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Redox-sensitive trace metals

Redox-sensitive trace metals. Interest in them as “paleo-proxies” for BW [O 2 ] and/or C org rain rate to the sea floor Because They are chemically stable in oxic seawater, but are insoluble in anoxic sediments. Marine Chemistry of U, Re, Mo. Uranium: conservative in seawater

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Redox-sensitive trace metals

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  1. Redox-sensitive trace metals Interest in them as “paleo-proxies” for BW [O2] and/or Corg rain rate to the sea floor Because They are chemically stable in oxic seawater, but are insoluble in anoxic sediments

  2. Marine Chemistry of U, Re, Mo Uranium: conservative in seawater sw concentration ~ 14 nmolo/kg at S=35 Residence time in the ocean: 250 - 500 ky Removal from the ocean: ** stable in oxic seawater, U(VI) as [UO2(CO3)3]4- In anoxic sediments: U(VI) ---> U(IV) as UO2 (s), insoluble either by -- inorganic reduction or -- microbially mediated reduction

  3. Marine Chemistry of U, Re, Mo Rhenium: conservative in seawater sw conc. ~ 39 - 45 pmol/kg sw residence time ~ 700 ky Removal from the ocean: ** stable in oxic sw, Re(VII) as ReO4- In anoxic sediments, reduction ---> insoluble species Note: solid phase Re enrichments in anoxic sediments easy to measure because of very low Re in terrigenous material

  4. Marine Chemistry of U, Re, Mo Molybdenum: conservative in seawater sw conc. ~ 105 nmol/kg sw residence time ~ 800 ky Removal from seawater: oxic sw: MoO42- …. Stable ? Appears to cycle with Mn ? Removal requires HS- ? Helz et al., 1996

  5. A hypothetical case: Source of RSM to sediments: diffusion from bottom waterRSM loss from pore water at a given depth At steady state: accumulation rate of solid phase metal = loss rate from pw Loss rate = Accum rate (A) = 3 x Accum rate (B)

  6. ==> IF source of RSM is diffusion from bottom water there is no postdepositional recycling of solid phase RSM THEN accumulation rate of RSM reflects depth at which it precipitates in the sediments. What determines depth of precipitation? ….. Sedimentary redox zonation e.g., depth of O2 penetration ~ X* depends on BOTH bw O2 concentration and Corg rain rate !!!

  7. Evidence from solid phase measurements - 1Morford & Emerson (1999) GCA 63, 1735-1750 Mo resembles Mn U, Re enrichment, O2 pen <1 cm

  8. Note use of local detrital values! Mo: little if any enrichment U, Re : enriched, esp. Re

  9. Mo enriched at anoxic site U, Re: highly enriched, & enrichment ihighest at anoxic site

  10. Summary Enrichment factors and oceanic fluxes

  11. Solid phase data -2Crusius et al., 1996, EPSL 145, 65-78 B. Pakistan margin Traversing O2 minimum A. Japan Sea

  12. Re/Mo ratio as indicator of “suboxic” conditions ? “suboxic” “intermittently anoxic” “anoxic”

  13. Solid phase data -3Post-depositional remobilization?Crusius et al. (2000) GCA 64, 2233-2242 Top of profile = top of turbidite Horizontal line = O2 penetration Higher resolution core

  14. Interpretation…

  15. Pore Water Data -1Uranium - continental slope sedimentsKlinkhammer & Palmer, 1991, GCA 55 1799-1806 Is there pore water evidence supporting Conclusion that U is removed from the Ocean in “suboxic” sediments?

  16. U removal from pore waters & enrichment In solid phase Station J Station M

  17. Pore water data - 2U removal vs. bottom water O2 and Corg ox.McManus et al., 2005 Study in the California borderland basins Sediment cores + in situ benthic flux chambers

  18. Pore water profiles -- shallower removal as BW O2 decreases -- apparent source = diffusion from bottom water

  19. In situ benthic flux chamber results Small signal, but in flux in reasonable agreement with fluxes calculated from pore water profiles Conc. In olw in chamber ? Time -- hours

  20. Interpretation… Removal rate increases as BW O2 decreases Removal rate increases as Corg ox rate and rain rates increase

  21. Pore water data - 2Comparative study: U, Mo, ReMorford et al., in progress Hingham Bay BW O2 = 250-325 Corg ox ~ 1600 -- Removal depths: U <~ Re < Mo -- consistent with diffusion from BW Removal confirmed by benthic Chamber data --- note Mo Removal rate > U rate

  22. Is H2S necessary for Mo removal? Low level of H2S present At apparent depth of Mo removal

  23. Mo -- evidence for cycling with MnSan Clemente Basin, CA Shaw et al., 1990 GCA 54, 1233-1246

  24. More pore water data(a less extreme coastal environment) Buzzards Bay -- BW O2 ~ 230-300, Corg ox ~ 600 Removal depths similar to Hingham Bay -- Re at interface? -- Remobilization of Mo and U at depth?

  25. Is diffusion from BW the only source of U to sediments for authigenic U formation?“Particulate non-lithogenic U”Zheng et al., 2002, GCA 66, 3085-3092 Santa Barbara Basin

  26. Particulate non-lithogenic Uranium PNU: well-preserved when BW O2 < 25 µM; not preserved in oxic sediments ? Makes up 40-70% of authigenic U in Santa Barbara Basin sediments? … contrast to McManus result

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