Elemental (C/N Ratios) Isotope Compositions (delta C-13(TOC) and delta N-15(TN)) of Surface Sediments from the Barrier Islands in the Nakdong River Estuary, South Korea

Abstract

The Nakdong River Estuary (NRE) in South Korea is a typical, artificially manipulated estuary that is blocked by two large dams. The estuary is characterized by a barrier-lagoon system with various subenvironments and a microtidal domain with a mean tidal range of 1.07-1.50 m. The stable isotopic values (delta C-13(TOC) and delta N-15(TN)) and elemental (C/N ratios) compositions of organic matter, including those of halophytes, from the barrier islands were investigated in the NRE. In May (n = 94) and August (n = 90) 2015, sediment samples were collected on and around three islands in the NRE. The mean grain size of the barrier island system in the NRE ranged from 1.1 to 8.9 Phi (average 3.9 Phi) and the sediment types included muddy sand, sandy mud, and mud. The relative low C/N ratios (average 5.88) imply that the deposited organic matter originated from autochthonous algae derived-sources. The delta C-13(TOC) ranged from -28.2 to -16.8 parts per thousand(VPDB) (average -22.5 parts per thousand(VPDB)), with delta N-15(TN) in the range 6.2-12.5 parts per thousand(AIR) (average 9.4 parts per thousand(AIR)). Two halophytes (Phragmites communis and Scirpus planiculmis) had delta C-13(TOC) values that ranged from -29.0 to -25.3 parts per thousand(VPDB) (average 26.5 parts per thousand(VPDB)) at ES-R and ES-5. The isotope and elemental composition of sedimentary organic matter (delta C-13(TOC) and delta N-15(TN)) indicated the deposition of algae-derived organic matter with limited input from terrestrial organic matter. These results suggest that the microphytobenthos contribution was greater than the inflow of surrounding land vegetation and sediments to the north of the barrier islands had a marine origin. To the south, the effect was considered mixed. Further studies should determine the effects of end-member discrimination on the origins of the organic matter, such as the advance signature proxy. It is necessary to continue measuring sediments and water in the brackish water zone around the barrier system with on-site monitoring of dam discharge, tidal currents, waves, and, etc.