Lecture 3Trace Metals in Seawater

What are trace elements?Why are they important?Principal of Oceanographic Consistency.Profiles shapes as clues for controlling processes.

A first look at spatial variation

What are the different “types” of elements?

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.

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

Classification of elementsConservative(or “bio-unlimited”)

Bio-limiting(and “biointermediate”) Scavenged

Some have a style of their own (e.g. O, Ar, Bi, Hg)

Shapes of Profiles – clues for controlsConservative - Cesium (Cs); Molybdenum (Mo) - under oxic conditionsNutrient Like – Biological control Shallow (soft parts) and Deep (hard parts) Regeneration

Zinc Cadmium Nickel Copper

BariumSurface Enrichment – Atm input, River/Coastal inputs

LeadManganese

Mid-depth Maximum – Hydrothermal inputs, Oxygen minimum Sediment Source

ManganeseIron

Near Bottom Enrichment – sediment sourceNorth Sea Metals (Cd, Cu, Mn)

Deep Depletion - scavengingLead-210AluminumManganese

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

Example: Comparison of vertical profiles of nutrients from the Atlantic and Pacific

PO4

ShallowSiDeep

Nutrient Like ExamplesCd, Zn, Cu, Ni

But what aboutMn, Pb ??

Ba and Si strongly correlated.

But Why??

Ba

Nutrient Like-Deep Regeneration-Hard Parts

Cd and PO4 stronglycorrelated.

But Why??

Cd

Nutrient Like-Shallow Regeneration-Soft Parts

Use the Cd-PO4 correlationas a tool to determine paleoPO4 concentrations.

Modern Data

Paleo Reconstruction

Al profilesMediterranean toAtlantic to Pacific

Al

Atmospheric InputandScavenging

Depth

(km)

Depth

(km)

Mid-depth Maximum (~200 – 1000m)

Mn

Murray et al (1981)

Dissolved

Total

Oxygen Minimum Zone - ETNP

MOR Hydrothermal System – Mid-Depth Maximum and Scavenging

Fe and Mn

Hydrothermal plume from the Juan de Fuca Ridge

Fe Mn

T anomalyparticles

Coale et al (1991) Nature, 352, 325

Mid-Depth Maximum (~2000m)

Atmospheric input

Pb in Greenland snow

Pb

Atmospheric InputAnthropogenic Origin

Pb

Surface Maximum

Flegal and Patterson, 1983

Sediment Source

High Trace Metal Concentrationson the Continental Shelf

Kremling (1983) Nature 303, 225

Cd

Cu

Mn

Si

PO4

S

pCu = - log Cu2+ Cutotal = Cu2+ + inorganic complexes + organic complexes

Metal Limitation and Toxicity – Cu – Role of Free Metal Ion

Cu SpeciationandPlankton Growth

Cu Speciation – Ocean Distributions

Total CopperStrong Organic Ligands

Free Cu2+

Total Cu

All Fe data as of 1997 (Johnson et al)

Quiz – What processes control the distribution of Fe in seawater?

Ocean Periodic Table (from Ken Johnson, MBARI)

http://www.mbari.org/chemsensor/pteo.htm

Then click on any element of interest for example profiles.