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| Titel |
Timing and intensity of anoxic-sulphidic redox conditions in Eastern Mediterranean Sapropels S1 and S5 deduced from metal isotopes and trace and major element variations. |
| VerfasserIn |
Alan Matthews, Irit Azrieli-Tal, Miryam Bar-Matthews, Ahuva Almogi-Labin, Derek Vance, Corey Archer, Nadya Teutsch |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250078729
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| Zusammenfassung |
| Redox is a critical control of organic carbon-rich sedimentation, especially for Eastern
Mediterranean (EM) sapropels where deepwater stagnation is related to increase freshwater
discharge into the EM Sea. We apply the metal isotope systems of Fe and Mo together with
redox-sulphide sensitive trace elements (RTSE) to the determination of paleoredox conditions
during the formation of Holocene S1 and Last Interglacial S5 EM sapropels (ODP core 967D;
2550m depth).
RTSE are asymmetrically distibuted in S1 sapropel, with peak enrichments occurring in
its lower part. Negative correlations between δ57Fe and Fe/Al and S wt% in the lower
sapropel are consistent with the benthic Fe shuttle model for anoxic-sulphidic basins1.
Correspondingly, Mo/U systematics are consistent with open marine conditions, with
sulphidic conditions in the lower sapropel compared to sub-oxic conditions in the upper
sapropel and enclosing sediments2. The Mo paleoredox model for highly sulphidic euxinic
marine systems envisages sediments acquiring Mo isotope signatures of sea water (δ98-95Mo
= 2.3 permil) due to quantitative Mo scavenging3. However, in keeping with several recent
studies of organic carbon-rich sediments, S1 profile shows ‘atypical’ light δ98-95Mo
values, which progressively decrease from sub-oxic values of ca 0 permil in the
underlying sediment to negative values of -0.9 permil in the lower sapropel, before
gradually rising through the sapropel to values of 0 to 0.4 permil in the post-sapropel
sediment. The lightest Mo isotopes occur with indicators of peak sapropel conditions -
high Fe/Al, Ba/Al, Mo/Al and S. A combination of source effects and isotopic
fractionation between sea water molybdate and particle reactive thiomolybdate species in
mildly sulphidic conditions can account for such isotopically light values. The proxy
methods thus reflect anoxic early S1 redox conditions in mildly sulphidic bottom
water, followed by a progressive waning and weakening to sub-oxic diagenetic
conditions.
Initial Sapropel S5 data are in marked contrast to the S1 data. RTSE and major elements
show stronger enrichments than for S1, and a trend toward peak values in the upper sapropel
section. Correspondingly, δ98-95Mo values, which reflect sub-oxic to anoxic conditions in the
sediments, show a rise to near-seawater values (1.9 to 2.2 permil) in the upper sapropel,
indicative of strong euxinic conditions. Mo/U ratios show seawater values at the S5 peak,
suggesting these elements are sequestered from sulphidic bottom waters. Thus, peak redox
(anoxic-sulphidic) conditions in S5 characterize the later stages and suggest a progressive
redox build-up.
The multiproxy use of RTSE, major elements and Fe and Mo isotopes has the potential to
reveal the strength and timing of the anoxic-sulphidic conditions during sapropel evolution.
Transient drops toward background sediment proxy levels within the sapropel record
hiatuses, as exemplified by the hiatus recording of the 8.2 ka event in S1 previously
recognised in other studies.
1Severmann et al. (2008) Geology 36, 487-90. 2Algeo and Tribollivard (2009) Chemical
Geology 68, 211-225. 3Arnold et al. (2004) Science 304, 87-90; Neubert et al. (2008)
Geology 35, 775-778. |
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