Page 23 - Carbonate Sedimentology and Sequence Stratigraphy

14 WOLFGANG SCHLAGER

The most conspicuous abiotic precipitate is cement
formed in the pore space during the early stages of diage- Autotrophic producers Heterotrophic producers
nesis when the deposit was still in the depositional environ-
ment. Burial cements are excluded from the abiotic carbon- Cyanobacteria (only bioti- Foraminifera
ate factory because they are not derived from sea water but cally induced precipitates) Archaeocyathans
largely from remobilized sedimentary material. The case for Coccolithophorid algae Sponges (e.g. pharetronids,
abiotic origin is particularly strong for acicular aragonite ce- (Haptophyceae) stromatoporoids, chaetetids)
ments. Acicular magnesian calcites may be biotically influ- Green algae (such as dasy- Ahermatypic corals (Sclerac-
enced (Morse and Mackenzie, 1990). cladaceans, codiaceans) tinia)
Associated with tropical skeletal carbonate we find abi-
Red algae Most bivalves
otic precipitates in the form of ooids. Ooids form in high-
Gastropods
energy environments by stepwise accretion on a nucleus.
Autotrophic production Cephalopods
Field observations and laboratory experiments indicate a
via symbionts Arthropods (e.g. trilobites,
growth history of alternating phases of accretion and rest.
ostracodes, barnacles)
The degree of organic influence on the precipitation pro- Many larger foraminifers Brachiopods
cess remains a matter of debate. However, two arguments Hermatypic corals (Sclerac- Bryozoans
tip the balance in favor of abiotic precipitation: (1) growth tinia) Echinoderms
of aragonitic, Bahama-type, ooids in the laboratory where Certain bivalves (Tridacnids,
the precipitation was essentially abiotic and organic matter rudists?)
had only a modulating effect (Davies et al., 1978) and (2) the
similarity of ooids and cements in terms of mineralogy and
chemical signature (Morse and Mackenzie, 1990). It seems Fig. 2.2.— Important autotrophic and heterotrophic carbonate
that the role of organisms and organic matter in the forma- producers.
tion of ooids is not sufficient to significantly alter the abiotic
controls (Morse and Mackenzie, 1990; see Reitner et al., 1997
for contrasting view on Great-Salt-Lake ooids). of the respective organisms (such as algae, foraminifera or
The origin of carbonate in whitings, clouds of carbonate corals); the organisms, in turn, are influenced by the con-
suspended in sea water, is a much debated issue. Morse and ditions of the sea they live in, particularly light, tempera-
Mackenzie (1990) conclude that abiotic precipitation (prob- ture and water chemistry (for instance the degree of carbon-
ably on nuclei of suspended sediment) is very likely for Ba- ate saturation of the sea water). To appreciate the effects
hamian whitings. However, in-situ experiments by Yates of various environmental factors we need to recall the two
and Robbins (1999) strongly suggest that blooms of unicel- fundamental types of metabolism introduced on p. 9. Au-
lular algae trigger the first precipitation, probably followed totrophic organisms nourish themselves by utilizing inor-
by extended abiotic growth of the original, biotically indu- ganic materials to synthesize their own living matter; het-
ced precipitates (Yates and Robbins, 1999, p. 135). I consider erotrophic organisms have to rely on organic material to do
the carbonate mud from whitings a mixture of biotically in- so. Autotrophic organisms among the carbonate produc-
duced and abiotic precipitation. ers are almost exclusively photo-autotrophic: they perform
The occurrence of acicular cements, ooids and whitings photosynthesis and thus depend on light for their liveli-
in modern oceans indicates strong control by inorganic ma- hood. Some carbonate-secreting organisms are themselves
rine chemistry of the sea water. In first approximation, oo- heterotrophs but live in symbiosis with autotrophic algae.
lites, aragonitic sea-floor cements and whitings occur in the As a result, the system of host plus symbiont becomes au-
zone of highest carbonate supersaturation in the ocean – totrophic. Fig. 2.2 gives an overview of important auto-
the mixed layer of the tropical seas. Ooids and whitings trophic and heterotrophic carbonate producers. Note that
are restricted to this zone, acicular cements are most abun- the metabolism of extinct groups can only be deduced from
dant there. In the temperate latitudes, sea-floor cementation circumstantial evidence.
is rare and destructive sea-floor diagenesis tends to dom-
inate. The correlation of needle cements, ooids and whit- Light is arguably the most important control on skeletal
ings with sea-water chemistry is very helpful in predicting carbonate precipitation because of the dominance of photo-
at least first-order trends in the geologic record and warrants autotrophic organisms in carbonate production – at least in
their classification as principally abiotic precipitates in spite the Cenozoic. Photosynthesis is a complex, and only partly
of evidence of some biotic influence. understood process. The basic reaction may be simplified
as
Biotically controlled precipitation
CO 2 +H 2 O + solar energy→ HCHO + O 2
The majority of carbonate material in modern oceans is
precipitated as highly structured skeletons of organisms. where HCHO represents a simple summary formula for or-
Precipitation is primarily controlled by the biochemistry ganic matter. The formula clearly illustrates the link be-