Jan 20, 1985 - Department of Oceanography, Florida State University, Tallahassee ... of Earth Sciences, Massachusetts Institute of Technology, Cambridge.
JOURNAL OF GEOPHYSICAL RESEARCH,VOL. 90, NO. C1, PAGES 1133-1141,JANUARY 20, 1985
The Geochemistryof Inorganic Germanium in Natural
Waters
PHILIP N. FROEL!CH,GORDONA. HAMBRICK,1MEINRAT O. ANDREAE,AND RICHARD A. MORTLOCK Departmentof Oceanography, Florida State University,Tallahassee JOHN M. EDMOND Departmentof Earth Sciences, Massachusetts Instituteof Technology,Cambridge We presentthe initial resultsfrom a surveyof the distributionof dissolvedinorganicgermaniumin rivers,in the oceans,and in hydrothermalventsand plumes.In the ocean,germaniumcyclingmimics that of silica,sincesiliceousorganisms incorporateGe asa traceconstituent in biogenicopal.The vertical and horizontal distributionsof Ge concentrationsin seawaterare thus identicalto thoseof silica (Ge/Si
- 0.7 x 10'•) andreflectuptakeintoanddissolution fromthefrustules of siliceous organisms. Germanium entersthe oceanvia dissolutionof continentaland seafloorsilicates.The naturally weatheredflux from continentsto oceanscarriesa dissolvedGe/Si atom ratio in riversof about0.7 x 10'6, whichreflectsthe ratio in averagecontinentalgranites.Riversdrainingindustrialregionswith significantcoal combustion are contaminatedwith Ge leached from fly ash particles,resulting in Ge/Si ratios up to tenfold above the naturallyweatheredbackground.The flux to the oceansvia high-temperature alterationof seafloor basalts(hot springs)producesa Ge anomalyin hydrothermalplumesovertheEast PacificRisecrest(20øS).
Combiningthe observedGe and 3Heanomaliesin plumeswith 3He/Si observations in hot springssuggeststhat the Ge/Si hydrothermalflux to the oceansis about4 x 10'6,nearthe ratio for averageoceanic basalts(--2.6 x 10'6).Vent effluentsfrom the Gal•tpagos hot springsmeasured directlydisplayGe/Si ratiosof 16 x 10'6, suggesting that the ratio is variablebetweenventfields.Metalliferoussediments accumulatingon theEPR crest(17øS)are enrichedin germanium,suggesting thata fractionof thehydrothermal germanluresourceon the risecrestisscavenged into theserapidlyaccumulating iron and manganese hydroxyoxide phas,,es.
INTRODUCTION
breakthroughsin analytical methodology now permit accurate and precisemeasurementsto be made at the levelsencountered
Germanium is a trace element in the earth's crust. Due to the
in
dosesimilaritiesin sizeand chemistryof germanium and silicon
atoms, thecrustal geochemistry of germanium isdominated by a strongtendencyto replacesiliconin the latticesitesof minerals (theprincipleof camouflage[Goldschmidt,!926, 1958]).The geochemical behaviorof Ge in many low-temperatureprocesses is analogous to that expectedfor a superheavystableisotope of silicon. It is only slightly fractionated from Si during magmaticdifferentiation;is ubiquitouslydispersedin silicate rocks,clays,and detrital sediments;and with a few exceptions displays concentrations in continentalcrustalrocksand minerals
the
environment.
For
the
first
time
we
can
extend
Goldschmidt's concept of the Ge/Si coherent pair to the chemistryof natural waters resultingdirectly from dissolution of silicates in the earth's crust. In this paper we present new data which demonstrate that Ge exhibits siliconlike behavior
during continental weathering, during mid-ocean ridge hydrothermal mobilization, and during diatom uptake and dissolution in the oceans. If it is assumed that silica entering the oceansvia rivers and mid-ocean ridge hydrothermal vents is balancedby burial of biogenicopal depositedon the seafloor
[DeMaster,1981],thentheGe/Si ratioburiedin opalmayreflect
of 1-2ppmGeandGe/Siatomratiosclose to 1x 10'6[Witt-
the flux-weighted input ratios [Froelich and Andreae, 1981].
man and HSrmann, 1976, and referencestherein]. Average
Changesin relativeratesof theseprocesses overgeologictime
oceanic basalts display ratiosnear2.6 x 10-6,significantlymay haveproducedvariationsin the compositionof seawater. enrichedin Ge over continentalrocks [de Argollo and Schil- Ge/Si ratiosin opal couldthusprovidea monitorof global ling, 1978a,b]. Sincethe earth's igneousrocksare severely fluxesof elements throughthe oceansand helpconstrainmodels depleted in Ge withrespect to solarsystem abundances (Ge/Si attemptingto explain the controls on the compositionof of C1chrondrites= 120x 10'6 [AndersandEb.ihara,1982]), seawaterand its geologichistory. it isgenerally presumed that mostof theearth'sGe resides in We havepresented elsewhere resultsfor the methylgermanium theNi.Fecoreandin themantle[WittmanandHiirmann,1976; species whichwe havedetected in seawater[Froelichetal., 1983, Chou,1978]. 1984a;AndreaeandFroelich, 1984].ThesespeciesbehaveconWhile thereisa greatdealof information ongermanium in servativelyin the oceansand in estuarieswith highdiatomprorocksandmineralsin the earth'scrust,thereis almostno data ductivity, displaying a constant Ge/salinity ratioof 9.4x.1042 ondissolved germaniumdue to i•tsultra-traceconcentration mo!-Ge/permil-salinityfor monomethylgermanium (MMGe)
rail-salinity for dimethylgermanium levels in naturalwaters(
