Hydrodynamic processes and source changes caused elevated 14C ages of organic carbon in the East China Sea over the last 14.3 kyr

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• Provenance of organic carbon (OC) in ECS 14–0 ka determined from δ 13 C & Δ 14 C. • 14 C ages of OC are older than co-deposited sediments by 1.9 to 5.5 kyr. • Old 14 C OC ages due to pre-aged terrigenous OC & aging during sediment transport. More than 80% of marine organic carbon (OC) burial occurs in sediments of marginal seas. Sedimentary OC 14 C ages up to several millennia older than co-deposited coastal and pelagic sediments have been well documented but the cause of this phenomenon remains uncertain. We measured 14 C and 13 C contents of OC, along with the sedimentary content of terrestrial and marine lipid biomarkers, in sediment cores from the East China Sea to evaluate and quantify processes controlling OC ages over the last 14.3 kyr. We find that 14 C ages of OC were persistently older than co-deposited sediments by 1930–5530 yr. Temporal variations of the calculated apparent initial radiocarbon ages of total OC (TOC-AIR) mirrored sea level changes, with higher values (4570 ± 1250 yr) during the transgression (14.1–7.8 kyr BP) and lower values (3170 ± 670 yr) during the mid-late Holocene (7.8–0 kyr BP), suggesting that transgression-induced coastline retreat reduced the transport of pre-aged terrestrial OC to the marginal sea. However, bulk OC 14 C ages were consistently older than those expected from ternary mixing of Changjiang (Yangtze River), Huanghe (Yellow River) and marine sources based on δ 13 C and Δ 14 C end-members. We therefore propose that hydrodynamic processes during sediment transport and the addition of pre-aged OC from land and submerged coast were the main factors contributing to these old 14 C ages of OC and their temporal variations. During the transgression, higher TOC-AIR values were observed during times of lower sea-level, which suggests that erosion of coastal deposits during transgression might have contributed old OC. Since sea level stabilized 7.8 kyr BP hydrodynamic processes were the primary cause of high 14 C ages of OC. The significant role for hydrodynamic aging processes during the mid-late Holocene is hypothesized to result from longer transport distances between river mouths and sediment depocenters which can accommodate additional deposition-resuspension loops.