Page 22 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 2

Principles of carbonate production

Three rules capture the peculiar nature of carbonate depo- c.1 controlled precipitates by photo-autotrophic or-
sitional systems – carbonate sediments are largely of organic ganisms that generate organic matter from dis-
origin, they can form wave-resistant structures and they are solved substances and sunlight, and
easily altered by diagenesis because the original minerals c.2 controlled precipitates by heterotrophic organisms
are metastable. The implications of these rules are perva- that are independent of light but require particu-
sive. We will encounter them throughout the chapters of late organic matter for food.
this book, starting with the present review of principles that Currently, organisms have the first hand on precipitation in
govern the production of sediments and the growth of reefs. most carbonate settings and abiotic precipitation will kick in
if biotic fixation is insufficient. Thus, abiotic precipitation is
a sort of “default setting” in the carbonate system of modern
MODES OF MARINE CARBONATE PRECIPITATION
oceans (term credit to Ron Perkins).

Precipitation of solid matter from the dissolved load of
the sea occurs either abiotically, governed by inorganic ther- MARINE CARBONATE PRECIPITATION
modynamics and reaction kinetics, or biotically as a conse-
quence of the metabolism of plants and animals. Precipita-
tion of marine evaporites is an example of an abiotic pro-
abiotic biotic
cess, precipitation of marine opal by diatoms or radiolaria ("default setting")
an example of a biotic one. Marine carbonate precipitation
proceeds along abiotic and biotic pathways and this makes
it particularly diverse and complex. The interplay of abi- biotically induced biotically controlled
otic and biotic processes during four billion years of organic (organomineralic + (skeletal)
biomineralic, mainly
evolution and environmental change has led to a stunning microbial, precipitates)
diversity of precipitation mechanisms that are far beyond
the scope of this book. However, even if one focuses on heterotrophic autotrophic
the practically relevant aspects of carbonate precipitation, (mainly photosynthetic)
the subdivision into abiotic and biotic precipitation is in-
adequate. Work in the past two decades in particular has Fig. 2.1.— Pathways of carbonate precipitation in aquatic envi-
shown that it is sedimentologically very advantageous to
ronments – a cascade of options governed by the degree of biotic
further subdivide the biotic category. I follow Lowenstam influence. After Schlager (2000), modified.
and Weiner (1989) who recognized three degrees of biotic
influence on precipitation in general and on carbonates in
particular: (Fig. 2.1)
a. Abiotic (or quasi-abiotic) precipitates where biotic ef-
fects are negligible. Abiotic marine carbonate precipitation
b. Biotically induced precipitates where the organism sets
the precipitation process in motion but organic influ- Organisms and organic matter are so common in depo-
ence on its course is marginal or absent. The reaction sitional environments and have so many ways of influenc-
takes place outside the cell and the product is very sim- ing carbonate precipitation that it is virtually impossible
ilar, often indistinguishable, from abiotic precipitates. to demonstrate that a particular carbonate precipitate from
c. Biotically controlled precipitates where the organism de- a natural environment is totally abiotic. However, there
termines location, beginninng and end of the process, are carbonate materials for which the organic influence, if
and commonly also composition and crystallography of present at all, is very subtle. Their texture and mineralogy
the mineral. All skeletal carbonate falls in this category. can be reproduced abiotically in the laboratory and their nat-
From an environmental perspective, it is important to ural occurrence is governed by first-order trends in ocean
further subdivide skeletal carbonates into: chemistry. These materials are included here.

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