Sedimentary organic carbon to phosphorus ratios as a redox proxy in long-term records from the Mediterranean

n this study, organic carbon to organic phosphorus (C_org/P_org) and total phosphorus (C_org/P_tot) 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 C_org/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 C_org/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 C_org/P_org 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 C_org/P_tot ratios in S5 are variable, reflecting short-term changes in marine conditions (productivity/redox) and sediment chemistry. The C_org/P_org and C_org/P_tot ratios thus provide valuable insight into paleoceanographic conditions and pathways of P cycling for (Mediterranean) sediment records extending back at least ~ 600–700 kyrs.