¹ GEOMAR Forschungszentrum für marine Geowissenschaften, Wischhofstr.
1−3, 24148 Kiel, Germany
² Mineralogisch-Petrographisches Institut, Universität Bern, Erlachstr. 9a, 3012 Bern, Switzerland
During the last years the precision of calcium isotope measurements (δ44Ca) has considerably improved due to
technical improvement in mass spectrometer instrumentation, and the application of the double spike technique. Recent progress in
Ca-isotope measurements showed that calcium isotopic variations can be used in order to reconstruct past sea surface temperature
(Nägler et al., 2000) and past global fluctuations of continental weathering fluxes (De La Rocha & DePaolo, 2000).
Here we present a rapid method for the determination of calcium isotope compositions by thermal ionisation
mass spectrometry (TIMS). The main improvement is the use of a multicollector analyzer and the use of a
43Ca/48Ca double spike. The analyses were performed on a MAT262 RPQ+ using a two step dynamic mode. In the
first step masses 40, 41, 42 and 43 are measured simultaneously and in the second step masses 44 and 48 are measured
simultaneously. Interferences of 40K are monitored on mass 41 and can generally be neglected. Comparison of the result
from this new method to results from simple peak jumping indicate that the new method show comparable and precision. Measured
ratios are corrected for the double spike by an external numerical algorithm using an iterative approach (Compston & Oversby
1969). The δ44Ca-ratios are normalized to the 44Ca/40Ca ratio of NIST SRM 915a (pure
CaCO3). The long term reproducibility was determined by repeated measurements of this standard and amounts to ±
0.08 ‰ (δ-units).
The new technique has been applied to δ44Ca variations of planktonic foraminifera (G.
bulloides) from ODP Leg 183 Site 1138 (Kerguelen Plateau). The results show that the δ44Ca ratios vary during
the last 22 Ma within a range of about 1 ‰. The δ44Ca values decrease from the present to a
δ44Ca-minimum at about 16 Ma. This observation can be interpreted in terms of global SST-cooling or an increase
of ocean calcium concentration due to enhanced rates of continental weathering.
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.
Nägler Th.F., Eisenhauer A., Müller A., Hemleben C., and Kramers J. (2000) The #948;44Ca-temperature calibration on fossil and cultured Globigerinoides sacculifer: New tool for econstruction of past sea surface temperatures. Geochem. Geophys. Geosyst. 1, 2000GC000091.
GEOMAR Forschungszentrum, Wischhofstr. 1−3, 24148 Kiel, Germany
The Kerguelen Plateau is a mostly drowned oceanic plateau in the Indian sector of the Southern Ocean. It formed during the
late Cretaceous as a temporarily subaerial volcanic plateau. Today most of the plateau is situated south of the polar front and
many parts are at water depths of less than 2000 m. Several holes drilled during ODP Legs 119, 120 and 183 offer the opportunity
to study Miocene sedimentary sequences from a southern high latitude setting.
The Miocene of the Kerguelen Plateau shows a pronounced facies shift from carbonate dominated sediments in
the Early and Middle Miocene to opaline and volcanoclastic domination in the Pliocene and Pleistocene. The facies transition
started during the Late Miocene, when around 11 Ma carbonate accumulation rates began to decline. A short period of lowered
carbonate contents at about 8.5 to 9.5 Ma may be an expression of the Late Miocene "carbonate crash". During the transitional
time hiatuses were widespread and there appears to exist no sedimentary record from about 4.5 to 7 Ma, when a major hiatus
formed at all investigated sites. Almost pure diatom oozes were deposited at the southern sites during the latest Miocene and in
the Pliocene. At the more northern sites, however, there is sufficient carbonate during this time to allow almost continuous
sampling of carbonate components from Early Miocene through Pleistocene. We collected planktonic foraminifera from the most
continuous record, cored at ODP Site 1138, at a resolution of about 1 Ma from Late Miocene to Pleistocene for determination of
calcium and strontium isotope ratios.
Our calcium isotope record correlates well with the data of De La Rocha & DePaolo (2000). Our better
temporal resolution allows to date the Middle Miocene calcium isotope minimum as 15 Ma. Comparison with a benthic foraminifera
oxygen isotope record from the nearby ODP site 747 (Wright & Miller, 1992) shows a parallel rise of calcium and oxygen
isotopes from the minimum at 15 Ma to a Late Miocene maximum at about 7 Ma. Comparing calcium and strontium isotope records, we
find that the the minimum further correlates with the well known Middle Miocene inflection in the seawater strontium isotope
curve.
During the Pliocene/Pleistocene cooling no correlation between oxygen and calcium isotopes can be observed.
Thus, it is not possible to explain our observations by local temperature effects. More likely, the calcium isotope record
reflects global changes in the oceanic calcium reservoir through changing weathering fluxes or dissolution and deposition of
calcium carbonates. If this is the case, global events like the Late Miocene "carbonate crash" or the Miocene/Pliocene lowering
of the carbonate compensation depth should have left traces in the calcium isotope record as recorded in planktonic
foraminifera.
