Mediterranean Sapropel Formation and Redox-Controled Preservation: the s1 Formation and its Interruption

Deposition of organic-rich units (sapropels) in Mediterraneran sediments is precession-related and associated with humid climate conditions. The last humid period from 11 - 5 ka 14C ago, occurred simultaneous with a sustained circum-Mediterranean wet period. The most recent sapropel (S1) formed synchronously between 9.8 and 5.7 14C ka BP at all water depths greater than a few hundred metres. As a consequence of increased fresh water (monsoon) input, surface waters had a reduced salinity and concomitantly the deep (> 1.8 km) eastern Mediterranean Sea was devoid of oxygen during 4,000 years of S1. This has resulted in a differential basin-wide preservation of S1 determined by water depth, as a result of different ventilation/climate-related redox conditions above and below 1.8 km. The end of this period is marked by a basin-wide high sedimentary manganese-oxide peak that represents an abrupt re-ventilation at 5.7 ka BP and remaining oxic conditions thereafter. Other short-term ventilation events appear to have occurred during sapropel S1, notably the 8.2 cal ka BP event. This potentially basin-wide event is particularly noticeable at relatively shallow near-coastal sites of high sedimentation rates. It marks a brief episode of not only re-oxygenated deep water thus reduced preservation, but also decreased primary productivity thus nutrient supply. This 8.2 cal ka BP interruption event is thought to be related to enhanced deep water formation in the Aegean due to a short period of sustained cold air fluxes from Polar regions. Sapropel formation mechanisms, therefore, seem related to a sensitive interplay between N-African monsoonal and northern climate systems.