Rolf Wehausen

The sapropel record of the eastern Mediterranean Sea — results of Ocean Drilling Program Leg 160

Research on sediments recovered during Ocean Drilling Leg 160 has concentrated on two issues: the first concerned the stratigraphy of sapropel formation, the second was oriented to clarify specific processes that explain sapropel origin. Progress has been made in the construction of stratigraphic composites out of sedimentary sequences from individual holes at each of the palaeoceanographic sites. On the composites, initial work has resulted in the establishment of high-resolution and intermediate-resolution stratigraphies for three sites (963, 964, 967); correlation of sedimentary cycles to astronomical (insolation) cycles extends the stratigraphies to Sites 969 and 966. The sapropel occurrences in the marine and land sequences over the entire Eastern Mediterranean are correlated; with the resolution that can be obtained from isotope studies, groups of sapropels occurred simultaneously over the entire basin. In detail, however, the temporal and facies patterns of sapropel sequences differ between individual sites and depositional basins. The differences may be related to effects of water depth, diagenesis, and post-depositional tectonic attenuation of sequences. Studies on the geochemistry and facies of sapropels agree that anoxic conditions favoured preservation of organic matter in sapropels, caused the enrichment of trace metals associated with sapropels, and helped to preserve primary sedimentary structures. Besides, all evidence is consistent with elevated fluxes of organic matter and associated elements during sapropel events.

Astronomical forcing of the East Asian monsoon mirrored by the composition of Pliocene South China Sea sediments

Abstract High-precision major and minor element records of Pliocene core intervals from Ocean Drilling Program (ODP) Site 1145 located in the northern part of the South China Sea were generated at a 5 cm (approximately 2 kyr) resolution. From the geochemical data distinct proxies, those indicating changes in detrital matter provenance as well as those indicating productivity variations, could be derived. Both the terrigenous input to the northern South China Sea (K/Si) and variations in productivity (Ba/Al) show a response to insolation-forced monsoon variability. While the fluvial input (K/Si) responds to changes in the summer monsoon, productivity increases, as documented by Ba enrichments, seem to reflect variations in winter monsoon intensity. A stronger winter monsoon may have increased nutrient availability via dust input and/or upwelling phenomena. According to our geochemical proxy records, summer and winter monsoons are approximately 180° out-of-phase. This may imply forcing of the winter monsoon through Southern Hemisphere summer insolation maxima. The K/Si ratio shows a linear correlation with the La90(1,0.5) Northern Hemisphere summer insolation record and has therefore been used to create an astronomical timescale for the investigated sediments of ODP Site 1145. Carbonate contents and corresponding color reflectances display 41 and 100 kyr cycles. We propose that these do not reflect glacial cycles, but are rather the result of an interference signal of dilution by fluvial and eolian terrigenous material and carbonate production (burial flux). A sudden onset of gradual changes in sediment composition occurs after 3.0 Ma. This is also documented by an increase in sedimentation rates and may be caused by enhanced fluvial input from the Pearl River due to intensified weathering rates on the Asian continent. This may be linked to tectonic processes around the Himalayan–Tibetan Complex and its consequences regarding land–sea heating and atmospheric circulation.

A new proxy for bottom-water ventilation in the eastern Mediterranean based on diagenetically controlled magnetic properties of sapropel-bearing sediments

Magnetic properties of eastern Mediterranean sediments recovered during Leg 160 of the Ocean Drilling Program (ODP) provide insight into non-steady-state diagenetic reactions associated with accumulation and degradation of organic matter in sapropels. According to their magnetic properties, sapropels can be classified as one of three types that correspond to increasingly anoxic conditions at the time of sapropel formation. A combination of magnetic and geochemical data suggests a causal relationship that enables determination of the relative role of bottom-water ventilation versus productivity in the resulting diagenetic stage reached for the three types of sapropels. It appears that increased productivity is a prerequisite for sapropel formation, but once organic matter is available in sufficient amounts, variable efficiencies in bottom-water ventilation are more important for modulating the diagenetic context in which different types of sapropels formed. Magnetic properties are more sensitive to variations in bottom-water ventilation than to productivity, and can be used to establish relative variations in bottom-water ventilation both at, and after, periods of sapropel formation. Magnetic results and the distribution and type of sapropels at ODP Site 966 (Eratosthenes Seamount) between 2.3 and 4.0 Ma suggest that bottom-water ventilation was modulated by the orbital eccentricity component, with ventilation being restricted during 400-kyr eccentricity maxima and enhanced during eccentricity minima. Enhanced ventilation during eccentricity minima, as indicated by magnetic data, is consistent with the occurrence of red intervals at Site 966 and at other eastern Mediterranean sites drilled during ODP Leg 160, and also with astronomically modulated variations in CaCO3 content found in Mediterranean land-sections. This suggests that variations in bottom-water ventilation modulated by 400-kyr eccentricity cycles operated at a basin-wide scale.

