Chemistry Reference
In-Depth Information
mechanism because Mg concentrations in seawater are 5 times higher then Ca. Hence this
subject requires careful further investigation, particularly since Mg content in foraminifera
shells is used now as a paleotemperature proxy (e.g., Lear et al. 2002). It should also be
remembered that the primary calcite is rich in Mg (up to 20 mole %), while the secondary
calcite is much lower in Mg. Different proportions of primary and secondary calcite may
bias the paleotemperature estimates based on Mg content. Or alternatively the proportions
between these to types of calcite is determined by temperature.
There are at least two possibilities to explain the exclusion of Mg from the shells.
One is the removal of Mg from the seawater vacuoles by an active process (a pump). The
other is discrimination against Mg during the formation of the mineral phase over and
above the normal distribution coefficient. Raz et al. (2000) did note that Ca was enriched
in the cores of spherulites composed of amorphous calcium carbonate, leaving the
periphery with more Mg relative to Ca. There may be other as yet undiscovered
processes, possibly involving the components of the organic matrix, that discriminate
against Mg and in favor of Ca.
Another observation may shed light on the removal of Mg at the site of calcification.
In inorganic and biogenic carbonates (including foraminifera) when Mg is higher, Sr is
also higher (Carpenter and Lohmann 1992). This suggests that Mg may be included in the
Rayleigh distillation model mentioned above for fractionation of divalent elements from
a semi-closed reservoir. However, in order to extend the model to Mg we need to lower
its activity in the solution by a factor of 10 for benthic foraminifera and 50 for planktonic
ones. It is highly unlikely that an electrogenic pump will achieve this. A more probable
solution is the involvement of some organic molecules that complex the Mg and do not
let it poison the growth of low Mg calcite.
One difference between corals and foraminifera is that the corals precipitate
aragonite, while foraminifera precipitate calcite. To precipitate aragonite from
supersaturated seawater there is no need to remove Mg because aragonite is the
kinetically preferred mineral. Corals precipitate their CaCO
3
without influence of
pseudopods in the space formed between the chalicoblastic ectoderm and the skeleton. In
the foraminifera, precipitation occurs under strong influence of pseudopodial network
(i.e., bilipid membranes). Is it possible that these membranes or some of their
components bind in some other way the Mg in the environment of crystal formation?
This may neutralize the inhibition effects of Mg on kinetics of calcite formation. Such a
mechanism may also explain why the foraminifera precipitate low Mg calcite (which is
also more supersaturated!). Is it possible that this is the role of vesicles that are rich in
fibrillar material (Angell 1967) from the electron dense bodies?
Finally it may be asked why most of the radial foraminifera (except of one family,
the aragonitic Robertinacea) precipitate calcite. One possibility is that they evolved in
periods where Mg:Ca ratio in the ocean was much lower than today. Such variations in
ocean chemistry have indeed been reported (e.g., Hardie 1996, Stanley et al. 1999)
suggesting that during the Phanerozoic there have been large variations in this ratio
ranging from 1:1 (e.g., most of the Cretaceous and the Paleogene) to 5:1 as observed in
the oceans today. These variations stem from changes in rates of sea floor spreading
which, when high, also enhance hydrothermal circulation at the ocean ridges. During this
process Mg is exchanged for Ca and this ratio decreases. It has been suggested that when
Mg:Ca is high, the oceans are dominated by aragonite producers, while calcite producers
are abundant during periods that have low Mg:Ca ratio. It is possible that when the
modern radial foraminifera evolved (later Cretaceous and the Paleogene), the Mg:Ca was
around 1:1, and the preferred CaCO
3
mineral was calcite. Calcite is advantageous
because of its lower solubility product relative to aragonite. It can be speculated that
