Bioturbation and sedimentation rate control of benthic microfossil taxon abundances in surface sediments: A theoretical approach to the analysis of species microhabitats

Abstract

Theoretical consideration of micro-organism shell production in depth stratified habitats within the bioturbated layer of the sediments shows that microfossil taxon abundances need not be constant throughout the sediment mixed layer. Rather, each taxon will have an abundance profile, through the mixed layer, whose shape will depend on taxon depth zonation in life and on sedimentation and bioturbation rates. A computer model integrating depth stratified taxon production with varying sedimentation and mixing rates yields the following observations: (1) For experiments run with sedimentation rates ranging from 20 to 2 cm/kyr. and mixing rates ranging from 40 to 150 cm 2/kyr., significant assemblage variation through the mixed layer results from taxon depth stratification in the sediments. (2) (2) Infaunal taxon fossil abundances increase non-linearly with depth in the bioturbated zone, down to the level of the taxon habitat, below which abundance is constant. (3) Surface sediment assemblages of benthic microfossils will rarely accurately represent total taxon production in the life zone of the sediments. Instead, surface sediment assemblages can be strongly affected by sedimentation rate and sediment mixing rate. The best assemblage representation of total taxon production occurs at the base of the bioturbated zone. (4) Sediment abundances of each taxon's shells can be used to determine long term taxon depth zonation within the sediments. This is because each taxon's shell abundance increases down to the life habitat depth horizon and then is constant below it. The model results are supported by data from a high sedimentation rate section collected in the western Mediterranean. Benthic foraminiferal taxon abundance profiles resemble those generated from theoretical calculations.