Source And Transport Of Sedimentary Organic Matter In The Jiulong River Estuary And Adjacent Western Taiwan Strait

As the interface of the land and sea,the estuarine-coastal margins are the major sinks of the river-derived sediments associated terrestrial organic matter(TOM).Sedimentary organic matter(SOM)buried in estuarine-coastal margins are extremely heterogeneous due to the the sensitive and vulnerable responses to impacts from the influences of land-sea interaction,adjacent anthropogenic activities and regional climate changes.Therefore,it is of great scientific significance to study the origins and transport of SOM in the dynamic estuarine and coastal margins.In present work,surface sediments collected from the subtropical Jiulong River Estuary(JRE)and adjacent Western Taiwan Strait(WTS)were investigated.A comprehensive analysis of total organic carbon(TOC),black carbon BC),normal alkanes(ALKs),and polycyclic aromatic hydrocarbons(PAHs)combined with their stable and radiocarbon isotopic compositions(?13C and ?14C)were conducted.According to the multiple biogeochemical analyses of bulk organic carbon and source-specific molecular biomarkers,we aim to elucidate the spatial distributions,sources,transport,and dispersal of different TOM delivered from the JRE to adjacent WTS under the impacts of recent human disturbances.This work has yielded the following research achievements.The concentrations of TOC and BC in surface sediments of the JRE and adjacent WTS also showed a decreasing trend with increasing distance from the estuary.The decreased TOC/TN and ?14CTOC,and increased ?13CTOC and ?13CBC from the estuary to the coastal area,suggested that the contributions of the Jiulong River-derived TOM(higher plantdebris,soil and fossil organic matter)gradually decreased due to continuous degradation,while the contribution of marine-derived OM relatively increased.The concentrations of n-alkanes and PAHs in sediments also decreased with the ascending distance from the JRE to adjacent WTS.The compositions and molecular diagnostic ratios of ALKs and PAHs revealed that,the contribution from terrestrial higher plants was decreased during the fluvial transport processes,while relatively homogeneous spatial distributions of even-numbered short-chain n-alkanes with increased proportions in the coastal areas were most likely microbial products.The fossil or petrogenic OM was ubiquitous in the JRE and adjacent WTS,which was related to the the soil erosion,sewage discharge and port shipping activities in surrounding areas.The land-based marker of long-chain n-alkanes(nC27+29+31)and perylene,anthropogenic PAHs significantly correlated with TOC and BC in sediments of the JRE and adjacent WTS,reflecting the TOM in SOM was related to the fluvial inputs carrying the higher plant debris,soil erosion and surface runoff affected by fossil fuel combustion.The compound-specific stable carbon isotopic compositions of n-alkanes in sediments showed that the SOM was mixed by terrestrial plant debris,weathered sedimentary rock and or fossil organic matter.The 14C ages of long-chain n-alkanes(nC27+29+31)and their spatial variations of ?14C were very close to those of TOC,indicating that TOM was mainly composed of pre-aged soil OM enriched in higher plant debris.Furthermore,the 14C ages of long-chain n-alkanes in the WTS were significantly older than those of the JRE,further indicating that TOM was largely degraded during depositing into the sea.The old 14C ages of short-chain alkanes indicated the contribution from weathered sedimentary rocks or ancient fossil sources in SOM.The analysis of the bulk and molecular characteristics and their ?14C compositions showed that,the compositions of BC and n-alkanes and the 14C age of TOC in the sediments of the southern WTS were quite similar to those in the lower reach of the JRE.The 14C age of TOC in the northern WTS was significantly older than that in the southern WTS,indicating that the degradation of OM in the northern WTS was higher.The 14C age of short-chain alkanes with even carbon predominance(nC14+16+18+20)which represent microbial activities in the southern WTS was quite close to the lower reach of the JRE,indicating the microbial degradation of riverine fossil OM.Whereas,the 14C age of nC14+16+18+20 in the northward WTS was significantly younger than the southward WTS and more close to the 14C age of TOC and nC27+29+31,indicated that microbial communities utilize the pre-aged soil OM as carbon source In general,according to above discussions,TOM delivered by Jiulong River showed a preferential southward dispersal and deposited in the southern WTS,while the SOM in the northern WTS was mainly from the long-range alongshore transport of the southward Yangtze River-derived pre-aged soil organic matter driven by the Fujian-Zhejiang Coastal Current.