Page 29 - Carbonate Sedimentology and Sequence Stratigraphy

20 WOLFGANG SCHLAGER

Latitudinal zonation of skeletal production. Skeletal carbon- duced by micro-organisms, mostly bacteria and cyanobacte-
ate production changes very significantly with latitude. ria. Micrite is a major, often dominant component of these
The differentiation into tropical and cool-water carbonates deposits. The term “(mud) mound” is commonly used as
is widely applied and often further subdivided (Figs 2.8, a field-geologic term (Wilson, 1975; James and Bourque,
2.10, 2.11; Lees, 1975; Tucker and Wright, 1990; James 1992). For the deposits themselves, the term “microbialite”
and Kendall, 1992; James, 1997). Tropical carbonates are is widely used. The drawback is that the word has a
dominated by photosynthetic organisms and usually in- strong genetic connotation. If one wishes to avoid this ex-
clude metazoan reefs, abundant green algae and larger plicit statement on genesis, the term “automicrite” is recom-
foraminifera. Cool-water carbonates lack these deposits and mended. It stands for autochthonous micrite as opposed to
consist mainly of skeletal sand and gravel derived from mol- allochthonous micrite that was transported and deposited
luscs, bryozoans, smaller foraminers and red algae. The con- as fine-grained sediment (Wolf, 1965). Whether the micrite
tribution of photo-autotrophs to cool-water carbonate pro- formed as a rigid precipitate can often be deduced from thin
duction is limited to red algae that are normally not the sections or polished slabs.
dominant component. Consequently, the depth window of The past decade brought enormous progress on the
cool-water carbonate production is much wider. orgin of mud mounds and other automicrite deposits. The
It should be noted that the zone of “tropical carbonates” combination of detailed field work, petrography and col-
◦
reaches to 30-35 of latitude and thus extends from the laboration with biologists and organic chemists has led to
humid tropics to the desert belt of the horse latitudes (Fig. detailed insight in a geologically very important carbonate
2.9). The cool-water realm extends over several climate precipitation mode that differs significantly from the more
zones, reaching from the northern limit of the desert belt to conspicuous skeletal mode (Monty et al., 1995; Reitner et
the polar regions (Fig. 2.11). The differences of the tropical al., 1995a; 1995b; Neuweiler et al., 2003).
and the cool-water realm are not restricted to the skeletal
material. Cool-water carbonates also are distinct by the The environmental controls on microbial precipitation are less
absence of mud, shallow reefs and oolitic sand shoals with well known than those of skeletal precipitation. An im-
early cementation. The lack of reefs and cemented shoals portant property of the microbial mode of precipitation is
has fundamental implications for the depositional anatomy. its near-independence of light. Microbial precipitates may
form in the photic zone or below, certainly to depths of 400
Nutrients. Contrary to common expectations, high-nutrient meters. On modern reefs, the microbial deposits are best
environments are unfavorable for many carbonate systems. developed in the forereef environment. However, stromato-
Nutrients, to be sure, are essential for all organic growth, lites in the uppermost photic zone (e.g. Reid et al. 2000) and
including that of carbonate-secreting benthos. However, automicrite in the interstices of coral framework (Camoin et
the carbonate communities dominated by autotrophs, such al. 1999) demonstrate that the microbial mode of carbonate
as reefs, are adapted to life in submarine deserts. They fixation finds its niches even in the prime domains of skele-
can produce their organic tissue with the aid of sunlight tal production.
from sea water with very low nutrient levels and are very An important chemical requirement is supply of alkalinity
efficient in recycling nutrients within the system. In high- in the form of the anions HCO and CO 2− . A likely source
−
3
3
nutrient settings, the carbonate producers are outpaced by of alkalinity is sulfate reduction combined with decay of or-
soft-bodied competitors such as fleshy algae, soft corals or ganic matter in oxygen-deficient layers of the ocean such as
sponges. Furthermore, the destruction of reef framework the oxygen minimum of the thermocline. The estimated wa-
by bio-erosion increases with increasing nutrient supply. ter depth and organic-rich ambient sediments of many mud
mounds support this assumption.
Salinity varies relatively little in the open-marine environ- Whether temperature sets practically relevant limits for
ment. The effects of these subtle variations on carbonate microbial carbonate precipitation is unclear. Mud mounds
production are not well known. Where access to the open
seem to be best developed in low latitudes. However, the
ocean is restricted, salinity varies greatly and significantly
paleo-latitude of many Paleozoic mounds is not well con-
affects the diversity of the biota (Fig. 1.16). The combined
strained and narrow latitudinal restriction is not to be ex-
effects of salinity and temperature variations allow one to
pected with a production system that demonstrably func-
subdivide carbonate environments (Fig. 2.12).
tions at low light levels and in indermediate water depths,
i.e. at temperatures significantly below tropical surface tem-
Biotically induced precipitation
peratures.
In the last two decades, it has been demonstrated that a
subdivision of shoal-water carbonates into abiotic and biot- Precipitation modes in comparison
ically controlled (skeletal) material is inadequate. A signifi-
cant portion of the non-skeletal carbonate material has been The boundaries of the three precipitation modes are gra-
precipitated under the influence of organisms and thus can- dational. The degree of biotic influence in the induced
not be classified as abiotic. Commonly precipitation is in- and controlled categories varies considerably and even the