INVESTIGATING A POTENTIAL VARVED RECORD FROM THE NORTHERN NEOTROPICS

Varved sediments provide continuous and annually-resolved paleoclimate records. Due to their high resolution, they provide invaluable information on past climate and are essential for comparison to climate models at key intervals in Earth’s history. Most of the published varved records, however, are from high latitudes, resulting in a spatial bias in annually-resolved records with minimal availability in tropical regions. Here we report on the sedimentology of sediment cores from Lake Izabal, eastern Guatemala, that contain a remarkably laminated section with a total thickness of 256 cm. We utilized radiocarbon-based age-depth models integrated with lithological, micro X-ray fluorescence scanning at 40µm and 500µm resolution, and multivariate statistical analyses to investigate the origin, composition, and texture of the laminated interval. Lithological analysis indicates that the laminated section consists of dark-light laminae couplets that are on average 0.07 and 0.06 cm thick, respectively. The dark laminae are characterized by a high abundance of lithogenic minerals (quartz, plagioclase), a high abundance of organic matter components (plant cuticles), a low abundance of opaline silica (diatoms, sponge spicules), a high abundance of lithogenic elements (Ti, Fe, Rb), low Si/Ti, and high S/Ti, suggesting high erosion/precipitation, low productivity, and high abundance of organic debris. In contrast, light laminae are characterized by a lower abundance of lithogenic minerals, lower organic matter constituents, higher abundance of opaline silica, lower abundance of lithogenic elements, higher Si/Ti, and lower S/Ti, suggesting low erosion/precipitation, high productivity, and a lower abundance of organic debris. To investigate the potential annual character of the laminae, we compared Bayesian age-depth models and laminae counts. Age-depth models were built through 1,000 Monte Carlo (MC) iterations and indicate that the laminated sequence spans from ca. 4,100 to 7,800 cal yr BP with a mean sedimentation rate ca. 0.8 mm/yr. The resulting MC iterations were compared with the cumulative thickness of 5,464 laminae. Preliminary results indicate a relatively good match between laminae counting and the age-depth models, suggesting that the couplets might represent yearly depositional events.