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Taphonomic signatures and patterns of test degradation on tropical, intertidal benthic foraminifera

Berkeley, Andrew and Perry, Christopher T. and Smithers, Scott G. (2009) Taphonomic signatures and patterns of test degradation on tropical, intertidal benthic foraminifera. ISSN 1872-6186

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Abstract

The preservational condition of benthic foraminifera from tropical intertidal sediments in Cleveland Bay, north Queensland, Australia was examined in order to provide systematic descriptions of taphonomic signatures and semi-quantitative estimates of test degradation. The primary aim was to improve understanding of the processes of test degradation experienced by intertidal foraminifera during early burial. Eight common tropical, intertidal foraminiferal species were examined: Arenoparrella mexicana, Miliammina fusca, Miliammina obliqua and Trochammina inflata from two upper mangrove stations, and Ammonia aoteana, Elphidium excavatum clavatum, Pararotalia venusta and Parrellina hispidula from two mudflat stations. Taphonomic alteration was broadly divided into surface/textural features and larger-scale, structural losses to tests. Surface alteration of calcareous tests was dominated by the etching and roughening associated with dissolution processes, with limited evidence for bioerosion and abrasion. Both surficial etching and larger-scale structural damage were closely related to test architecture, with degradation most intense on porous chamber walls, as opposed to non-porous-areas, interlocular walls and chamber tips. Furthermore, all calcareous species exhibited consistent pathways of degradation whereby the most recently formed chambers were the most vulnerable to degradation. The correspondence between larger-scale test loss and these patterns of surface textural alteration, suggests calcareous test degradation is primarily driven by dissolution. Degradation of agglutinated tests contrasts in a number of ways. Pristine tests of all four species examined exhibited a smooth, outer organic coating, which may be missing from other specimens. The extent to which this initial loss effects further test breakdown is uncertain, but appears to differ between species. M. obliqua and some specimens of A. mexicana exhibited structural damage which corresponded to clear cement losses and agglutinant disaggregation. More generally, agglutinated species displayed fracturing and test breakages which bore no conspicuous relation to cement loss. Furthermore, neither surface alteration nor structural loss affected any part of agglutinated tests preferentially, and therefore pathways of degradation were inconsistent across specimens. The fact that agglutinated tests degrade in a more arbitrary manner, compared with calcareous tests, is taken as evidence that their degradation is (1) not entirely controlled by test architecture, and (2) at least partially driven by physical/mechanical processes. We suggest that, if agglutinated tests are not already inherently weaker than their calcareous counterparts, the loss of organic cements may sufficiently weaken agglutinated tests so as to increase susceptibility to physical damage. Tests were also graded according to the extent to which they were structurally degraded. Average taphonomic grades and taphonomic variability (i.e. standard deviation) for all species at Cocoa Creek were statistically invariant with depth, contrary to expectations based on the concept of progressive burial through a taphonomically-active zone. This indicates that shallow sub-surface sediments are significantly homogenised, and that sediment mixing and infaunal test production imparts a considerable effect on the development of surface and sub-surface dead assemblages at the study site.

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