On the origin of aged sedimentary organic matter along a river-shelf-deep ocean transect

Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Biogeosciences 124(8), (2019): 2582-2594, doi: 10.1029/2019JG005107.To assess the influences of carbon sources and transport processes on the 14C age of organic matter (OM) in continental margin sediments, we examined a suite of samples collected along a river‐shelf‐deep ocean transect in the East China Sea (ECS). Ramped pyrolysis‐oxidiation was conducted on suspended particulate matter in the Yangtze River and on surface sediments from the ECS shelf and northern Okinawa Trough. 14C ages were determined on OM decomposition products within different temperature windows. These measurements suggest that extensive amounts of pre‐old (i.e., millennial age) organic carbon (OC) are subject to degradation within and beyond the Yangtze River Delta, and this process is accompanied by an exchange of terrestrial and marine OM. These results, combined with fatty acid concentration data, suggest that both the nature and extent of OM preservation/degradation as well as the modes of transport influence the 14C ages of sedimentary OM. Additionally, we find that the age of (thermally) refractory OC increases during across‐shelf transport and that the age offset between the lowest and highest temperature OC decomposition fractions also increases along the shelf‐to‐trough transect. Amplified interfraction spread or 14C heterogeneity is the greatest in the Okinawa Trough. Aged sedimentary OM across the transect may be a consequence of several reasons including fossil OC input, selective degradation of younger OC, hydrodynamic sorting processes, and aging during lateral transport. Consequently, each of them should be considered in assessing the 14C results of sedimentary OM and its implications for the carbon cycle and interpretation of sedimentary records.This study was supported by Doc. Mobility Fellowship (P1EZP2_159064; R. B.) from the Swiss National Science Foundation (SNSF). This study was also supported by SNF “CAPS‐LOCK” project 200021_140850 (T. I. E.), by the National Natural Science Foundation of China (NSFC; grants 41520104009 and 41630966, M. Z.), and by the “111” project (B13030). We are grateful for support of the NOSAMS staff in the execution of this project. We also appreciate the assistance from Yushuang Zhang (Ocean University of China) at NOSAMS and members of the Laboratory for Ion Beam Physics at ETH Zurich for AMS measurements. We acknowledge Lei Xing, Haidong Zhang, Guodong Song, Meng Yu, Yonghao Jia, and Shanshan Duan (Ocean University of China) for sampling assistance on the cruises. Assistance at sea by the crews of R/V Dongfanghong II and R/V Hakuhu Maru is also acknowledged. Readers can access or find the data from figures and tables in the supporting information