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 sw concentration ~ 14 nmolo/kg at S=35 Residence time in the ocean: ky Removal from the ocean: ** stable in oxic seawater, U(VI) as [UO 2 (CO 3 ) 3 ] 4- In anoxic sediments: U(VI) ---> U(IV) as UO 2 (s), insoluble either by -- inorganic reduction or -- microbially mediated reduction

Marine Chemistry of U, Re, Mo Rhenium: conservative in seawater sw conc. ~ pmol/kg sw residence time ~ 700 ky Removal from the ocean: ** stable in oxic sw, Re(VII) as ReO 4 - 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

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: MoO 4 2- …. Stable ? Appears to cycle with Mn ? Removal requires HS - ? Helz et al., 1996

Where do redox reactions of U, Re, and Mo fit on the “pe ladder”?

A hypothetical case: Source of RSM to sediments: diffusion from bottom water RSM 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)

==> 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 O 2 penetration ~ X* depends on BOTH bw O 2 concentration and C org rain rate !!!

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

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

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

Summary Enrichment factors and oceanic fluxes Relating enrichments to O 2 penetration Implications for oceanic budgets

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

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

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

Data examples: Me / Al ratios central California margin & Mexico / Peru

And “paleoproxy potential” For C org rain rate For BW O 2

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

Interpretation…

Under different conditions… BW O µM C org ox 400 (av. BB) (av. HB)

Comparing the 2 coastal sites

Interpreting the pore water profiles

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

A general view of RSM cycling during early diagenesis

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.