A historical perspective on recent biogeochemical trends in a high altitude lake from the central-Eastern Pyrenees (Lake Montmalus, Andorra)

The singularity of current environmental change in high mountain environments can only be tested by comparison with long-term time series beyond anthropogenic impact. Here, we present a 4-ka sedimentary record from Lake Montmalús (Andorra, 2,433 m a.s.l.), obtained through the integration of sedimentology, grain-size end-member modelling, bulk geochemistry, and the analysis of stable isotopes in organic matter (δ13CV-PDB, δ15NAIR) and heavy metals (Pb, Hg, As), alongside monitoring data. Pre-industrial fluctuations in detrital delivery and organic carbon burial largely reflect hydroclimatic variability, with increased transport occurring during the humid phases of the Iberian-Roman period (200 BCE-300 CE) and the Late Antiquity Little Ice Age (400-765 CE). In contrast, the last two centuries show unprecedented, synchronous changes driven by industrialisation and climate warming. Since around 1850 CE, and particularly after around 1960 CE, organic carbon burial has increased as C/N declined. Meanwhile, δ13CV-PDB and δ15NAIR have become more negative, indicating a stronger algal contribution to the carbon budget, intensified respiration processes, and longer summer stratification. The late 20th century marks a lake regime shift, with the onset of finely laminated deposition in the deep areas and persistently anoxic deep waters (Mn/Fe rise). Heavy metals (Pb and Hg) peaked in the industrial era (early 20th century) and declined after environmental regulations were enforced in the late 20th century. Arsenic shows a distinct late-modern enrichment after 1995 CE, reaching its maximum during the 2000s decade, decoupled from runoff proxies, and likely related to the activity of a garbage burning treatment plant nearby. Our results highlight the vulnerability of high-alpine lakes to the combined effects of warming, changing precipitation seasonality, and atmospheric pollution. They show that the lake regime change in the mid-20th century had no precedent in the previous 4000 years. The range of natural changes during millennia prior to industrial times also provide a baseline against which to evaluate resilience, and to design targeted monitoring of lake dynamics and contaminant deposition in these rapidly changing mountain environments.