TERRESTRIAL VEGETATION CHANGE INFERRED FROM N-ALKANE DELTA-C-13 ANALYSIS IN THE MARINE-ENVIRONMENT

We report gas chromatography-isotope ratio monitoring-mass spectrometr y (GC-IRM-MS) measurements of the delta(13)C values of individual biom arker compounds (n-alkanes) extracted from a 3 m marine sediment core taken near the mouth of the Johnstone River, North Queensland, Austral ia. The technique allows a purely terrestrial isotope signal to be dis cerned despite mixing of terrestrial and marine-derived carbon. The re sults indicate that there has been a 2 parts per thousand increase in the delta(13)C values of terrestrially derived n-alkanes (C-29-C-33) s ince clearing of the forested Johnstone River drainage basin for sugar cane and pasture began in the late 19th century. A much slower similar to 1 parts per thousand increase in delta(13)C values after similar t o 6,000 years BP and prior to European settlement may be related to a decrease in rainfall in the basin, or to an increase in the abundance of C-4 plants as a result of increased aboriginal burning. The results from the sediment core are consistent with data obtained for modern r iver sediments from forested and cleared subcatchments within the basi n, and demonstrate that the delta(13)C values of terrestrially derived n-alkanes in the marine environment can be used to assess basin-wide vegetation changes in adjacent river catchments on geological timescal es.