Historical shifts in mercury deposition in northeastern China: From vegetation to human activity

Mercury (Hg) pollution is a pervasive environmental issue that greatly threatens ecosystem and human health. However, the primary natural factors and human-induced phase shifts affecting Hg deposition in typical areas remain unclear. With its long history of industrialization, northeastern China is an important area for Hg pollution research. Here, we constructed an accurate chronological framework using varve counting combined with Hg concentration measurements to reconstruct the high-resolution Hg depositional history of Sihailongwan Maar Lake (SHML) in northeastern China over the past 1400 years. High (low) Hg flux in the SHML sediments was closely linked to an increase (decrease) in broadleaved tree coverage. During the Medieval Climate Anomaly, warming promoted the expansion of broadleaved trees, increasing Hg flux. Conversely, colder climates hindered broadleaved tree growth during the Little Ice Age, reducing Hg flux. However, Hg flux has significantly increased since the Current Warm Period (∼1850 CE) and was strongly correlated with regional population growth, causing a gradual increase in Hg deposition with increasing human activities and decreased natural input. Four-phase regime shifts in Hg flux influenced by anthropogenic activities were identified using the Sequential T-test Analysis of Regime-Shifts algorithm, highlighting the influence of government policies, economic development, and social changes on Hg deposition. This study elucidated the role of vegetation in northeastern China before large-scale human activities and the complex effects of human activities on Hg deposition.