Young death assemblages with limited time-averaging in rocky and
 Posidonia oceanica
 habitats in the Mediterranean Sea


 
 Death assemblages (DAs) are increasingly recognized as a valuable source to reconstruct past ecological baselines, due to the accumulation of skeletal material of non-contemporaneous cohorts. We here quantify the age and time-averaging of DAs on shallow subtidal (5–25 m) rocky substrates and in meadows of
 Posidonia oceanica
 in the eastern Mediterranean. We show that such DAs are very young–median ages 9–56 years–with limited time-averaging, one to two orders of magnitude less than on even nearby soft substrates. On rocky substrates, out-of-habitat transport is likely the main cause of loss of older shells. In
 Posidonia oceanica
 meadows, the root and rhizome system creates a dense structure–the
 matte
 –that quickly entangles and buries shells and limits the potential for bioturbation. The
 matte
 is, however, a peculiar feature of
 Posidonia oceanica
 , and age and time-averaging in meadows of other seagrass species may be different. The young age of DAs in these habitats requires a careful consideration of their appropriateness as baselines. The large difference in DA age between soft substrates, subject to numerous studies, and hard and seagrass substrates, rarely inspected with geochronological techniques, implies that DA dating is important for studies aiming at using DAs as baselines.
 
 
 Supplementary material at
 https://doi.org/10.6084/m9.figshare.c.6315678
