Quantifying sediment supply at the end of the last glaciation: Dynamic reconstruction of an alpine debris-flow fan

In this paper we quantify the sediment dynamics in the formerly glaciated Ziel- bach catchment in the Italian Alps from the end of the Last Glacial Maximum (LGM) until today. As a basis for our quantifica- tion, we use the stratigraphic record offered by a 3.5 km2 large fan that we explore with a seismic survey, stratigraphic analyses of drillhole material, and 14C ages measured on organic matter encountered in these drill- ings. In addition, we calculate past denuda- tion rate variability in the fan deposits using concentrations of cosmogenic 10Be. We merge this information into a scenario of how the sediment flux has changed through time and how this variability can be related to cli- matic variations, framed within well-known paraglacial models. The results document a highly complex natural system. From the LGM to the very early Holocene, ice-melted discharge and climate variability promoted a high sediment flux (sedimentation rate up to 40 mm/yr). This flux then dramatically decreased toward interglacial values (0.8 mm/yr at 5–4 calibrated kyr B.P.). However, in contrast to the trend of classic paraglacial models, the flux recorded at Zielbach shows secondary peaks at 6.5 ka and 2.5 ka, with values of 13 mm/yr and 1.5 mm/yr, respec- tively. Paleo-denudation rates also decrease from ~33 mm/yr at the beginning of the Holocene to 0.42 mm/yr at 5 ka, with peaks of ~6 mm/yr and 1.1 mm/yr at 6.5 ka and 2.5 ka. High-amplitude climate change is the most likely cause of the secondary peaks, but anthropogenic activities may have contrib- uted as well. The good correlation between paleo-sedimentation and paleo-denudation rates suggests that the majority of the degla- ciated material destocked from the Zielbach catchment is stored in the alluvial fan.

In this paper we quantify the sediment dynamics in the formerly glaciated Zielbach catchment in the Italian Alps from the end of the Last Glacial Maximum (LGM) until today. As a basis for our quantification, we use the stratigraphic record offered by a 3.5 km2 large fan that we explore with a seismic survey, stratigraphic analyses of drillhole material, and 14C ages measured on organic matter encountered in these drillings. In addition, we calculate past denudation rate variability in the fan deposits using concentrations of cosmogenic 10Be. We merge this information into a scenario of how the sediment flux has changed through time and how this variability can be related to climatic variations, framed within well-known paraglacial models. The results document a highly complex natural system. From the LGM to the very early Holocene, ice-melted discharge and climate variability promoted a high sediment flux (sedimentation rate up to 40 mm/yr). This flux then dramatically decreased toward interglacial values (0.8 mm/yr at 5-4 calibrated kyr B.P.). However, in contrast to the trend of classic paraglacial models, the flux recorded at Zielbach shows secondary peaks at 6.5 ka and 2.5 ka, with values of 13 mm/yr and 1.5 mm/yr, respectively. Paleo-denudation rates also decrease from ̃33 mm/yr at the beginning of the Holo cene to 0.42 mm/yr at 5 ka, with peaks of ̃6 mm/yr and 1.1 mm/yr at 6.5 ka and 2.5 ka. High-amplitude climate change is the most likely cause of the secondary peaks, but anthro pogenic activities may have contributed as well. The good correlation between paleo-sedimentation and paleo-denudation rates suggests that the majority of the deglaciated material destocked from the Zielbach catchment is stored in the alluvial fan. © 2014 Geological Society of America.