 |
| Titel |
Changing paleoceanographic and paleoclimatic parameters during sapropel formation in the Aegean Sea |
| VerfasserIn |
E. B. Isler, A. E. Aksu, R. N. Hiscott |
| Konferenz |
EGU General Assembly 2009
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250023728
|
|
|
|
|
|
| Zusammenfassung |
| A high-resolution record of paleoenvironmental changes during the last 80,000 years,
monitored using a mean temporal sampling interval of ~900 years, has been determined for
two 8-meter long piston cores collected from North Skiros and North Ikaria basins in the
northern Aegean Sea. Paleoceanographic and paleoclimatic trends are revealed by
geochemical, stable-isotope and trace-element analyses and by micropaleontological studies.
Trace-element concentrations were obtained from the carbonate tests of the planktonic
foraminifer Globigerinoides ruber using laser-ablation ICP-MS. The data are intended to
evaluate the role of two theoretical factors, organic-matter preservation and biological
productivity, in the formation of sapropels.
On the basis of total organic carbon (TOC) content, visual examination and color, four
distinct sapropel units (S1, S2, S3, and S4) were identified throughout the cores. The
chronostratigraphy of the cored successions is based on the ages of identified tephra layers
and correlation of oxygen-isotope curves for the benthic foraminifer Uvigerina mediterranea
with the global oxygen-isotope curve.
Mg incorporation into foraminiferal calcite is predominantly regulated by temperature.
Mg/Ca ratios are used to calculate sea surface temperature (SST) and sea surface salinity
(SSS) variations. The latter variable is obtained from the oxygen-isotopic composition of sea
water (δ18Ow):salinity relationship where δ18Ow can be extracted from the paleotemperature
equation based on the independent calculation of temperature. SST, SSS and δ18Ow
plots show that the surface waters were predominantly cool and less saline prior
to the onset of sapropel deposition, but became progressively warmer and more
saline during and after sapropel deposition. The observed fluctuations in salinity
(and possibly temperature) demonstrate that there was an increased influx of fresh
and/or brackish waters to the Aegean Sea, probably originating from the Black Sea,
supplemented by local influx from nearby rivers. Before the initiation of sapropel
deposition, low-density surface waters must have acted as a cap, causing stratification
within the water column. Accordingly, bottom-water ventilation was diminished.
Respiration then resulted in a progressive decline in the dissolved oxygen levels, creating
favorable conditions for organic carbon preservation on the seafloor. During times of
sapropel formation, δ34S and C/S profiles display excursions toward lower values
signifying that bottom waters were dysoxic but never anoxic. This interpretation is
further supported by the sustained presence of benthic foraminifera in the cored
succession.
Cd/Ca and inorganic δ13C (primary productivity indicators) increased during times of
sapropel formation in the N. Skiros Basin – highest values occurred during accumulation of
sapropels S3 and S4. In contrast, primary productivity in the N. Ikaria Basin was low except
during accumulation of sapropel S3. This interpretation suggests that primary productivity
fluctuated both temporally and spatially and played a comparatively more important role in
the N. Skiros Basin, particularly for the two lower sapropels. Both water-column
stratification and primary productivity contributed to sapropel formation in the
Aegean Sea, to varying degrees. Thus, these two hypothesis may not be mutually
exclusive. |
|
|
|
|
|
|