Late Pleistocene sedimentation in the Western Mediterranean Sea: implications for productivity changes and climatic conditions in the catchment areas

Abstract Sediment cores from the Western Mediterranean Sea (WMS) have been analyzed for their bulk element composition, δ18O values of planktic foraminiferal tests, and 87Sr/86Sr and 143Nd/144Nd ratios of their bulk lithogenic components. The investigated time interval comprises the last 215 kyr. Si/Al and Ti/Al ratios as well as radiogenic isotope compositions indicate changes in the provenance of the lithogenic components between glacial intervals and interglacial phases. Comparison with modern data indicates that detrital input from the northwestern and northeastern Sahara may have dominated during interglacial phases. In contrast, during glacial periods the accumulation rate of terrigenous sediment is high and changes in the sediment source areas are evident that may be related to changes in the prevailing atmospheric circulation over the basin and its source areas. A productivity reconstruction based on bio-mediated barium accumulation rates reveals increased surface productivity during glacial phases. Intervals time-equivalent to sapropel formation in the Eastern Mediterranean Sea (EMS) show no changes in surface productivity compared to the intervening intervals. Comparison of the productivity patterns between the WMS and EMS suggests a decoupling during Late Pleistocene sapropel formation and highlights the importance of more localized factors such as the freshwater drainage basin.

Geochemistry of sediments from the connection between the western and the eastern Mediterranean Sea (Strait of Sicily, ODP Site 963)

Abstract A geochemical and stable isotope geochemical study was carried out on sediments and pore waters from the Strait of Sicily at the connection between the western and the eastern Mediterranean Sea (ODP Site 963). Organic-matter-rich layers (ORLs), which are proposed to be time-equivalent to eastern Mediterranean sapropels, only occur in sediments older than 550 kyr, most likely because of a strong dilution of late Pleistocene and Holocene sediments by terrigenous material. The ORLs have total organic carbon (TOC) contents of up to 1.6% and are enriched in pyrite and trace metals. TOC/pyrite ratios indicate the at least temporary presence of an anoxic water column during sediment formation, which is in agreement with the observed sulfur isotope discrimination. These conditions induced the enrichment in sulfide-forming and redox-sensitive trace elements. Trace metal enrichment is lower than at other sites in the Mediterranean Sea, which may be attributed to a less reducing or a less stable anoxic environment that was less favorable for the accumulation of redox-sensitive trace elements and organic matter. However, during sapropel events sedimentary conditions in the Strait of Sicily, based on manganese and barium geochemistry, must have been largely similar to those at other sites in the Mediterranean Sea. According to elemental, Rock-Eval, and stable carbon isotope analyses the organic matter in the ORLs is mainly of marine origin, which appears to have been overprinted significantly during diagenesis. A significant part of the organic matter in sediments from ODP Site 963 may also originate from uplifted land sections eroded during weathering and may primarily also be of marine origin. The results of pore water analyses illustrate the presence of an active deep biosphere in the sediments of Site 963, reflected for example by sulfate, methane and iodine concentrations. A brine, probably originating from underlying Messinian evaporites, was detected, but is only of minor influence.

