THE STRUCTURE AND FORMATION OF TIME-AVERAGED FOSSIL RECORDS: AN EMPIRICAL MODEL FROM AN OPEN SILICICLASTIC SHELF

Here, we assess how skeletal elements are incorporated into “ordinary” sedimentary records, i.e., records that have accumulated under fully oxygenated conditions and without exceptional burial events, using 14C-calibrated AAR dating of bivalve shells from surface and core samples along gradients of water depth, sedimentation, and bioturbation on the southern California siliciclastic shelf. The net effects of these factors has always been difficult to predict, even assuming constant skeletal input. Environmental energy favors both exhumation and destruction of bioclasts (and can thus both promote and reduce their time-averaging); lower sedimentation rates are similarly double-edged (delay in permanent burial), as is bioturbation (can retain bioclasts in the surface mixed layer but promotes their destruction via sediment aeration). Studies including ours have shown that assemblages shift down-core from (a) strongly right-skewed age-frequency distributions in the surface mixed layer (most bioclasts are young, but with a long tail of old shells) to (b) flatter and more normal age distributions with older median ages. Our modeling has already shown the role of (a) rapid initial postmortem rates of shell loss, which slow as shells age (making the L-shape) and (b) burial of an assemblage beyond the reach of bioturbation, starving it of new shells while older cohorts dwindle. We now show that (i) sedimentation rate is the primary determinant of the duration of time-averaging, despite the perils of delayed burial; (ii) deep and/or intense bioturbation also net increases time-averaging, and moreover (iii) decreases the temporal distinctiveness of successive core increments, as expected if the negative effects of bio-irrigation are minor. These new results thus underscore (iv) the importance to a permanent fossil record of shells becoming less reactive as they age within surficial increments, i.e. of becoming better able to withstand the taphonomic rigors of time-averaging. Such permanent, fully buried assemblages capture ≤1% of all skeletal input to the surface mixed layer (differential preservation of species or ontogenetic age classes resides there), and they have coarser temporal resolution than surficial death assemblages, a qualitatively different source of paleontologic information.