Upward decrease of organic C/N ratios in the Okinawa Trough cores: proxy for tracing the post-glacial retreat of the continental shore line

Six piston cores were collected along the NW–SE line in the Okinawa Trough close to the Chinese continental shelf edge and dated by 23 AMS 14C measurements, along with planktonic δ18O stratigraphy and an ash layer dated as ∼6.3 ka. On a time scale to ∼19 ka, the ratio of total organic carbon to total nitrogen concentration (C/N ratio) displays gradual decreases toward the present and suggests the decrease of vascular plant fragment input to the trough from continental rivers. This suggestion is confirmed by similar decreases of lignin phenol content and ratio of C28 fatty acid to C16 fatty acid content in core RN93-PC3. The gradual decrease of land plant fragment input is interpreted to be a consequence of the retreat of the continental coastline during the deglacial period.