A new proxy for bottom-water ventilation in the eastern Mediterranean based on diagenetically controlled magnetic properties of sapropel-bearing sediments

Magnetic properties of eastern Mediterranean sediments recovered during Leg 160 of the Ocean Drilling Program (ODP) provide insight into non-steady-state diagenetic reactions associated with accumulation and degradation of organic matter in sapropels. According to their magnetic properties, sapropels can be classified as one of three types that correspond to increasingly anoxic conditions at the time of sapropel formation. A combination of magnetic and geochemical data suggests a causal relationship that enables determination of the relative role of bottom-water ventilation versus productivity in the resulting diagenetic stage reached for the three types of sapropels. It appears that increased productivity is a prerequisite for sapropel formation, but once organic matter is available in sufficient amounts, variable efficiencies in bottom-water ventilation are more important for modulating the diagenetic context in which different types of sapropels formed. Magnetic properties are more sensitive to variations in bottom-water ventilation than to productivity, and can be used to establish relative variations in bottom-water ventilation both at, and after, periods of sapropel formation. Magnetic results and the distribution and type of sapropels at ODP Site 966 (Eratosthenes Seamount) between 2.3 and 4.0 Ma suggest that bottom-water ventilation was modulated by the orbital eccentricity component, with ventilation being restricted during 400-kyr eccentricity maxima and enhanced during eccentricity minima. Enhanced ventilation during eccentricity minima, as indicated by magnetic data, is consistent with the occurrence of red intervals at Site 966 and at other eastern Mediterranean sites drilled during ODP Leg 160, and also with astronomically modulated variations in CaCO₃ content found in Mediterranean land-sections. This suggests that variations in bottom-water ventilation modulated by 400-kyr eccentricity cycles operated at a basin-wide scale.