Changes in carbon and nitrogen cycling during tree-line retreat recorded in the isotopic content of lacustrine organic matter,western Taimyr Peninsula,Russia

Bulk organic and cellulose stable carbon isotope and bulk organic nitrogen isotope profiles from the sediments of a small tundra lake on the western Taimyr Peninsula, Russia, show changes that are correlated with climate cooling and treeline retreat at c. 4000 ¹⁴C years BP. Increased soil organic matter decomposition, combined with a moist climate, probably provided a ¹³C-depleted source of CO_2(aq) to lake phytoplankton thriving under favourable conditions during the forest interval. Increased concentration of CO_2(aq) and re-utilization of respired CO_2(aq) from organic matter settling through the water column may have also contributed to the relatively low δ¹³C values in the lower part of the lake sediment record. Bulk organic and cellulose carbon are more enriched in ¹³C after the boreal forest retreated probably because CO₂ from the atmosphere became the dominant source of carbon to the lake as soil organic decomposition rates declined, the lake became more oligotrophic, and the climate became colder and drier. Reduced concentration of CO_2(aq) may have also led to ¹³C-enrichment. Interpretation of the bulk organic δ¹⁵N record is somewhat more speculative but changes in drainage basin terrestrial vegetation, soil decomposition and hydrology also appear to have strongly influenced the lake water nitrogen cycling. Although considerably more effort is required to assess modern carbon and nitrogen isotope systematics, these promising results suggest that lake sediment organic matter δ¹³C, δ¹⁵N and cellulose δ¹³C combine to form useful tracers of past nutrient cycling in boreal tree-line watersheds.