Geology Reference
In-Depth Information
shallow seas, but is also important in cool-water skel-
etal shelf carbonates (Akpan and Farrow 1984; Young
and Nelson 1988).
Microborers are studied in order to understand
• destructive processes resulting in the degradation of
skeletal elements and the production and accumulation
of fine-grained carbonate sediment,
• processes responsible for the formation of micrite
envelopes (cortoids; Pl. 52/6),
• distributional patterns that can be used to differenti-
ate modern and ancient marine environments (e.g. in-
tertidal and supratidal sequences, Hoffman 1985),
• the role of microborers as (paleo-)bathymetric and
(paleo-)ecologic indicators,
• the role of increased nutrient input to reefs that might
result in reef demise (Hallock 1988; Wood 1993).
Fig. 9.12.
Recent cold-water microborings
produced by ma-
rine fungi attacking the deep-water coral
Lophelia
. Microbor-
ings are not restricted to shallow-marine environments (see
Pl. 51/1-4), but also occur in the deep sea where bacteria,
fungi and bryozoans act as borers (Freiwald and Wilson 1998).
The abundance of fungal borings increases with water depth
from deeper subtidal to bathyal environments (see Fig. 9.16).
The SEM photograph of the resin cast shows linear fungal
borings and globular sporocysts developed within 70
m be-
low the surface of the coral. Propeller Mound, Northern At-
lantic, west of Ireland. Water depth about 700 m, water tem-
perature 10 °C. Courtesy of L. Beuck, Erlangen.
Microboring groups
Microborers include bacteria, algae and fungi that
penetrate substrate by etching. Boring foraminifera and
bryozoans are attributed to microborers, because the
dimensions of the trace fossils are smaller than 100
m.
Bacteria:
Non-photosynthetic
bacteria
and photo-
synthetic
cyanobacteria
occur in marine, limnic, flu-
vial and terrestrial environments. Both groups include
the oldest microborers known since the Precambrian
and recorded in carbonate rocks as old as 1700 Ma
(Zhang and Golubic 1987). Bacteria have various
shapes (spheres, bars, bent bars and spirals). The size
of non-photosynthetic bacteria ranges between 1 and
5
m with diameters between 0.2 and 1
m. Modern
endolithic cyanobacteria are represented by more than
20 genera (Budd and Perkins 1980).
Green algae
occur in marine and non-marine envi-
ronments. Some chlorophycean algae live within car-
bonate substrates or have endolithic stages (Golubic et
al. 1975); they are already recorded from Early Paleo-
zoic skeletal grains and ooids (Podhalanska 1984) and
possibly also from Precambrian rocks.
Red algae
are predominantly marine. Endolithic life
stages are known from only a few marine genera that
already occur in Early Paleozoic rocks.
Fungi
are heterotroph organisms without photosyn-
thetic pigments. Boring fungi occur in shallow and deep
marine environments down to 5000 m. Their trace fos-
sils are differentiated by the shape, size, mode of branch-
ing and sporangia (Glaub 1994). The oldest records of
boring fungi are from the Middle Ordovician.
Schönfeld 1996). They occur in groups characterized
by agglutinated and calcareous perforate tests; they are
known from cold-water and warm-water environments,
and are reported from Jurassic to Quaternary carbon-
ates (Venec-Pyre 1987; Baumfalk et al. 1983; Smyth
1988; Cherchi et al. 1990; Cherchi and Schroeder 1991).
Bryozoans:
Trace fossils of boring bryozoans can
be easily recognized in SEM images, because the trace
closely resembles the producer morphology and reflects
the shape of the body fossil. The trace corresponds to a
network that connects elongated chambers. These bryo-
zoans are differentiated by the shape, size and position
of the zooids, the relation between zooids and the
stolonial system, and the size of surface openings
(Pohowsky 1978). The diameter of the non-calcified
slender structural elements is less than 10
m. Most
boring bryozoans known from Jurassic to Quaternary
limestones are attributed to the Ctenostomata. The Pa-
leozoic record of boring bryozoans is scanty.
9.3.2 Recent and Fossil Macroborers
Macroborings occur in marine and non-marine envi-
ronments and contribute considerably to bioerosion in
marine shallow- and deeper-water, warm- and cold-
water settings (Ekdale et al. 1989). The most effective
Foraminifera:
Boring endolithic foraminifera appear
to be of greater importance for the degradation of skel-
etal grains than previously assumed (Freiwald and
