GEOCHEMICAL CYCLES IN LAKE IZABAL, EASTERN GUATEMALA, IN RESPONSE TO MARINE INUNDATION

Global warming has triggered significant ice-sheet loss in polar regions with potential changes in global sea level. Low-lying coastal areas are at risk of marine inundation, yet the response of these biodiverse freshwater ecosystems to marine flooding is poorly studied. To explore potential biogeochemical changes in coastal freshwater ecosystems, we collected sediment cores from Lake Izabal, which is currently connected to the Caribbean Sea via the Dulce River. Sediment cores were dated using radiocarbon methods and analyzed using several proxies, including sedimentological descriptions, core scanning, stable isotopes (δ13C, δ15N), total sulfur (TS), and total organic carbon (TOC). We focus here on chemical processes recorded in Core 5, which has the best chronology. Marine flooding due to Early Holocene sea-level rise took place at ~8,300 cal yr BP, establishing a chemocline in the lake that lasted until ~4,800 cal yr BP. During this time, the sediments in Izabal are characterized by sub-mm laminae and minimal bioturbation. Total sulfur, elevated Cl/Ti, Fe/Ti, and Fe/Mn, along with an overall positive trend in both δ13C and δ15N values indicate hypersaline, alkaline, and reducing conditions. However, the δ13C record exhibits a pronounced negative trend from 6,500 to 5,300 cal yr BP, succeeded by a subsequent positive trend extending until ~4,800 cal yr BP, indicating a lake mixing event. Total organic carbon decreased also decreased substantially, followed by a subsequent recovery after 4,800 cal yr BP. An increase in the mineralization of organic carbon caused by high sulfate reduction rates resulted in a decrease in sediment TOC, suggesting a reduction in the potential carbon storage in the system. Our results indicate that marine flooding of freshwater ecosystems will likely result in significant changes in geochemical processes and reduce the system’s capacity to store carbon and that perturbed freshwater environments may necessitate thousands of years to recover once marine inundation occurs.