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| Titel |
Silicon and oxygen isotopic trends in Mesozoic radiolarites |
| VerfasserIn |
Maximlien Bôle, Peter O. Baumgartner, Lukas Baumgartner, Anne-Sophie Bouvier, Hori Rie, Ikeda Masayuki |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250134584
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| Publikation (Nr.) |
 EGU/EGU2016-15328.pdf |
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| Zusammenfassung |
| Silicon and oxygen isotopes (δ30Si and δ18O) of siliceous tests (diatoms, sponges and
radiolarians) preserve environmental signatures in unconsolidated sediments, but few studies
show such signatures for ancient biosilicieous rocks. In Precambrian cherts from greenstone
belts, small scaled isotopic variations were interpreted as a primary diagenetic feature. They
were used, coupled to mean δ18O, to reconstruct seawater temperature at which cherts
precipitated. Here, we examine stable isotopes in Mesozoic biogenic cherts that may also
preserve an environmental signature.
We measured δ30Si and δ18O in situ by SIMS, in the chalcedony of individual radiolarian
tests preserved in Mesozoic radiolarites. Microanalysis of chalcedony, rather than the bulk
rock isotopic composition, is likely to reveal a palaeoenvironmental signal, since it is derived
from biogenic opal, the most mobile silica phase during earliest diagenesis. Our data reveal
clear trends through several Mesozoic radiolarite sections from Panthalassa (Kiso River,
Japan) and Western Tethys (Sogno, Italy).
δ18O records measured in radiolarites show a relatively good correlation to
δ18O-variations of Mesozoic low magnesium calcite shells, which are commonly used as a
palaeotemperature proxy. Once these variations, attributed to seawater temperature, are
removed, the residual δ18O trends are opposite to the δ30Si trends. δ30Si increases from
Middle Triassic to Early Jurassic in the Kiso River sections and decrease during
the Middle Jurassic in the Sogno section. The observed d30Si-trends are likely to
represent a palaeoenvironmental signal, because they are not compatible with simple
models of progressive diagenesis along P/T-paths (or depth below sea bottom in drill
holes).
Among the palaeoenvironmental factors that may have influenced these trends are the
oceanic silica cycle changing though time, oceanic circulation and/or the palaeogeographic
location of each studied site. Siliceous organisms are the main extractors of light silicon in
the oceans and lead to a δ30Si increase of the remaining ocean water reservoir. Rivers furnish
the main supply of fresh silicon to the ocean but climate and exposed source rocks may
determine the silicon isotopic composition of river water. Measurements of δ30Si in
ocean water depth profiles were successful in characterizing watermasses of the
Antarctic Ocean. Finally, proximity from continents may also contribute to the
observed trends. Near continents, high productivity may lead to high δ30Si, whereas
light, recycled silicon may be more important in locations far from continents. This
recycled silicon should be relatively light considering that it results from biogenic and
dissolution fractionation which are both negative. These factors might affect also the
difference in δ30Si-trends of radiolarites between Panthalassa and western Tethys. |
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