Mediterranean climate and oceanography, and the periodic development of anoxic events (sapropels)

Abstract

Mediterranean sapropels are layers with elevated organic carbon concentrations that contrast with surrounding sediments, which are organic poor. Sapropels occur (quasi-) periodically in sedimentary sequences of the last 13.5 million years, and exist both in the eastern and western Mediterranean sub-basins. They have been the subject of extensive study, based on records from both short (conventional) and long (Ocean Drilling Program) sediment cores, and from a wide variety of uplifted marine sediment sequences on the basin margins and islands. Previous syntheses in the 1990s and 2000s have discussed how the formation of sapropels is commonly ascribed to deep-sea anoxia, enhanced export productivity, or a combination of these effects. However, a wealth of new evidence and insights has emerged during the past 1–2 decades, based on traditional and novel proxy data as well as modelling, which has revealed intriguing new aspects and nuances to the reconstructed conditions. Hence, it is timely to present a new synthesis of current understanding of the processes behind the formation of sapropels, which have over the past decade also become a matter of commercial interest in sub-salt hydrocarbon exploration. In this review, we present a context of modern Mediterranean climate and oceanography, followed by an integrated assessment of the growing understanding of climatological and ocean circulation changes that were associated with sapropel deposition. We find that sapropels predominantly formed during (astronomically timed) episodes when climatic and oceanographic conditions and ecological responses broadly preconditioned the basin for sapropel deposition. There is strong correspondence with times of monsoon intensification, fuelling runoff from North Africa into the Mediterranean Sea, while preconditioning due to sea-level rise, and regional precipitation and runoff may have contributed as well. Within these broad episodes of surface buoyancy gain and resultant decline in deep-water ventilation, specific deposition under dysoxic, anoxic, or even euxinic conditions occurred within a clearly dynamic system that was characterised by complex spatial and depth-dependent patterns/gradients, with distinct temporal variability on (at least) decadal to centennial–millennial timescales. In the final section, we evaluate the implications of different modes of deep-water removal from silled basins, to investigate why sapropels are more frequently and often more intensely developed in the eastern Mediterranean than in the western Mediterranean.