A GEOCHEMICAL RECORD OF PRECESSION-INDUCED CYCLIC EASTERN MEDITERRANEAN SEDIMENTATION : IMPLICATIONS FOR NORTHERN SAHARA HUMIDITY DURING THE PLIOCENE

Zusammenfassung Der Chemismus lithogener Komponenten pliozäner Sedimente des östlichen Mittelmeers weist periodische Änderungen auf, die mit den Orbitalparametern der Erde und dadurch hervorgerufenen Variationen der Sonneneinstrahlung im Bereich der Nordhemisphäre einhergehen. Dadurch entstehen zwei gegensätzliche paläoklimatische bzw. paläozeanographische Szenarien. In Zeiten mit geringen Niederschlägen und oligotrophen Bedingungen im Mittelmeer, wie sie auch heute vorherrschen, ist die Sedimentation sehr stark von äolischen Staubeinträgen angrenzender Wüstengebiete gekennzeichnet. Demgegenüber dominiert in feuchten Klimaperioden die Sedimentation von fluviatilem Material, das durch den Nil und Flüsse der nördlich an das Mittelmeer grenzenden Gebiete eingetragen wird. Die durch Zirkulationsänderungen sowie den Nährstoffeintrag der Flüsse induzierten eutrophen Bedingungen führten schließlich zur Ausbildung von Wassersäulen-Anoxia und der Ablagerung von Sapropelen im östlichen Mittelmeer. Unsere Untersuchungen unterstützen frühere Forschungsergebnisse, wonach in weiten Teilen der nördlichen Sahara ein häufigerer Wechsel zwischen ariden und humideren Klimabedingungen stattfand. Nach unseren Ergebnissen folgte dieser rhythmische Wechsel im Pliozän dem Insolationszyklus von ca. 22 ka.Abstract. The chemical composition of lithogenic components in Pliocene sediments from the eastern Mediterranean displays periodic variations that are related to earths orbital parameters owing to changes in insolation of the northern hemisphere. This can be explained by two different paleoclimatic/palaeoceanographic scenarios. During oligotrophic periods, similar to those persisting in the modern Mediterranean, sedimentation is rather uniform, and sediments receive high proportions of eolian material from the surrounding deserts. During more humid intervals sedimentation is dominated by fluviatile material that has been introduced by the Nile and northern borderland rivers. Higher nutrient loads associated with enhanced riverine input and a change in water circulation led to eutrophication, water column anoxia, and sapropel deposition in the eastern Mediterranean. Our investigations confirm earlier works claiming that in large parts of the northern Sahara palaeoclimatic conditions changed frequently from arid to more humid. Our results suggest that these changes in the Pliocene match with the approximately 22-ka insolation cyclicity.

Determination of major and minor ions in brines by x‐ray fluorescence spectrometry: comparison with other common analytical methods

A method is presented for the determination of major and some minor ions (Cl−, SO42−, Br−; Na+, K+, Ca2+, Mg2+, Sr2+) in marine pore waters. X-ray fluorescence spectrometry provides the possibility of analysing large numbers of samples within a short time interval (5 min per sample) using a simple sample preparation. Calibration graphs were obtained from diluted salt or standard solutions. The precision and accuracy were found to be comparable to those of common titrimetric, colorimetric, spectrometric and ion chromatographic methods which are generally used for such matrices. Copyright

Late Pleistocene sedimentation in the Western Mediterranean Sea

Sediment cores from the Western Mediterranean Sea (WMS) have been analyzed for their bulk element composition, N18O values of planktic foraminiferal tests, and 87Sr/86Sr and 143Nd/144Nd ratios of their bulk lithogenic components. The investigated time interval comprises the last 215 kyr. Si/Al and Ti/Al ratios as well as radiogenic isotope compositions indicate changes in the provenance of the lithogenic components between glacial intervals and interglacial phases. Comparison with modern data indicates that detrital input from the northwestern and northeastern Sahara may have dominated during interglacial phases. In contrast, during glacial periods the accumulation rate of terrigenous sediment is high and changes in the sediment source areas are evident that may be related to changes in the prevailing atmospheric circulation over the basin and its source areas. A productivity reconstruction based on bio-mediated barium accumulation rates reveals increased surface productivity during glacial phases. Intervals time-equivalent to sapropel formation in the Eastern Mediterranean Sea (EMS) show no changes in surface productivity compared to the intervening intervals. Comparison of the productivity patterns between the WMS and EMS suggests a decoupling during Late Pleistocene sapropel formation and highlights the importance of more localized factors such as the freshwater drainage basin. ; 2002 Elsevier Science B.V. All rights reserved.