Coccolith δ18O and alkenone records in middle Pliocene orbitally controlled deposits: High-frequency temperature and salinity variations of sea surface water
Résumé
The calcareous nannoplankton flora is strongly controlled by water masses and is sensitive to ambient nutrient levels. In consequence, fossil remains of calcareous nannoplankton are useful tools for paleoceanography. Although they are the major producers of carbonates in the pelagic realm, their use as a seawater d 18 O recorder is not easy due to their small size and the difficulty in isolating them from the bulk carbonate. Using a granulometric separation technique (Minoletti et al., 2001), it is now possible to concentrate coccoliths in monospecific fractions. We applied this method on three precession cycles (insolation cycles i-288 to i-282) from the mid-Pliocene Punta Piccola section (South Sicily), which led to an almost pure Pseudoemiliana lacunosa fraction. It was then possible to measure the isotopic signatures (d 18 O and d 13 C) and alkenone unsaturation ratio on the same recorder: P. lacunosa. This permits a precise reconstruction of environmental conditions (temperature and salinity) that prevailed in the upper photic zone during the cyclic deposition of these sediments. Our data provide a clear confirmation that sapropelic marl formation occurred during a wetter climate compared to the deposition of the marly limestones. Moreover, these results point out that salinity variations are not, as previously thought, always the primary factor that controlled the deposition of these cyclic events. This study is a step forward in the utilization of calcareous nannoplankton isotopic records in paleoenvironmental reconstructions.
Domaines
Océanographie
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