Evidence of fire in Australian Cenozoic rainforests

https://doi.org/10.1016/j.palaeo.2018.11.023Get rights and content

Highlights

  • The paleoenvironment controls the distribution of charcoal in brown coals.

  • Charcoal abundance in brown coals is not related to drier climates.

  • There is no evidence of climatic drying in the Cenozoic Latrobe Group peatlands.

  • Modern sclerophyllous flora likely evolved on fire-prone fringes of peatlands.

Abstract

New palynological analysis of the Middle Eocene to Middle Miocene Latrobe Group coals of the Gippsland Basin in Australia sheds new light on fire adaptation in Australia's modern flora. The distribution of charcoal and fire-prone flora within brown coals is entirely controlled by facies and the paleoenvironments within the peatland, and does not result from drier climates as has been previously suggested. There is therefore, no evidence of climatic drying from this Cenozoic peatland record. Charcoal and fire-prone floras are associated with emergent and meadow marsh environments that produce darker coal lithotypes. Counter-intuitively, the low-nutrient and fire-prone environments that fringed the ever-wet rainforests of the Latrobe Group peatlands may have represented an ideal setting for southeastern Australia's modern fire-adapted and sclerophyllous flora (i.e., Eucalyptus and Banksia) to evolve in.

Introduction

Australia's remarkable arid-adapted modern flora stands in stark contrast to its Cenozoic record, which is dominated by warm and ever-wet rainforest communities (Macphail et al., 1994; Martin, 2006). The timing of this major climatic transition and the origin of Australia's dryland flora within a rainforest-dominated continent remain contentious (Crisp et al., 2004; Hill, 2004). Traditionally, the onset of aridity, and the associated increased frequency of burning, was suggested to commence in the mid-late Miocene (Martin, 1990; Kershaw et al., 1994; Martin, 2006) in southeastern Australia, with evidence (i.e., charcoal abundance) derived from studies on the Yallourn brown coals of the Latrobe Group (Blackburn and Sluiter, 1994). The onset of aridity reportedly terminated coal deposition in the Gippsland Basin (Sluiter and Kershaw, 1982) and limited Late Miocene sedimentation in the region (Kershaw et al., 1994). Recent evidence does however suggest that the Australian scleromorphic angiosperm vegetation may be older (Carpenter et al., 2015) and more persistent through the Eocene (Carpenter et al., 2014) in other parts of Australia.

This Cenozoic history of wet climates is also reflected in Australia's (particularly southeastern Australia's) geological record, with a long and semi-continuous record of forested peatland environments in regions such as the Gippsland Basin. Throughout the world, large peatlands and coal-forming systems are restricted to regions of very high rainfall and/or humidity (Stach et al., 1982), and in the Gippsland Basin, coals are deposited throughout the early Cenozoic, persisting into the Middle Miocene (Korasidis et al., 2018). The widespread occurrence of coals and coaly sediments in many of southeastern Australia's basins up to the mid-Miocene (Holdgate, 2003) is further suggestive that a wet climatic regime existed in the region well into the Neogene. Several authors however, suggest that seasonally drying climates were present in the mid-Miocene of the Gippsland Basin (e.g., Blackburn and Sluiter, 1994; Holdgate et al., 2007), while others propose that the onset of aridification and drying in Australia occurred during the late Neogene, as late as 1.5 Ma (McLaren and Wallace, 2010). In this study, we use new quantitative palynological, charcoal and colourimetry analysis to determine if the Latrobe Group peatlands record any evidence of fire and climatic drying from the Middle Eocene to Middle Miocene. The evolution of floral successions represented by the extensive brown coal lithotype cycles of the Latrobe Group through the Early Oligocene and Middle Miocene are also investigated in this study.

Section snippets

Geological setting

The Latrobe Valley of Australia (Fig. 1) contains Eocene to Miocene low ash (<5%) brown coals, with individual seams > 100 m thick and vertical multi-seam levels without parallel globally (e.g. up to 700 m thick) (George and Mackay, 1991; Holdgate et al., 1995). The Traralgon, Morwell and Yallourn formations are the main coal-bearing sequences, which within the Gippsland Basin are further subdivided into individual seams. In order of decreasing age, these are the T2, T1, T0, M2C, M2B, M2A, M1B,

Methodology

Six hundred and eighty four (684) new samples collected from the Loy Yang and Yallourn Open Cut Mines were quantitatively analysed using colourimetry measurements as outlined in Holdgate et al. (2014). New quantitative colourimetry analysis was completed on the upper M1B seam (78 samples), M1A seam (314 samples) and Y seam (292 samples). This new colourimetry analysis was completed because every palynology sample processed could then be accurately assigned to a particular lithotype based on its

Colourimetry analysis

The stratigraphic sections of the M2A, M1B, M1A and Y seams consist of a series of lithotype cycles (Fig. 3) of variable thickness (5–30 m). The lithotypes are cyclic on a coarse scale and exhibit oscillations on a fine scale. Overall lithotype cycles lighten upwards (see Supplementary material 1), as supported by statistical analyses conducted by Mackay et al. (1985). Cycle boundaries are characterised by light or pale coals of the underlying cycle being overlain abruptly by laminated dark or

Depositional environments of the Latrobe Group lithotypes

New palynological and charcoal data confirms that each brown coal lithotype has a distinct floral component that is controlled by the peat-forming environment (Holdgate et al., 2014; Korasidis et al., 2016, Korasidis et al., 2017a). The laminated dark lithotypes of the Early Oligocene to Middle Miocene Morwell and Yallourn brown coal seams are characterised by high but fluctuating relative abundances of Cyatheaceae and Gleicheniaceae fern spores and of Banksieaeidites spp., Cyperaceae,

Conclusions

New palynological analysis of the Latrobe Group coals of the Gippsland Basin in Australia suggests that the distribution of charcoal and fire-adapted flora within brown coals is entirely controlled by facies and the paleoenvironments within the peatland, and does not result from drier climates as has been previously suggested. Charcoal and fire-tolerant floras are closely associated with emergent and meadow marsh environments that produced darker lithotypes. The low-nutrient and fire-prone

Acknowledgments

AGL Loy Yang and GHD are gratefully acknowledged for enabling the sampling of the M2A, M1B and M1A seams from the Loy Yang Open Cut Mine. Energy Australia is also gratefully acknowledged for enabling the sampling of the Yallourn seam from the Yallourn Open Cut Mine. We are also grateful to Dr. Alan Partridge for his insights into the identification of palynomorphs. The contributions of John Korasidis to the processing of colourimetry samples are also appreciated. This work was supported by an

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