Organic Carbon Dynamics beyond the Perspective of Monitoring: Impact of Historical Landscape Utilization on the Past Lake-Water Carbon Trajectory in Central Boreal Sweden

Abstract:

To date, the key drivers behind the recent observed increase in organic carbon (OC) concentrations in surface waters are still controversial. The lack of long-term monitoring data – over centuries and millennia – leaves us with an ambiguous understanding of the past trajectory of OC concentrations in surface waters, and inhibits a better mechanistic understanding of past and a reliable prediction of future changes in OC levels.

By using a paleolimnological approach, we reconstructed past lake-water total organic carbon (TOC) concentrations in lakes across the boreal landscape of central Sweden. Reconstructions are based on a transfer function between visible near-infrared spectra of surface sediments and the corresponding TOC concentration in the water column. Potential drivers behind changes in TOC were determined by a multi-proxy analysis of one of the studied lake sediment records including organic and inorganic geochemistry as well as biological proxies (pollen, diatoms).

Our results show a significant decrease in lake-water TOC beginning already ~550 years ago. This decline continued until the mid-20^th century when TOC concentrations started to increase again. These dynamics in TOC coincide with changes in proxies indicating catchment disturbance by human activities. The chronology of these changes corresponds to the expansion and decline of a landscape-wide system of summer forest grazing and farming in central Sweden from the 15^th century to the turn of the 20^th century. Frequent grazing and exploitation of forests and mires reduce aboveground vegetation and physically disturb soils. This further affects the carbon cycling by enhancing carbon turnover, reducing the thickness of organic soils and consequently altering the transport of OC from the catchment to lakes.

Our findings suggest that recent changes in lake-water TOC in Sweden are strongly associated with historical patterns in land use and not only on-going changes in climate or sulfur deposition.