Reconstructing tropical cyclone frequency using hydrogen isotope ratios of sedimentary n-alkanes in northern Queensland, Australia

Abstract

A peat record from Quincan Crater (Queensland, Australia), spanning the past 200years, was used to test if hydrogen isotope ratios of leaf wax long-chain n-alkanes derived of higher plants can be used to reconstruct past tropical cyclone activity. Queensland is frequently impacted by tropical cyclones, with on average 1–2 hits per year. The most abundant n-alkanes in the peat are C29 and C31. Possible sources for long chain n-alkanes in the peat core are ferns and grasses, which grow directly on the peat layer, and the tropical forest growing on the crater rim. Hydrogen isotope ratios of C27, C29 and C31n-alkanes vary between −155 and −185‰ (VSMOW), with the largest variability in the upper 30cm of the record. For the period 1950–2000AD the variability in δD of C29 alkanes resembles a smoothed record of historical tropical cyclone frequency occurring within a 500km radius from the site. This suggests that the high number of tropical cyclones occurring in this period strongly impacted the δD signal and on average resulted in more depleted values of precipitation. In the period before 1900AD, the variability in the hydrogen isotope record is relatively small compared to the period 1950–2000AD. This might be the result of lower variability of tropical cyclones during this time period. More likely, however, is that it results from the increasing age span per sampled interval resulting in a lower temporal resolution. Average δD values between 1900 and 2000AD are around −167‰, which is similar to average values found for the period between 1800 and 1900AD. This suggests that on average tropical cyclone frequency did not change during the past 200years. This study demonstrates the potential of stable hydrogen isotope ratios of long chain n-alkanes for the reconstruction of past tropical cyclone frequency.