Decadal to millennial time scale climate variability in the Central Mediterranean during the Holocene: a reconstruction based on geochemical proxies from high resolution sedimentary records

To assess potential anthropogenic contributions to future climate change it is necessary to understand natural climate variability. This can be achieved by studying climate variability during the Holocene, when similar basic climate boundary conditions persisted as today. During this period climate is characterized by millennial to centennial scale events such as the ‘little ice age’. To gain more insight in possible triggers and forcers of such climate variability, high resolution records are needed from climate sensitive areas. The Mediterranean climate is affected by the sub-tropical high pressure belt in summer, whereas mid-latitude westerlies as well as outbreaks of northern cold air dominate during winter. Climate archives from this region, therefore, record both high- and low- latitude climate change. In addition, high accumulation rates along the western Adriatic Sea (AS) and Gulf of Taranto (GoT) permit high resolution paleoclimate studies. Hence, sedimentary records from this area are ideal for studying Holocene climate variability. In chapter one of this thesis the elemental composition of surface samples in this region, and of samples from a variety of rivers from the Italian hinterland are studied. This composition together with other proxies is used to derive the sediment provenance, which for GoT sediments appears to be for ~ 80% of northern Italian provenance. In chapter two, this elemental composition is used to build a stratigraphic framework . By combining the latter with 14-C and 210-Pb dating, and applying the provenance indicators from chapter one, environmental changes during the last 16.000 years have been reconstructed. It appears that from ~7 ka BP, detrital input from Adriatic mud belt provenance can be found in the GoT. From this period onwards millennial scale events of high detrital input appear which concord with changes in moisture transport by the westerlies. By combining bulk-sediment elemental composition and elemental composition of tests of two kinds of foraminifera ( H. balthic and G. ruber), climate variability can be reconstructed for summer and winter separately, and thus seasonality. It appears that during the Bronze Age, simultaneous with the down-fall of several Mediterranean civilizations, both winter and summer are dry, causing major year round aridity. In addition, there was a major seasonal temperature contrast, i.e. cold winters, hot summers. Variations in low- and high- latitude climate patterns appear to be partly controlled by changes in Atlantic Ocean sea surface temperatures. The effect of the latter on winter climate, however, is restricted to periods with increased occurrence of El Nino events or a relatively warm North Atlantic Ocean. Not only millennial to centennial time-scale climate variability plays an important role during the Holocene. In the last chapter, the laminated sediments of the most recent sapropel S1 are studied. It is generally assumed that during the S1 period decreased ventilation and increased primary productivity in the Mediterranean resulted in enhanced preservation of organic matter, thus in deposition of distinctly organic-rich sediments. This study shows that decadal interruptions of anoxic conditions occurred during the formation of S1. Outbreaks of cold air from the poles and associated deep-water formation are thought to cause these interruptions.