Depth as an overarching environmental variable modulating preservation potential and temporal resolution of shelly taphofacies

Abstract

In the marine realm, the interpretation of taphofacies relies heavily on how oceanographic and sedimentary conditions affect the preservation state of fossils. Several taphonomic variables either covary with depth or are directly influenced by depth. Facies-level factors rather than broad, basin-scale parameters influence the taphonomic profile of mollusc death assemblages according to actualistic and experimental evidence. To determine the possible relation between depth and the taphonomic conditions of multiple species of bivalve remains, we used seven samples gathered over a comprehensive bathymetric gradient (from 7 to 150 m below mean sea level; topmost 10- to 20-cm layer, roughly corresponding to the taphonomically active zone). We selected samples from predominantly muddy facies on the southern Brazilian shelf (SBS). The taphonomic damage profile (TDP) was measured using site samples based on a standard taphonomic analysis (categorical scoring system) of shells and fragments larger than 4 mm, to identify site damage patterns. Restricting the sedimentary grain size (samples from fine sediments) enabled the determination of the variation in damage with depth among the samples. Constrained analysis of proximities (CAP) revealed that up to 46% of the taphonomic variation observed was related to variation in depth (with approximately 28% unexplained by environmental factors). Part of the unexplained fraction was due to the effect of temporal mixing, which is predictable along large-scale patches but is inversely linked to the TDP. Our results show that taphonomic analysis, considering large spatial scales in recent environments, can explain the variations present in shell beds that formed during distinct time periods of the evolution of a Quaternary sedimentary basin.