Biophilic elements in core sediments as records of coastal eutrophication in the Seto Inland Sea, Japan

This study presents vertical profiles of biophilic elements in core sediments from the Seto Inland Sea, Japan. We determined total organic carbon (TOC), total nitrogen (TN), total phosphorus (TP), biogenic silica (BSi), and phaeopigments in sediments from 80-cm-long cores collected from Hiuchi-Nada (Site A) and Harima-Nada (Site B). The vertical profiles are generally similar: all concentrations decrease downward from the top layer and show maximum values in the top layers for each parameter at both study areas. Among these biophilic elements, the correlation between BSi and TOC is particularly high, suggesting that diatoms are the main source of TOC in the sediments. The degradation rates at Site A in increasing order, as estimated by a steady-state diagenetic model (one-G model), are TOC approximate to BSi < TP < TN approximate to phaeopigments. However, these rates differ substantially from those at Site B, suggesting that the degradation rates might differ among sedimentation processes and environments. Using the one-G model and molecular composition (C:N) ratios, we estimate the origin (terrestrial and marine) for the amount of carbon before decomposition and followed its variation over the past approximately 200 years. We found that marine carbon is a major component and tends to be higher during the eutrophication period (1960-1990) at Site B, although not so much at Site A. At Site B the marine carbon content increases by about 25% during the eutrophication period, but at Site A no remarkable increase can be confirmed. These results are consistent with previous studies showing that the trend in primary production in the Seto Inland Sea since the eutrophication period has not been regionally homogeneous. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

This study presents vertical profiles of biophilic elements in core sediments from the Seto Inland Sea, Japan. The study found that diatoms are the main source of total organic carbon (TOC) in sediments. Degradation rates vary significantly between different sites, suggesting differences in sedimentation processes and environments. Marine carbon content tends to be higher during the eutrophication period at Site B, but not at Site A, indicating regional heterogeneity in primary production trends in the Seto Inland Sea.