Particulate trace metals

Phoebe Lam

Marine Bioinorganic Chemistry lecture

October 5, 2009

outline

• Why are particles important

• How do we sample for particulate trace metals (suspended, sinking)

• Techniques for analysis

• Sample profiles (bulk)

• Sample profiles (speciation)

Why are particles important to trace metal (TM) cycling?

• Source of lithogenic TMs (dust, mobilization of continental margin and benthic sediments)

• Participate in internal cycling of TMs: release some TMs into solution, provide surfaces for scavenging TMs out of solution; biological uptake and remineralization

• Are the ultimate sink of dissolved trace metals (vertical particle export and removal to sediments)

Sampling for suspended particles

McLane battery-operated in-situ pump: <1000L, size fractionated

MULVFS: Multiple Unit Large Volume in-situ Filtration System (ship power): <12,000L, 3 flow paths, size fractionated(Jim Bishop)

GO-Flo filtration: 10L, size fractions hard

Gas line to over-pressure

47mm or 25mm filter holder goes here

142mm filter holder142mm

293mm

47mm

Sampling for sinking particles

PIT-style surface-tethered sediment trap, adapted for trace metal clean collection (Carl Lamborg)

Using 234Th/238U disequilibrium and particulate 234Th:TM ratios (Weinstein and Moran 2005)

The basic analysis: applying crustal ratios to total digests

• Total digests using (sub)boiling strong acids with HF to dissolve aluminosilicates

Sherrell and Boyle, 1992, after Taylor 1964 GCA

Nutrient(-like) dissolved profiles have mirror image particulate profiles

Nozaki 2001Dissolved profiles from N.Pacific

Sherrell and Boyle 1992Particulate profiles, BATS

Al, Fe: The “Major minors (nM)”

Dissolved Al, Fe from BATS in 2008 (GEOTRACES IC1, Bruland website)

Al Fe

Particulate Al, Fe from BATS in 1987 (Sherrell and Boyle, 1992)

Dissimilar dissolved profile shapes but similar particulate profile shapes--increase until ~1000m, then constant until nepheloid layer at bottom

Strong nepheloid layers with concentrations 7x higher than water column profile

Mn, Co, Pb, Zn, Cu, Ni: the “Minor minors (pM)”

(Sherrell and Boyle, 1992)

•Similar profiles: Generally low at the surface, increasing to max at 500m•Authigenic Mn as host phase for scavenged metals?•Nepheloid layers in most pTMs (Mn, Co, Zn, Ni), but not Pb, Cu, and not nearly as strong as for Fe, Al

Lithogenic contribution to pTMs

(Sherrell and Boyle, 1992)

% particulateAl: <10%Fe: ~50%Mn: <25%Co: <10%Zn: <5%Cu: <5%Ni: <5%Cd: <5%Pb: <5%

•Lithogenics are strong sources for Al, Fe everywhere, moderate for Mn and Co, not at all for Zn, Cu, Ni (?), Cd, Pb•Fe has the highest %particulate

Modelling scavenging and removal (I)

(Sherrell and Boyle, 1992)

Use slope of particulate 230Th profile to estimate the mean particle sinking speed, S; p=D/S

FT=FS+FR

FR=(MeP*D)/p= MeP*S

How much of total flux is due to sinking from the surface vs. repacking in the water column?

Modelling scavenging and removal (II)

(Sherrell and Boyle, 1992)

-repackaging flux (FR) provides ~30% of total flux out of surface (except Cd: 80%, Zn: 10%); i.e. Most of total flux due to flux out of surface (FS)

Pools of particulate trace metalsBiologicalSurface adsorbedAuthigenic particlesLithogenic particles

Simplified Fe cycle

Terrigenous(clays (dust), oceanic crustal material, volcanic sediments)

Biota

Atmospheric deposition

Dissolved(Fe-L)

Lateral transport (from rivers, continental margin) Authigenic

(hydroxides)

Uptake/scavenging

Sinking

Dissolved Pool

Particulate Pool

Remineral-ization

How to distinguish between different pools??

Leaching methods (not exhaustive!):“biogenic”: weak acid+mild reductant+heat (Berger et al. 2007); total-lithogenic (Frew et al. 2006)“surface adsorbed”: oxalate wash (Tovar-Sanchez et al. 2004)“authigenic”: mild reductant+acid (eg. Poulton and Canfield 2005) “lithogenic”: strong acid digest (w/ HF) and crustal Al:TM ratio (eg. Sherrell and Boyle 1992; Frew et al. 2006)

BiologicalSurface adsorbedAuthigenic particlesLithogenic particles

Transformation between pools?

Frew et al. 2006

Frew et al. applied a crustal Al:Fe ratio to total pFe (HNO3/HF) to partition between “lithogenic” and “biogenic”. Surface samples were 80% “lithogenic”; trap samples were only 50% “lithogenic”Conclude biologically-mediated conversion of “lithogenic” to “biogenic” pFe

X-Ray Fluorescence (XRF) microprobe: spatial distribution of elements

Incident

x-rays Sample

Detector

Fluor-escent x-rays

Wikipedia

•Incident beam of 10keV

Synchrotron X-Ray microprobe: spatial distribution of pTM

71m1 mm

Red=FeBlue=Ca

Lam et al. GBC 2006

Silicoflagellate (scale bar = 20 m)

c/o Ben Twining

Cellular scale Aggregate scale

Speciation from X-Ray Absorption Spectroscopy: valence

EXAFS region

Energy (eV)Absorption

XANES region

Position of edge depends on valence

Energy (eV)

Absorption

Fe

Speciation from X-Ray Absorption Spectroscopy: mineralogy

Fe

EXAFS region

Energy (eV)Absorption

XANES region

ClayOlivineHydroxideOrganic Fe

Chemical mapping combines XRF and XAS

all 3 species more or less equal at 7160eV

at 7122eV, Fe3+ is significantly lower than Fe2+ or pyrite

at 7117eV, pyrite is signicantly higher than Fe2+ or Fe3+

This set of energies minimizes error estimates

7105: everyone is low7117: pyrite only is high7122: pyrite,Fe2+ are high (Fe3+ is low)7160: everyone is high

Figure 3: Preliminary x-ray fluorescence maps showing the relative abundance of Fe3+ oxides (blue), Fe2+ silicates (green), and pyrite (red) in end member (aerosol and sediment core) samples. Aerosol samples are a mix of Fe3+ oxides and Fe2+ silicates, whereas core top samples have abundant pyrite. Scale bar is 200um.

PyriteFe2+Fe3+1-21k0.1-20.1k.7-20.7kGamma=0.69

SIA14C aerosol--OUT SIRENA Core Top--OUT

SIM87T d11 125m--IN

SIM84T d10 20m--IN

SIM89T d11 200m--IN, low RGB

12 3 4 65

PyriteFe2+Fe3+1-21k0.1-20.1k.7-20.7kGamma=0.69

References