New radiocarbon age constraints for the 120 km-long Toomba flow, north Queensland, Australia

Abstract

The Toomba flow is the youngest flow of the Nulla volcanic province, located in north Queensland. This 120 km-long flow has yielded a published ⁴⁰Ar/³⁹Ar age of 21,000 ± 3000 years. In contrast, seven published conventional radiocarbon (¹⁴C) analyses of carbon-bearing material beneath the flow yielded radiocarbon ages of 16,000 to <2500 BP. These radiocarbon ages are younger than the ⁴⁰Ar/³⁹Ar age, potentially due to contamination of the charcoal by younger carbon that was not removed by the acid–base pre-treatment. We have re-examined the radiocarbon age of Toomba flow using newly sampled charcoal buried beneath the Toomba flow in combination with hydrogen pyrolysis pre-treatment and accelerated mass spectrometer (AMS) measurements. We determined a calibrated radiocarbon age of 20,815–19,726 cal BP (2σ) for the material beneath the Toomba flow. Our radiocarbon age, therefore: (1) is older than previous radiocarbon ages for the Toomba flow, (2) provides the most precise age yet available for the Toomba flow, (3) is in agreement with the ⁴⁰Ar/³⁹Ar age, and (4) validates that hydrogen pyrolysis is a robust and effective pre-treatment method, for subtropical conditions where samples are susceptible to contamination by younger carbon. The Toomba flow erupted during the Last Glacial Maximum, but the preserved surface suggests that the rate of weathering and soil formation has been almost negligible in this region, despite being situated in a subtropical climate that experiences highly variable often intense rainfall.

Keywords:

• Toomba
• ¹⁴C
• hydrogen pyrolysis
• radiocarbon dating
• young basalts
• Queensland

Acknowledgements

We are thankful to the members of Geosciences Department, Centre for Tropical Environmental and Sustainability Science, Australian Research Council Centre of Excellence for Biodiversity and Heritage (CABAH), and Economic Geology and Research Centre (EGRU) for their support. We would especially like to thank Emeritus Professor Bob Henderson for his support and guidance about the sampling location and the regional geology, and Rainy Comely for assistance in the lab. We are also thankful to Dr Benjamin Cohen and an anonymous reviewer for their valuable suggestions as reviewers that helped to significantly improve the paper.

Disclosure statement

No potential conflict of interest was reported by the authors.