Abstract
In this thesis diagenetic processes in both organic-poor and organic-rich eastern Mediterranean sediments are studied. Furthermore, (present-day) fluxes and the variations in several proxies are discussed.
Chapter 2 deals with the aerobic mineralisation of Corg in surface sediments of the deep eastern Mediterranean Sea. Quantification was done with the modeling environment
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for early diagenetic problems MEDIA. The reactive fraction comprises 60-80% of the total Corg reaching the sediments and is largely oxidized within the surficial 10 cm. An non-reactive fraction dominates at depths > 10 cm, and makes up 20-40% of the total flux. Total mineralization rates in the surface sediments are typical for oligotrophic, deep-sea environments. The low fluxes and rapid remineralisation of Corg are accompanied by 210Pbexcess surface mixed layers which are only 2 cm deep, among the thinnest reported for oxygenated marine sediments. Paleaoproductivity estimates calculated from the sediment data yield low surface productivities.
In Chapter 3 a semi-quantitative study is made of sedimentary salt fluxes into the eastern Mediterranean deep water. Different sources can be distinguished with mud diapirs/volcanoes and normal pore waters as the most important ones. Compared with the "salt input" by evaporation, the sedimentary salt input is rather small and has little influence on the models used for the water- and salt budgets. Salt input from the sediment is too low to initiate stagnation of the water column, but will enhance the stagnation process once it is initiated by other processes.
Chapter 4 investigates the diagenetic signals above the most recent sapropel S1. Generally, two Mn-rich layers are observed above this sapropel, one immediately above the sapropel top, and one a few centimeters closer to the sediment surface. Pore water profiles show that the lower Mn peak is actively forming at this moment. Paleaoproductivity profiles are used to calculate the initial organic carbon profile of the S1 sapropel. The profiles generated by this method demonstrate that the original sapropel unit was bounded by the upper Mn peak. The initially deposited Corg has been oxidized by a progressive downwards-moving oxidation front and is still ongoing. As the upper Mn peak marks the end of sapropel formation, the deposition of sapropel sediment has ceased more recently than is indicated by radiocarbon dating of the visible top of S1.
The concept of oxidation of sapropels is extended in Chapter 5. The occurrence of sapropels appears to be associated with maxima in the summer insolation target curve. However, sapropels are not observed at each maximum. Geochemical and magnetic signals allow for the detection of completely oxidized or "missing" sapropels. An important parameter to reveal missing sapropels is Ba. At least 8 of 11 sampled intervals around insolation maxima contain evidence of a completely oxidized sapropel: Ba, trace metal distributions and magnetic parameters are similar to those observed around visible sapropels.
Chapter 6 discusses the different types of sediment found in the eastern Mediterranean, making use of a very long piston core. The composition of the turbidites is similar to the sediments in which they are embedded, pointing to a local source area. A comparison between sapropel and normal sediments show that these are remarkably similar. Climatic variations that cause the formation of sapropels, do not have a very significant impact on the sediment composition on this location. Climate related cyclic variations have not been found in this core.
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