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
Wright J.D. and Miller K.G. (1992) Miocene stable isotope stratigraphy, Site 747, Kerguelen Plateau. Proc. ODP Sci. Res. 120, 855−866
¹ GEOMAR, Forschungszentrum für marine Geowissenschaften, Wischhofstr. 1−3, 24148 Kiel, Germany
² Department of Earth Sciences, University of Bristol, Queens Road, Bristol BS8 1RJ, United Kingdom
³ Institut für Isotopengeologie der Universität Bern, Erlachstr. 9a, 3012 Bern, Switzerland
We present a comparison of the calcium isotope record (δ44Ca) of different species of planktonic foraminifers
from the southern Indian Ocean (ODP Site 1138; G. bulloides) and the western Equatorial Pacific (ODP Site 871A and 872C;
G. ruber and G. trilobus). For the latter two sites most recently a detailed δ11B-record was
presented by Pearson et al., 2000.
The δ44Ca records of both cores show decreasing δ44Ca values between 21 Ma
and 16 Ma. The minimum at 16 Ma is followed by rapidly increasing values until 13 Ma and then a slow increase to the present
seawater values. These results are in general accord with earlier findings of De La Rocha & DePaolo (2000), although there
are some discrepancies concerning core chronologies. Unfortunately, direct comparison of the δ44Ca values is
hindered by the use of different standard materials and normalizing procedures.
Because of our high temporal resolution of about 1 sample/Ma details of the calcium isotope evolution of the
oceans can be seen. The G. bulloides record shows a drop of the δ44Ca of 0.5 ‰ whereas the G.
ruber record shows a drop of 0.3 ‰ at about 3 Ma. Another drop occurred between 10 and 9 Ma but with a smaller
amplitude (0.15 ‰ G. ruber and 0.2 ‰ G. bulloides). Presumably, the amplitude of the observed
variations depends on foraminiferal species and/or location. The δ44Ca of G. ruber and of G.
bulloides show maximum variations of 0.6 ‰ and about 0.7 ‰, respectively, over the studied time interval. First
measurements of G. trilobus show a δ44Ca increase of about 1.0 ‰ between 16 and 12 Ma. The records
of G. ruber and G. trilobus (ODP sites 871A and 872C) show the same trend and similar δ44Ca values
between 19 and 16 Ma. Between 4 Ma and today the trends are similar, but the δ44Ca values show an offset of about
0.25 ‰.
The trends of our δ44Ca records can be interpreted in terms of changing global conditions,
e.g. changing weathering rates, global temperature changes, ocean pH-variations and of oscillations of input and output calcium
fluxes.
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.
Pearson P.N. and Palmer M.R. (2000) Atmospheric carbon dioxide concentrations over the past 60 million years. Nature 406, 695−699.
¹ GEOMAR, Forschungszentrum für marine
Geowissenschaften, Wischhofstr. 1&8722;3, 24148 Kiel, Germany
² Department of Earth Sciences, University of Bristol, Queens Road,
Bristol BS8 1RJ, United Kingdom
We present δ44Ca records of three different foraminifera species (G. ruber/subquadratus, G. trilobus
and Globigerinella spp.) from the western equatorial Pacific (ODP Site 871 and 872) corresponding to the last 23 Ma.
Assuming a constant calcium isotope fractionation factor (α) between seawater and the foraminiferal calcium carbonate the
δ44Ca of the past seawater (δ44Casw) can be reconstructed. The
δ44Casw records of G. ruber/subquadratus and of Globigerinella spp. are similar. The
two records show a decrease of the δ44Casw between 25 and 16 Ma of about 0.5 ‰ followed by an
increase of about 0.5 ‰ between 16 and 3 Ma. Between 3 Ma and the present the δ44Casw decreases
again by about 0.25 ‰. The δ44Casw calculated from G. trilobus record shows a similar
trend between 22 and 6 Ma but is isotopically lighter by about ~0.2 ‰ compared to the other two records: The
δ44Casw of Site 871 is positively related to its 87Sr/86Sr record. This
indicates that the δ44Casw is triggered by the balance of the input of continental weathering
products and submarine volcanism and the output of biogenically driven calcium carbonate precipitation.
From G. sacculifer being closely related to G. trilobus it is known that calcium isotope
fractionation is temperature dependent (Nägler et al. 2000; Gussone et al. 2002). Thus the differences between the
δ44Casw from G. trilobus and the mean of the δ44Casw of G.
ruber/subquadratus and Globigerinella spp. can be interpreted as a temperature related signal. Using the known
correlation of the fractionation factor (α) and temperature (T) of G. sacculifer the calculated temperature varies
between 26.5 °C and 29 °C over the past 23 Ma. The calculated temperatures the G. trilobus record shows a
cooling trend between 23 and 16 Ma followed by slight warming between 16 and 3 Ma. Between 3 and 1.5 Ma a rapid warming can be
observed. These observed temperature variations are in the order of 1−2 °C. In contrast, the δ18O
record of Site 871 and the global evolution of the δ18O record of benthic foraminifers show a cooling between
16 Ma and 1.5 Ma being in contradiction to the δ44Ca based temperature reconstructions of the G. trilobus
record.
Gussone, N., Eisenhauer, A., Dietzel, M., Heuser, A., Spero, H., Bijma, J., Böhm, F., Nägler, Th.F., Geophysical Research Abstracts 4, 2002 (EGS02-A-02944)
Nägler Th.F., Eisenhauer A., Müller 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
Seite zuletzt geändert am 02.10.2004 um 14:42 Uhr.