Differential preservation of gravelsized bioclasts in Alpheid-versus callianassid-bioturbated muddy reefal sediments

Infaunal bivalve shells were extracted from sediment cores taken in shallow seas off Phuket, South Thailand, and were scored for four types of taphonomic degradation to assess the effect that different types of bioturbation have on the preservation of such bioclasts. This gravel-sized fraction of off-reef biogenic (bivalve) sediments showed variation in the extent of degradation, depending on whether it was found in an area bioturbated by callianassid or alpheid shrimps. Cores taken in the alpheid area have a higher proportion of grains with poorly defined shape and ornamentation, a loss of surface luster and high levels of microbioerosion. In contrast, those from the callianassid area have a higher proportion of shiny, sharply defined fragments and a lower level of microbioerosion than those from the alpheid area. It is concluded that the different nature of bioturbation in the two areas accounts for the difference in preservation. In working the sediment, callianassids bury the gravel fraction, while alpheids repeatedly bring it to the sediment surface, thus exposing it to higher levels of physical and microbiological erosion. The shrimps' activities result in a difference in preservation potential of fine and coarse sediments, and this taphonomic bias will affect taphofacies interpretation. Rapid burial of gravel by callianassids also has implications for time-averaging, as a shell fragment's condition is more indicative of its residence time at the surface-water interface than its age.