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 similar to 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 similar to 5500 C-14 yr BP and Taxodium expansion at 2500 C-14 yr BP) and coincident increases in the Floridan water table. The delta C-13 values for total sedimentary organic carbon showed a shift from terrestrial and aquatic macrophyte sources (-27.8 parts per thousand, in sediments dated at similar to 5400 C-14 yr BP) within the basal peat, to a cyanobacterial-dominated sapropel (-18.1 parts per thousand) at the surface. A comparison of the delta C-13 value, of bulk sediment and biomarkers representative of cyanobacterial and algal input (e.g. 7- and 8-methylheptadecane and the n-alkane C-17) indicates that the present shallow lake was fully developed by similar to 2400 C-14 yr BP. Differences among the delta C-13 values of specific biomarkers derived from vascular plants (C-29 and C-31 n-alkanes and CuO lignin oxidation products) and from cyanobacteria and algae are nearly equivalent in magnitude to the shift recorded in delta C-13 of TOC, indicating their effective use as source proxies in this system. (C) 2001 Published by Elsevier Science Ltd.