Sources of Tsunami Deposits on the Pacific Coasts of Iwate, Miyagi, and Fukushima Areas by the 2011 off the Pacific Coast of Tohoku Earthquake, Northeast Japan Inferred from Geochemical Signatures (TOC/TN Ratio and Stable Sulfur Isotope)

Abstract

 Geochemical analyses (total nitrogen: TN, total organic carbon: TOC, total sulfur: TS, and stable sulfur isotope ratio: δ³⁴S) were performed on tsunami deposits by the 2011 off the Pacific coast of Tohoku Earthquake obtained from Iwate, Minamisanriku, Kesen-numa, and Sendai Bay areas for a total of 133 samples. Although TOC/TN ratios ranged widely (0.7–90), most of the samples (more than 80% of samples) ranged from 10–30, which were similar results to those for marine surface sediment in the Pacific coast area. Low δ³⁴S values (from -29.6‰ to -11.3‰) from Iwate, Minami Sanriku, and Sendai Bay tsunami deposits were observed. Generally, such low δ³⁴S values result from large isotope fractionation caused by the activity of sulfate-reducing bacteria in marine sediment under a sulfate (SO₄^2-) rich environment. These results suggest that the tsunami deposits were mainly derived from marine sediment. High correlations between TS and As contents (r=0.958) were observed in tsunami deposits from the Kesen-numa area. Furthermore, the high δ³⁴S values (from -3.1‰ to -0.7‰) in the tsunami deposits from the Kesen-numa area correspond well with those of hydrothermal ore samples from the Kesen-numa area. These results indicate that sulfur and arsenic in tsunami deposits from the Kesen-numa area originated in slag from the mines.