Bioturbation increases time averaging despite promoting shell disintegration: a test using anthropogenic gradients in sediment accumulation and burrowing on the southern California shelf

Bioturbation can increase time averaging by downward and upward movements
of young and old shells within the entire mixed layer and by accelerating
the burial of shells into a sequestration zone (SZ), allowing them to
bypass the uppermost taphonomically active zone (TAZ). However,
bioturbation can increase shell disintegration concurrently, neutralizing
the positive effects of mixing on time averaging. Bioirrigation by
oxygenated pore water promotes carbonate dissolution in the TAZ, and
biomixing itself can mill shells weakened by dissolution or microbial
maceration, and/or expose them to damage at the sediment-water interface.
Here, we fit transition rate matrices to bivalve age-frequency
distributions from four sediment cores from the southern California middle
shelf (50-75 m) to assess the competing effects of bioturbation on
disintegration and time averaging, exploiting a strong gradient in rates
of sediment accumulation and bioturbation created by historic wastewater
pollution. We find that disintegration covaries positively with mixing at
all four sites, in accord with the scenario where bioturbation ultimately
fuels carbonate disintegration. Both mixing and disintegration rates
decline abruptly at the base of the 20-40 cm-thick, age-homogenized
surface mixed layer at the three well-bioturbated sites, despite different
rates of sediment accumulation. In contrast, mixing and disintegration
rates are very low in the upper 25 cm at an effluent site with legacy
sediment toxicity, despite recolonization by bioirrigating lucinid
bivalves. Assemblages that formed during maximum wastewater emissions vary
strongly in time averaging, with millennial scales at the low-sediment
accumulation non-effluent sites, a centennial scale at the effluent site
where sediment accumulation was high but bioturbation recovered quickly,
and a decadal scale at the second high-sedimentation effluent site where
bioturbation remained low for decades. Thus, even though disintegration
rates covary positively with mixing rates, reducing postmortem shell
survival, bioturbation has the net effect of increasing the time averaging
of skeletal remains on this warm-temperate siliciclastic shelf.