Geology Reference
In-Depth Information
Fig. 9.10. Distribution patterns of micro-encruster associations in Late Jurassic reefs developed in different parts of carbon-
ate ramps. Modified from Schmid (1996).
cyanobacteria, rhodophycean algae, various groups of
sessile foraminifera (e.g. acervulinids), corals, hydro-
zoans, sponges including stromatoporoids and chaet-
etids, bryozoans, and microproblematica (e.g. Litho-
codium and Bacinella ).
al. 1983), and affects carbonate rocks, life or dead skel-
etons as well as hardened sea floors. Endolithic borers
(algae, polychaetes) and grazers (e.g. herbivorous fishes
and sea urchins) modify and redistribute carbonate pro-
duced by reef-building organisms. The relative impor-
tance of endolithic borers and grazers varies between
reef environments and with time of exposure (Kiene
and Hutchins 1994). Macroborers are responsible for
extensive substrate erosion, sediment production and
the generation of secondary porosity in reef frame-
works.
Bioerosion occurs in marine warm-water and cold-
water carbonate environments and is controlled by il-
lumination, nutrients, sedimentation rate, siliciclastic
influx and water depth, as drastically shown by the com-
positional difference of boring associations in shallow
warm and deep cold-water environments. The latter,
exemplified by deep-water Lophelia reefs in the Atlan-
tic are characterized by the abundance of non-auto-
trophic bacteria, heterotrophic fungi as well as clionid
sponges and bryozoans. Sediment production is high
(Freiwald 1998), sometimes competing with figures
known from shallow tropical environments (up to 0.5
to 2.5 cm/year).
The interest in bioerosion and boring organisms has
increased significantly during the last few decades. A
bibliography of papers dealing with modern and fossil
micro- and macroborers includes more than 800 refer-
ences (Radtke et al. 1997).
9.3 Bioerosion, Boring and Grazing
Organisms
This chapter deals with the destruction of carbonates
substrate by boring organisms and with the nature, in-
tensity and significance of bioerosion. The term bio-
erosion (Neumann 1966) describes the processes by
which biological activity destroys hard substrates. Bio-
erosion contributes significantly to sediment produc-
tion, controls early diagenetic processes, and the tapho-
nomic history of fossils.
Many fine-grained carbonate particles, seen in thin
sections, may be the result of biodegradation by bor-
ing. Especially particles in the silt-size range are can-
didates for this interpretation, as shown by SEM stud-
ies. In coral reefs, bioerosion depends on numerous
environmental factors such as light availability, nutri-
ent supply, and water depth. Boring occurs in shallow-
and deep-marine, freshwater and terrestrial environ-
ments (contributing to phytokarst formation and the
degradation of limestones and marbles used as build-
ing stones and for art sculptures (e.g. Anagnostidis et
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