Stable oxygen isotopes of Thoracosphaera heimii (Dinophyceae) in relationship to temperature; a culture experiment
Introduction
The isotopic composition and elemental chemistry of calcareous microfossils form often the backbone of palaeoceanographic and palaeoclimatic studies. Foraminifera have been most widely used as a result of their abundance in the sediments and the ease in which monospecific samples can be isolated from the sediment. However, biological factors such as the migration of several planktonic species through different water masses, the influence of calcite shell isotopic composition by photosynthetic activity of symbionts and consumption of other organisms, the species specific seasonality of shell production combined with calcification at different water depths and the ontogeny of individual species can complicate the interpretation of these signals (see overview in e.g. Bemis et al., 1998, King and Howard, 2003). The study of the stable oxygen isotope signal of photosynthetic species that do not contain symbionts and that do form their calcite walls at a stable position within the water column, might overcome some of these problems. Recent studies for the use of primary producers such as coccolithophores give promising results despite the difficulties in extracting a monospecific assemblage from sediments due to their small sizes (Stoll et al., 2001, Stoll et al., 2002, Stoll and Ziveri, 2002). This latter is a requirement for obtaining a clear isotopic signal since individual coccolithophorid species have large differences in so called “non-equilibrium” effects (Ziveri et al., 2003). The photosynthetic dinoflagellate cyst T. heimii (in former days thought to be a coccolithophore) can relatively easily be isolated from sediments. Calculated temperatures based on the palaeotemperature equation for inorganic calcite precipitation generally reflect mean annual temperatures at thermocline (Deep Chlorophyll Maximum) depths, which represent its preferred depth habitat (Zonneveld, 2004). However, to date only extremely little information is available about the relationship between the isotopic composition of its shell and temperature of the surface waters (Dudley et al., 1986, Friedrich and Meier, 2003). To enhance this information we have grown T. heimii under different temperatures at stable salinity conditions using a temperature gradient box. We compared the stable oxygen isotope composition to cyst production rates, pH and the stable carbon isotope composition of the shells to estimate the possible effects of population growth and metabolic and kinetic processes on the oxygen isotopic composition of the shell walls. This paper forms the basis of future studies on the cause and usability in palaeo-environmental studies of the vital effects that influence calcite precipitation in T. heimii.
Section snippets
Material and methods
Thoracosphaera heimii shells have been isolated from ocean surface water from the equatorial Atlantic (strain GeoB 92) and Eastern Mediterranean (strain GeoB 116) at 3°43.9′N – 42°45.4′W and 34°18.1′N – 19°53.9′E during Meteor cruises M38/2 and M40/4 respectively. Unicellular cultures have been established in 24-chambers microwells (Corning Inc., Corning, NY, USA; Costar 3524) containing ∼ 1.0 mL of culture medium (K medium–Si). The basis of the medium consist of seawater collected at Station
Results
Both strains of T. heimii that originate from the equatorial Atlantic Ocean and eastern Mediterranean, were growing between 12 °C and 30 °C. Cultures did not produce cell amounts high enough to enable isotopic measurements at 32 °C. The two studied strains showed a similar relationship between temperature and the δ18O and δ13C signals of their cysts (Fig. 1).
Significant relationships between the stable oxygen isotope signal of T. heimii and temperature can be observed (Fig. 1). The relationship
Discussion
The present results show that there is a clear relationship between the oxygen isotopic composition of T. heimii cysts and temperature. Both strains showed similar relationships indicating that interspecific differences did not affect the results of this study. The oxygen isotopic ratio's of T. heimii are lower (isotopically “lighter”) than the equilibrium temperature relation predicted for inorganic calcite (Fig. 1). It differs slightly from the relationship found for T. heimii by Dudley et
Concluding remarks
This study documents a clear relationship between the oxygen isotope composition of T. heimii cysts and temperature that subscribes its potential to be a useful tool to reconstruct palaeotemperatures of the upper water column, notably the deep chlorophyll maximum (Karwath, 2000, Karwath et al., 2000b). Although there exist already a considerable amount of palaeotemperature proxies based on isotopic, elemental or biomarker composition, the usability of these proxies can be hampered by the
Acknowledgements
Thanks are given to Monika Segl for stable isotope measurements. Monika Kirsch, Nicole Kniebel and Angelica Fresemann are thanked for technical support. This study was financed by the German Science foundation, DFG as subprojects of the International Graduate College: Proxies in Earth History (EUROPROX) and the research Centre Ocean Margins (RCOM).
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