Sedimentary records of the extent and impact of atmospheric contamination from a remote Siberian highland lake

The Central Asian Khamar Daban Mountain Range (SE Siberia) is a remote region with softwater lakes that probably receive atmospheric pollution from local industry as well as via long-distance transport. Palaeolimnological evidence of the impact of atmospheric pollution at one isolated and otherwise undisturbed highland site in the Khamar Daban, Lake Kholodnoye, was obtained. Short sediment cores were analysed for radio-isotopes, to establish a sediment chronology. The sediment accumulation rate in core KOL1 is slow (4 mg dry sediment cm^-2 yr^-1, or 0.3 mm yr^-1) but trace metal (notably Pb and Zn) and spheroidal carbonaceous particle profiles show increases beginning at 2-cm (1920s) and 1.3-cm (1940s) depth, respectively. The trace metal profiles are essentially corroborated by KOL2, a replicate core analysed at a Russian laboratory. Diatom analysis revealed no evidence of recent acidification or indeed of any marked floristic change during the twentieth century (upper 2.5 cm of sediment). Lack of catchment disturbance was indicated by very uniform pollen frequencies throughout the core. Lake-water chemistry indicated an excess of sulphate. Both the diatom and base cation stratigraphies showed a small but sustained change over the entire 28 cm of the core. Maximum floristic change in sedimentary diatom assemblages occurred around 7 cm sediment depth, approximately dated to the mid-eighteenth century. Small but gradual up-core increases in Na, K and Mg indicate a disturbance in the delivery of minerals from the catchment to the lake. The sedimentary profiles of trace metals and spheroidal carbonaceous particles show that the site is contaminated by low-level but nevertheless significant atmospheric pollution. This contamination occurred during the twentieth century. Small changes in the diatom and other geochemical sedimentary records predate these industrial impacts and so could reflect longer-term catchment weathering processes and/or indirect effects of climate change.