INFERRED ENVIRONMENTAL CHANGES FROM GEOCHEMICAL AND ISOTOPIC ANALYSIS: PRELIMINARY STUDY OF A SEDIMENT CORE FROM LAKE IZABAL, GUATEMALA

Stable carbon and nitrogen isotopes (δ¹³C, δ¹⁵N) on organic matter, total organic carbon (TOC) and total nitrogen (TN) have been measured on a five meter sediment core from Lake Izabal, eastern Guatemala to reconstruct past environmental conditions. The core was taken from the interdistributary bay of the Polochic delta. The core was dated using radiocarbon dating of organic matter from three samples. The five meter core records ca. 225 years of sediment accumulation. The core is characterized by muddy homogeneous beds with intervals of thinly laminated and thinly bedded silty and sandy mud. The chemistry of organic matter in the core allows us to infer conditions of the lake system over a range of human population densities in the area surrounding the lake and its watershed, from low densities in the older times through current high densities. Geochemical and isotopic analyses of the organic matter reveal two general trends. First, from 503 cm (ca. 1800 A.C.) to 100 cm (ca. 1970 A.C.), there is a general decrease in TOC and TN values, and a trend toward more positive δ¹⁵N and δ¹³C values. Then, from 100 cm to the surface, there is an increase in TOC and in TN, and a negative trend in δ¹⁵N and δ¹³C values. C:N values decrease toward the top of the core. The observed increase in TOC, TN, and the decrease of δ¹³C in the uppermost 100 cm of sediment suggest increased productivity in the lake system. However, the trend toward lower δ¹⁵N values contradicts this interpretation and is likely a result of significant increase in TN observed in the upper part of the core. Our preliminary results indicate that Lake Izabal has undergone significant environmental changes, possibly as a result of an increase in human-derived organic matter delivered to the lake and a significant increase in use of fertilizer along the Polochic River. Our results may be useful to others trying to protect and manage the lake and its ecosystem.