In:
Biogeosciences, Copernicus GmbH, Vol. 17, No. 7 ( 2020-04-03), p. 1745-1763
Abstract:
Abstract. Benthic fluxes of dissolved silicon (Si) from sediments into the water
column are driven by the dissolution of biogenic silica (bSiO2) and
terrigenous Si minerals and modulated by the precipitation of authigenic Si
phases. Each of these processes has a specific effect on the isotopic
composition of silicon dissolved in sediment pore fluids, such that the
determination of pore fluid δ30Si values can help to decipher
the complex Si cycle in surface sediments. In this study, the δ30Si signatures of pore fluids and bSiO2 in the Guaymas Basin
(Gulf of California) were analyzed, which is characterized by high
bSiO2 accumulation and hydrothermal activity. The δ30Si
signatures were investigated in the deep basin, in the vicinity of a
hydrothermal vent field, and at an anoxic site located within the pronounced
oxygen minimum zone (OMZ). The pore fluid δ30Sipf
signatures differ significantly depending on the ambient conditions. Within
the basin, δ30Sipf is essentially uniform, averaging
+1.2±0.1 ‰ (1 SD). Pore fluid δ30Sipf values from within the OMZ are significantly lower
(0.0±0.5 ‰, 1 SD), while pore fluids close to the
hydrothermal vent field are higher (+2.0±0.2 ‰,
1SD). Reactive transport modeling results show that the δ30Sipf is mainly controlled by silica dissolution (bSiO2 and
terrigenous phases) and Si precipitation (authigenic aluminosilicates).
Precipitation processes cause a shift to high pore fluid δ30Sipf signatures, most pronounced at the hydrothermal site.
Within the OMZ, however, additional dissolution of isotopically depleted Si
minerals (e.g., clays) facilitated by high mass accumulation rates of
terrigenous material (MARterr) is required to promote the low δ30Sipf signatures, while precipitation of authigenic
aluminosilicates seems to be hampered by high water ∕ rock ratios. Guaymas OMZ
δ30Sipf values are markedly different from those of the
Peruvian OMZ, the only other marine OMZ setting where Si isotopes have been
investigated to constrain early diagenetic processes. These differences
highlight the fact that δ30Sipf signals in OMZs worldwide
are not alike and each setting can result in a range of δ30Sipf values as a function of the environmental conditions. We
conclude that the benthic silicon cycle is more complex than previously
thought and that additional Si isotope studies are needed to decipher the
controls on Si turnover in marine sediment and the role of sediments in the
marine silicon cycle.
Type of Medium:
Online Resource
ISSN:
1726-4189
DOI:
10.5194/bg-17-1745-2020
DOI:
10.5194/bg-17-1745-2020-supplement
Language:
English
Publisher:
Copernicus GmbH
Publication Date:
2020
detail.hit.zdb_id:
2158181-2
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