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

Authors

V Ramaswamy, National Institute of Oceanography, India
Birgit Gaye, University of Hamburg
P V. Shirodkar, National Institute of Oceanography, India
P S. Rao, National Institute of Oceanography, India
Allan Chivas, University of WollongongFollow
David J. Wheeler, University of WollongongFollow
Swe Thwin, University of Mawlamyine

RIS ID

24656

Publication Details

Ramaswamy, V., Gaye, B., Shirodkar, P. V., Rao, P. S., Chivas, A., Wheeler, D. & Thwin, S. (2008). Distribution and sources of organic carbon, nitrogen and their isotopic signatures in sediments from the Ayeyarwady (Irrawaddy) continental shelf, northern Andaman Sea. Marine Chemistry, 111 (3-4), 137-150.

Abstract

Total organic carbon (TOC), total nitrogen (TN) and their δ¹³C and δ¹⁵N 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 δ¹³C and δ¹⁵N 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 δ¹³C 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 δ¹⁵N 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.