Isotope paleohydrology at the northern boreal treeline, Canada and Russia

Abstract

To improve our understanding of the relationship between the northern treeline region and climate change, a Canadian-Russian research project (Paleoecological Analysis of Circumpolar Treeline) was established in 1993. Research strategies primarily include analysis of lake sediment cores using a wide variety of sedimentological, biological and geochemical techniques. Oxygen and carbon isotope analysis on fine-grained lake sediment cellulose comprises an integral part of these multidisciplinary investigations, and have provided novel insight into Holocene paleohydrology and watershed carbon cycling in arctic Canada and Russia.

Separation of oxygen isotope effects caused by changes in lake water balance from the oxygen isotope composition of precipitation (018Op) in the lake sediment cellulose 018O records indicates that periods of treeline advance and climate warming in central Canada (~= 5000-3000 14C years BP) and central and eastern Russia (~= 8000-4000 14C years BP) were characterized by distinct changes in moisture conditions. During these intervals, summer relative humidity increased by about 10 to 15% in central Canada; central Russia also became wetter whereas a drier climate is associated with treeline advance in eastern Russia. Reconstruction of 018Op in central Canada displays a straightforward relationship with expected temperature change during the mid- to late Holocene. However, high 018Op values during the early Holocene, when mean annual temperature was probably similar to present, may instead reflect a small reduction in distillation of moisture in Pacific air masses traversing the Cordillera, perhaps associated with a higher summer:winter precipitation ratio.

Carbon isotope records in lake sediment cellulose suggest that lake carbon reservoirs at boreal treeline were strongly regulated by catchment hydrology as well as soil and vegetation development. During the moist periods of forest expansion in central Canada and Russia, terrestrial input of dissolved inorganic carbon to downstream aquatic ecosystems was an important process, supplying phytoplankton with non-limiting quantities of carbon. Exploratory investigations suggested that the nitrogen isotope composition of lacustrine bulk organic matter may also be useful for reconstructing nutrient dynamics in these watersheds.

Overall, these results confirm that lake sediment organic isotope tracers are sensitive to changing hydrologic conditions and are an effective approach for paleoenvironmental reconstruction. Notably, new information acquired from these studies concerning the response of watershed hydrology and carbon balance to natural climate variation provides a fundamental baseline for evaluating the impact of future climate change in northern regions.