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Sedimentology
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Understanding the patterns and characteristics of sedimentary deposits on the conjugate Australian-Antarctic margins is critical to reveal the Cretaceous-Cenozoic tectonic, oceanographic and climatic conditions in the basin. However, unravelling its evolution has remained difficult due to the different seismic stratigraphic interpretations on each margin and sparse drill sites. Here, for the first time, we collate all available seismic reflection profiles on both margins and use newly available offshore drilling data, to develop a consistent seismic stratigraphic framework across the Australian-Antarctic basins. We find sedimentation patterns similar in structure and thickness, prior to the onset of Antarctic glaciation, enabling the basin-wide correlation of four major sedimentary units and their depositional history. We interpret that during the warm and humid Late Cretaceous (~83-65 Ma), large onshore river systems on both Australia and Antarctica resulted in deltaic sediment deposition offshore. We interpret that the onset of clockwise bottom currents during the Early Paleogene (~58-48 Ma) formed prominent sediment drift deposits along both continental rises. We suggest that these currents strengthened and progressed farther east through the Eocene. Coevally, global cooling (<48 Ma) and progressive aridification led to a large-scale decrease in sediment input from both continents. Two major Eocene hiatuses recovered by the IODP site U1356A at the Antarctic continental slope likely formed during this pre-glacial phase of low sedimentation and strong bottom currents. Our results can be used to constrain future paleo-oceanographic modelling of this region and aid understanding of the oceanographic changes accompanying the transition from a greenhouse to icehouse world.
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Over the past fifty years, Eastern Tasmanian waters have experienced rapid warming, primarily due to the extension of the East Australian Current. This has driven expansion of warm-water biota and decline of those adapted to cooler conditions, including phytoplankton. Presently, plankton monitoring, including diatoms along Eastern Tasmania, spans <100 years. This study reconstructed diatom communities throughout a sediment core spanning 9,000 years before present (9 kyrs BP), using microfossil analysis and molecular techniques, including sedimentary ancient DNA (sedaDNA) and NRS 18S rRNA from a 10-year water column archive at the Maria Island IMOS NRS mooring. Microfossil analysis revealed a dominance of strongly silicified benthic taxa (Campylodiscus, Diploneis, Paralia, Pyxidicula, Triceratium). Notably, Paralia sulcata showed a shift ~6 kyrs BP from small to larger cells, possibly reflecting a transition from a coastal to shelf ecosystem. However, microfossils underrepresented lightly silicified planktonic diatoms. Molecular methods detected higher diatom diversity, though up to 50% of sedaDNA reads remained unclassified due to reference library limitations. Lightly silicified planktonic genera (Chaetoceros, Corethron, Lithodesmium, Rhizosolenia) were identified only via molecular approaches and comprised 73% of sedaDNA and 88% of 18S rRNA records. Of 10 shared diatom families, 5, 15, and 4 were unique to microscopy, sedaDNA, and 18S rRNA, respectively. SedaDNA also captured greater benthic diversity. Our findings revealed limitations in reconstructing historic diatom assemblages from sediment cores. Microfossils faced constraints due to difficulties in morphological identification and preservation biases. In contrast, sedaDNA analysis yielded finer taxonomic resolution, provided access to high-quality reference sequence libraries were available.