Carbon cycling in a complex lake: a novel use of Δ14C

Abstract:

Changing deposition chemistry, land use and climate induced impacts on hydrology will affect soil biogeochemistry and terrestrial C export and hence lake ecology with potential feedbacks for regional and global C cycling. Globally lakes bury and remineralise significant quantities of terrestrial C, and the associated flux of terrestrial C strongly influences their functioning. Primary production (autochthonous production) in lakes is based on dissolved inorganic C (DIC). DIC in alkaline lakes is partially derived from weathering of carbonaceous bedrock, a proportion of which is ¹⁴C-free. The low ¹⁴C activity yields an artificial age offset leading samples to appear hundreds to thousands of years older than their actual age. Dissolved organic carbon (DOC) and particulate organic carbon (POC) can contain terrestrial inputs. The terrestrial inputs can be labile or detrital; their age depends on their depth in catchment soil/peat stocks. We present a pilot study that uses the radiocarbon (∆¹⁴C) method to determine the source of carbon buried in the surface sediment of Lough Erne, a humic, alkaline lake in northwest Ireland. ∆¹⁴C, δ¹³C and δ¹⁵N values were measured from phytoplankton and other biota, dissolved inorganic, dissolved organic and particulate organic carbon. A novel radiocarbon method, Stepped Combustion¹ was used to estimate the degree of the burial of terrestrial carbon in surface sediment. The ∆¹⁴C values of the low temperature fractions were comparable to algal ∆¹⁴C, while the high temperature fractions were ¹⁴C-depleted (older than bulk sediment). The ∆¹⁴C end-member model indicated that ~64% of carbon in surface sediment was derived from detrital terrestrial carbon. The use of ∆¹⁴C in conjunction with stepped combustion allows the quantification of the pathways of terrestrial carbon in the system, which has implications for regional and global carbon burial.

¹McGeehin, J., Burr, G.S., Jull, A.J.T., Reines, D., Gosse, J., Davis, P.T., Muhs, D., and Southon, J.R., 2001, Stepped-combustion C-14 dating of sediment: A comparison with established techniques: Radiocarbon, v. 43, p. 255-261.