Calcium (Ca) isotopes show only little fractionation in nature (~6 ‰ ) and mass spectrometric determination of
calcium isotopic ratios is difficult (e.g. Russell et al., 1978). One of the goals of this thesis was to improve analytical precision of Ca isotope
measurements. Measurements of Ca isotopes were carried out using a thermal ionization mass spectrometer (TIMS) using a two step
dynamic mode (Heuser et al., 2002). In the
first sequence masses 40, 41, 42 and 43 are measured simultaneously and in the second sequence the masses 44 and 48 are measured
simultaneously. About 200 ng of Ca is needed for a single measurement. In order to correct for the fractionation during the
measurement I added a 43Ca/48Ca double spike to the sample. For the correction of the added double spike I
developed an iterative routine based on the algorithm of Compston and Oversby (1969). This new "multi-collector" technique improves sample throughput by a factor of 3
compared to "single-collector" measurements. The variations of Ca isotopes are presented as δ44Ca:
δ44Ca=[(44Ca/40Ca)sample/(44Ca/40Ca)standard
−1]×1000 using the measured 44Ca/40Ca of NIST SRM 915a calcium carbonate as
(44Ca/40Ca)standard.
The Ca isotope compositions of several different standard materials have been analyzed: natural
CaF2 (+1.41‰), IAPSO seawater salinity standard (+1.83 ‰) and Johnsen Matthey calcium carbonate Lots 4064
and 9912 (+0.58 and −11.46 ‰).
Measurements of δ44Ca of foraminifers (Orbulina universa), cultured at different
temperatures show a weak temperature dependent Ca isotope fractionation of 0.019 ‰ / °C (Gussone et al., 2003) being in contrast to the study of Nägler et al. (2000) who reported a
temperature dependent Ca isotope fractionation of Globigerinoides sacculifer of 0.24 ‰ / °C . This different
fractionation behaviour can be attributed to a difference of the calcification process. In O. universa Ca is transported
in a hydrated form (Ca2+-aquocomplex). The relative mass difference between a 40Ca2+-aquocomplex
and a 44Ca2+-aquocomplex with masses of about 500 amu is very small compared to the mass difference of pure
40Ca2+- and 44Ca2+-ions.
The δ44Ca of four different foraminifera (Globigerinoides trilobus, Globigerinoides
ruber, Globigerinella spp. and Globigerina bulloides) were measured in order to reconstruct the δ44Ca
of seawater (δ44Casw) of the past 24 Ma. The samples are from the western equatorial Pacific Ocean
(ODP Leg 144, Sites 871 & 872) and from the southern Indian Ocean (ODP Leg 183, Site 1138).
The fractionation of Ca isotopes between foraminiferal calcite (cc) and seawater (sw) is expressed by the
fractionation factor α (α=(44Ca/40Ca)cc/(44Ca/40Ca)sw) and seawater temperature
changes. Assuming a constant α, the δ44Casw can be reconstructed from the foraminiferal
δ44Ca. Changes of the seawater pH do not affect the fractionation between foraminiferal calcium
carbonate and seawater of the studied foraminifera.
A plot of α-values and corresponding mixed layer water temperatures for the youngest foraminiferal
samples suggests that G. bulloides and G. trilobus fractionate Ca isotopes independent of seawater temperature
changes and G. ruber/subquadratus and Globigerinella spp. do fractionate Ca isotope depending on seawater
temperature changes. This enables a reconstruction of δ44Casw using the G. bulloides and G.
trilobus records and to reconstruct the temporal evolution of the western equatorial Pacific using the δ44Ca
records of G. ruber/subquadratus and Globigerinella spp.
The reconstructed δ44Casw records are in good agreement with previously
published δ44Ca data of marine carbonates (De La Rocha and DePaolo, 2000). As our sample series have a higher temporal resolution (~1
Ma) some more details of the seawater δ44Ca evolution can be seen. Additional to a minimum of
δ44Casw at about 16 Ma, a minimum at about 4 Ma can be observed.
From the δ44Ca of G. ruber/subquadratus and Globigerinella spp. we can
estimate temperature changes while at the same time changes of the δ44Casw can be considered. The
calculated temperatures of the two records vary between 28 to 31 °C over the studied time period. The most prominent
feature of both trends is a cooling of about 1 to 2 °C between 4 and 1.5 Ma. During 15 and 7 Ma temperatures remained
constant at about 29 °C. The evolution of temperatures between 24 and 15 Ma is ambiguous as the two records differ. The
calculated temperatures are in general agreement with global benthic δ18O data and local planktic
δ18O data giving further support to a temperature dependent Ca isotope fractionation of G.
ruber/subquadratus and Globigerinella spp.
Although there are strong indications of a temperature dependent Ca isotope fractionation of G.
ruber/subquadratus and Globigerinella spp. an alternative model can be proposed assuming temperature independent Ca
isotope fractionation of these two species. The calculated α-values from this study as well as data from literature suggest
that α is species-specific. A change of the calcite precipitation mechanism in the course of evolution of a foraminiferal
species could lead to a change of the fractionation factor. A change of the α-values of Globigerinella spp. and G.
ruber/subquadratus between 3 and 1.5 Ma leads to a unique data set of the calculated δ44Casw of
all four studied species.
Compston W. and Oversby V. (1969) Lead Isotopic Analysis Using A Double Spike. J. Geophys. Res. 74, 4338−4348.
De La Rocha C.L. and DePaolo D.J. (2000) Isotopic Evidence for Variations in the marine Calcium Cycle Over the Cenozoic. Science 289, 1176−1178.
Gussone N., Eisenhauer A., Heuser A., Dietzel M., Bock B., Böhm F., Spero H.J., Lea D. W., Bijma J., Zeebe R. and Nägler T.F. (2003) Model for Kinetic Effects on Calcium Isotope Fractionation (δ44Ca) in Inorganic Aragonite and Cultured Foraminifer (Orbulina universa and Globigerinoides sacculifer). Geochim. Cosmochim. Acta. 63, 1375−1382
Heuser A., Eisenhauer A., Gussone N., Bock B., Hansen B.T., and Nägler Th.F. (2002) Measurement of Calcium Isotopes (δ44Ca) Using a Multicollector TIMS Technique. Int. J. Mass Spec. 220, 387−399.
Nägler Th.F., Eisenhauer A., Müuller A., Hemleben C., and Kramers J. (2000) The δ44Ca-temperature calibration on fossil and cultured Globigerinoides sacculifer: New tool for reconstruction of past sea surface temperatures. Geochem. Geophys. Geosyst. 1, 2000GC000091.
Russell W.A., Papanastassiou D.A., and Tombrello T.A. (1978) Ca isotope fractionation on the Earth and other solar system materials. Geochim. Cosmochim. Acta 42, 1075−1090.
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