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| Titel |
Quantitative 'deep-time' palaeoclimate reconstruction using large benthic foraminifera |
| VerfasserIn |
D. Evans, W. Müller, W. Renema |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250058767
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| Zusammenfassung |
| Despite some promising work demonstrating that the geochemistry of large benthic
foraminifera (LBF) faithfully records their environment [e.g. 1], the full potential of these
organisms for palaeoclimate reconstruction has not been realised. Planktic foraminifera are
routinely used to assess the temperature and pH (amongst other parameters) of the past
oceans but are short lived and currently offer only limited information regarding seasonal
change. In contrast, LBF may live for several years, particularly the large and widespread
Paleogene genera such as Nummulites [2]. These foraminifera therefore offer the potential for
quantitative reconstruction of seasonal changes of ambient seawater temperature and
chemistry.
In order to investigate the use of LBF for seasonal palaeoclimate and palaeoceanographic
reconstruction we analysed both recent and Eocene Operculina ammonoides (from five
different reefs in SE Asia and the Great Barrier Reef) as well as Eocene Nummulites
djokdjokartae and N. laevigatus (from Java and England respectively). Our results are
obtained using the LA-ICPMS system at RHUL featuring a two-volume LA cell
characterised by uniform signal response and rapid washout [3], enabling intratest
compositional variability to be assessed on a μm scale. Our results show that recent O.
ammonoides modifies its calcite test chemistry according to its environment. Moreover, our
data demonstrate that, as in planktic foraminifera, a systematic relationship exists
between Mg/Ca and temperature thus enabling the use of LBF for palaeotemperature
reconstructions. Because the Nummulitids have a similar peak abundance range
to planktic species usually considered to be surface dwelling (20-40 m), results
derived from LBF can be considered to be representative of sea surface temperatures.
Because the relationship between seawater Mg/Ca and test Mg/Ca has also been
calibrated for a Nummulitid (Heterostegina depressa) [4], our data can also be used to
better constrain the Mg/Ca ratio of Eocene seawater. Knowledge of this variable
is vital for accurate palaeoclimate reconstruction using the Mg/Ca temperature
proxy.
In addition to Mg/Ca, we have also investigated the use of these organisms as recorders of
other important palaeoenvironmental and ocean-chemistry archives. Preliminary data
appear to suggest that the use of B/Ca as a pH proxy appears to be viable in the
Nummulitids. Furthermore, it appears that inter-ocean differences in chemistry can be
evaluated. Finally, the variability of certain trace element systems in LBF calcite can
be related to salinity, potentially offering a new and quantitative palaeosalinity
proxy.
[1] Wefer & Berger, 1980, Science, 209:803. [2] Purton & Brasier, 1999, Geology, 27(8):
711. [3] Müller et al., 2009, JAAS, 24: 209. [4] Segev & Erez, 2006, Geochem. Geophys.
Geosyst., 7(2). |
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