Fjord sedimentary signature of the last surging phase of Pio XI Glacier (Chilean Patagonia)

Proglacial sediments hold continuous and high-resolution records of past glacier dynamics. In this study, we examine the sediments of Eyre Fjord (Chilean Patagonia, 49°S), which is fed by Pio XI Glacier, to gain a better understanding of how the surging phase of a growing glacier is recorded in marine sediments. Pio XI Glacier has experienced a net advance of >10 km since 1945 and has had several surging phases that each last 2 – 3 years and occur every ~14 years. The last reported surging phases happened between 1976 – 1979, 1997 – 1998, and 2014 – 2018. Thirty CTD profiles taken in the fjord along one longitudinal and three transverse transects in February 2019 show that sediment transfer through the fjord during a quiescent (i.e., non-surging) phase in summer primarily happens by means of widespread (unchannelised) hyperpycnal flows that are 20 – 100 m thick. To assess spatiotemporal variability in sedimentation throughout the fjord, nine sediment cores were collected in 2019. Concentrations of short-lived radionuclides suggest a sedimentation rate of 3 – 20 cm/year. Sediment grain size and magnetic susceptibility, (CT) density, and inorganic geochemistry (Fe, Ti, K, Mn, Zr, Zn, Rb, Sr), which were obtained at higher resolution, were used as proxies for hydrodynamic conditions in the fjord. The longest sediment record holds fine glacial mud and low density sediments between 135 – 70 cm and in the upper 10 cm, which indicate relatively low hydrodynamic activity, likely corresponding to the quiescent phases before 2014 after 2018. Between 70 cm and 10 cm, the overall denser and coarser sediments intercalated with cm-thick sandy layers indicate higher hydrodynamic activity and flood events, most likely representing the most recent (2014 – 2018) surging phase of the glacier. Interestingly, the thickest and coarsest flood deposits seem to be concentrated at the top of the sediment unit that represents the surging phase suggesting that the floods are the most intense towards the end of the surging phase. Overall, these results show that quiescent and surging phases of surge-type glaciers leave distinct sedimentary signatures in fjord sediments, offering the possibility to use longer sediment records to identify former surging phases.