Structural features of humic acid of the coastal sediment in Ariake Sea tidelands: Use of humic acid as an environmental indicator for river basins and coastal regions

The structural features of humic acid (HA) at the sediment surface of the tideland at the Hayatsuegawa-river mouth at the Ariake Sea were investigated for the utilization of HA toward an environmental indicator of the features of the river basin and coastal region. ¹H NMR analysis revealed a high-content hydrocarbon residue with a similar type of terrigenous HA. Direct and methylation-pyrolysis-GC analysis suggested the incorporation of long-chain carboxylate in HA in the tidelands. The incorporation of branched-chain carboxylate residues in HA is the result of the microbial decomposition of detritus; these residues could be one of the characteristic structural features of HA in this area, which is rich in biodiversity and microbial activity. Because the structural features of coastal zone HA appear to reveal the characteristics and activities of the biological environment, these findings suggest the possibility of becoming an indicator of the detailed analysis of the structural features of coastal zone HA.