How do recent depositional processes changes in high altitude Pyrenean lakes compare with Late Holocene variability? The case of Montmalús Lake (Principality of Andorra, Eastern Pyrenees)

High mountain lakes are pristine ecosystems that archive in their sediments high-resolution records of watershed and lake evolution. Understanding how they have responded to Holocene climate fluctuations and anthropic impacts provides essential information to put into a historical context the magnitude and unique features of the current global change.

The REPLIM project funded by the Interreg program (POCTEFA 2014-2020) has implemented a network of lakes in Spain, France and Andorra to study current and past climate, environmental and anthropic changes in lakes. In August 2017, eight short sediment cores were recovered in Montmalús Lake (2433 m a.s.l., Andorra). We present a paleolimnological reconstruction based on sedimentary facies, chemical (main and trace element) analyses and δ¹³C and δ¹⁵N of bulk organic matter. We developed a robust age model based on ²¹⁰Pb, ¹³⁷Cs and ¹⁴C dating for the last 2000 years. Moreover, modern lake dynamics have been characterized with continuous water temperature measurements at various depths, periodical water sampling and sediment traps.

The results show large depositional fluctuations in the lake dynamics, especially during the Little Ice Age, with changes in organic matter accumulation, bioproductivity and sources and increased sediment delivery. Sedimentological and geochemical indicators point to the onset of high human impact in the landscape around the 11^th century. Also, medieval mining and metallurgic activity from 11^th to 14^th centuries increased Pb deposition.

The 20^th century is characterized by a recovery in organic accumulation and bioproductivity rates. Heavy metal deposition also increased during the late 20^th century and started to decrease in the 2010’s decade, without reaching the background values

This study provides the first data on recent changes in lakes from the Principality of Andorra. The results highlight the uniqueness of current global change impacts in alpine lakes and underline the interplay of Great Acceleration and Global Warming processes in these fragile and sensitive depositional systems.