200 Year interruption of Holocene sapropel formation in the Adriatic Sea

Abstract. An interruption of Holocene sapropel S₁ is found in cores from various subbasins of the eastern Mediterranean. In core IN68-9 from the Adriatic Sea, sapropel S₁ is dated between 8300 and 6340 BP, interrupted between 7100 and 6900 BP (¹⁴C years uncorrected for reservoir age). Lithology and variations in the foraminiferal faunas suggest that the interruption is genuine, and not the result of resedimentation. The results indicate that S₁ was deposited within a period of enhanced levels of productivity (resulting from increased seasonal contrasts) which started around 9300 BP and ended around 5200 BP. The onset, interruption, and final ending of S₁ deposition in the Adriatic Sea, however, appear to have been triggered by changes in ventilation of the basin related to changes in sea surface temperature (SST). Although the rough estimates of SST change are relatively small (< 2°C), they still are significant when compared with the relative SST changes considered necessary to upset convection in the Adriatic. Moreover, recent studies show that the influence of the inferred temperature changes should be viewed in combination with that of reduced salinities due to (1) the deglaciation, and (2) increased humidity in the eastern Mediterranean area during the deposition of S₁. The lithological and benthic foraminiferal evidence that sapropel formation in the Adriatic Sea ended around 6340 BP contrasts with the conclusion from a recent geochemical study that sapropel formation in the open eastern Mediterranean would have ended as late as 5000 BP. More significantly, the results of the present study combined with other reports on sapropel interruptions suggest that the process of sapropel formation is not a very stable mode in the basin, but that it may be relatively easily interrupted in response to subtle rearrangements in the balance between productivity and, especially, deep water ventilation.