Historical trends of metals and metalloids into lake and coastal sediments of Halong Bay (Vietnam).

Halong Bay (northern Vietnam) is heavily affected by human activities. Metals and metalloids (e.g., Al, Cu, Hg, Ni, Sb, Pb, V; MM) were used as proxies to assess the origin and transport of anthropogenic inputs. The chronology of inputs was examined according to different particle transport pathways using two sediment cores, one from coastal waters (HL) exposed to both atmospheric deposition and the Red River drainage, and the other from a small isolated lake on an islet (HT) receiving only atmospheric deposition. Higher accumulation rates were observed in HL (0.449 g cm⁻² year⁻¹) than in HT (0.069 g cm⁻² year⁻¹). In both cores, all anthropogenic MM fluxes are controlled by sedimentation processes regardless of their origin and transport pathways. Results show that coastal waters are strongly influenced by Red River drainage with specific events recorded in 1947 (Indochina War), 1960 (first five-year plan), 1974 and 1985 (flash flooding) and 2002 (Tuyen Quang Dam landslides). In HL core, the highest variabilities were observed for Cr, Hg, Ni, Pb and V with surface maxima of 67.7, 0.107, 26.0, 31.4, 82.2 μg g⁻¹, respectively. Geochemical ratios (Pb/Hg and V/Ni) showed specific signatures related to fossil fuel combustion. Estimated Pb deposition fluxes follow the increase in CO₂ emissions in the region.