Natural and anthropogenic sources of organic matter across Liao River delta: A combination of lipid biomarkers and isotope analyses

Organic matter (OM) cycling between coastal wetlands and their connected rivers is poorly constrained due to difficulties in assessing the composition of different OM sources (natural vs. anthropogenic). Bulk characteristics and lipid biomarkers were analyzed to distinguish different sources of OM in the sediments and soils of Liao River Delta, Northeast China, including Liao River Wetland, its connected Liao River, and nearby Daliao River. A similar range of stable carbon isotopic values (δ13Corg) was observed in wetland soils (−27.8‰ to −22.6‰) and river sediments (−26.0‰ to −23.3‰). In contrast, significantly higher stable nitrogen isotopic values (δ15N) were observed in Daliao River sediments (5.8‰–7.7‰) than both Liao River and wetland soils. Lipid biomarkers, especially n-alkyl lipids, phytosterols, triterpenoids, isoprenoids, monoalkyl glycerol ethers, and monoacylglycerols, indicated that the natural OM input in Liao River Delta was mainly of terrestrial origin, followed by in situ aquatic and microbial inputs. In addition to natural OM, anthropogenic influences in the form of sewage and petroleum inputs were evidenced by the detection of fecal sterols, plasticizers, and petrogenic biomarkers. Biomarker distributions in samples from Liao River and Liao River Wetland suggested similar OM sources or close interaction between them, which may be caused by lateral transport considering the low elevation delta exposed to strong tidal effects. In contrast, significantly higher anthropogenic inputs were detected in Daliao River, with no connectivity to the Liao River wetland. Taking advantage of isotopic and biomarker data, the principal component analysis further suggests that both the natural wetland distribution and anthropogenic activities may affect the OM sources and distribution in coastal rivers, which serve as an important transit of OM to coastal oceans.