Are ice-contact lacustrine sediments in the Eastern Alps capable of constraining pre-LGM ice-stream network dynamics?

Mountain glaciers represent sensitive recorders of climate variability across a wide range of temporal and spatial scales. While their Lateglacial and Holocene dynamics are comparatively well constrained, glacier extent and behaviour prior to the Last Glacial Maximum (LGM) remain poorly documented in many mountain regions. In the Eastern Alps, the timing and magnitude of glacier advances during Marine Isotope Stage (MIS) 4 and MIS 3 are still debated, despite numerical glacier models predicting repeated pre-LGM advances into the Alpine foreland. This knowledge gap largely reflects (i) the scarcity of suitable terrestrial archives capable of recording such advances, (ii) the fact that organic material for radiocarbon dating is generally rare or absent in glacial settings and (iii) geomorphological evidence such as moraines is commonly eroded, reworked, or overprinted by subsequent glacier advances.

This study examines successions of ice-dammed glaciolacustrine sediments preserved in Alpine tributary valleys as alternative terrestrial archives for reconstructing pre-LGM glacier dynamics in the Eastern Alps. Ice-dammed lakes form when advancing trunk glaciers block the outlet of smaller tributary glaciers, creating temporary sediment traps that enable glaciolacustrine deposition. These ice-contact sediments record glacier advances and can survive multiple glacial cycles.

We focus on glaciolacustrine successions as ice-margin indicators and present a research approach that combines detailed sedimentological investigations with luminescence geochronology. The sedimentary architecture and stratigraphic relationships of ice-dammed sediments and associated delta complexes provide spatial constraints on an interconnected system of valley glaciers, including minimum ice-surface elevations and relative glacier extent. Chronological control is obtained using optically stimulated luminescence (OSL) and infrared stimulated luminescence (IRSL) dating of fine-grained quartz and feldspar (4–11 μm), respectively. Methodological challenges related to partial signal resetting are addressed using adapted bleaching-plateau tests [1], increasing confidence in the luminescence-based age constraints.

Our initial results from inneralpine sites indicate glacier advances which potentially reached into the Eastern Alpine foreland during MIS 4 and eventually MIS 3c, implying that glacier extent prior to the LGM was potentially more dynamic and spatially extensive than previously assumed. These results are consistent with numerical glacier model predictions [2].

From a methodological point of view, we conclude that (i) precise dating of ice-dammed lacustrine sediments enables reconstruction of the spatiotemporal dynamics of the associated ice-stream network, and (ii) luminescence-based lacustrine dating might complement geomorphological and cosmogenic-nuclide methods focused on lateral and frontal moraines used to delineate former ice margins.

 

References

1. Reimann, Tony; Notenboom, Paul D.; De Schipper, Matthieu A.; Wallinga, Jakob (2015): Testing for sufficient signal resetting during sediment transport using a polymineral multiple-signal luminescence approach. In: Quaternary Geochronology 25, S. 26–36. DOI: 10.1016/j.quageo.2014.09.002.

2. Jouvet, Guillaume; Cohen, Denis; Russo, Emmanuele; Buzan, Jonathan; Raible, Christoph C.; Haeberli, Wilfried et al. (2023): Coupled climate-glacier modelling of the last glaciation in the Alps. In: J. Glaciol. 69 (278), S. 1956–1970. DOI: 10.1017/jog.2023.74.