OF DROUGHT AND FLOODING RAINS: HOLOCENE PRECIPITATION VARIABILITY IN ARID AUSTRALIA FROM SPELEOTHEM AND ALLUVIAL RECORDS

We explore changes in the frequency, magnitude and seasonal distribution of rain in the Holocene by coupling new high-resolution MC-ICPMS U/Th ages and C and O isotopic analyses from an extinct, intra-catchment speleothem with luminescence and ¹⁴C ages of alluvial outwash gravels in arid Australia. The speleothem record provides evidence for increased effective moisture at ca. 11.5 ka and 8.0-5.2 ka, peak moisture at 7-6 ka, and the onset of modern arid conditions by ca. 5 ka. Mid-Holocene δ¹⁸O and δ¹³C time-series data exhibit marked (>+1‰) contemporaneous excursions over base-line values of -5.3‰ and -11.0‰, respectively, suggesting increased moisture variability between 7-6 ka. Unlike flood records from nearby lakes (eg. Lake Eyre) the speleothem provides a record of in-situ precipitation over the catchment area, and it is therefore highly likely that the presently ephemeral stream system draining this catchment was perennial at this time. Optically stimulated luminescence and ¹⁴C ages from alluvial fan deposits in the region indicate an increase in large flood recurrence interval after 5 ka, from ca. 7 kyr recurrence intervals between 29-5 ka to ca. 0.9 kyr recurrence intervals from 4 ka to present. We interpret the speleothem and alluvial records to record significant changes in rainfall frequency-magnitude distributions through the Holocene. The mid-Holocene ‘wet period’ is characterised by increased rainfall and wetting/drying episodes and an apparent absence of large magnitude floods, interpreted to indicate increased southward penetration of the Australian summer monsoon into the arid continental interior. This coincides with an interval of globally decreased ENSO intensity, suggesting ENSO weakening may have allowed other weather systems to exert a greater influence on precipitation regimes over the Australian continent. The onset of aridity and coeval increase in large flood frequency at ca. 4-5 ka is interpreted to indicate the re-establishment of an ENSO-like climate in the late Holocene.