Mercury Isotope Variability in Pyrenean Lake Sediments during the Late Holocene: Sources, Deposition, and Environmental Controls

Abstract

Atmospheric mercury (Hg) emissions represent a persistent global threat to ecosystems and human health. Stable Hg isotopes have emerged as powerful tools to trace historical pollution sources and reconstruct depositional pathways in natural archives. In this study, we present a 4000-year reconstruction of Hg isotopic composition from two Pyrenean lake sediment records (Lake Marboré and Lake Estanya) located along an altitudinal gradient and compare them with those of a nearby ombrotrophic peatland (Estibere mire). Both lakes exhibit a long-term increase in Hg accumulation rates and shifts in isotope values since the onset of the Modern Period (∼16th century), consistent with intensified anthropogenic emissions. However, the isotopic patterns differ: Lake Estanya, located in a lowland area with historical land-use changes, reflects a more localized Hg signal, whereas the high-elevation, remote Lake Marboré preserves a broader regional atmospheric imprint, dominated by wet deposition. The comparison with Estibere mirepristine and situated within the same air mass trajectory as Marboréreveals a consistent offset in Δ¹⁹⁹Hg values yet strikingly similar temporal trends, indicating a shared regional source signal modulated by ecosystem-specific processes. This multiarchive and multialtitude framework provides a rare opportunity to disentangle Hg source signatures from depositional and postdepositional transformations. Moreover, variations in even-MIF (Δ²⁰⁰Hg) in the alpine lake show the potential to reflect past climate phases, highlighting the additional value of Hg isotopes as paleoclimatic proxies. Our results underscore the importance of integrating different ecosystem archives to improve reconstructions of atmospheric Hg dynamics and to refine interpretations of legacy pollution and climate interactions.

Keywords: Pyrenees; isotopes; lake sediments; legacy pollution; mercury.

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Location of the study sites; (A) Map of Europe; (B) Digital Elevation Model for the Central and Eastern Pyrenees showing the location of Lakes Estanya (42°02′N; 0°32′E, 670 m asl) and Marboré (42°41′N; 0°2′E, 2612 m asl) and other Hg records mentioned in the text; (C, D) Photographs of the lakes and their watersheds.

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From top to bottom, Late Holocene mercury accumulation rates variability (HgAR) and Hg isotopic composition variability (δ²⁰²Hg, Δ¹⁹⁹Hg and Δ²⁰⁰Hg) in Lake Marboré and Lake Estanya. Color bands represent the main historical periods).

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δ²⁰²Hg vs Δ¹⁹⁹Hg plot for both Lake Marboré and Lake Estanya together with literature data: both MDF and odd-MIF increase along with contamination.

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(A) δ²⁰²Hg vs Δ²⁰⁰Hg diagram for both Lake Marboré and Lake Estanya together with typical wet (cloud waters and precipitations) and dry (GEM) deposition Hg isotope signatures in the Central Pyrenees; (B) Δ²⁰⁰Hg box plot with the mean Lake Estanya and Lake Marbore isotopic values and wet and dry deposition isotopic ranges in the Pyrenees. Wet deposition includes data from Precipitation-Pinet and from Cloud water–Pic du Midi. Dry deposition includes data from GEM-Pinet and from GEM-Pic du Midi.

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Δ¹⁹⁹Hg variability during the last millennium in Lake Marboré (blue) and Peat Estibere (from ref ) (purple) compared with estimations of mercury production in Spanish mines (from Hylander and Meili (2003)). Main historical and climatic phases are also indicated.