University of Tasmania
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Living and fossil calcareous nannoplankton from the Australian sector of the Southern Ocean: Implications for Paleoceanography

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posted on 2023-05-27, 06:47 authored by Findlay, CS
This study documents the distribution of calcareous nannoplankton in the waters and surface sediments of the Australian Sector of the Southern Ocean, and applies the information to core samples from the region to infer past changes in the ocean between 41 os and 64¬¨‚àûS. The preservation of calcite plates produced by these phytoplankton are preserved in pelagic sediments and are useful in paleoceanography. Water column samples show that calcareous nannoplankton can be separated into five distinct assemblages associated with properties of the water mass, i.e., temperature, salinity, light and nutrients. In general the abundance and diversity of nannoplankton decrease poleward from subtropical to polar waters. The surface sediments show an abundance and diversity of calcareous nannoplankton different from living assemblages in the water column. Surface sediments are dominated by a single assemblage including C. pelagicus, a species not found in water column samples. The absence of C. pelagicus suggests a1recent extinction in the Southern Ocean. Of 45 surface sediment s~mples, only eight were identified as younger than 73 ka BP based on currently recognised biostratigraphy, indicating erosion and disturbance of sediments in the region. Preferential preservation of larger, more robust species of nannoplankton in the surface sediments suggests that chemical dissolution of calcite is significant. Calcareous nannoplankton biost~atigraphy from a 5.1-metre core (GC07; 45¬¨‚àûS; 146¬¨‚àûE; 3307m water depth), coupled with 14C dates, oxygen isotope ratios and �C03 data show that the core spans the interval of about 129 ky (from the beginning of the last interglacial) to Late Holocene. Changes in fossil assemblages with time are related to glacial and interglacial intervals, suggesting that the nannoplankton are useful as paleoclimatic indicators. A change from dominance by Gephyrocapsa muellerae to dominance by Emiliania huxleyi occurred at about 11 ka BP, suggesting that the commonly used date for this reversal (73 ka BP) is not applicable for the Sub-Antarctic. The presence of Miocene and Pliocene species in the core samples indicates that reworking of sediments is commori in the region.


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