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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