Coccolithophores in sediments from offshore Australia : pre and post-industrial species distribution and calcification

• Main Author: Brentegani, Luna
• Format: Thesis
• Language: English
• Published: Canberra, ACT : The Australian National University 2015
• Subjects: Indian; Ocean acidification
• Online Access: https://dx.doi.org/10.25911/5d4d4d02865a1; https://openresearch-repository.anu.edu.au/handle/1885/155154

The waters offshore southern and eastern Australia are characterised by the presence of warm boundary currents, the Leeuwin Current (LC) and the East Australian Current (EAC) respectively. These currents experience physical changes associated with climatic events such as the Southern Annular Mode (SAM), the El Nino Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). In order to explore these changes and compare present to past environmental conditions in southern and eastern Australia, coccolithophore species composition, abundance and calcification of sediment assemblages were investigated in several short cores. Coccolithophores are primary producers and they are fundamental for producing organic carbon and carbonate in the marine realm. Because they can be affected by environment change, there is the need to understand if the coccolithophore community has varied in the Australian waters during the last centuries. Previous research focusing on coccolith assemblages from the sea floor offshore Australia is limited and does not provide high resolution information across the recent past. In this thesis, multicores were used to investigate the fossil coccolith record of marine sediments covering 370 years of sedimentation from about 1635 and 2005 AD. This time-frame allowed a comparison of species-specific coccolith abundances and masses (as a calcification rate indicator) before and after the industrial revolution (1850 AD). The research presented in this thesis deals with three aspects of coccolithophore distribution. The first aspect focused on present-day oceanographic conditions and relative nannofossil assemblages to determine, using multivariate analyses, the current biogeographic distribution of coccoliths in relation to the studied area. The work provided the calibration and proxies, based on selected coccolith species, for down core reconstructions. The second aspect of this research was aimed at tracing changes through time in coccolith assemblages and derived proxies resulting from the environmental variability of south-eastern Australia. The short time-frame covered by the multicore sediments allowed the data to be tested via two calibrations: one based on historical instrumental data and the other based on the core-top coccolith distributions. Finally, key selected coccolithophore species and the whole assemblages were examined using morphometric and size distribution analyses, to obtain a degree of the coccolith calcification status through time. Some of the significant findings confirmed the role of F. profunda as a palaeoproductivity proxy. Both coccolith assemblages from southern and eastern Australia showed signs of the LC and EAC over the sites. Temperature and nutrients were responsible for potential coccolith mass changes. In conclusion, coccolith assemblages from sea floor sediments appear to be reliable indicators of water mass and current circulation characteristics in south-eastern Australia. For the time period investigated, coccolithophores seemed to respond with resilience and adaptation to the changing environmental conditions. Considering the recent ocean acidification and the response of other marine calcifiers, these properties may strengthen the role already played by coccolithophores as major contributors to the oceanic carbon cycle.