Sedimentary organic carbon to phosphorus ratios as a redox proxy in Quaternary records from the Mediterranean

In this study, organic carbon to organic phosphorus (Corg/Porg) and total phosphorus (Corg/Ptot) ratios as bottom water redox proxies are explored for two eastern Mediterranean sediment records extending back ~ 600–700 kyrs. Cores KC19C (19.6 m long, 2750 m water depth) and KC01B (37.0 m long, 3643 m water depth) contain numerous organic-rich layers (sapropels). Throughout the two cores, variations found in the Corg/P ratios correspond to those for other redox proxies such as total S contents and V/Al ratios. Accordingly, these are interpreted as variations in bottom water oxygenation and inter-sapropel redox variability. In core KC19C, the original Corg/P signals seem to have been preserved despite intense post-depositional sulfidization. High-resolution P fractionation data of sapropel S5 from a third core (PS25PC, 3304 m water depth) showing high and relatively stable Corg/Porg ratios, indicate that sapropel S5 at this deep location was deposited under permanently reducing conditions. While P regeneration from organic matter was intense, net P burial increased in the sapropel due to enhanced burial of inorganic calcium phosphate, likely biogenic Ca–P. Sediment P contents and Corg/Ptot ratios in S5 are variable, reflecting short-term changes in marine conditions (productivity/redox) and sediment chemistry. The Corg/Porg and Corg/Ptot ratios thus provide valuable insight into paleoceanographic conditions and pathways of P cycling for (Mediterranean) sediment records extending back at least ~ 600–700 kyrs.

Research Highlights
► Sedimentary organic C to P ratios, especially the combined use of organic C/organic P and organic C/total P, provide a valuable record of redox conditions in Mediterranean bottom waters during sapropel formation.
► Post-depositional diagenesis does not seem to strongly affect the organic C to P ratios in sapropels on timescales up to at least 700 kyrs.
► The intensity of the African monsoon, delivering fresh water and nutrients for enhanced stratification and productivity in the Mediterranean, plays a crucial role in controlling redox conditions in the bottom waters during sapropel formation.
► Combined use of organic C to P ratios and high-resolution P fractionation indicates that the paleoceanographic conditions during formation of sapropel S5 were characterized by high variability in biological productivity in surface waters on short timescales, while bottom waters remained strongly reducing throughout this period.