CLIMATIC AND PALEOMAGNETIC FORCING OF BERYLLIUM ISOTOPE RATIOS OF LAKE SELINA, TASMANIA FROM 125,000 YEARS AGO TO THE PRESENT

Long, continuous records of glacial-interglacial cycles in the Southern Hemisphere are rare when compared to Northern Hemisphere records. Past studies have documented the effects of glaciation during the last glacial period in parts of Australia, though Tasmania is not as well-documented as the mainland. Tasmania was extensively glaciated during glacial periods; however, Lake Selina in the West Coast Range was not covered with ice during the Last Glacial Maximum, and therefore contains a record of climatic changes from ca. 125 ka BP to the present. Due to Lake Selina’s location on the west coast of Tasmania, there is high correlation between wind speed and precipitation. Thus, Lake Selina is an ideal location to research correlations between the advance/retreat of glaciers and variations in precipitation and wind regime. Here, the longest known continuous record of ice advance and retreat in Tasmania during the last glacial period, including the Last Glacial Maximum, is assessed using beryllium-10 and beryllium-9 isotope ratios (¹⁰Be/⁹Be).

Beryllium isotope ratios are often used as a proxy for changes in geomagnetic field strength, with the ratio cancelling out climatic forcings which affect both isotopes and leaving behind the paleomagnetic signal. Here we present an examination of the balance between climatic and paleomagnetic influences on beryllium isotope ratios of Lake Selina, Tasmania from ca. 125 ka BP to the present. Lake Selina ⁹Be and ¹⁰Be are closely correlated during this time period, indicating both isotopes enter the lacustrine system through the same, or closely related, forcing mechanism(s). The ¹⁰Be/⁹Be trend is similar to that of Northern Hemisphere summer insolation, suggesting climatic influence is stronger in the beryllium record than signals from the geomagnetic field intensity. However, this alignment between ¹⁰Be/⁹Be and insolation only occurs during the cold glacial period. In the warmer interglacial and transition period, there is little to no correlation between ¹⁰Be/⁹Be and other climate proxies, and it is during this period we may see a stronger influence of geomagnetic field strength variations on the ¹⁰Be/⁹Be. The record presented here may contain the first known recording of the Blake excursion (ca. 120 ka BP) recovered from Tasmania.