Geochemical record of anthropogenic impacts on Lake Valencia, Venezuela

摘要

Bulk geochemical parameters and organic matter biomarkers in a short, high resolution gravity core (Lake Valencia, Venezuela) were examined to reconstruct anthropogenic impacts on the lake's conditions. During the period of ca. 1840-1990, sedimentary organic matter was characterized by high contents of total organic C (TOC) and total N (TN), low TOC/TN values as well as relatively enriched delta C-13 and delta N-15 signals, suggesting a primary autochthonous (algae and macrophytes) organic matter origin. The occurrence of large amounts Of C-23 and C-25 relative to C-29 and C-31 n-alkanes indicated substantial inputs from submerged/floating macrophytes. The variations of C-32 15-keto-ol, tetrahymanol, diploptene, C-32 bishomohopanol, 2-methylhopane, dinosterol and isoarborinol concentrations over the investigated period record changes in the planktonic community structure, including Botryococcus braunii, bacteriavore ciliates, cyanobacteria, Eustigmatophytes and dinoflagellates. A principal shift occurred in the 1910s when cyanobacteria and dinoflagellates became more abundant at the expense and decline of B. braunii and Eustigmatophytes, likely related to increasing anthropogenic activity around the lake. A second shift (less obvious) occurred in the 1960s when cyanobacteria became the sole predominant planktonic class, coinciding with further deterioration of lake conditions.