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 balk organic nitrogen isotope profiles from the sediments of a small tundra lake on the western T aimyr Peninsula, Russia, show changes that are correlated with climate cool ing and treeline retreat at c. 4000 C-14 years BP. Increased soil organic m atter decomposition, combined with a moist climate. probably provided a C-1 3-depleted source of CO2(aq) to lake phytoplankton thriving under favourabl e conditions during the for-rst interval. Increased concentration of CO2(aq ) and re-utilization of respired CO2(taq) from organic matter settling thro ugh the water column may have also contributed to the relatively low delta( 13)C values in the lower part of the lake sediment record. Bulk organic and cellulose carbon are more enriched in C-13 after the boreal forest retreat ed probably because CO, from the atmosphere became the dominant source of c arbon to the lake as soil organic decomposition rates declined, the lake be came more oligotrophic, and the climate became colder and drier. Reduced co ncentration of CO2(aq) may have also led to C-13-enrichment. Interpretation of thr bull; organic delta(15)N record is somewhat more speculative but ch anges in drainage basin terrestrial vegetation. soil decomposition and hydr ology also appear to have strongly influenced the lake water nitrogen cycli ng. Although considerably more effort is required to assess modern carbon a nd nitrogen isotope systematics, these promising results suggest that lake sediment organic matter delta(13)C, delta(15)N and cellulose delta(13)C com bine to form useful tracers of past nutrient cycling in boreal tree-line wa tersheds.