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Evolutionary Control on Ca Isotope Fractionation Factors?

The proposed temperature controlled δ⁴⁴Ca of G. ruber/subquadratus and Globigerinella is mainly based on the "one point evidence" of Fig. 5.3. The data from this study as well as literature data of δ⁴⁴Ca of foraminifers suggest that the fractionation factor α is species specific (Tab. 6.1 and Fig. 6.1). This lets me hypothesize that the fractionation factors α are not constant throughout time rather than dependent on relative changes of the individual calcification process of the species. Biochemically controlled calcification is determined by the foraminifer genes which may have changed due to selection and mutation with their evolution throughout times.

Tab. 6.1: Ca isotope fractionation factors (α_{calcite-fluid}) of various foraminifers.

species	α	T (°C)	type	reference
G. trilobus	0.99905	28.6	natural	this study
G. ruber	0.99881	28.6	natural	this study
Globigerinella spp.	0.99862	28.6	natural	this study
G. bulloides	0.99898	2	natural	this study
G. sacculifer	0.99796	26.5	cultured	Nägler et al. (2000)
G. sacculifer	0.99897	29.3	cultured	Nägler et al. (2000)
G. sacculifer	0.99914		natural	Zhu and Macdougall (1998)
G. sacculifer	0.99855		natural	Zhu and Macdougall (1998)
O. universa	0.99904	22	cultured	Zhu (1999)
O. universa	0.99903	22	cultured	Gussone et al. (2003)
O. universa	0.99919	29.3	cultured	Gussone et al. (2003)
N. pachyderma (s)1	0.99836	3.4	natural	Hippler (pers. comm)
N. pachyderma (s)2	0.99852	3.4	natural	Hippler (pers. comm)
N. pachyderma (s)	0.99846	0.28	natural	Gussone (prev. unpublished)
N. pachyderma (s)	0.99863		natural	Zhu and Macdougall (1998)
N. pachyderma (s)	0.99904		natural	Zhu and Macdougall (1998)
Alveolinella sp.	0.99957		natural	Skulan et al. (1997)
G. ornatissima	0.99877	9.8	natural	De La Rocha and DePaolo (2000)

¹arctic; ² antarctic

In contrast to other isotope systems like boron, oxygen and carbon Ca isotope fractionation may be very sensitive to changes of the calcium carbonate precipitation mechanism because it is controlled by kinetic isotope fractionation rather than by equilibrium fractionation (Heumann and Lieser, 1973; O'Neil et al., 1986, Gussone et al., 2003). Equilibrium fractionation for C and O isotopes is associated with covalent bonding and occurs already at the formation and transition of the dissolved carbonate species like CO₂(aq), HCO₃⁻ and CO₃²⁻ in the bulk solution. Isotope fractionation of C and O isotopes is relatively large when these molecules form in a distinct bulk solution. For example at the transition from CO²(aq) to HC₃⁻ oxygen isotope fractionation is in the order of about 23 ‰ and O isotope fractionation at the transition from HCO₃⁻ to CO₃²⁻ is 16 ‰ (Zeebe, 1999). However, kinetic O isotope fractionation during CaCO₃ precipitation is relatively small. That C and O isotope fractionation is less controlled by mineralogical effects during precipitation is also supported by the observation that O and C isotopic difference between calcite and aragonite only amounts to about 1 ‰ and 1.7 ‰ , respectively (Böhm et al., 2000; Romanek et al., 1992). This is much less than the fractionation processes related to the formation of HCO₃⁻ and CO₃²⁻ molecules in the bulk solution.
   In the bulk solution Ca isotopes occur as hydrated Ca²⁺ ions and no isotope fractionation occurs as a function of varying speciation. Because Ca isotopes are controlled by kinetic fractionation only during CaCO₃ precipitation from the bulk solution Ca isotope fraction reacts much more sensitive to changes of the calcification process. This has been extensively discussed in Gussone et al. (2003) who showed that different biochemical calcification processes in foraminifer control their fractionation factor differing by one order of magnitude between G. sacculifer and O. universa. Pearson et al. (1997) used the fossil record from Site 871 to date this separation of G. trilobus and O. universa from their ancestral lineage back to the middle Miocene (~15.1 Ma). From a genetic point of view, both foraminifers evolved from a common ancestor at about 19 Ma ( de Vargas et al., 1997). Evolutionary changes of the biochemical processes controlling CaCO₃ precipitation in foraminifer are therefore a likely candidate to influence Ca isotope fractionation.
   Evolutionary changes of foraminiferal species are well known to have occurred throughout the Phanerozoic (e.g. Loeblich and Tappan, 1964; Pflug 1965, Kennett and Srinivasan, 1983). However, to our knowledge no information are available about the evolutionary history of changes of biochemical controlled calcification processes and their effects on the Ca-isotope fractionation.
   It seems possible that the Miocene G. subquadratus being the ancestor of G. ruber ( Kennett and Srinivasan, 1983) had a different Ca fractionation factor than modern G. ruber. From Globigerinella spp. it is known that there was an evolution since the Oligocene (Kennett and Srinivasan, 1983). G. praesiphonifera evolved from G. obesa at the transition from the Oligocene to Miocene and further evolved into modern G. siphonifera (= G. aequilateralis) during the late Miocene.
   Currently, foraminiferal species are distinguished primarily on morphological concepts although several studies showed that this classical species definition is critical with regards to molecular approaches (De Vargas et al., 1999; De Vargas et al., 2002; Darling et al. 1999; Huber et al. 1997; Kucera and Darling, 2002). De Vargas et al. (2002) found four genotypes for Globigerinella siphonifera. The extend of the genetic differences between these four genotypes is in the range of species level. De Vargas et al. (2002) detected that each of these genotypes is adapted to specific environment characterized by typical ranges of e.g. salinity, temperature and pH. Genotypes with significant genetic differences have also been reported for G. ruber (Darling et al., 1997). Differing from G. siphonifera and G. ruber there are only small genetic differences within the G. sacculifer cluster. This is also true for G. bulloides, where only slight genetic differences between transitional zone and subtropical specimens were observed (Darling et al., 1999). A good overview on the relations between morphologic evolution and genetic evolution of foraminifera was given by De Vargas et al. (1997).

Our data indicate a divergence of the records starting at about 3 Ma. We propose that a change of the fractionation factor at this time leads to a shift between the δ⁴⁴Ca_sw record of G. trilobus relative to the δ⁴⁴Ca_sw records of G. ruber/subquadratus and Globigerinella spp.
   In order to reconcile the four records the fractionation factor for Globigerinella in the time interval before 3 Ma had to be about 0.99894, slightly higher than in the time interval from the present to about 3 Ma (α = 0.99863)(Fig. 6.2). Similarly, for G. ruber/subquadratus we assume that the fractionation factor dropped at about 3 Ma from 0.99910 to 0.99881 (Fig. 6.2). In contrast to these species we infer that the fractionation factors for G. trilobus and G. bulloides must have remained constant for the last 24 Ma. Applying this corrections to the Globigerinella and G. ruber records the δ⁴⁴Ca_sw} records are consistent within their statistical uncertainties (Fig. 6.3). However, records still show discrepant values before about 16 Ma. It may be speculated that this observation also reflects an evolutionary change of the calcification processes resulting in an earlier change of the fractionation factor α.

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