1 Introduction and Goals

Calcium is the fifth most abundant element in the Earth's crust (including hydrosphere and biosphere) and has six stable naturally occurring isotopes with masses ranging from 40 to 48 atomic mass units (amu). Therefore, investigations on variations of calcium isotopes have been performed since the late 1950's up to the end of the 1970's (e.g. Herzog et al., 1954; Backus, 1955; Backus et al., 1964; Corless, 1968; Stahl and Wendt, 1968; Coleman, 1971; Moore and Machlan, 1972; Russell et al., 1978). These early studies mainly focused on the search for calcium isotope variations in terrestrial (Artemov et al., 1966; Corless, 1968; Heumann and Luecke, 1973; Stahl, 1968; Stahl and Wendt, 1968) and extra-terrestrial samples (Backus et al., 1964; Russell et al., 1978) and the use of radiogenic 40Ca (from the β− decay of 40K) as a geochronometer (Coleman, 1971; Heumann et al., 1977). A good overview of these early studies is given by Platzner (1997).
   One major result of these early studies was that calcium isotope variations are very small compared to oxygen or carbon isotope variations. Mass spectrometers were not able to measure Ca isotopes with the required precision. A mile-stone was the work of Russell et al. (1978) who used a 42Ca/48Ca double spike. The use of a double spike improved the precision of mass spectrometric calcium isotope analysis. Differences in the 40Ca/44Ca of about 0.5 ‰ became clearly resolvable. However, the interest in calcium isotope variations disappeared. Through the 1980's up and the 1990's only a few investigations on calcium isotopes were published (Marshall and DePaolo, 1982; Niederer and Papanastassiou, 1984; Jungck et al., 1984; Marshall and DePaolo, 1989).
   Recent advancements in mass spectrometry lead to a revivication of the interest in calcium isotope analysis. It was the study of Zhu and Macdougall (1998) showing that calcium isotope variations of foraminifers can be used as a proxy for paleo sea surface temperatures (SST) which led to the more detailed study of Nägler et al. (2000). Zhu and Macdougall (1998) also showed that the oceanic calcium isotope composition is not in steady state caused by an imbalance of Ca input and Ca output. This suggests the possibility to use δ44Ca variations of seawater as a weathering proxy.
   Widely used proxies for the reconstruction of past climate and environmental conditions are inter alia the oxygen isotope composition (δ18O) and element/calcium ratios (e.g. Mg/Ca, Sr/Ca, U/Ca) of biogenic (esp. foraminiferal) calcium carbonate. The major difficulties of the above proxies are:

  1. The variations are caused by more than one process: It is known that δ18O variations of foraminifers are caused by temperature changes, changes in the volume of polar and continental ice sheets, changes of the seawater pH and changes of salinity (e.g. Billups and Schrag, 2002; Rohling and Bigg, 1998; Zeebe, 1999).
  2. econdary factors affect Mg/Ca and Sr/Ca in foraminiferal calcium carbonate. These factors are: shell growth rate, pH and salinity, and species-specific effects (e.g. Delaney et al., 1985; Elderfield et al., 1996; Lea et al., 1999; Rosenthal et al., 1997).
  3. 3. The original records/signals preserved in the foraminifers are altered through time by geochemical processes. Element/calcium ratios can be altered by diagenetic processes (especially recrystallization), secondary mineralizations of calcium carbonate and leaching/dissolution of calcium carbonates (e.g. Brown and Elderfield, 1996).

Calcium isotopes are less sensitive to alterations of the original record and variations of δ44Ca of foraminifers are mainly caused by temperature variations (Nägler et al., 2000; Zhu and Macdougall, 1998) and secular changes of the seawater Ca isotopic composition.
   The profit of a better knowledge of the secular variations of calcium isotopes in the oceans is twofold. (1) A neglect of secular changes can lead to over- and underestimations of temperature changes indicated by δ44Ca variations of foraminifers. (2) Seawater δ44Ca variations can be used as a proxy for continental weathering (Zhu and Macdougall, 1998; De La Rocha and DePaolo, 2000) as these variations are closely linked to the balance between Ca input and Ca output in the oceanic Ca cycle.
   A reconstruction of the oceanic Ca cycle using Ca isotope variations can also lead to better models of the global carbon cycle as calcium carbonates (CaCO3) are the main sink for oceanic Ca.

The major goals of this work were:
  1. Development of a method for precise and fast measurements of calcium isotopes by thermal ionization mass spectrometry (TIMS) using a 43Ca/48Ca double spike.
  2. Calibration of different calcium isotope standard materials (seawater, calcium carbonate and CaF2) for inhouse use and for interlaboratory comparisons.
  3. Measurements of the calcium isotope composition of foraminiferal calcium carbonate to reconstruct the calcium isotope composition of seawater through the entire Miocene.

Chapter 2 presents a new TIMS multicollector method which I developed for the measurements of calcium isotopes. This method is now used for routine Ca isotope measurements at the GEOMAR research center, Kiel. The main advantage of this method is a higher sample throughput compared to the previously used "single collector" method without a significant loss of precision. I have published this new method in the "International Journal of Mass Spectrometry" (Heuser et al., 2002).
   Chapter 3 is a manuscript written by D. Hippler from Berne for publication in Geo-Standards Newsletter. To this comparative study of the δ44Ca of different Ca standard materials I contributed about 60% of the presented data from Kiel. The δ44Ca values of NIST SRM 915a calcium carbonate, natural CaF2, a seawater salinity standard (IAPSO), and two Johnsen Matthey calcium carbonate standards are presented and compared between the laboratories of Kiel, Berne and Strasbourg. The results show a good agreement between the laboratories and indicate that the multicollector measurements are as precise as single collector measurements.
   In chapter 4 results of δ44Ca measurements of cultured O. universa and inorganically precipitated aragonite are presented showing only a weak temperature dependent Ca isotope fractionation. This is in contrast to the previously published data of Nägler et al., (2000) who reported a strong temperature dependent Ca isotope fractionation (0.24 ‰/°C) for Globigerinoides sacculifer. A model is presented explaining the mechanisms which lead to the observed differences of temperature dependent Ca isotope fractionation. This manuscript was written by N. Gussone and is now accepted for publication in Geochimica et Cosmochimica Acta. I was involved in this study by numerous discussions, significant input to the development of the model and support of Ca isotope measurements.
   In chapter 5, I present δ44Ca ratios of four different foraminifera from the western equatorial Pacific Ocean (ODP Leg 144) and from the southern Indian Ocean (ODP Leg 183). I performed these measurements in order to reconstruct the δ44Ca of seawater over the past 24 Ma. The samples were provided by Paul Pearson (Bristol, ODP Leg 144 samples) and by Florian Böhm (ODP Leg 183 samples). This is the first study comparing the δ44Ca of different foraminifera over a time interval of 24 Ma. From the δ44Ca of the foraminifers the δ44Ca of the past seawater is reconstructed. Additionally it is possible to use the δ44Ca data for a reconstruction of the evolution of seawater temperatures in the western equatorial Pacific.
   In chapter 6, an alternative interpretation of the data of Chapter 5 is presented. Currently there is no direct evidence supporting this 'evolutionary' concept but it shows that other factors than temperature or seawater chemistry might be important to interpret fossil foraminiferal δ44Ca records.


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