Abstract
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.
Highlights
Tropical brackish ecosystems such as mangrove swamps, intertidal/subtidal mudflats, and coastal peatlands play an important role in the global biogeochemical cycles (Yu et al, 2010; Page et al, 2011; Gallego-Sala et al, 2018)
Negombo Lagoon sediments record mollusk shells, wood fragments, and black carbon. This observation suggests that sedimentary organic matter can mainly derive from allochthonous sediments that are transported by longshore currents along the western coast of Sri Lanka (Amalan et al, 2018; Ratnayake et al, 2019b)
The humification index is decreased slightly with the depth suggesting a slow rate of transformation of complex organic matter into simple carbon substances
Summary
Tropical brackish ecosystems such as mangrove swamps, intertidal/subtidal mudflats, and coastal peatlands play an important role in the global biogeochemical cycles (Yu et al, 2010; Page et al, 2011; Gallego-Sala et al, 2018). Limnological studies provide proxy information to reconstruct the delivery and decomposition changes in the burial of sedimentary organic matter during early diagenesis. Tropical subtidal sediments have received little attention in limnological studies on the modern time scale compared to mid- to high-latitude intertidal and subtidal sediments. The present study focuses to understand carbon sink potential, depositional, and decomposition characteristics of tropical subtidal sediments in Negombo Lagoon of Sri Lanka (Figure 1). The island of Sri Lanka consists of about 1770 km long coastal tract with diverse geomorphological features such as lagoons, bays, lakes, wetlands, and peatlands. Negombo Lagoon is one of the largest estuarine lagoons in Sri Lanka.
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