Palaeoenvironmental reconstruction of the Late Pleistocene to Early Holocene Robertson Cave sedimentary deposit, Naracoorte, South Australia

Abstract

Sedimentological investigations and geochemical analyses of the Robertson Cave deposit, 7 km south of Naracoorte, South Australia, document a palaeoenvironmental record for the Late Pleistocene and Early Holocene. Three distinct depositional episodes spanning the interval 32 – 8 ka are represented by over 4 m of sediments exposed in the chamber 2 excavation pit. The oldest of the three, the lower unit (>30 ka BP) is dominated by reddish and brown sandy silts. A 20 cm band of red silt horizons that cap this unit contain enriched δ¹⁵N values (12 – 15‰), low C/N ratios, significant SO₃ (20%) and CaO (18.5%) contents, a high alkyl-C component and phosphorus-rich mineralogy. These unique sediment horizons are interpreted as being derived from the accumulation of bat guano and degraded cave rock fragments. Their presence suggests a period of low sediment input into the cave, possibly reflecting the existence of a stable geomorphic environment prior to 30 ka. The time period 30 – 27 ka saw the rapid deposition of the middle unit, a sequence of coarser homogeneous sands that resulted in the entrance to chamber 2 becoming blocked. These horizon-less sediments, with clear aeolian features, are most likely representative of the drier conditions associated with the Last Glacial Maximum, which peaked at 20 – 17 ka. Deposition into the chamber recommenced at approximately 13 ka and continued until 8 ka, resulting in the accumulation of the upper unit, a sequence of silt, charcoal and organic matter rich horizons. Here, a shift to more ¹³C-depleted values (−25‰ to −27‰) is evident in the bulk soil organic matter and charcoal between 11 and 10 ka, possibly reflecting a shift from the dry Last Glacial Maximum to the wetter environs of the Holocene. The upper unit may also be a record of burning practices associated with Aboriginal migration to the area.

Key words:

• geochemistry
• guano
• Holocene
• Naracoorte Caves
• palaeoenvironment
• Pleistocene
• sediments

Acknowledgements

We thank S. Bourne, L. Reed and all the staff at the Naracoorte Caves Conservation Park for help with access to the cave deposits. We are grateful to M. Raven at CSIRO Land and Water for XRD analysis, J. Stanley at the department of Earth and Environmental Sciences, University of Adelaide for XRF analysis, P. Russ for help with petrographic thin-section photography and I. Lewis for his contributions on cave formation in the Naracoorte area. We also thank A. Mee for her editorial contributions, J. Skjemstad of CSIRO Land and Water for his expert contributions regarding the ¹³C-NMR spectroscopy and A. Osborne for his scientific and editorial contributions during the reviewing process. Finally, this study would not have been possible without the funding provided by the Commonwealth Department of Environment and Heritage.