EVIDENCE FOR THE EVOLUTION OF AN OXYGEN MINIMUM LAYER AT THE BEGINNING OF S-1 SAPROPEL DEPOSITION IN THE EASTERN MEDITERRANEAN

The sediments of the eastern Mediterranean basin contain a series of o rganic-rich sapropels intercalated with organic-poor nannofossil oozes . Until recently the timing of the onset of sapropel formation was not known accurately because of the low resolution achievable by conventi onal radiocarbon dating. Compilation of all available C-14-AMS dates s how that the base of S-1 (the most recent sapropel) was initiated 8800 years B.P. (C-14 age corrected by 400 years for reservoir effect) und er a 500 m water column and moved progressively into deeper water reac hing depths of 3500 m at 8200 years B.P. The linear correlation betwee n the age of S-1 onset and water depth (r = 0.71) suggests that format ion of sapropels moved into deeper water al a rate of similar to 1000 m/200 year. A model is suggested in which export production which sank below the well-mixed surface layers (similar to 500 m) was respired c onsuming dissolved oxygen in the Levantine deep water until a threshol d value was reached when sapropels began to be preserved in the sedime nt. This resulted in a progressively deepening oxygen minimum zone wit h time until eventually the entire deep water in the basin was oxygen depleted. Assuming that the threshold value for sapropel formation was complete anoxia, it was calculated that primary productivity in the b asin during the deposition of S-1 was a factor of 5 greater than that found at present. (C) 1997 Elsevier Science B.V.