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Titel |
Terrigenous organic matter input to the Black Sea originating from different hinterland regimes |
VerfasserIn |
S. Kusch, J. Rethemeyer, G. Mollenhauer |
Konferenz |
EGU General Assembly 2009
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 11 (2009) |
Datensatznummer |
250027557
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Zusammenfassung |
The Black Sea as the world’s largest anoxic basin has been shown to be a significant sink of
terrigenous and phytoplankton derived organic material. The north-western part is
dominated by a large shelf area, while in the SW and E Black Sea, steep slopes plunge
into the anoxic zone at short distances to the shore. Major rivers draining into the
Black Sea include the Danube River, the Dniester River, the Kuban and the Don
River. These rivers and their tributaries transport huge amounts of suspended load to
the Black Sea, eroded from mountain ranges including the Alps, the Carpathian
Mountains and the Caucasus Mountains. However, the size, climate and ecology of the
respective drainage areas and the near-shore topography differ substantially between the
rivers.
We show geochemical proxy data, bulk radiocarbon (14C) ages and compound-specific
14C ages of terrigenous biomarkers from core-top samples collected along three sample
transects in front of the Danube and the Dniester river mouths in the NW Black Sea off
Rumania and Ukraine, draining the Alps and the Carpathian Mountains, and just south of
the Strait of Kerch, connecting the Black Sea to the Sea of Azov (drainage of the
Caucasus Mountains). Two further core locations are situated in front of the Çoruh and
Acharistsgali river mouths in the SE Black Sea off Georgia (Eastern Pontic Mountains) and
north of the Gülüç and ÇatalaÄci river mouths in the SW Black Sea off Turkey
(Western Pontic Mountains), respectively. The samples range from the oxygenated
surface waters to the anoxic deep basin and form transects along specific transport
pathways.
The Branched and Isoprenoid Tetraether index (BIT) is used to trace the terrigenous
organic matter in marine sediments, and it is thought to represent mainly soil-derived
materials. BIT-values show the expected pattern of high terrigenous input in front of the
river mouths and decreasing values further offshore along the sampled transport
trajectories. Proxies indicative of organic matter derived from higher land plants
exhibit corresponding patterns: Average chain length (ACL) of high molecular
weight hydrocarbons (n-alkanes) ranges from 29.06 to 29.79, and n-alkanes show a
typical odd-over-even-predominance. This is illustrated by the Carbon Preference
Index (CPI) values, which vary between 4 to 7 at the Western Black Sea stations,
typical for ”fresh” terrigenous material. CPI values from the Eastern Black Sea core
locations show a range between 3 to 4, which indicates either enhanced marine
phytoplankton-derived input, or potentially a slight oil contamination due to oil seepage in
that area.
Carbon to nitrogen (C/N) ratios, which range between 7-10 at all stations, also reflect
input of terrigenous organic matter defined by less molecular nitrogen compared to marine
organic matter. For the Western transects C/N ratios are decreasing from the stations close to
the river mouths towards the offshore core locations, but show increasing values
towards the anoxic stations again. This might be explained by an influence of the
quasi-permanent Black Sea rim current, probably partly eroding terrestrial organic
matter.
Conventional 14C ages of total organic carbon correlate well with the BIT index values.
Highest 14C ages of 1410±20, 1350±30 and 400±30 14C years are observed in front of the
river mouths, indicating an input of pre-aged terrigenous organic matter. The radiocarbon
contents are increasing further offshore along the transects with modern 14C ages (bomb-14C
contribution) at those core locations where organic matter is dominated by marine
production. For a core south of the Strait of Kerch, where compound-specific 14C ages of
terrestrially derived long-chain fatty acids (FA) are available, our results show increasing 14C
ages with increasing chain-length from n-C24 (post-1950 values) and n-C26 (654±110
14C years) to n-C28 and n-C30-FA (1092±110 14C years), probably indicating
higher resistance to degradation. TOC 14C ages and radiocarbon ages of n-C26-FA
(1000±30 and 907±140 14C years) and n-C28-FA (767±30 and 768±120 14C
years) of two cores from the SW and NW Black Sea agree well, confirming the
presence of pre-aged terrestrial material. The n-C26-FAs are slightly younger (400
14C years) than the n-C28and n-C30-FAs, but are older than marine biomarkers
(bomb-14C contribution). In our presentation, we will discuss the implications of our
observations for the timescales of transport affecting terrigenous organic matter. |
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