New constraints on climate forcing and variability in the circum-Mediterranean region from magnetic and geochemical observations of sapropels S1, S5 and S6

摘要

Eastern Mediterranean sapropels are organic-rich layers whose formation was triggered during boreal summer insolation maxima (precession minima). Sapropel formation is associated with increased precipitation and runoff from Europe and North Africa, although the relative influence of high- and low-latitude climate mechanisms is not fully resolved. Likewise, it remains uncertain how centennial-scale interruptions in conditions that give rise to sapropels may reflect atmospheric circulation changes in low latitudes. We present magnetic, geochemical, and diffuse reflectance spectroscopy data from three sapropels that are representative of glacial (S6) and interglacial (S1, S5) conditions in the eastern Mediterranean Sea to assess environmental changes associated with sapropel formation. The extent of diagenetic magnetite dissolution and authigenic formation of fine-grained greigite in these sapropels and their underlying dissolution intervals are linked to enhanced sulphidic conditions during sapropel formation. Aeolian hematite and goethite are largely unaffected by this reductive dissolution except within the interglacial sapropel S5 and its underlying dissolution interval, which formed under relatively stronger sulphidic conditions. Nevertheless, low hematite contents indicate that the three studied sapropels accumulated under reduced aeolian dust inputs in response to an intensified African monsoon, which resulted in expansion of savannah landscapes into NE Saharan dust source areas. Small variations in goethite contents across sapropels indicate additional aeolian entrainment of goethite that formed under previous wet phases in the NE Sahara or in subtropical savannahs located further south. We link short-lived dust abundance peaks within sapropels S1 and S6 to centennial-scale periods of enhanced bottom-water circulation reported previously for these sapropels. Although these sapropel interruptions are driven by high-latitude cooling events, our results indicate that such centennial-scale episodes of atmospheric reorganization affected not only the eastern Mediterranean northern borderlands, but also subtropical North Africa. Overall, our results point to a dominant low-latitude forcing on sapropel formation via boreal summer insolation maxima and intensification of the African monsoon.