Cross shelf transport of terrigenous organic matter in surface sediments from outer shelf to Okinawa Trough in East China Sea

Abstract

Identification of sources and distribution of terrigenous organic matter (TOM) are significant to understand the carbon transport in East China Sea (ECS). However, sources of TOM buried in the ECS are diverse and complex, which are likely derived from the Changjiang River, Yellow River and/or small rivers from China mainland and Taiwan. In order to evaluate the fate of TOM offshore in the ECS, the study sampled 80 surface sediments from outer shelf-slope-Okinawa Trough (OT) region in the ECS. Both terrigenous and marine organic biomarkers were detected. The concentration (ng/g dry weight sediment) of terrigenous biomarkers including ƩC27 + C29 + C31 n-alkanes ranged from 12.2 to 990.9; while the content of marine biomarkers varied from 1.3 to 3891.2 for C37 alkenones, 2.6 to 922.6 for brassicasterol and 7.1 to 1535.8 for dinosterol. Bulk organic carbon and TOM distributions were controlled by the hydrodynamic conditions. The terrigenous and marine biomarker ratio (TMBR) (ƩC27 + C29 + C31 n-alkanes)/(ƩC27 + C29 + C31 n-alkanes + (brassicasterol + dinosterol +alkenones)) increased first and decreased from the outer shelf to OT spatially, showing the distribution pattern of relative proportion of TOM. Hotspots of significant deposition of TOM were observed in the ECS by δ13C and TMBR proxies that the mechanism was unclear yet. Binary model revealed higher percentage of TOM based on TMBR than δ13C for both slope (32.1% vs. 20.0%) and trough (23.1% vs. 15.2%) region. An underestimation of C4 vegetation contribution likely biased the bulk carbon model. Carbon preference index suggested a zonation distribution of TOM that outer shelf had different sources from the slope and OT attributed to cross shelf transport of terrigenous organic matter.