Geochemistry of Negombo Lagoon sediments in Sri Lanka: implications for environmental monitoring

Negombo Lagoon is one of the largest lagoons in Sri Lanka. This study focuses on the geochemical evaluation of poorly understood tropical subtidal sediments of the western coast, Sri Lanka. Geochemical characteristics of sediments were examined using δ¹³C and δ¹⁵N stable isotopes, X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FTIR) analyses. The chronology was determined using accelerated mass spectrometry (AMS) ¹⁴C dates for undamaged two shells. Stratigraphic observations suggested grey to blackish fine-grained muddy clay sediments in the subtidal mudflat. The calculated organic matter and carbonate contents decrease slightly from bottom to surface sediments. Bulk C/N ratios and δ¹³C values suggested terrestrial organic matter dominant sediments such as from mangrove swamps mixed with marine dissolved organic carbon. XRD identified the crystalline phases present in sediments and thereby chemical composition can be identified by the software as illite, montmorillonite, and quartz. It indicates the deposition of limited chemical weathering materials from near distance sources. FTIR identified several functional groups such as carboxylic, polysaccharide, carboxylate ions, and aliphatic substances in the lagoon sediments. The variations of carboxylic groups/carboxylate ions indicate a slow rate of decomposition of organic matter during early diagenesis. Humification and decomposition proxies also indicate slow rates of decomposition under an anoxic condition. However, Negombo Lagoon still acts as a healthy ecosystem under the minor influence of sewage/agricultural nutrient contamination.