SEDIMENTOLOGICAL CHARACTERISTICS OF A LAMINATED CORE FROM LAKE IZABAL, EASTERN GUATEMALA

A radiocarbon dated sediment core from the central part of Lake Izabal, eastern Guatemala, contains a 310 cm laminated interval that covers the time interval between ~ 4200 to ~ 7500 cal yr BP. Such a record is rare in a shallow (~ 15 m), polymictic, tropical lake. We carried out a detailed sedimentological analysis of the entire core, focusing on 80 cm of the laminated interval to characterize the texture, structure, composition, and thickness of the laminations. We utilized a GEOTEK multi-sensor core logger to obtain high-resolution photographs, and density and magnetic susceptibility measurements along with an ITRAX X-ray fluorescence (XRF) scanner to obtain a high-resolution (0.5 mm) elemental composition of each lamina. The first two meters of the core (185 to 385 cm) are composed of brown homogeneous mud. X-radiographic images reveal faint laminations and complex bioturbation patterns. From 385 to 741 cm, the core is composed of light and dark laminations that vary in thickness, sedimentary texture, composition, and color. Light laminations contain clay size grains, scattered amorphous organic matter, and a few sponge spicules. Dark laminations contain clay size grains with scattered silt size grains, abundant amorphous organic matter, and a large amount of sponge spicules. XRF data reveals significant variability in composition between the laminations. For the 80 cm interval, we counted, labeled, and measured the light and dark laminations. Results show that the interval contains 3,960 laminations that range in thickness between ~ 0.1 and ~ 1.0 mm; light laminations are ~ 0.1 to ~ 0.9 mm thick and dark laminations are ~ 0.1 to ~ 1.0 mm thick. This study provides the preliminary observations of a remarkable record in the shallow sediments of Lake Izabal. Future studies, especially improving core chronology, will allow us to better understand the origin of such laminations.