The deposition of Late Pleistocene and Holocene sediments in the
high-altitude lake Meidsee (located at an altitude of 2661 m a.s.l. in
the Southwestern Alps) strikingly coincided with global ice-sheet and
mountain-glacier decay in the Alpine forelands and the formation of
perialpine lakes. Radiocarbon ages of bottom-core sediments point out
(pre-) Holocene ice retreat below 2700 m a.s.l., at about 16, 13, 10,
and 9 cal. kyr BP. The Meidsee sedimentary record therefore provides
information about the high-altitude Alpine landscape evolution since the
Late Pleistocene/Holocene deglaciation in the Swiss Southwestern Alps.
Prior to 5 cal. kyr BP, the C/N ratio and the isotopic composition of
sedimentary organic matter (delta N-15(org), delta C-13(org)) indicate
the deposition of algal-derived organic matter with limited input of
terrestrial organic matter. The early Holocene and the Holocene climatic
optimum (between 7.0 and 5.5 cal. kyr BP) were characterized by low
erosion (decreasing magnetic susceptibility, chi) and high content of
organic matter (C-org > 13 wt.%), enriched in C-13(org) (>-18 parts per
thousand) with a low C/N (similar to 10) ratio, typical of modern algal
matter derived from in situ production. During the late Holocene, there
was a long-term increasing contribution of terrestrial organic matter
into the lake (C/N > 11), with maxima between 2.4 and 0.9 cal. kyr BP. A
major environmental change took place 800 years ago, with an abrupt
decrease in the relative contribution of terrestrial organic material
into the lake compared with aquatic organic material which subsequently
largely dominated (C/N drop from 16 to 10). Nonetheless, this event was
marked by a rise in soil erosion (chi), in nutrients input (N and P
contents) and in anthropogenic lead deposition, suggesting a human
disturbance of Alpine ecosystems 800 years ago. Indeed, this time period
coincided with the migration of the Walser Alemannic people in the
region, who settled at relatively high altitude in the Southwestern Alps
for farming and maintaining Alpine passes.