Abstract
The Mediterranean Sea is a semi-enclosed basin surrounded by catchment areas characterized by different climate regimes, as it lies at the interface between the African tropical and European temperate zones. Moreover, the Mediterranean has a dynamic thermohaline circulation, making it particularly sensitive to changes in the hydrological cycle. This sensitivity
... read more
is best witnessed by the occurrences of sapropel – organic-rich layers, at an astronomically determined cyclicity throughout the last 13.5 million years. Sapropels occurred more frequently in the eastern Mediterranean Sea (EMS) and only during the precession-forced summer insolation maxima in the northern hemisphere, coupled with increased seasonal contrast and river runoff. The enhanced freshwater and nutrients brought in by the runoff stimulated a pronounced density stratification of the water column and/or an increased primary productivity in the surface waters, which ultimately led to stagnant deep-water conditions and sapropel formation. Regardless of the relative importance of deep-water stagnation versus surface-ocean productivity, sapropel deposition can thus be viewed as directly related to freshwater forcing and its hydrographic response. Furthermore, it has been shown that with a stronger freshwater forcing, the strength of the deep-water stagnation increases; and the changes in the freshwater source will modulate this effect. This is in line with the interplay between physical circulation and deep-water oxygen consumption, as suggested by ocean-biogeochemical modeling. However, the exact sources of freshwater and associated hydroclimate, including precipitation/evaporation balance and changes in regional convection, remain elusive. Moreover, the complex interactions between climatic and paleoceanographic processes for sapropel formation are still highly debated. This PhD study deals with these challenges via three research routes, see General Introduction and Outline (Chapter 1). First of all, under “the freshwater sources and associated hydroclimate changes”, we aim to reconstruct the riverine supplies into the EMS (Chapters 2–4). The approaches involve the combined use of major and trace elements, Sr and Nd isotopes, clay mineralogy, grain size end-member modeling, etc., which are mostly done on the terrigenous detrital component of the marine sediments. The samples are taken from a geographical and bathymetric coverage of the EMS, including the sediments of Holocene sapropel S1 and of last interglacial sapropel S5. Specifically, our study is started from the sapropel S1 period, mainly focusing on a well-dated sediment core in the central Mediterranean (Chapter 2). Then it is extended to the past 18,000 years in comparison with existing North-African hydroclimate records on a sub-continental scale (Chapter 3). After that, the knowledge is applied to the last interglacial sapropel S5 to investigate the source and distribution of river-borne materials on a basin-wide EMS scale (Chapter 4). During this study it appeared necessary to “evaluate and examine the application of various detrital provenance proxies”. One prominent problem is the potential barite-associated Sr remaining in decarbonated sediments, as the not fully removed barite-Sr may largely affect the detrital Sr isotope composition. This problem was ignored or just overlooked in published studies. The intention and set-up for Chapter 5 is thus to evaluate the contribution of barite-associated Sr for provenance studies using Sr isotopes and concentration. Apart from the studies on marine detrital sediments, we use seawater-derived fractions to explore the changes in the thermohaline circulation and water column of the Mediterranean Sea during sapropel formation. For the purpose of a better understanding of “the complex interplay between the paleoceanographic and paleoclimatic processes”, for the first time, a basin-wide EMS reconstruction of seawater Nd isotopic ratios during sapropel S1 deposition is given (Chapter 6).
show less
