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| Titel |
Barium in Black Sea sediments: a reliable indicator for marine primary productivity or the migration of the sulphate/methane transition zone? |
| VerfasserIn |
Susann Henkel, Kerstin Nöthen, Christine Franke, Kara Bogus, Eric Robin, André Bahr, Martin Blumenberg, Thomas Pape, Richard Seifert, Christian März, Gert J. De Lange, Sabine Kasten |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250040764
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| Zusammenfassung |
| Sedimentary Ba profiles are often used to trace paleoproductivity since the decay of organic
matter in the water column coincides with precipitation of “biogenic” barite. However, Ba is
diagenetically overprinted if sediments are buried below the sulphate/methane transition zone
(SMTZ). To gain insight into the geochemical behaviour and cycling of Ba in ocean areas
characterized by an anoxic water column and a shallow sedimentary SMTZ, gravity cores
from two sites in the northwestern Black Sea (GC 755, 501 m water depth and GC
214, 1686 m water depth) were examined. The main objective of this study was to
evaluate the applicability of Ba as an indicator for marine primary productivity and/or
migrations of the SMTZ in such exceptional (modern and ancient) environments. This is
the first study that combines solid phase Ba data from the Black Sea with pore
water Ba2+ profiles to identify current depths of diagenetic barite dissolution and
precipitation. Within the uppermost ~1.5 m (above the current SMTZ) a positive
correlation between solid Ba and total organic carbon (TOC) exists and suggests a
contribution of “biogenic” barite to the bulk Ba concentration. In contrast, around the
SMTZ and below, Ba and TOC are completely decoupled due to the diagenetic
dissolution and reprecipitation of BaSO4. Authigenic barite fronts are indicated by
pronounced peaks in particulate Ba slightly above the SMTZ (at about 1.7 and 1.5
m, respectively). The current zone of diagenetic barite precipitation corresponds
to the overall solid-phase maximum of Ba at the shallow site (755), while at the
deeper site (214) the current Ba2+ sink is situated some tens of centimetres above
the distinct Ba maximum. This offset indicates an upward shift of the SMTZ at
site 214 in the recent past, possibly induced by complete burial of the sapropel
(stratigraphic Unit II) below the initial SMTZ depth and subsequent in situ methanogenesis
within this TOC-rich section. Our findings indicate that the decoupling of TOC and
solid phase Ba in Black Sea sediments results from diagenetic recycling of Ba at
shallow sediment depths. This non-correlation between Ba and TOC is different to the
one found in Mediterranean sediments, where the initial close positive correlation
between these two components is affected by post-sedimentary aerobic degradation of
TOC.
The numerical transport/reaction model CoTReM was used to simulate the downward
migration of the SMTZ induced by the strong increase in bottom water sulphate
concentrations after the Holocene intrusion of Mediterranean seawater into the formerly
limnic Black Sea (~9 kyrs BP). The simulation showed that the pore water system reached a
steady-state situation (in terms of stable depth of the SMTZ) about 3.8 kyrs after the
first flooding. The diffusion of sulphate into the sediment column was faster than
the burial of a particular horizon according to accumulation rates. Therefore, the
SMTZ was located within the limnic deposits (Unit III) for several thousands of
years. At both sites, the marine sapropel of Unit II was buried below the SMTZ
only ~1-2 kyrs BP, which confines the time available for barite recycling since
the limnic sediments below probably did not contain significant concentrations of
BaSO4.
Scanning electron microscopic analyses revealed the presence of two classes of barite in
surface sediments at site 214: ellipsoidal “biogenic” and irregularly shaped “non-biogenic”
particles. In addition to diagenetic and productivity-related barite formation, precipitation due
to oversaturated deep water must therefore be taken into account. This study demonstrates
that Ba cannot serve as a productivity tracer in the Black Sea or similar ancient marine
environments, such as sediments deposited during Oceanic Anoxic Events (OAEs). |
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