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Lecture 3Trace Metals in Seawater
What are trace elements?Why are they important?Principal of Oceanographic Consistency.Profiles shapes as clues for controlling processes.
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A first look at spatial variation
What are the different “types” of elements?
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Trace elements in seawaterDefinition: Those elements that do not contribute to salinityAll elements less than 1 mg kg-1. But usually use mole units!
Why are they important?1. many are micronutrients (e.g. Fe, Cu) – speciation is important2. others are toxic (e.g. Cu, Hg)3. some are tracers for redox conditions (Mn, Fe, Cr, I, Re, Mo, V, U)4. some are enriched in economic deposits such as manganese nodules (e.g. Cu, Co, Ni, Cd)5. some have man made sources and are tracers of pollution (e.g. Pb, Pu, Ag)
** Difficult to collect samples for without contamination and difficult to analyze.
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Oceanographic consistencyAcceptance of data must satisfy two criteria:
1. Vertical profiles should be smooth, not spiky. Ocean mixing produces smooth profiles2. Correlations should exist with other elements that share the same controlling mechanisms.
First Example – Cu in surface waters south of New Zealand (Boyle and Edmond, 1975, Nature, 253, 107)
SST
-
Si
PO4
NO3
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Classification of elementsConservative(or “bio-unlimited”)
Bio-limiting(and “biointermediate”) Scavenged
Some have a style of their own (e.g. O, Ar, Bi, Hg)
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Shapes of Profiles – clues for controlsConservative - Cesium (Cs); Molybdenum (Mo) - under oxic conditions
Nutrient Like – Biological control Shallow (soft parts) and Deep (hard parts) Regeneration
Zinc Cadmium Nickel Copper
Barium
Surface Enrichment – Atm input, River/Coastal inputsLeadManganese
Mid-depth Maximum – Hydrothermal inputs, Oxygen minimum Sediment Source
ManganeseIron
Near Bottom Enrichment – sediment sourceNorth Sea Metals (Cd, Cu, Mn)
Deep Depletion - scavengingLead-210AluminumManganese
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Superposition of vertical biological flux on horizontal circulation
Results in low surface water and highdeep water concentrations.
Results in higher concentrations inthe older deep Pacific than the younger deep Atlantic
Nutrient Like Profiles
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Example: Comparison of vertical profiles of nutrients from the Atlantic and Pacific
PO4
ShallowSiDeep
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Nutrient Like ExamplesCd, Zn, Cu, Ni
But what aboutMn, Pb ??
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Ba and Si strongly correlated.
But Why??
Ba
Nutrient Like-Deep Regeneration-Hard Parts
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Cd and PO4 stronglycorrelated.
But Why??
Cd
Nutrient Like-Shallow Regeneration-Soft Parts
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Use the Cd-PO4 correlationas a tool to determine paleoPO4 concentrations.
Modern Data
Paleo Reconstruction
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Al profilesMediterranean toAtlantic to Pacific
Al
Atmospheric InputandScavenging
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Depth
(km)
Depth
(km)
Mid-depth Maximum (~200 – 1000m)
Mn
Murray et al (1981)
Dissolved
Total
Oxygen Minimum Zone - ETNP
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MOR Hydrothermal System – Mid-Depth Maximum and Scavenging
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Fe and Mn
Hydrothermal plume from the Juan de Fuca Ridge
Fe Mn
T anomaly
particles
Coale et al (1991) Nature, 352, 325
Mid-Depth Maximum (~2000m)
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Atmospheric input
Pb in Greenland snow
Pb
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Atmospheric InputAnthropogenic Origin
Pb
Surface Maximum
Flegal and Patterson, 1983
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Sediment Source
High Trace Metal Concentrationson the Continental Shelf
Kremling (1983) Nature 303, 225
Cd
Cu
Mn
Si
PO4
S
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pCu = - log Cu2+ Cutotal = Cu2+ + inorganic complexes + organic complexes
Metal Limitation and Toxicity – Cu – Role of Free Metal Ion
Cu SpeciationandPlankton Growth
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Cu Speciation – Ocean Distributions
Total CopperStrong Organic Ligands
Free Cu2+
Total Cu
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All Fe data as of 1997 (Johnson et al)
Quiz – What processes control the distribution of Fe in seawater?
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Ocean Periodic Table (from Ken Johnson, MBARI)
http://www.mbari.org/chemsensor/pteo.htm
Then click on any element of interest for example profiles.
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