An isotopic biogeochemical assessment of shifts in organic matter input toHolocene 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 t o 5500 years to molecular and isotopic proxies for biological sources of or ganic matter to the lake sediments. Temporal shifts in the source of organi c matter to the sediment, as determined by the stable carbon-isotope compos ition of bulk organic matter and biomarkers, correspond with previously def ined regional scale transitions in forest ecosystems (Quercus to Pinus at s imilar to 5500 C-14 yr BP and Taxodium expansion at 2500 C-14 yr BP) and co incident increases in the Floridan water table. The delta C-13 values for t otal sedimentary organic carbon showed a shift from terrestrial and aquatic macrophyte sources (-27.8 parts per thousand, in sediments dated at simila r 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 del ta C-13 value, of bulk sediment and biomarkers representative of cyanobacte rial 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 bioma rkers 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 e ffective use as source proxies in this system. (C) 2001 Published by Elsevi er Science Ltd.