Particulate trace metals Phoebe Lam Marine Bioinorganic Chemistry lecture November 22, 2011

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 holders 142mm 293mm 47mm

Sampling for sinking particles PIT-style surface-tethered sediment trap, adapted for trace metal clean collection (Carl Lamborg) Using 234 Th/ 238 U disequilibrium and particulate 234 Th: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 2001 Dissolved profiles from N.Pacific Sherrell and Boyle 1992 Particulate 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) At BATS, similar profiles: Generally low at the surface, increasing to max at 500m Authigenic Mn as host phase for scavenged metals? (not everywhere, see next slide) Nepheloid layers in most pTMs (Mn, Co, Zn, Ni), but not Pb, Cu, and not nearly as strong as for Fe, Al Mn CoPbZnCuNi

Mn, Co, (Pb,) Zn, Cu, Ni: the “Minor minors (pM)” Ohnemus et al. unpublished GEOTRACES data Off NW Africa, “minor minors” group differently: pMn and pCu min at surface, subsurface max (more like lithogenic) pCo, pZn, pNi max at surface (biogenic)

Lithogenic Tracers Bulk comparisons within the data Ohnemus et al. unpublished GEOTRACES data

Lithogenic contribution to pTMs (Sherrell and Boyle, 1992) % particulate Al: <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

Motivation for size-fractionation: Particle dynamics (via geochemistry, USGT NAZT stn 11) Depth (m) Part. PhosphorusPart. Barium Production of 1-51um P by photosynthesis near surface >51um P near surface from aggregate formation μm-sized barite particles formed in micro- environments of aggregates near surface  max in >51um Ba near surface As organic matter remineralizes (gradient in P), aggregates fragment and release barite into small size fraction Maximum in remineralization in P coincides with max in 1- 51um Ba Suspended (1- 51um) size fraction Sinking(>51u m) size fraction Unpublished US GT NAZT data from Dan Ohnemus

Modelling scavenging and removal (I) Small suspended particles Large fast- sinking particles Remineralization (biogenic particles only) Aggregation/repa ckaging disaggregation “Activity” on large particles “Activity” on small particles Dissolved “activity” adsorption desorption remineralization Aggregation/repa ckaging disaggregation sinking Marchal and Lam, submitted; after Murnane et al. 1990

Modelling scavenging and removal (II) (Sherrell and Boyle, 1992) Use slope of particulate 230 Th profile to estimate the mean particle sinking speed, S; Residence time, τ p =D/S F T =F S +F R F R =(Me P *D)/τ p = Me P *S How much of total flux (F T ) is due to sinking from the surface (F S ) vs. repacking (F R ) in the water column? D Suspended particle residence time

Modelling scavenging and removal (III) (Sherrell and Boyle, 1992) -repackaging flux (F R ) 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 (F S )

Pools of particulate trace metals Biological Surface adsorbed Authigenic particles Lithogenic 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) Biological Surface adsorbed Authigenic particles Lithogenic particles

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 71m 1 mm Red=Fe Blue=Ca Lam et al. GBC 2006 Silicoflagellate (scale bar = 20 um) c/o Ben Twining Cellular scaleAggregate 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 Clay Olivine Hydroxide Organic Fe

Chemical species mapping combines μXRF and XAS 7105eV 7117eV 7124eV7180eV 7105eV: background defined for all species 7117eV: pyrite (iron sulfide) has a distinct shoulder and is significantly higher than biotite (Fe(II)-silicate) or Ferrihydrite-2L (Fe(III)) 7124eV: Fe(III) is significantly lower than Fe(II) or pyrite 7180eV: all species are equal 7105eV 7117eV7124eV7180eV Lam et al. GCA, in revision

Northwest Pacific, Stn K2 550km from coast Eastern Tropical Atlantic Stn 3 550km from coast Fe(II) is much more important in the NW Pac Fe(III) is much more important in the E. Atl Total pFe Fe(III) Fe(II) Pyrite Acid-leachable pFe Total pFe Fe(III) Fe(II) Pyrite Acid-leachable pFe Lam et al. GCA, in revision

Chemical maps of potential endmembers Mauritanian sediments 39%pyrite, 20%Fe(II), 41%Fe(III) Saharan Dust 1%pyrite, 14%Fe(II), 85%Fe(III) Pyrite is a unique tracer of sedimentary source of Fe Lam et al. GCA, in revision

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