Dissolution behaviour of bryozoan sediments: taphonomic implications for nontropical shelf carbonates

A. M. Smith, C. S. Nelson, P. J. Danaher

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Diagenesis of carbonate sediments on temperate marine shelves is commonly destructive, in contrast to a dominantly constructive regime in tropical waters. In some cases dissolution can be important in temperate shelf waters, particularly affecting the metastable carbonate minerals. If dissolution does occur, it can be an important taphonomic process, reducing skeletal remains available for fossilization and biasing fossil assemblages. The dissolution behaviour of bryozoans has been investigated in an acid bath laboratory experiment using nontropical carbonate sediments from the New Zealand shelf. Bryozoans of various shapes (growth forms) and compositions (low-to high-Mg calcite and aragonite) were extracted and their weight loss over time was measured. Results show that variations in stability of carbonate mineralogies are insufficient to predict relative dissolution rates. If shelf dissolution occurs, even if only periodically, one might anticipate that the less stable aragonite and high-Mg. calcite skeletons would dissolve first. In fact, this experiments shows that small delicate skeletons with non-compact morphologies dissolve most rapidly, apparently with little regard for mineralogy. Fossil bryozoan assemblages from temperate marine shelf environments may therefore not only reflect the preferential loss of certain aragonitic species, but also the loss of small delicate species. New Zealand's nontropical Tertiary bryomol limestones often lack aragonitic skeletons; they may also be relatively depleted in small, delicately branched bryozoans. In contrast, tropical shelf carbonate sediments can retain more delicate skeletons through early cementation and lack of dissolution. A difference in character between tropical and temperate carbonate shelf deposits may therefore be in the proportion of small delicate bryozoan colonies.

Original languageEnglish
Pages (from-to)213-226
Number of pages14
JournalPalaeogeography, Palaeoclimatology, Palaeoecology
Issue number3-4
Publication statusPublished - 1 Jan 1992
Externally publishedYes

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