Geochemistry of a tropical lake (Lake Leopoldo) on pseudo-karst topography within the Roraima Group, Guayana Shield, Venezuela

A geochemical assessment was performed on Lake Leopoldo, a tropical lake located on the Guayana Shield, Venezuela. The lake water was characterized (Ca2+, Mg2+, Na+, K+, Cl−, image, image, image, image, Fe, and P), lacustrine setting the origin, transport and fate of sedimentary material in this highly unusual lacustrine setting. Lake sediments were analyzed for textural, mineralogical and chemical composition, including the rare earth element (REE) contents. In addition to %TOC, δ13C, δ15N and elemental C/N ratios, the molecular distributions of several biomarker compounds were determined and used to assess organic matter source and transport to the lake. Lake Leopoldo developed on a sequence of quartz arenites of the Precambrian Roraima Group. The area conforms to the definition of pseudo-karst morphology. The results show that the lake is oligotrophic where primary production seems to be limited by nutrients. In the sediments, organic matter shows an overwhelming higher plant-derived origin. Certain elements (K, V, Cu, Th, Ba, Hg, and LREE) displayed enrichments with organic matter, while others (Zr, Hf, Si, Sn, Pb, and Ta) displayed an association with the mineral fraction. The results obtained in this study suggest that the lake water characteristics and the affinities of trace elements for sediment organic fractions are controlled by biogeochemical processes within the surrounding forest soil environment, prior to the transport of the terrestrially-derived organic matter to the lake. The most significant geochemical finding is that organic matter can play an important role in the distribution of trace metals in such highly oligotrophic terrains of humid, tropical environments.