Sapropel burn-down and ichnological response to late Quaternary sapropel formation in two ∼ 400 ky records from the eastern Mediterranean Sea

Two approximately 400 ky long sedimentary records from the eastern Mediterranean Sea were used to study the factors determining why some sapropels are more or less completely erased while others are preserved. X-ray radiographs, bulk density, organic carbon content, Fe / Al and Mn / Al ratios were used to reconstruct initial thickness and burn-down ratios of sapropels S1 to S11. The results show that, at the low-sedimentation rate, open ocean settings studied, initial carbon content of the sapropel, as well as post-sapropel sedimentation rate and carbon flux have had only limited influence on the preservation of sapropels. In contrast, the regional position relative to the deep-water source controlling bottom water current intensity and bottom water oxygenation played a major role in the preservation of the sapropels. Furthermore, orbitally controlled variations in the intensity of subsequent summer insolation maxima also seem to play an important role in the preservation potential of sapropel layers. Subsequent strong insolation maxima resulting in thick sapropels tend to act as a lid, preventing further burn-down of the older sapropels below. The long and well-dated records also offer an opportunity to study the response of the benthic fauna to the drastic environmental changes related to the formation of sapropels.The sediment in the two cores is generally characterized by mottled burrows, with only scarce occurrences of distinct trace fossils such as Scolicia, Thalassinoides, Chondrites, and Trichichnus. The meager ichnofauna is attributed to the well-oxygenated and warm bottom waters in an oligotrophic environment typical for non-sapropel times. The sparse ichnofauna and the lack of tiered ichnocoenosis render existing models of bioturbational response to changes in bottom water oxygenation unsuitable for this kind of environments. However, observations on individual traces support earlier studies that suggest that the trace fossils Trichichnus and Chondrites are the result of chemosymbiotic activities that benefit from reducing conditions in the pore water, and that the producers of Scolicia have a preference for well-oxygenated conditions and sandy substrates.