1 / 25

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

natasha
Download Presentation

Redox-sensitive trace metals

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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 identify 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. Where do redox reactions of U, Re, and Mo fiton the “pe ladder”?

  6. 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)

  7. ==> 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 !!!

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

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

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

  11. Summary Enrichment factors and oceanic fluxes Relating enrichments to O2 penetration Implications for oceanic budgets

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

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

  14. U and Mo accumulation:”paleoproxy potential” Bw O2 33, 56, 133 Cox 25 - 90 Bw O2 4-52, + 133 Cox 15-70; Corg 1-4% Bw O2 0 Cox 40-90 Corg 6.6-8% Bw O2 <10-70, + 148 Cox 90-365 Corg 1.5-2.5% McManus et al., 2006: Measure: (1) solid phase U and Mo conc., (2) sed. MAR, in regions where bw [O2] is known, and sed. Corg ox. Rate has been measured

  15. Data examples: Me / Al ratioscentral California margin & Mexico / Peru

  16. And “paleoproxy potential” For BW O2 For Corg rain rate

  17. 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

  18. Interpretation…

  19. Under different conditions… BW O2 240-370 µM Corg ox 400 (av. BB) - 850 (av. HB)

  20. Comparing the 2 coastal sites

  21. Interpreting the pore water profiles

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

  23. A general view of RSM cycling during earlydiagenesis

  24. Conclude… Authigenic RSM accumulation is quite closely related to: Organic matter oxidation rate And somewhat on bottom water oxygen concentration Because: The accumulation of U and Re appears to require O2 depletion by organic matter oxidation in sediments The accumulation of Mo appears to depend on the occurrence of sulfate reduction Caveat: There may be - at least under well-oxygenated bottom water - complicated cycling of RSM between solid and dissolved phases, plus dependence on irrigation, that affect the ultimate accumulation rates.

More Related