Eocene-miocene Carbon-isotope And Floral Record From Brown Coal Seams In The Gippsland Basin Of Southeast Australia

摘要

The carbon-isotope and palynological record through 580 m thick almost continuous brown coal in southeast Australia's Gippsland Basin is a relatively comprehensive southern hemisphere Middle Eocene to Middle Miocene record for terrestrial change. The carbon isotope δ~(13)C_(coal) values of these coals range from -27.7‰ to -23.2. This isotopic variability follows gymnosperm/angiosperm fluctuations, where higher ratios coincide with heavier δ~(13) values. There is also long-term variability in carbon isotopes through time. From the Eocene greenhouse world of high gymnosperm-heavier δ~(13)C_(coal) values, there is a progressive shift to lighter δ~(13)C_(coal) values that follows the earliest (Oi1?) glacial events around 33 Ma (Early Oligocene). The overlying Oligocene-Early Miocene brown coals have lower gymnosperm abundance, associated with increased % Nothofagus (angiosperm), and lightening of isotopes during Oligocene cooler conditions. The Miocene palynological and carbon-isotope record supports a continuation to the Oligocene trends until around the late Early Miocene (circa 19 Ma) when a warming commenced, followed by an even stronger isotope shift around 16 Ma that peaked in the Middle Miocene when higher gymnosperm abundance and heavier isotopes prevailed. The cycle between the two major warm peaks of Middle Eocene and Middle Miocene was circa 30 Ma long. This change corresponds to a fall in inferred pCO_2 levels for the same period. The Gippsland data suggest a link between gymnosperm abundance, long-term plant δ~(13)C composition, climatic change, and atmospheric pCO_2. Climatic deterioration in the Late Miocene terminated peat accumulation in the Gippsland Basin and no further significant coals formed in southeast Australia. The poor correspondence between this terrestrial isotope data and the marine isotope record is explained by the dominant control on δ~(13)C by the gymnosperm/angiosperm abundance, although in turn this poor correspondence may reflect palaeoclimate control. From the brown coal seam dating, the coal appears to have accumulated during a considerable part of the allocated 30 Ma Cenozoic time period. These brown coal carbon isotope and palynological data appear to record a more gradual atmospheric carbon isotope change compared to the marine record.