.
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
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.
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 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 HPO42- 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)2o,
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
Methylation rate
constant
Demethylation rate
constant
From Heyes et al. Marine Chemistry 102 (2006) 134–147
Heyes et al 2006