Diagenetic fractionation of Ge and Si in reducing sediments

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Hammond, D. E., McManus, J., Berelson, W. M., Meredith, C., Klinkhammer, G. P., & Coale, K. H. (2000). Diagenetic fractionation of Ge and Si in reducing sediments: The missing Ge sink and a possible mechanism to cause glacial/interglacial variations in oceanic Ge/Si. Geochimica et Cosmochimica Acta, 64(14), 2453-2465. doi:10.1016/S0016-7037(00)00362-8
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TitleDiagenetic fractionation of Ge and Si in reducing sediments
AuthorsD. Hammond, J. McManus, W. Berelson, C. Meredith, G. Klinkhammer, K. Coale
AbstractThe average Ge/Si ratio in the ocean is determined by the budgets for each of these elements. Previous budget formulations have assumed that the only important sink for both elements is burial as opal, based on studies of the Si cycle and the close oceanic coupling observed between inorganic Ge and Si distributions. However, these budgets implied two paradoxes: (1) hydrothermal flow through ocean ridges is smaller than predicted by other tracers, and (2) the lower Ge/Si ratio of opal deposited during glacial times compared to that deposited during interglacial times required enhanced weathering during cooler, drier climates. Both paradoxes could be resolved if a significant sedimentary sink for Ge other than opal burial could be identified, and the objective of this study was to search for one. Two pore water profiles collected in Equatorial Pacific sediments show that Ge and Si behave similarly in the upper 10 cm of sediment, indicating no evidence for a significant non-opal sink for Ge in oxic sediments. By contrast, profiles in several cores from the California Margin demonstrate that in reducing sediments, Ge diagenesis is poorly coupled to Si diagenesis: significant Ge removal is evident, both downcore and sometimes in the near-surface. Benthic flux chamber measurements at three continental slope stations, all with an oxic layer less than 1 cm thick and large iron gradients in near-surface pore waters, showed that 55 ± 9% of the Ge released by opal dissolution is sequestered. However, at two locations with anoxic sediments but little pore water Fe+2 in the upper 2 cm, flux measurements indicated little fractionation from the oceanic ratio during diagnesis, implicating the importance of iron for fractionating Ge from Si during diagenesis. If the Ge sequestration observed in the iron-rich CA margin sediments is typical of all slope sediments (using a depth range of 200-1000 m), then the Ge sink is sufficient to bring the hydrothermal budget based on Ge into concurrence with that based on other tracers. The temporal variation in oceanic Ge/Si could be explained if Ge and Si inputs remain constant and the effective diagenetic fractionation of Ge increases by a factor of 2-3 during glacial times. Increased fractionation would require that glacial periods are characterized by increased opal dissolution in iron-rich reducing sediments; this could be caused by (1) thinning of the oxygenated sediment layer in response to decreased bottom water oxygen concentrations or increased rain of organics to the sea floor, (2) increased rain of iron-rich detrital sediments in areas receiving high opal rain, (3) increased rain of opal to sediments in margin areas. If the oceanic Ge/Si ratio reflects increased rain of diatom opal or organic carbon in margin areas during glacial periods, it may indicate an increase in the efficiency of the biological pump for CO2 during glacial times. Copyright (C) 2000 Elsevier Science Ltd.
JournalGeochimica et Cosmochimica Acta
Date2000
Volume64
Issue14
Start page2453
End page2465
ISSN0016-7037
Subjectsdiagenesis, fractionation, germanium, glacial-interglacial cycle, redox conditions, seawater, silicon
NoteCited By (since 1996):38, Oceanography

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