Carbonate–organic ¹⁴C age offsets in the East Sea shelf sediment

Radiocarbon ages of bulk sedimentary organic carbon (bulk OC) are often used on continental shelf. However, bulk OC ¹⁴C ages can be older than those of co-deposited carbonates. We examined this issue using two mud-dominated sediment cores from the East Sea shelf area: HI-24-05 GPC1 from the Korea Strait Shelf Mud (KSSM) (water depth, 110 m) and 22HP-01 from the Hupo Basin (water depth, 204 m). We measured AMS ¹⁴C ages of planktonic foraminifera (Globigerinoides sacculifer). Then, they were compared with bulk OC ¹⁴C ages from the same or adjacent depth intervals. We also measured TOC, δ¹³C_TOC, and TOC/TON. The sediments of two cores cover the Holocene (approximately 8 kyr BP). In both core records, ¹⁴C ages of bulk OC are consistently older than those of foraminiferal shell. Bulk OC ¹⁴C ages are ~800–1300 years older than those of foraminiferal carbonate. The offset is larger in the KSSM core than in the Hupo Basin core. In contrast, δ¹³C_TOC (approximately −21‰) and TOC/TON (7–9) vary within narrow ranges. We evaluate several possibilities that may cause the carbonate–OC ¹⁴C offsets, including organic matter source, bioturbation, particle residence time, and sediment redistribution on the shelf. The results may provide practical criteria for diagnosing carbonate–OC offsets and help to use radiocarbon for precise age control of sedimentary sequence.