Chemistry Reference
In-Depth Information
involved in skeleton formation and the states of preservation of the fossil material being
analyzed. Much attention has been paid to the latter when it comes to fossils extracted
from strata now on the continent, but surprisingly little attention seems to be paid to the
preservation states of shells of marine organisms in deep-sea sediments. We will focus,
however, on the many open questions that concern the signals embedded in biogenic
minerals and in particular questions related to the biological processes overriding the
environmental signals—the so-called vital effect (Urey et al. 1951).
The first quantitative use of biogenic minerals for extracting information about the
environment of formation was the demonstration that the stable oxygen isotopic
composition of some taxa (mollusks in particular) reflects in part the temperature of the
water in which the mollusks lived (Urey et al. 1951). Epstein et al. (1951) noted however
that not all biogenic minerals are deposited in isotopic equilibrium and that “the presence of
a physiological effect in the case of certain groups of animals such as the echinoderms and
corals, and plants such as a coralline algae, has seemed probable ….” (p. 424). This so-
called “physiological effect” in time came to be referred to by the geochemical community
as a “vital effect.” Furthermore, because most geochemists investigated biogenic materials
to reconstruct the paleoenvironments in which animals lived, the vital effect, just like post-
burial diagenetic effects, represented a severe hindrance to their goals.
To this day, the most common approach used to resolve the problem involves
identifying the taxa that faithfully record environmental parameters while avoiding the
others. This empirical approach has the obvious advantage that while one uses these
apparently “reliable” organisms, there is no need to worry about the other organisms with
their associated complications. The problem is that in the absence of a deep
understanding of the vital effects, it is well nigh impossible to know when the recording
is really faithful, particularly when differentiating between equilibrium and small non-
equilibrium effects. This problem has become acute in the last decade with the realization
that the climate is changing and that one of our most promising means of assessing the
significance of this change is to understand the fine details of past climatic changes.
Understanding vital effects is therefore of considerable current importance.
At this point, we do not have the answers, and a perusal of this topic describing the
current state of the art in the field of biomineralization will probably not provide many
answers. This is not because the subject matter is too difficult to resolve, but because the
interests of most of the scientists investigating the field of biomineralization are
elsewhere. Very few members of the geochemical community have made this their major
interest, but those who have made very significant contributions (e.g., Erez 1978;
McConnaughey 1989). A much more concerted effort, however, is needed.
Two basic categories of vital effects
Kinetic effects.
McCrea (1950) investigated disequilibrium effects in an
in vitro
system in which calcium carbonate was precipitated under varying conditions. The
greatest departures from equilibrium were found when the precipitates formed rapidly at
temperatures below 15ºC or above 60ºC. Epstein et al. (1953) identified this potential
problem in biogenic calcium carbonate when they noted that the calcium carbonate
deposited by abalones, in an attempt to fill up holes that had been drilled in their shells,
was not in oxygen isotopic equilibrium with the water in which the abalones lived (see
also Epstein and Lowenstam 1954). Although the effect was small, it was clearly
identifiable. They attributed it to the rapidity with which the material was laid down.
Since then many different observations of diverse organisms have been reported in which
mineral laid down rapidly appears to be out of isotopic equilibrium (Weber and
Woodhead 1970; Land et al. 1975; Erez 1978; McConnaughey 1989a; Ziveri et al. 2003).
