An isotopic biogeochemical assessment of shifts in organic matter input to Holocene sediments from Mud Lake, Florida

A molecular and isotopic study of sediment cores from a sinkhole lake, Mud Lake, Florida, USA, was performed in order to relate documented changes in the regional terrestrial vegetation and water table over the last ∼5500 years to molecular and isotopic proxies for biological sources of organic matter to the lake sediments. Temporal shifts in the source of organic matter to the sediment, as determined by the stable carbon-isotope composition of bulk organic matter and biomarkers, correspond with previously defined regional scale transitions in forest ecosystems (Quercus to Pinus at ∼5500 ¹⁴C yr BP and Taxodium expansion at 2500 ¹⁴C yr BP) and coincident increases in the Floridan water table. The δ¹³C values for total sedimentary organic carbon showed a shift from terrestrial and aquatic macrophyte sources (-27.8‰, in sediments dated at ∼5400 ¹⁴C yr BP) within the basal peat, to a cyanobacterial-dominated sapropel (-18.1‰) at the surface. A comparison of the δ¹³C values of bulk sediment and biomarkers representative of cyanobacterial and algal input (e.g. 7- and 8-methylheptadecane and the n-alkane C₁₇) indicates that the present shallow lake was fully developed by ∼2400 ¹⁴C yr BP. Differences among the δ¹³C values of specific biomarkers derived from vascular plants (C₂₉ and C₃₁ n-alkanes and CuO lignin oxidation products) and from cyanobacteria and algae are nearly equivalent in magnitude to the shift recorded in δ¹³C of TOC, indicating their effective use as source proxies in this system.