HR: 17:30h
AN: V54B-07    [Abstracts]
TI: Deep Marine Sediment Diagenesis of Germanium, Silica, Lithium and Lithium Isotopes in ODP-177: The "Missing Oceanic Ge Sink"
AU: * Froelich, P N
EM: froelich@magnet.fsu.edu
AF: Philip N. Froelich, Dept. of Oceanography, National High Magnetic Field Lab, Florida State University, Tallahassee, FL 32310
AU: King, S L
EM: sk383@cornell.edu
AF: Stagg L. King, Dept. of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY 14583
AB: Understanding sediment diagenetic alterations of the crustal Ge/Si ratio is important as a clue to identifying the phases and processes responsible for burying the "missing" portion of the input Ge-flux (rivers and MOR vents) that can not be ascribed to diatom Ge/Si burial. In the ocean, in diatoms, and in most low-temperature biogeochemical processes, Ge behaves much like a trace cogenor of Si, substituting for Si and displaying Ge/Si ratios that don't deviate much from crustal (10-6 mol/mol). Significant fractionation of the ratio is now recognized for continental weathering processes and hydrothermal basalt reactions. During early diagenesis in the shallow suboxic zone of marine sediments, where non-silicate phases may dominate Ge-cycling (e.g., FeOOH), pore water [Ge] also reflects uptake into authigenic phases but seldom exceeds several hundred picomolar (10-12 Molar). In this paper we present the first deep pore water Ge data from Sub-Antarctic South Atlantic Sites drilled during ODP Leg 177 across the Southern Ocean biosiliceous ooze belt. In all sites (ODP 1088-1094), pore water Ge displays steep gradients and maxima and minima that are unrelated to opal diagenesis. Deep maxima Ge-concentrations approach 120 nM, 1000-fold higher than seawater (0.1 nM). The shapes of these profiles require local diagenetic sources and sinks for Ge that are very large, and both upward and downward fluxes that far exceed those estimated for local Ge-burial in diatomaceous opal. Thus there must be other non-biosiliceous phases that carry Ge to the seafloor that are activated during deep burial diagenesis. Temperature does not seem to be a factor: deep borehole temperatures do not exceed 46-deg C. So far we have been unable to identify these reactive phases, although we suspect some combination of chert formation and clay alteration as likely culprits. In contrast, Si profiles are featureless, exhibiting near-zero Si gradients over most intervals, with Si typical of opal saturation (1-2 mM) diluted by opal/detrital sediment ratio. Pore water lithium [Li] and del-7Li display both uptake and release profiles related to aluminosilicate clay formation and destruction. Del-7Li varies from +5 to minus 25 per mil (ref seawater), while [Li] varies from sligtly below seawater values (25 uM) to about 250 uM. The downcore data are consistent with release of Li from clay minerals carrying light (7Li depleted) cations. These Li and Li-isotope data are internally consistent, but do not shed any light on the diagenetic processes affecting deep Ge diagenesis.
DE: 4805 Biogeochemical cycles (1615)
DE: 4870 Stable isotopes
DE: 1040 Isotopic composition/chemistry
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting