Effects of disintegration on the incompleteness of the Holocene fossil record

The mechanisms controlling incompleteness and time averaging of fossil assemblages include (1) sediment accumulation, (2) mixing, and (3) skeletal disintegration. Although sediment accumulation is a major factor controlling these attributes of the fossil record, predicting the effects of mixing and disintegration depends on their interaction and can be counterintuitive. Stochastic models of fossil preservation show that even when disintegration is fast in the taphonomically-active zone (surface well-mixed layer, SML), its effect on time averaging and incompleteness in the historical layer can be minimized when shells can be sequestered by burrowers into the lower parts of the incompletely-mixed layer. To assess the prevalence of this sequestration effect, we estimate the covariation between incompleteness on one hand and disintegration, burial and exhumation on the other hand, fitting age-frequency distributions from 18 Holocene sediment cores to stochastic transition-rate matrices. The model has five parameters, including burial below the SML (a function of both sediment accumulation and downward mixing), exhumation into the SML, disintegration in the SML and below it, and disintegration of diagenetically-stabilized shells that were exhumed to the SML. We find that the majority of cores show a major decline in disintegration within the upper decimeters and exhibit the persistence of very old shells in the surface seabed, indicating some role of their diagenetic stabilization below the SML. Both burial and exhumation positively covary with disintegration in the SML. However, the incompleteness is negatively related to burial (varying between 90-99% at sites with slow sediment accumulation and between 50-90% at sites with fast sediment accumulation) but does not correlate with disintegration in the SML. These results indicate that although bioturbation positively covaries with disintegration in the SML, it can also increase preservation potential of some shells by transferring them below the SML.