The Hidden Threat of Glacier Melt: Rising Mercury Levels in French Alpine Lakes

The melting rate of glaciers in Europe has accelerated continuously since the 1980s and continues today at an unprecedented pace. Such rapid and sustained glacier retreat has important implications for the release of legacy contaminants stored in ice. Since the onset of the Industrial Revolution, atmospheric deposition has led to the accumulation of pollutants, including mercury (Hg)—a highly toxic element with well-documented impacts on ecosystems and human health—within glaciers. Ongoing climate-driven melting can remobilize these long-term contaminant reservoirs.
To investigate this process, we examined Hg accumulation rates (Hg AR) in sediments from two neighboring high-altitude lakes in the French Alps. One lake represents a reference system receiving only atmospheric inputs, while the other is influenced by both atmospheric deposition and meltwater from a shrinking glacier. Comparing the two sedimentary records allowed us to isolate the signal associated with cryospheric change.
In the reference lake, Hg AR is controlled by regional atmospheric Hg emissions and follows the expected anthropogenic pattern, with maxima during World War II and in the 1970s, followed by a steady decline in recent decades. In contrast, the glacier-fed lake shows a steadily increasing Hg AR from the early 1900s to the present, with a doubling of accumulation rates over the past several decades.
Using estimates of glacier volume loss over the last 50 years together with Hg concentrations in glacier ice and cryoconite reported in the literature, we demonstrate that the recent acceleration of Hg AR is consistent with enhanced Hg release driven by glacier shrinkage. These results indicate that glacier melt represents an additional and climate-sensitive source of legacy Hg to downstream aquatic systems, compounding the environmental impacts of cryospheric change alongside pressures such as freshwater scarcity.