¹⁰Be moraine chronologies from the Austrian Alps and their correlation with climatic events during the Late Glacial and the Holocene

Full text: Late Glacial (LG) moraines in the European Alps, especially those deposited during the Younger Dryas (YD, c. 12.9-11.7 ka), are subject of numerous paleoclimatic studies. Nevertheless, there are still many periods before and after the YD that are controversial in terms of glacier extents and ice dynamics, e.g., the Bølling- Allerød (BA, ca. 14.6–12.9 ka) and the Early Holocene (EH, ca. 11.7–9 ka). At the beginning of the BA, glaciers in the Alps were much more extensive compared to the beginning of the EH. However, the same climatic pattern of rapid temperature increase occurred during both periods, and lead to fast deglaciation. For the period between ca. 12–10 ka, moraine chronologies from different places in the Alps, including data presented in this study, have shown that ice retreat and therefore warming was not steady, but was interrupted by centennial-scale cooling. In contrast, it is still unclear whether ice decay during the BA was fast enough to cause continuous glacier retreat to the highest cirques, or whether glaciers remained larger during that time. To contribute to this question, we mapped LG and EH moraine sequences in a glacial valley of the Austrian Alps, the Jamtal, and applied ¹⁰Be surface exposure dating to selected landforms. Fifteen ¹⁰Be exposure ages collected along five different moraines suggest the following glacier history: the last major glacier advance occurred during the Little Ice Age (LIA, ca. 1250-1850 CE). Just outboard the historical moraine are two lateral ridges that date to c. 11 ka and that are attributed to Preboreal Oscillation (ca. 11,150–11,300 ka). Boulders from a lateral moraine located c. 100 m higher relative to the EH moraines, indicate a much higher glacier hypsometry in the valley and yield ages that fall into the YD period. A moraine preserved c. 70 m above the YD moraine dates to c. 14.7–14 ka, which overlaps with a centennial-scale cold interval during the BA termed the Older Dryas. The location and age of this moraine indicate that the glacier in this valley was not vanished during the BA, but was even more extensive than during the YD. Moraine formation ca. 14.7–14 ka is consistent with recent studies from Norway and Scotland (e.g. Ballantyne and Stone, 2012; Wittmeier et al., 2020) and may be correlated with periods of low δ¹⁸O analyzed in Greenland ice cores, so-called "sub-events" (Rasmussen et al., 2014). We note that the moraine age coincides with the Antarctic Cold Reversal (ca. 14.7-12.9 ka) and hypothesize that cooling at that time may have occurred beyond the Southern Hemisphere. More geochronological data of LG moraine sequences are needed for testing this hypothesis.