Chemistry Reference
In-Depth Information
Geological aspects of rock-forming organisms
It should be pointed out that foraminifera have played a major role in the
sedimentation history of the oceans and seas and have left impressive rock formations
built almost entirely of their shells. Notable are the Mesozoic and Cenozoic chalks in
Europe and the Middle Eastern deposits composed mainly of planktonic foraminifera and
coccolithophores. Numulitic limestones of the Eocene are made almost entirely of very
large benthic foraminifera (that may have had symbiotic algae), and fusulinids produced
high accumulations of limestone in the Permian. Deposition of these formations must
have modified the global carbon cycle of their times and are an important part of the
sedimentary rock record.
BIOMINERALIZATION IN FORAMINIFERA
Introduction
Foraminifera are unicellular calcifying ameba, taxonomically part of the Protista
(Loeblich and Tappan 1964, 1986, 1987). They are strictly marine organisms, although
recently there have been some reports of fresh water non-calcified granoreticulate ameba
that may be considered as foraminifera (Holzmann and Pawlowski 2002; Holzmann et al.
2003). Their evolution started in the mid-Paleozoic some 400 my ago, and their species
diversity is very high. Over 40,000 species have been identified in the fossil record, while
roughly 4,000 species are extant. These data suggest that the average rate of evolution per
species is 1/10,000 yr. This high rate of evolution, the coexistence of many species, and
the good preservation of their calcitic shells provide very detailed biostratigraphic
division of the geological record based on fossil foraminifera (Loeblich and Tappan
1964, 1986, 1987). As a result, much attention has been given to their shell structure and
texture and with the advent of SEM and TEM a large body of knowledge accumulated on
these subjects (e.g., Towe and Cifelli 1967; Hansen and Reiss 1972). Biostratigraphic
methods that are based upon foraminiferal zoning became the main tool for correlations
and reconstruction of subsurface geology that were highly needed for oil and gas
prospecting. This entire field has benefited greatly from the Deep Sea Drilling Project
(DSDP) and its successor the Ocean Drilling Project (ODP). These projects provided
continuous well-preserved records of oceanic sediments through the Cenozoic (e.g.,
Vincent and Berger 1981). In addition to obtaining well-preserved foraminifera, these
projects greatly assisted in the transformation from biostratigraphy to absolute
chronostratigraphic time scale (e.g., Berggren et al. 1995).
The process of biomineralization is a major modifier of the paleoceanographic
proxies stored in their shells. More reliable and new information will be extracted from
foraminiferal shells if their biomineralization process is well understood. In addition, the
foraminifera are very large eukaryotic cells that can be cultured in the laboratory and
provide a very informative experimental preparation to study biomineralization at the
cellular level. As will be shown below, these organisms are particularly suitable for light
and confocal microscopy. Finally, it should be mentioned that while calcification studies
in foraminifera can teach us a great deal about the interpretation of their trace elements
and isotopes, the latter may also teach us a most interesting lesson on biomineralization
mechanisms in these organisms.
Recent work (e.g., Langer et al. 1993, Darling et al. 2000, 2003; Huber et al.1997; de
Vargas et al. 2002; Pawlowski et al.2002) has shown that extant foraminiferal DNA
produces a taxonomy that in general fits very well the classical one based on shell
structure. Obviously new insights are obtained from these studies, but in general they
confirm the strong genetic control on shell structure and possibly function.
