Distribution and sources of organic carbon, nitrogen and their isotopic signatures in sediments from the Ayeyarwady (Irrawaddy) continental shelf, northern Andaman Sea

Abstract

Total organic carbon (TOC), total nitrogen (TN) and their δ 13 C and δ 15 N values were determined from 110 sediment samples from the Ayeyarwady (Irrawaddy) continental shelf, northern Andaman Sea to decipher the concentration and source of organic matter. Comparatively higher TOC and TN concentrations are found in the inner-shelf mud belt, and on the continental slope sediments, whereas the outer-shelf sediments, composed mostly of relict sands, are low in TOC. The TOC contents are positively correlated with the abundance of fine-grained sediments. The TOC:TN ratios and δ 13 C and δ 15 N values show low variability within the modern inner-shelf mud belt and Gulf of Martaban, indicating similar source. The TOC:TN ratios are mostly between 6 and 8 in the inner-shelf mud belt and these values are similar to the suspended sediments in the Ayeyarwady and Salween rivers. The δ 13 C values of organic matter increase from − 25‰ in the Gulf of Martaban to about − 22‰ in the slope regions indicating decreasing terrestrial input away from the coast. The δ 15 N values on the Ayeyarwady shelf are rather low (+ 3.3 to + 4.8‰), especially off the mouths of the Ayeyarwady River mouths, reflecting greater influence of freshwater and terrigenous sediment discharge. A simple two end-member carbon mixing model applied to the Ayeyarwady shelf region indicates that terrigenous sources contribute more than 70% of the organic carbon in the modern mud belt in the inner shelf and Gulf of Martaban. Terrigenous organic carbon percentages reduce gradually offshore, reducing to less than 60% near the continental shelf edge. A strong terrigenous signal is preserved in the inner shelf and Gulf of Martaban sediments probably because organic matter from the source region is not subject to intensive processing and replacement in the floodplains and deltaic regions as well as rapid burial at sea.

Determination of rare earth, major and trace elements in authigenic fraction of Andaman Sea (Northeastern Indian Ocean) sediments by Inductively Coupled Plasma-Mass Spectrometry

“…5). The sediments of Andaman Sea are invariably fine-grained derived from Myanmar Rivers (Irrawaddy, Salween) and are responsible for the organic enrichments along the western Andaman Sea [27]. High TOC observed during the isotopic stages 2 and 3 are probably due to the enhanced productivity.…”

A geochemical record of environmental changes in sediments from Sishili Bay, northern Yellow Sea, China: Anthropogenic influence on organic matter sources and composition over the last 100years

“…Stable carbon and nitrogen isotope (d 13 C and d 15 N) ratios and the ratio of total organic carbon (TOC) to total nitrogen (TN) in sediments (TOC:TN) have been widely used to elucidate the source and fate of organic matter in the marine environment (Fry and Sherr, 1984;Mishima et al, 1999;Ogrinc et al, 2004;Ramaswamy et al, 2008). The TOC:TN ratio is an important indicator of the origin of organic matter, as there are obvious differences between different organic matter sources.…”

“…In contrast to the TOC:TN ratio, d 13 C only changes slightly during the sedimentation, degradation and transformation of organic matter in marine environments; thus, d 13 C has become a powerful tool in deciphering the origin of organic matter (Andrews et al, 1998;Wu et al, 2002;Ramaswamy et al, 2008). In addition, d 15 N in the sediments can reflect the deposition rate of terrestrial organic matter, the nitrogen source types for phytoplankton and decomposition processes (e.g., denitrification).…”

“…However, the fractionation of d 15 N during the absorption and assimilation of phytoplankton depends on the concentration and species of inorganic nitrogen (Owens, 1985;Mishima et al, 1999;Cohen and Fong, 2005). Normally, d 15 N is used as a subsidiary factor for special events; for example, an increase in organic matter d 15 N values can record deliveries of isotopically heavy nitrate (d 15 N = 10-25‰) from farm runoff and human sewage (Teranes and Bernasconi, 2000;Ramaswamy et al, 2008;Sampaio et al, 2010).…”

“…In comparison, the combined use of d 13 C, d 15 N and TOC:TN ratios can help improve the discrimination of organic matter sources from the confusion caused by various biogeochemical processes (e.g., microbial activity, denitrification), particularly in estuaries and coasts with a high mixture of terrestrial and marine matter (Matson and Brinson, 1990;Thornton and McManus, 1994;Andrews et al, 1998;Graham et al, 2001;Ogrinc et al, 2004;Ramaswamy et al, 2008).…”