Persistently high efficiencies of terrestrial organic carbon burial in Chinese marginal sea sediments over the last 200 years

Constraining the origins, transport history, and burial efficiency of terrestrial organic carbon (OCterr) accumulating in marine sediments is of fundamental importance for understanding the carbon cycle on a range of spatial and temporal scales. While there is abundant evidence that OC composition and age influences the sequestration of OCterr in surface sediments, little is known about longer-term controls on OCterr sources and burial efficiencies in response to natural and anthropogenic processes that influence marginal sea sediments. Here, we use bulk and molecular-level carbon isotopic (δ13C and Δ14C) measurements to examine depth-related variations in the sources, ages and burial efficiency of OCterr in a sediment core that captures deposition over the last 200 years in the central Yellow Sea mud area, the largest mud deposit in eastern Chinese marginal seas. The similar 14C ages of terrestrial higher plant long-chain fatty acid biomarkers (1830–2700 yr) to those of sedimentary OC (1890–3360 yr) suggest the continuous and dominant supply of pre-aged OC (OCpre-aged). Two carbon isotopic mixing models are used to apportion contributions from different terrestrially-derived OC pools. A dual carbon mixing model based on bulk OC and molecular δ13C and Δ14C values showed OCpre-aged and fossil OC (OCfossil) accounted for 52 ± 3% and 13 ± 2% of sedimentary OC, respectively; while a binary mixing model based on bulk δ13C values showed OCterr accounted for 45 ± 3% of sedimentary OC. Estimates of high burial efficiency for OC (~60%) and especially for the different terrestrial OC pools (>80%) over the last 200 years highlight that refractory millennial-aged terrestrial OC inputs from the Yellow River combined with stable depositional conditions promote OC preservation in the Yellow Sea. Notably, OCpre-aged and OCfossil exhibited contrasting fates, with higher burial efficiency for the former. These observations are attributed to differences in mineral associations, transport pathways as well as changes in sediment provenance and hydrodynamic regime. Furthermore, our results suggest that initial sequestration of OCterr in surface sediments presages long-term burial in deeper sediments despite the existence of temporal variability. Although not directly influenced by riverine discharge, persistent and efficient sequestration of millennial-aged terrestrial OC renders the distal marginal sea mud area a long-term carbon sink on (at least) centennial to millennial timescales.