EARTH SCIENCE | CLIMATE INDICATORS | PALEOCLIMATE INDICATORS
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Ice cores from Mount Brown South (MBS), East Antarctica, were drilled to help understand the past atmospheric circulation variability in the southern Indian Ocean and southwest Pacific Ocean. There are visible bubble-free layers occurring frequently multiple times a year, and the origin of these features is still unknown. This project aims to determine whether the bubble-free layers in the MBS ice core can be related to atmospheric processes. ERA-5 data, including surface (skin) temperature, 2 metre air temperature, wind at 10 metre height, the mean surface downward short-wave radiation flux and snowfall, is used to assess the target climate variables from 1979 to 2017 at the ice core sites.
This dataset is a compilation of published records of 230Thorium - normalised lithogenic and biogenic fluxes from the Southern Ocean, south of 30S. All age models and derived fluxes were taken as published. Lithogenic fluxes are based on 232Th concentrations. Opal and carbonate fluxes are also included where available. In some cases fluxes had to be derived from published data. LGM values for each core represent an average of observations between 28 - 18 ka BP and Holocene values represent an average of observations from 10 - 0 ka BP. These data were collated as part of modelling study of the Southern Ocean during the LGM (Saini et al, Southern Ocean ecosystem response to Last Glacial Maximum boundary conditions, Submitted to Paleoceanography and Paleoclimatology, 2021)
Predicting phytoplankton impacts in response to a changing climate on Tasmania's east coast is presently based on short-term plankton data sets (~75 years). However, given the vital contribution coccolithophores make to oceanic carbon pumps, it is crucial to understand longer-term assemblage trends better. Here, we expand the archive of calcareous nannoplankton in southeast Australian waters by analysing coccolithophore microfossils in a ~2.68m long marine sediment core from the climate hotspot of Maria Island, Tasmania, using polarising light and scanning electron microscopy techniques in combination with analysis of sedimentary ancient DNA (sedaDNA). Coccolith assemblages at this site represent the complex interplay between the East Australian Current, Subantarctic incursions, and the hydrodynamics driving Subtropical Front positioning. Microfossil analysis revealed a shift from a cold to warm-adapted assemblage ~8 200 years BP, expressed by a striking transition from assemblages dominated by the cold-water species Gephyrocapsa muellerae to warmer water species Emiliania huxleyi. This transition compares with similar occurrences in the literature at ~11 000 years reported in the Southern Ocean and 12 000 years in the Tasman Sea, reflecting a broad shift of the Subtropical Front. E. huxleyi microfossils displayed the highest relative abundance, but less abundant larger taxa (including Calcidiscus, Coccolithus, Helicosphaera) accounted for >50% of coccolith CaCO3 sequestration, indicating large densely calcified species do the 'heavy-lifting' in terms of carbon cycling within mixed coccolithophore assemblages. Analysis of sedaDNA showed coccolithophores comprised the largest number of eukaryote molecular sequences recovered (~44%), far exceeding diatoms and dinoflagellates.
Declining atmospheric CO2 concentrations are considered the primary driver for the Cenozoic Greenhouse-Icehouse transition, ~34 million years ago. A role for tectonically opening Southern Ocean gateways, initiating the onset of a thermally isolating Antarctic Circumpolar Current, has been disputed as ocean models have not reproduced expected heat transport to the Antarctic coast. Here we use high-resolution ocean simulations with detailed paleobathymetry to demonstrate that tectonics did play a fundamental role in reorganising Southern Ocean circulation patterns and heat transport, consistent with available proxy data. When at least one gateway (Tasmanian or Drake) is shallow (300 m), gyres transport warm waters towards Antarctica. When the second gateway subsides below 300 m, these gyres weaken and cause a dramatic cooling (average of 2–4°C, up to 5°C) of Antarctic surface waters whilst the ACC remains weak. Our results demonstrate that tectonic changes are crucial for Southern Ocean climate change and should be carefully considered in constraining long-term climate sensitivity to CO2.
Southeastern Australia's marine waters are undergoing a trend of increased warming, surpassing the global average. This area has emerged as an alluring location for research on planktic microfossils, particularly dinoflagellate cysts, which are abundant in contemporary and Late Quaternary sediments. The composition of dinoflagellate cyst assemblages offers valuable information about the physical and biogeochemical properties of mid-latitude waters in this region. This study presents an analysis of cyst assemblages from marine sediment cores from waters inshore and offshore Maria Island, Tasmania, southeast Australia, up to 9 kyrs BP. The dominant cysts were Protoceratium reticulatum, Protoperidinium spp. (P. avellana, P. conicum, P.minutum, P. oblongum, P. subinerme, P. shanghaiense) and Spiniferites spp. (S. bulloideus, S. hyperacanthus, S. membranaceus, S. mirabilis, S. pachydermus, and S. ramosus). Inshore, Spiniferites spp. were more abundant (up to 61%), while P. reticulatum was dominant (up to 80%) at the offshore site. Impagidinium spp. and Nematosphaeropsis labyrinthus were exclusively detected offshore, with their increasing occurrence from 6 kyrs BP to present suggesting a transition from shallow coastal to stable deep-water habitat. Cysts of the Alexandrium tamarense complex were detected over the past 140 years and 9 kyrs BP at the inshore and offshore sites respectively, indicating an endemic long-term presence. Low abundances of Gymnodinium catenatum cysts were detected exclusively inshore from 50 years ago to present, suggesting recent bloom events. The limited southward penetration of the East Australian Current is indicated by the lack of warm-water cyst taxa such as Lingulodinium machaerophorum. Unlike coccolithophores, previously studied in the same sediment core, no discernible shift from cold to warm-water dinoflagellate cyst species was observed. The documentation of dinoflagellate cyst assemblages presented in this study will aid in predicting the effects of climate change, eutrophication, and introduction of novel species on local and broader community dynamics.